WO2013044978A1

WO2013044978A1 - Method of building an offshore power storage facility and corresponding offshore power storage facility

Info

Publication number: WO2013044978A1
Application number: PCT/EP2011/067124
Authority: WO
Inventors: Lars Stig Nielsen; Janus MÜNSTER-SWENDSEN
Original assignee: Seahorn Energy Holding ApS
Priority date: 2011-09-30
Filing date: 2011-09-30
Publication date: 2013-04-04

Abstract

An offshore power storage facility with a reservoir (701), wherein the reservoir encloses a volume (702) and is configured to withstand water pressure from surrounding sea water (703); wherein the offshore reservoir is configured with a pump (706) for draining water from the reservoir by use of energy and a turbine for generating energy when filling surrounding seawater into the reservoir. The reservoir comprises: a reservoir wall (801) that comprises multiple wall elements (107); a coherent foundation (102) comprising drilled interlocking columns (201), said interlocking columns comprising a mixture of seabed material and a cement material, where the foundation extends vertically from a level substantially corresponding to the seabed level and down into the seabed; and multiple piles (106) cast into the foundation and protruding from the foundation up into the reservoir wall, where the piles are interconnected to the reservoir wall. Further, there is provided a method of building such an offshore facility.

Description

METHOD OF BUILDING AN OFFSHORE POWER STORAGE FACILITY AND CORRESPONDING OFFSHORE POWER STORAGE FACILITY

This invention relates to the construction of an offshore power storage facility, the foundation for an offshore wall, a reservoir and a method of erecting the offshore wall on the foundation for an offshore power storage facility.

Background

Renewable energy is receiving much focus due to the dependency on fossil fuels of the current power supply and the mitigation of man-made climate changes. Renewable energy output from wind turbines, wave energy converters, solar panels and other renewable energy converters is intermittent or irregular depending on the presence and strength of wind, waves and sun light. Conventional fossil fuel operated power plants generate power regulated to meet an estimated demand by the consumers. Power is delivered to the consumers via existing distribution systems. However, in frequently occurring situations the demand from the consumers does not match the presence and strength of wind, waves and sun light. This causes difficulties since it is not possible to store significant amounts of power in conventional distribution systems.

Conventional fossil fuel operated power plants are regulated to constantly balance consumption and production of power. Further, a stable power supply is a necessity for a modern society and the power consumption is highly inflexible and follows, to a large extend, a daily cycle. Therefore, it is challenging to meet the power demand with energy from intermittent renewable resources. Power storage is a viable solution to this challenge. Power can be stored when renewable energy production is high and demand is low and be supplied to the power system when renewable energy production is low and demand is high. Thus power storage enables a higher share of intermittent renewable energy in a power system and reduces the need for backup capacity of conventional fossil fuel power production.

The storage of power as hydro potential is a well known and proven technology. Bui the conventional hydro power solutions including storage require an area to be dammed up, causing severe changes to the local environment. In many places the conventional hydropower has limited resources as it is competing for the land with for instance buildings and recreational areas. Further, there is an increasing pressure for renewable energy converters such as wind turbines to be located away from human living and recreational areas. But offshore constructions are often difficult and costly to construct, which can lead to increased prices of the energy delivered by the offshore facility. An offshore environment is less controllable than an on shore environment and weather is an important factor in offshore construction work. Often bad weather causes so-called downtime, where construction work has to be called off. Costly measures has to be taken to secure accurate handling and placement during construction work Thus, conventionally, many forces work against cost efficient production, conversion and storage of renewable energy offshore. Related prior art

WO2009123465 discloses the working principle of an offshore power plant. However, this document fails to disclose how to build such an offshore power plant in an efficient manner. Summary

There is provided a method of building at least a portion of an offshore reservoir for energy storage, where the reservoir encloses a volume and is configured to withstand water pressure from surrounding sea water. The offshore reservoir is configured with a pump for draining water from the reservoir by use of energy and a turbine for generating energy when filling surrounding seawater into the reservoir. The method of building at least a portion of an offshore reservoir for energy storage comprises: preparing, on an offshore location, a foundation that extends into the seabed; ferrying wall elements to the offshore location; lowering a respective wall element and placing it for interconnection with the foundation; erecting a plurality of said wall elements and interconnecting them to form at least a portion of a reservoir. The foundation is prepared by a series of operations comprising: lowering a drill into the seabed to loosen seabed material within a first bore, retracting the drill upwards, and adding cement material to mix with the seabed material to thereby make a first column with a first volume; making a second column with a second volume such that the first column and the second column interlock in a coherent foundation structure when the material within the first and second volumes cures; while the foundation is not fully cured, inserting piles down into the foundation and ending the insertion of a respective pile such that the pile protrudes from an upper portion of the foundation. The series of operations preparing the foundation are repeated such that multiple columns in the foundation interlock with a neighbouring column to form a coherent foundation. The step of erecting multiple wall elements comprises securing the wall element to a portion of a pile that protrudes from the upper portion of the foundation.

This configuration enables a fast and cost efficient erection of an offshore reservoir for storing energy. The in situ construction of the interlocking columns ensures that the foundation is a coherent structure, embedded within the naturally varying seabed. The second column is constructed the same way as the first column. The foundation is thus a coherent succession of multiple interlocking columns placed beneath the wall elements. When viewed from above, the columns of the foundation extend along a line, where said line is e.g. curved or linear or piecewise linear or a combination thereof. The foundation can be regarded as a line foundation. The coherent foundation has a reduced permeability compared to the original seabed, due to the mixing process of cement material and seabed material and the following curing process. When the material within the interlocking columns is cured, a foundation of interlocking columns is formed. The coherent foundation is thereby impeding the flow of water through the seabed underneath the reservoir. Impeding flow of water into the reservoir reduces the loss of the energy stored within the reservoir and thus increases the ability of the reservoir to store energy. Further, the reduced flow of water underneath the reservoir also reduces the risks of scouring around and underneath the reservoir wall, as there is less flow to wash away material. This is advantageous, as scouring can lead to foundation failure. Thus, the coherent foundation extending into the seabed reduces the costs of scouring protection of the offshore reservoir.

Further, the in situ construction of the foundation enables the piles to be inserted into the foundation, while this is not fully cured, e.g. within 24 hours from the construction of the foundation or before the foundation reaches about 40% of the so-called 28-days strength. Inserting the piles while the foundation is not fully cured is a cost-efficient way of establishing a connection with good load transferring capabilities between the piles and the foundation. The piles protruding from the foundation also provide a simple and fast method for interconnecting the wall elements and the foundation, by securing the wall elements to the piles; thereby, the time and costs of erecting a wall element are reduced. By securing is intended fastening the wall element to the piles such that the wall element and the piles cannot move relative to each other and such that the connection between the wall element and the foundation is impermeable to water, e.g. by an in situ casting process. This fast and simple erection method enables the cost saving measure of prefabricating or precasting the wall elements.

By precasting the wall elements on shore, it is possible to manufacture the wall elements by weather protected series production with conventional manufacturing means. The standardized manufacturing process of precast constructions ensures low cost and homogenous quality of an offshore reservoir comprising precast wall elements. The prefabricated or precast wall elements are then transported to an offshore erection site for onsite assembly and installation. By prefabricated is intended that components of the wall elements are manufactured and/or assembled in an onshore or near- shore facility, such as inside a harbour, and that the wall element is then transported to the erection site of the offshore reservoir. By precast is intended that the wall elements cast in an onshore or near-shore facility, such as inside a harbour, and then transported to the erection site of the offshore reservoir. Prefabricated and/or precast wall elements are still subjected to assembly work at the erection site of the reservoir, such as in situ grouting or casting. The terms 'wall element' and 'precast wall element' are used interchangeably in the present application.

Further, the piles, protruding from the foundation, work as shear keys and transfer the large forces acting on the wall element from damming up water to the foundation. The piles can be inserted in many ways, e.g. by lowering, pressing, hammering, vibrating, drilling or a combination thereof. As the piles extend into the foundation, the forces transferred through the piles are distributed to a large portion of the foundation, thereby reducing load concentrations within the foundation which enables the foundation to carry a larger load. As the foundation extends into the seabed, the forces transferred through the piles are transferred deep into the seabed where the strength of the seabed increases compared to near the seabed surface. Further, as the foundation utilizes the deeper layers of the seabed for support, the foundation has a smaller footprint at the seabed surface. This is in contrast to offshore foundations utilizing a large area of the seabed surface to distribute the forces on to e.g. a gravel bed foundation. Foundation methods with a large seabed surface footprint require extensive seabed preparation such as trenching and construction of a levelled gravel bed. These are costly operations that are reduced or avoided with the present invention. Further, the construction method of the foundation of the present invention enables the level of the top of the foundation to be adapted to the level of the naturally varying seabed.

Thus, the coherent foundation integrates the functionality of foundation and water sealing within the seabed into one coherent structure with minimum footprint that is constructed in the naturally varying seabed. The piles transfer the large forces acting on the reservoir wall to the foundation, thus enabling the reservoir wall to dam up water.

Utilizing on-site seabed material as an aggregate material for mixing with cement material is a cost-efficient method of constructing an offshore foundation, where the production process and installation process are combined into one operation, utilizing the surrounding seabed as mould for curing of the foundation. Constructing the reservoir from prefabricated wall elements enables onshore series production of the wall elements. Thereby costs are reduced and quality improved compared to wall elements constructed in situ at the offshore location, where weather downtime and offshore conditions increase costs. The wall elements are interconnected to form a reservoir wall. The interconnection comprises establishment of a sealed contact between neighbouring wall elements, where the contact reaches at least from the bottom of the wall elements to sea level, such that the reservoir wall, comprising the interconnected wall elements, is able to dam up water. A reservoir wall can be constructed from similar wall elements or from different types of wall elements. By 'sealed' is intended impermeable to water under a pressure corresponding to that at a water depth equal to the height of the offshore reservoir wall. By 'reservoir wall' is intended the wall that forms the reservoir, constructed from wall elements and possibly other supporting structures. The pump configured for draining water from the reservoir using energy and the turbine configured for generating energy when letting water into the reservoir enable the reservoir to store energy. The energy for the pump can be produced e.g. from electricity by an electrical motor, and the energy from the turbine can e.g. be converted into electricity by an electrical generator, thereby enabling the reservoir to store electricity.

The offshore reservoir can be operated e.g. with alternating periods: with a period where the reservoir is drained by the pump, where the pump consumes energy; and a period where the reservoir remains in an at least partially drained state; and a period where the reservoir is filled, where the turbine generates energy. These periods can e.g. be controlled by energy prices and/or energy demand.

Thus, the coherent foundation with protruding piles is a cost-efficient foundation for an offshore reservoir for damming up water, and the offshore reservoir for storing energy can be erected in a fast and cost efficient way.

In an embodiment, the foundation is constructed in a continuous process. Thereby, a neighbouring column is constructed while the material within the first column is still wet, such that material within both columns is mixed to form a coherent foundation structure when the material cures. This ensures a strong foundation that impedes flow of water through the seabed along the entire length of the foundation. As the columns of the foundation are constructed by a drill, the columns are substantially cylindrical. In an embodiment, all the columns of the foundation are substantially equal in diameter. Thereby, the columns of the foundation have substantially similar properties and can be created by drills with the same diameter, which makes the construction process simpler. In an embodiment, preparing the foundation comprises creation of multiple neighbouring interlocking columns simultaneously by use of multiple overlapping drills, e.g. 2 or 4 or 6 or 8 overlapping drills. Thereby good connection between said multiple neighbouring columns is ensured and construction speed increased resulting in lower costs. By overlapping drills is intended that the distance between the centreline of the drills is less than the drill diameter, such that the bores created by the overlapping drills share a common volume.

In an embodiment, the insertion of the piles, designated for connection with a respective wall element, is ended such that the tops of the piles are substantially at the same level, e.g. with a difference of less than 1 cm or 5 cm or 10 cm or 20 cm. Thereby, the construction of the wall element is simpler, as the piles with which it is designated to be interconnected are substantially at the same level.

In an embodiment, a pile is inserted into every column of a coherent foundation, thereby providing shear key connections that are distributed along the length of the wail element. This reduces load concentration in the reservoir wall near the piles compared to a lower number of piles. Further, as the loads on the wall element from damming up water are distributed along the length of the wall element, shear keys distributed along the wall element provide good resistance towards these loads.

In an embodiment, the portion of the piles protruding from the foundation acts as male parts, and the wall elements contain hollow compartments in the bottom surface acting as female parts, in the interconnection between the wall element and the foundation. Thereby, a strong and durable interconnection is ensured such that the forces acting on the wall element are efficiently transferred to the foundation. Further, the designated position of the wall element is defined by the piles protruding from the foundation. Aligning the hollow compartments with the piles protruding from the foundation ensures a correct position of the wall element. This makes correct positioning of the wall element easier, as it is clear when the wall element is in correct position.

In an embodiment, both the interlocking columns forming the foundation and the piles protruding from the foundation are substantially vertical. This makes the reservoir construction simpler, as the interlocking foundation columns are easier to construct when they are substantially vertical, and the wall elements are easier to interconnect with substantially vertical piles.

In an embodiment, a wall element is interconnected with the foundation, when the foundation is sufficiently cured to support the wall element. This can be e.g. after about 28 days or 20 days or 15 days. The longer the period, the higher is the strength of the foundation, but waiting prolongs the construction period and thus increases the costs.

In an embodiment, the length of the coherent foundation is at least the length of the wall element. Thereby, it is ensured that there is foundation along the entire wall element. Further, a coherent foundation running underneath the entire length of the wall element will impede the flow of water underneath the wall element in the entire length of the wall element, such that the wall element can effectively dam up water when the reservoir is finished.

In an embodiment, the step of ferrying a wall element to the offshore location comprises: towing the wall element with boats; where the wall element is floated by enclosures within the wall element filled with air or by external buoyancy means or a combination of the two. Said external buoyancy means are floating devices attached to the wall element before the ferrying, e.g. a buoy or a rigid shell structure. Towing a floating wall element makes the process of getting the wall element from the manufacturing facility to the offshore location easier and less costly compared to conventional offshore transportation on a boat or a barge. This is because the heavy wall element does not need to be lifted onto a boat or a barge at the manufacturing site, and off again at the offshore location; both of which operations require expensive high-capacity lifting equipment. Other means of transportation than a floating wall element are possible, e.g. ferrying the wall element on a boat or a barge.

In an embodiment, external buoyancy means are attached to a wall element by use of cables and the wall element is floating in an upright position, where the lowering of the wall element comprises: slackening said cables such that the wall element slowly descends towards the seabed. Thereby, the external floating means perform the dual purpose of floating and lowering of the wall element. Further, the wall element is lowered to its designated position on the foundation in an upright position. Thereby the need for expensive heavy-duty offshore vessels for handling and lowering the wall element is reduced. By 'designated position on the foundation' is intended 'the position that the wall element is in when it is ready for interconnection with the foundation'. In an embodiment, the step of positioning a wall element is performed by a catamaran vessel, where the wall element is positioned between the hulls of the catamaran vessel during the positioning process. Thereby, the wall element is somewhat protected from wave impacts during the positioning as the catamaran vessel extends down both sides of the wall element. Further, the hulls of the catamaran vessel can connect to both sides of previously erected wall elements, which helps to keep the catamaran vessel in a position for positioning the wall element correctly. By 'catamaran vessel' is intended a vessel which has two hulls that are connected such that a space is created between the hulls, wherein a wall element can enter into said space in a floating position from at least one end of the catamaran vessel.

In an embodiment, placing a wall element for interconnection with the piles comprises: cleaning the top portion of the foundation from seabed material, by use of e.g. a jet of water. Thereby, a good connection is ensured, as the connection is not weakened by unwanted seabed material.

In an embodiment, the interconnection of a wall element with a portion of the pile that protrudes from the upper portion of the foundation comprises an in situ cast interconnection which connects the wall element to said protruding portion of the pile and to the top of the foundation.

In an embodiment, a wall element has a height that makes it extend above the sea level, at the location of the reservoir, when it is placed at its designated position on the foundation. Thereby, the wall element separates the sea into a sea side and a reservoir side when the wall element is installed on the foundation. This enables a reservoir constructed from such wall elements to dam up water.

In an embodiment, a wall element is configured for a water impermeable connection with an upper wall element, along a substantially horizontal division, when the upper wall element is placed on top of the first wall element. Thereby, both the wall element and the upper wall element are able to have a height that is lower than the depth of the sea at the designated position of the wall element. This makes the manufacturing, transportation and installation process of the both the wall element and the upper wall element easier and less costly. The water impermeable connection enables the reservoir wall to be constructed from multiple wall elements stacked on top of each other while retaining its ability to dam up water.

In an embodiment, the wall element comprises a lower portion which is substantially vertical and an upper portion which is inclined relative to the vertical, where said upper portion is configured to break the waves of the sea. Thereby, the peak load magnitude on the wall element from waves is reduced since waves impacting on the inclined portion of the wall will have a reduced peak load magnitude. The upper portion inclined relative to the vertical is smaller than the substantially vertical portion, e.g. having a vertical height of about 5%, 10%, 15% or 20% of the height of the substantially vertical portion. The inclination angle of the inclined portion is less than 45 degrees relative to the vertical, e.g. about 15 degrees, 25 degrees or 35 degrees. In an embodiment, a wall element or a portion thereof is made from reinforced concrete. Thereby, the wall element is constructed from materials with high strength and durability and relatively low cost. Further, concrete is suitable for the corrosive offshore environment and thus provides good protection for any reinforcement not well suited for the corrosive environment, e.g. steel, that is embedded within the concrete.

In an embodiment, a pile protruding from the foundation is made from steel. Thereby, it has high strength with regard to shear, tension and compression and a high ductility. This enables good load transfer through the pile and thus a good load transfer between the wall element and the foundation. The cross section of the pile is one that provides a high stiffness to material ratio e.g. a hollow pile, a hollow cylinder, an I-beam shape or an H-beam shape.

In an embodiment, a pile protruding from the foundation is made from reinforced concrete. Thereby, the pile is constructed from relatively inexpensive materials. Further concrete is relatively insensitive to the corrosion of an offshore environment. A pile of reinforced concrete can be assembled e.g. from multiple shorter pile pieces with post tensioning techniques. By 'post tensioning' is intended applying tension to cables or rods running through channels in the structure after the structure is constructed. Thereby, the structure is held together and reinforced by the cable or rods. The channels can subsequently be filled with a filler material, e.g. grout. in an embodiment, a pile protruding from the foundation is made from fibre reinforced polymer. Thereby, the pile has a high strength to weight ratio which is advantageous during transportation and installation of the pile. Further, a fibre-reinforced polymer pile is well suited for the corrosive offshore environment. The fibres can e.g. be glass of carbon.

In an embodiment, multiple piles are interconnected before they are inserted down into the foundation, e.g. 2 or 4 or 6 or 8 piles are interconnected. Thereby, the process of inserting the piles is fast as more piles are inserted simultaneously, and the piles can support each other while the foundation material is curing.

In an embodiment, neighbouring wall elements are positioned with adjacent respective ends and the neighbouring wall elements are joined together by means of a wet or dry interconnection to establish a joint. Generally, joints can be classified as wet and dry. Wet joints are constructed with cast-in- place concrete poured between the precast wall elements. To ensure structural continuity, protruding reinforcing bars from the wall elements (dowels) are welded, looped, or otherwise connected in the joint region before the concrete is placed. Dry joints are constructed e.g. by bolting or welding together steel plates or other steel inserts cast into the ends of the precast wall elements for this purpose. Wet joints more closely approximate cast-in-place construction, whereas the force transfer in structures with dry joints is accomplished at discrete points. Combinations of wet and dry joints are also viable for interconnecting wall elements. Joining the neighbouring wall elements to each other makes the erection of the reservoir fast and simple as no additional elements are needed to connect the neighbouring elements in a sealed joint. The sealed joint may comprise e.g. gaskets or a casting process or sealing plates or flexible bags filled with a fluid filler material with a curing ability or a combination of these. In an embodiment, neighbouring wall elements are overlapping. Thereby, the interconnection of the wall elements is made simpler as the wall elements have a larger connection area.

In an embodiment, the wall elements are straight and the coherent foundation consists of multiple straight portions, each substantially the length of one or more wall elements. Thereby, lowering reservoir costs, as a coherent foundation consisting of straight portion has lower construction costs than a curved coherent foundation and straight wall elements have lower manufacturing costs compared to curved wall elements.

In an embodiment, the offshore reservoir substantially forms a regular polygon when viewed from above. Consequently, the reservoir provides the highest possible reservoir volume for a given reservoir wall length, when considering straight wall elements. Straight wall elements are easier and thus less costly, to manufacture, compared to wall elements describing an arc. A higher reservoir volume means that more energy can be stored within the reservoir. In an embodiment, the substantially regular polygon formed by the reservoir has more than 12 sides, e.g. about 14 sides or 20 sides or 30 sides.

In an embodiment, the reservoir wall has a height of at least 15 meters. Thereby, the reservoir wall enables the construction of an offshore reservoir with a possible height difference of more than 10 meters between the inside reservoir and the surrounding body of water, which is necessary for the reservoir to be of commercial interest. In an embodiment, a wall element is longer than 20 meters or 40 meters or 60 meters or 80 meters. Thereby, the total reservoir costs are reduced as the longer the lengths of the wall elements, the fewer wall element transportations and installations are necessary for a given total reservoir wall length. Transportation constraints on size and weight are less important on sea compared to land transportation. However, the longer the lengths of the wall elements, the more difficult they are to handle during manufacturing, transport and installation. Thus the length of a wall element depends on manufacturing facilities and transport and installation equipment available. In an embodiment, a wall element consists of multiple shorter wall segments assembled to a single wall element by post tensioning at the manufacturing site. Thereby, the manufacturing process is easier and thus less expensive as the shorter segments can be handled better during the manufacturing process and can more easily be cast in one continuous process, which ensures a strong element

In an embodiment, the offshore reservoir substantially forms a circle when viewed from above. Consequently the reservoir is providing the highest possible reservoir volume for a given reservoir wall length. The wall elements are arched, with an arc radius substantially equal to the reservoir radius. The wall element can for instance be arched as disclosed in connection with co- pending application with the title "A wall element system for an offshore power storage facility", filed on the same day by the applicant of this present application, e.g. in connection with figure 6a-6d and the description thereof on page 19-21 and 41-42.

In an embodiment, a wall element is arched such that it bulges out towards the surrounding sea and the portion of the coherent foundation, which the wall element connects to, substantially follows a line with the same curvature as the wall element. Thereby, the wall element is stronger towards the forces from the surrounding sea, as an arched wall has less internal shear forces compared to a straight wall.

In an embodiment, the cement material is added to mix with the seabed material through the shaft of the drill. By 'drill' is intended an apparatus that penetrates into the seabed soil and rotates.

In an embodiment, the drill comprises means to mix the cement material with the seabed material. Thereby, a more homogenous, and thus stronger, foundation is ensured. Said means to mix can be e.g. mixing blades, mixing paddles, auger flights or other things protruding from the shaft of the drill to increase mixing of the materials.

In an embodiment, the foundation extends at least 5 m into the seabed, e.g. 10 m, 20 m, 30 m or 40 m. Thereby, a foundation with sufficient foundation capacity and sufficient sealing within the seabed is ensured. Further, a foundation that extends deep into the seabed reduces the risks of scouring around and underneath the reservoir wall. Thus, costs of additional scour protection are reduced. In an embodiment, the foundation extends into the seabed to a depth where the surrounding seabed material has a hydraulic conductivity of 10^("7) m/s or lower. Thereby the natural low impermeability of that layer is used to advantage to keep the flow of water underneath the reservoir to a minimum. in an embodiment, the foundation is prepared from a jack-up vessel, where the same jack-up vessel is inserting the piles into the foundation. Thereby, the steps of preparing the foundation and inserting the piles are performed by the same vessel, where the vessel remains in position due to the jack-up system. This ensures the quality of the coherent foundation while keeping the offshore installation costs down by performing multiple steps of building the reservoir by the same vessel kept in the same position. in an embodiment, a pump for draining water from of the reservoir and a turbine for generating energy when filling water into the reservoir are configured to handle more than 50 m³ of water per second.

In an embodiment, a pump for draining water from the reservoir and a turbine for generating energy when filling water into the reservoir are integrated into one pump/turbine unit. Thereby the use of space and materials is reduced as fewer units are required. This reduces the total reservoir costs.

There is also provided a method of building at least a portion of an offshore reservoir for energy storage, said method comprising: preparing a further coherent foundation in the seabed, such that the reservoir has a first and a second coherent foundation. The second coherent foundation is located on a reservoir side of the first coherent foundation, and the reservoir wall comprises means to convert at least a portion of the overturning moment, acting on a sea side of the reservoir when damming up water, to a substantially downwards force on the second coherent foundation. This configuration provides additional foundation capacity for the finished reservoir wall. The reservoir wall must counteract the large overturning moment acting on it in order to stay upright. The large overturning moment arises from the hydrostatic pressure of damming up water and the waves of the surrounding sea. Counteracting a large overturning moment typically results in large load concentrations within slender structures, as the lever arm for counteracting the overturning moment is small. The second coherent foundation prepared on the reservoir side of the first coherent foundation enables the reservoir wall to comprise a larger lever arm to counteract the overturning moment. Thereby, a better distribution of material within the reservoir wall is enabled, which results in material savings and lower reservoir wall costs. By 'reservoir side' of the first coherent foundation is intended the side of the first coherent foundation facing towards the inside of the reservoir. The other side of the first coherent foundation is referred to as the sea side, as it is facing the surrounding sea.

The means for converting at least a portion of the overturning moment to a substantially downwards force on the second coherent foundation provides the connection between the wall element and second coherent foundation. The mixture of seabed material and cement material within the coherent foundations of the offshore reservoir is stronger in compression than in tension and shear. Therefore, the means for converting at least a portion of the overturning moment to a substantially downwards force on the second coherent foundation makes better use of the foundation. At least a portion of the overturning moment is converted into compression forces in the second coherent foundation, which is the type of load that the second coherent foundation is best at handling. Thereby, a foundation with a lower capacity is enabled. Said means for converting at least a portion of the overturning moment to a substantially downwards force on the second coherent foundation is a rigid structure extending from the wall element to the second coherent foundation. Said means can be either an integral part of the wall element or connected to the wall element at the offshore reservoir site. The means for converting at least a portion of the overturning moment to a substantially downwards force on the second coherent foundation extends from an upper portion of the wall element and can be e.g.: a truss structure, a shell structure or a solid structure and can be made from e.g. steel, concrete, reinforced concrete or a combination thereof. This could be e.g. an independent prefabricated concrete structure or a buttress structure integrated in the wall element or an inclined wall surface integrated in the wall element and extending down towards the second coherent foundation. The marginal costs of the second coherent foundation are lower than for the first coherent foundation, as the construction method and the installation equipment are the same. The buttress structure is for instance as disclosed in connection with co-pending application with the title "A wall element system for an offshore power storage facility", filed on the same day, by the applicant of this present application, in connection with figure 4 and the description thereof on page 14-17 and page 38-40. In an embodiment, the second coherent foundation comprises multiple interlocking columns and is substantially parallel to the first coherent foundation. Preparing the second coherent foundation such that it is substantially parallel to the first coherent foundation enables simpler manufacturing and thus lower manufacturing costs of the reservoir wall that is connecting to both the first and the second coherent foundation. Further, a second coherent foundation, substantially parallel to the first coherent foundation, ensures a substantially evenly distributed load on the second coherent foundation, as the columns are located in substantially the same distance from the first coherent foundation. Additionally, substantially parallel coherent foundations enable both coherent foundations to be constructed from the same vessel, without the vessel changing position. By parallel is meant that theoretical planes going through the centrelines of the cylinders of each coherent foundation are substantially parallel. Thereby, parallel can also refer to two curved foundations, such as two concentric rings of foundation for e.g. a circular reservoir. The second coherent foundation is parallel with a portion of the first coherent foundation of at least similar length and located in a transverse position of the second coherent foundation.

The second coherent foundation may comprise any number of columns such as 1 , 2, 3, 5, 10 or any other number of columns The term 'coherent foundation' is predominantly used to designate that columns integrate with each other, but the term is also used when only one column is present.

In an embodiment, both the first coherent foundation and the second coherent foundation are prepared from a single jack up vessel, without the jack up vessel changing location. Thereby, the cost of preparing the second coherent foundation is lowered, as no time-consuming and costly relocation of the jack up vessel are needed for the preparation of the second coherent foundation.

In an embodiment, the second coherent foundation is prepared at a distance of at least 4 meters from the first coherent foundation, e.g. about 6 m or 8 m or 10 m. Thereby, the iever arm from the first to the second coherent foundation is sufficiently large for the reservoir wall to counteract the overturning moment acting on it.

In an embodiment, the second coherent foundation is prepared at a distance of between 20-50% of the reservoir wall height from the first coherent foundation. This is a good trade-off between ability to counteract overturning moments and the added material consumption of the reservoir wall.

In an embodiment, piles are inserted into both the first and the second coherent foundation. Thereby both foundations are reinforced with piles, thus enabling them to carry a larger load and act as shear keys. In an embodiment, a wall element is placed for interconnection with piles protruding from the first coherent foundation and for interconnection with piles protruding from the second coherent foundation. Thereby, the ability of the wall element to counteract the shear forces acting on it from damming up water is increased. Further, a wall element configured for interconnection with both the first and the second coherent foundation reduces the number of offshore operations, as there is no need for ferrying and placement of an individual element for connecting the wall element and the second coherent foundation.

In an embodiment, the length of the second coherent foundation is at least the length of a wall element, and the wall element is placed for interconnection with piles protruding from the first coherent foundation and for interconnection with piles protruding from the second coherent foundation. Thereby load concentrations within the reservoir wall are reduced as the increased ability to withstand overturning moments is added all along the wall element, compared to it being added only at some places along the wall element. Further, the second coherent foundation will further impede the flow of water through the seabed underneath the reservoir, thus further reducing loss of the energy stored within the reservoir from damming up water.

In an embodiment, the reservoir comprises multiple second coherent foundations of substantially equal length, where a wall element is configured for interconnection with piles protruding from the first coherent foundation and for interconnection with piles protruding from a second coherent foundation, and where the wall element comprises multiple buttress support sections, corresponding to the respective multiple second coherent foundation. Thereby, the wall has a high resistance towards overturning moments while keeping material consumption to a minimum. In an embodiment, the means for converting at least a portion of the overturning moment, acting on the sea side of the reservoir when damming up water, to a substantially downwards force on the second coherent foundation, extends in a transverse direction from the wall element. Thereby, said means are most efficient towards normal forces acting on the wall element, such as hydrostatic forces from damming up water.

In an embodiment, a portion of the offshore reservoir comprises a first and a second coherent foundation and a portion of the reservoir comprises only a first coherent foundation. Thereby, materials savings for the second coherent foundation are achieved, as a second coherent foundation is added only to the portion of the reservoir where it is needed, e.g. a more wave beaten portion of the reservoir. In an embodiment, the method of building at least a portion of an offshore reservoir for energy storage comprises: ferrying a wall supporting element to the offshore location; placing the wall supporting element for interconnection with piles protruding from the first coherent foundation and with piles protruding from the second coherent foundation. Further, ferrying a wall element to the offshore location; and placing the wall element for interconnection with piles protruding from the first coherent foundation and for interconnection with the wall supporting element. This configuration enables the use of wall elements connecting only to the first coherent foundation, while still having the increased resistance towards overturning moment of a reservoir wall connecting to a second coherent foundation. Thereby the cost per unit length of reservoir wall is reduced, as the wall elements connecting only to the first coherent foundation have reduced material consumption and size compared to a wall element of similar length connecting to both coherent foundations. The smaller wall elements are easier to handle during manufacturing, transport and installation which reduce equipment requirements and thus lower costs. By a 'wall supporting element' is intended an element that braces a wall element on the reservoir side to counteract the forces acting on the wall element when the reservoir is damming up water. The wall supporting element connects to the wall element at least at an upper portion of the wall element. The wall supporting element is a rigid structure capable of converting at least a portion of the overturning moment, acting on the sea side of the reservoir when damming up water, to a substantially downwards force on the second coherent foundation. The wall supporting element can be e.g. a truss structure or a shell structure or a solid structure made e.g. from steel, concrete, reinforced concrete or a combination thereof. The wall supporting elements are made shorter than the wall elements, as they do not need to brace the wall elements in the entire length of the wall elements. This means that the wall supporting element does not increase equipment requirements for handling during manufacturing, transporting and installation compared to the wall element, even though the wall supporting elements are connecting to both the first and the second coherent foundation. The piles in the second coherent foundation are inserted in a way similar to the piles in the first coherent foundation. The wall element interconnects with both the first coherent foundation and the wall supporting element with a sealed connection to seal the reservoir and thus make it capable of damming up water.

In an embodiment, the first coherent foundation extends further along the seabed than the wall supporting element, and the wall supporting element covers the second coherent foundation when viewed from above. The first coherent foundation extends along the seabed beyond the wall supporting element to create a seal that impedes flow of water through the seabed underneath the reservoir. The second coherent foundation is configured with a length such that the wall supporting element covers the second foundation when viewed from above. Thereby, material reductions are achieved as the second coherent foundation is constructed only where it is needed to support a wall supporting element. The reservoir sealing is handled by the first coherent foundation.

In an embodiment, the reservoir comprises one first coherent foundation and multiple second coherent foundations, where multiple wall supporting elements are connected to the first coherent foundation and a second coherent foundation respectively. Thereby, the reservoir wall comprises multiple wall supporting elements to brace it, which enables the wall elements to be of a lighter and thinner construction, which reduces material consumption for the wall element resulting in lower manufacturing, transportation and installation costs.

In an embodiment, a wall supporting element is placed where two adjacent wall elements are joined, such that the wall supporting element serves as bracing for both wall elements. Thereby, the connection between the wall elements is made easier, as the wall supporting element overlaps both wall elements. Further, the wall supporting element adds strength to the connection between the wall elements, thereby creating a stronger reservoir wall. The connection between the wall elements and the wall supporting element can be done e.g. as disclosed in connection with co-pending application with the title "A set of building elements for an offshore power storage facility", filed on the same day, by the applicant of this present application, in connection with figure 4a-4c and the description thereof on page 30-31 and page 51 -53, wherein the term 'wall panel' generally designates a wall element of the present application.

In an embodiment, a wall supporting element is configured to have a wind turbine installed on top. Thereby, achieving material savings, compared to installing the wind turbine on top of a wall element. As the wall supporting element is already dimensioned to support the wall element it can be constructed to support a wind turbine as well. The wall element can thus be constructed optimally to dam up water without considering the additional load coming from a wind turbine. This is more cost-efficient as the wall element can remain a slender structure, while only the wall supporting element is further reinforced to handle the loads from a wind turbine. By 'wind turbine' is intended a wind turbine in the megawatt range, e.g. about 2 MW, 3.6 MW, 5 MW, 7 MW or 10 MW in rated power.

In an embodiment, the method of building at least a portion of an offshore reservoir for energy storage comprises: placing a wall element such that a retracted underside of the wall element is located above a top portion of the protruding piles, and a bottom underside of the wall element is located below the top of the protruding piles; such that the wall element between the retracted underside and the bottom underside forms a wall section that extends below the top of the protruding piles, at least on the side of the piles facing the surrounding sea when the reservoir is formed. Said wall section is reinforced for transferring forces acting on the wall element to the piles. This configuration enables the forces acting on the wall element, from the damming up of water, to be transferred to the piles through a section of the wall element especially reinforced for transferring these forces. The piles are transferring the forces from the wall element to the foundation. By 'top portion of the piles' is intended that uppermost portion of the pile, where it is configured for carrying at least a portion of the load of the wall element, e.g. at the uppermost 5 m of the pile or 3 m or 1 m. The wall section that extends below the top of the protruding piles, at least on the sea side of the piles, enables the forces to be transferred to the piles as normal compression forces. This increases the load transferring capability compared to transferring the forces as shear forces or normal tension forces. Further, said wall section increases the contact area for the load transfer by extending down below the top of the piles; thereby, the load concentrations in the piles and the wall element are reduced. This reduces requirements of both piles and wall elements and thus the costs. By a 'retracted underside' is intended an underside positioned at a higher level than the bottom underside of the wall element, when the wall element is in an upright position. By 'bottom underside' is intended the surface located at the lowest level and facing downwards, when the wall element is in an upright position.

In an embodiment, the vertical distance between the retracted underside and the bottom underside of a wall element is at least 1 m, e.g. about 3 m or 5 m or 7 m. Thereby, the wall section that extends below the top of the protruding piles is large to ensure a connection between the wall element and the piles with good load transfer and minimum load concentrations.

In an embodiment, the retracted underside of a wall element is configured to carry the weight of the wall element, thus making it a bearing underside of the wall element. A bearing underside has a strength that makes it capable of carrying the wall element: it may come e.g. from local reinforcement of the retracted underside with a material of higher strength. Thereby, the wall element is able to rest on top of the piles during the process of interconnecting the wall element and the piles, e.g. by a grouting process. This makes the process of interconnecting the wall element and the piles easier, as the means used for positioning the wall element are not needed for carrying the weight of the wall element.

In an embodiment, the retracted underside of a wall element is configured to carry the weight of the wall element and forms the top of multiple hollow compartments in the wall element, such that there is one hollow compartment for each protruding pile designated for interconnection with the wall element. Thereby, a strong wail element structure is ensured near the interconnection to the protruding piles, where the loads on the wall element are transferred to the foundation. This reduces the risks of excessive load concentrations near the interconnection between the wall element and the protruding piles. In an embodiment, the distance between the foundation and the bottom underside of a wall element is smaller than the distance between the bottom underside of the wail element and the retracted underside of the wall element. Thereby the contact area is increased between the wall element and the piles where the forces are transferred as normal compression forces, and thus the load concentrations within the wall element and the piles are reduced.

In an embodiment, placing a wall element comprises: lowering said wall element in a substantially vertical direction down over the piles, such that the bottom underside of the wall element circumscribes the piles designated for connection with the respective wall element. Thereby minimum space is left to be sealed off between the foundation and the wall element in order for a casting process to interconnect the wail element with the piles and the foundation. This enables a fast and cost effective method of connecting the wall element to the piles and the foundation.

In an embodiment, placing a wall element comprises: moving the wall element in a substantially horizontal direction, until the retracted underside of the wall element is located above a top portion of the protruding piles, and the bottom underside of the wall element is located below the top of the protruding piles. This enables the wall element to be placed over the foundation sideways. Thereby, it is possible to place the wall element over the foundation by use of tow boats. This is an inexpensive way to position the wall element. Further, as the wall element can be positioned sideways, it can also be removed from its manufacturing position sideways. This reduces the requirements for heavy lifting equipment at the manufacturing site for lifting the wall element out of its manufacturing position. In an embodiment, the method of building at least a portion of an offshore reservoir for energy storage comprises: positioning a wall element such that it is hanging on the portion of the piles protruding from the upper portion of the foundation. This configuration ensures that the piles carry a substantial portion of the load of the wall element. As the piles are inserted into the foundation, the load of the wall element is distributed into the foundation instead of being delivered on the top of the foundation. This means that the foundation can carry a larger load, as this load is distributed to a larger portion of the foundation.

Hanging the wall element on the piles means that a gap is created between the bottom of the wall element and the top of the foundation. It is easier to seal this gap than to manufacture the wall element to correspond to the top of the foundation at the designated position of the respective wall element. The gap can vary in size, but is e.g. about 20 cm or 50 cm or 1 m where it is shortest. Thereby, it is possible to manufacture the wall element with a substantially levelled bottom even for a foundation following the variations of a natural seabed. When the piles are carrying the wall element, there is no need for the bottom underside of the wall element and the top of the foundation to correspond to each other, and the bottom underside of the wall element can thus be substantially levelled, while the top of the foundation follows the variations of a natural seabed. Manufacturing a wall element with a substantially levelled bottom is easier and has lower costs than a wall element with a sloping bottom or a bottom that varies in level, customized for the variations in the seabed at the specific location of the respective wall element. Further, hanging the wall element on the piles enables the wail element to stand for itself, e.g. while means of interconnecting the wall element and the foundation are curing. Thereby vessels and equipment for handling and positioning the wall element are free for use for other assignments, resulting in a faster reservoir erection and thus lower reservoir erection costs. By 'hanging a wall element' is intended that a substantial portion of the load of the wall element is carried by a portion of the wall element retracted from the bottom underside of the wall element. In an embodiment, the piles protruding from the foundation comprise a top plate. Thereby, the surface area that the wall element is hanging on is enlarged, thus reducing load concentrations within the piles and the wall element near said surface area.

In an embodiment, the insertion of the piles is ended such that the top surfaces of the piles are substantially levelled. Thereby, the wail element becomes capable of hanging on the piles without being customized for the specific levels of the top of the individual piles, whereby the manufacturing costs of the wall element are reduced. Further, any costs associated with specifying the specific levels of the top of the individual piles are obviated.

In an embodiment, the insertion of the piles is ended such that the top surfaces of the piles are substantially levelled, where the piles subsequently are supported by a temporary support structure while the foundation is curing. Thereby it is ensured that the piles do not settle unintentionally and thus cause increased load concentrations in the connection between wall element and piles due to misaligned piles. Said temporary structure can be e.g. a lever beam suspended from previously inserted piles or a support structure resting on the surrounding seabed or the vessel installing the piles holding on to the piles or submerged floating means with positive buoyancy, e.g. a buoy. In an embodiment, the top surfaces of the piles are cut off when the foundation has cured enough to make sure that no additional settling of the piles occur to the effect that the top surfaces of the piles are substantially levelled. Thereby, it is ensured that the top surfaces of the piles are substantially levelled, even if the piles have settled differently during the curing of the foundation. In an embodiment, the wall element comprises a hollow compartment with a downward opening and with an upwardly extending channel, and the method of building at least a portion of an offshore reservoir for energy storage comprises: placing the wall element such that the hollow compartment accommodates a portion of the piles protruding from the top of the foundation. A gasket is placed between the underside of the compartment and the top of the foundation, such that the gasket is at least partially compressed by the load of the wall element. Grout or concrete is poured into the hollow compartment through the upwardly extending channel and the gasket is configured to seal the hollow compartment and withhold the grout or concrete. The at least partly compressed gasket abuts the top of the foundation, whereby a sealed contact between the underside of the compartment and the top of the foundation is established capable of withholding grout or concrete. This configuration ensures a simple and fast interconnection between a wall element and the foundation by a casting process of the sealed hollow compartment, where said hollow compartment is sealed off by placing the wall element. Thereby, the operation of interconnecting the wall element and the foundation is fast, which reduces the risks of weather and offshore conditions, such as waves and marine currents, delaying the erection of the wall elements. A fast erection of the wall elements results in a shorter time period from erecting the first wall element to completion of the reservoir. In the period of erecting the wall elements, the wall elements are more vulnerable as they are not interconnected to form a reservoir. The short erection time enables the erection process to be planned for periods with more favourable offshore conditions, which reduces the risk of costly delays due to weather and other offshore conditions.

By 'hollow compartment with a downward opening' is intended a cavity within the wall element that is open to the environment outside the wall element in a downwards direction. Thereby, a portion of the piles protruding from the top of the foundation is able to pass up into the hollow compartment, when the wall element is lowered in a downwards direction. If the top of the foundation is substantially plane and levelled, the wall element can be manufactured such that the underside of the compartment is substantially plane and levelled. Thereby, the manufacturing costs of the wall element are lowered. By 'underside of the compartment' is intended a solid surface on the wall element near the downwards opening of the compartment in the wall element. The underside of the compartment can be e.g. the bottom underside of the wall element, and the top of the hollow compartment can be e.g. the retracted underside, both mentioned earlier on in this application.

Further, the wall element fully encapsulates the portion of the piles protruding from the foundation, which ensures good load transferring capabilities. The grout or concrete has a large contact surface on both the piles protruding from the foundation and the inside of the compartment, thereby making the connection strong. As the protruding portion of the piles is fully enclosed within the sealed compartment filled with grout or concrete, they are protected against the corrosive environment of the surrounding saltwater. By "pouring grout or concrete" is intended that it is accomplished by means of pumping or gravity feed. The upwardly extending channel can be a pipe system embedded within the wall element, e.g. a tremie pipe for the grout or concrete.

By 'top of foundation' is intended the top of cured cement and seabed material mixture, or a plate pressed down on to the cement and seabed material mixture, while this was still wet, or in situ cast portion on top of the cement and seabed material mixture. The piles extend further above the foundation than the top of the foundation.

In an embodiment, the step of inserting the piles down into the foundation comprises pressing a substantially levelled plate down onto the top of the foundation and removing it again. Thereby, it is ensured that the top of the foundation is substantially levelled, while the costs are kept at a minimum as the plate is removed again.

In an embodiment, plates are inserted into the top of the foundation while it is still wet such that these plates form a substantially levelled surface integrated with the foundation near the top of the foundation. Thereby, a substantially levelled surface for the gaskets to seal off against is provided by the plates. Said plates can be integrated with the piles inserted into the foundation, e.g. by a welding process.

In an embodiment, a box is inserted into the top of the foundation while it is still wet such that an enclosure is formed, configured for an in situ casting process, where the top of the box is substantially levelled, and an in situ casting process is performed in the box. Thereby a substantially levelled surface for the gaskets to seal off against is provided by the in situ cast portion when it is cured. The in situ cast portion can carry the weight of the wall element when the foundation is cured. The box can be either with or without a top plate. The box can be integrated with the piles. In an embodiment, the sealed compartment is pressurized before the pouring of the grout or cement. Thereby, the sealing of the compartment is checked by controlling that the pressure in the sealed compartment does not drop due to sea water escaping the compartment through the seal. This control will reveal whether the compartment is fully sealed and thus can withhold grout or cement.

In an embodiment, the method of building at least a portion of an offshore reservoir for energy storage comprises: inserting rigid skirts, attached to the wall element, into the seabed on both sides of the coherent foundation, along the length of a wall element. Said rigid skirts form a part of a sealed enclosure established between the wail element and the seabed on both sides of the coherent foundation. Grout or cement is poured into the sealed enclosure through a pipe. This configuration enables the wall element to have a substantially plane bottom underside and still seal off against a naturally varying seabed. Thereby the nature of the seabed which can be penetrated by a rigid skirt is utilized to create a seal between a wall element with a substantially plane bottom and the naturally varying seabed. Thereby, no customization of the skirt is needed to match the skirt to the seabed variations on the erection site of the specific wail element. This enables the foundation of the wall element to be constructed such that the level of the top of the foundation substantially follows the level of the naturally varying seabed. When the level of the top of the foundation substantially follows the level of the naturally varying seabed, it provides a solid top surface of the foundation for the casting process to seal off against. That means that the casting process of the sealed enclosure that will interconnect the foundation and the wall element will form a connection impermeable to water; and the wall that is created is coherent and impermeable to water from the bottom of the foundation to the top of the wall element.

Further, the manufacturing process of the wall element is simplified as a wall element with a substantially plane bottom underside is easy to manufacture, and the wall element does not need to be customized for the seabed variations on the erection site of the specific wall element. Additionally, the skirt reduces the risk of scouring around the reservoir foundation, thereby reducing the costs for additional scouring protection. By 'both sides of the coherent foundation' is intended the side facing the reservoir and the side facing the surrounding sea. By 'length of wall element' is intended in a direction along the seabed when the wall element is in its designated position on the foundation. The rigid skirts are attached to the wall element, e.g. by means of nuts and bolts or by a friction coupling or by a welding process or in tracks installed on the wall element or by a casting process such that the skirts form an integral part of the wall element. In order for said enclosure to be fully sealed off from the surrounding sea water, the enclosure must be sealed near the respective ends of the wall element, where a skirt cannot penetrate into the seabed, due to the location of the coherent foundation, which is extending beyond the wall element at both ends. Fully sealing off of the enclosure enables grout or cement to be placed within the enclosure through one or more pipes, e.g. tremie pipes, thereby interconnecting the wall element and the foundation when the grout or cement is cured.

In an embodiment, the enclosure is sealed near the end by means of a flexible bag attached to the wall element, where the flexible bag is filled with a filler material, e.g. grout, after the wall element has been placed at its designated position. The filler material causes the flexible bag to expand and seal off against the wall element and the coherent foundation and the skirts on both sides of the coherent foundation and the seabed between the coherent foundation and the skirts. Thereby the seal has the flexibility to adapt to both the seabed and the foundation and can be filled from the top of the wall element through preinstal!ed pipes. in an embodiment, the skirts are attached to the wall element in their final position, e.g. by a casting process or with nuts and bolts or by a welding process, before the wall element is ferried to the erection site. At the erection site, the wall element is lowered substantially vertically down to reach its designated position. Thereby, the skirts are inserted into the seabed by the weight of the wall element when it is lowered into its designated position.

In an embodiment, the skirts are attached to the wall element such that they can slide down relative to the wall element, e.g. by means of rails or tracks. The skirts are inserted into the seabed after the wall element has reached its designated position. Thereby, the skirts can be attached to the wall element in a retracted position, which reduces the risk of damage to the skirts during transportation, lowering and positioning of the wall element.

In an embodiment, the wall element comprises a hollow compartment that forms a part of the sealed enclosure that the skirts inserted into the seabed also form a part of; wherein the hollow compartment accommodates a portion of the piles protruding from the top of the foundation. Thereby, a good interconnection between the wall element and the foundation is ensured, where the interconnection is capable of transferring the large loads from the damming up of water.

In an embodiment, the skirts are constructed from sheet piling. Thereby, the well known and well proven sheet piling elements are used to construct the skirts.

In an embodiment, the pipe through which the grout or cement is poured is preinstalled within the wall element. Thereby, easy access to the sealed enclosure is enabled, and grout or cement can be poured into said sealed enclosure without further operations or measures. By 'sealed enclosure' is intended sealed off towards the surrounding sea. Thus, the sealed enclosure can still have openings to the air, e.g. through a pipe connection to the top of the wall element for pouring grout or cement.

In an embodiment, the individual wall elements are configured with a height that corresponds to the sea depth at the designated position of the respective wall element. Thereby, the height of the skirt is minimized, which also minimizes the amount of cement material needed for the in situ casting process of the sealed enclosure. Further, it is advantageous to make as large a portion of the reservoir structure onshore as possible, as the manufacturing process is more controlled onshore, and e.g. placement of reinforcement is easier to control.

In an embodiment, the placing of a wall element comprises: positioning a wall element by means of first positioning means, such that a bearing underside of the wall element is positioned above a top portion of the piles designated for connection with the respective wall element, such that a gap is created between the bearing underside of the wall element and the top portion of the piles. Activating second positioning means in order to bridge the gap and establish a contact between the top portion of the piles and the bearing underside; where the second positioning means are configured to carry the load of the wall element. Releasing the first positioning means such that the second positioning means carry a substantial amount of the load of the wall element. This configuration performs an in situ fitting between the wall element and the piles protruding from the in situ constructed foundation. The bearing underside of the wall element is resting on the second positioning means which are resting on the top of the portion of piles protruding from the foundation. As the second positioning means are configured to span the gap, it is ensured that the weight of the wall element is substantially evenly distributed onto the piles designated for connection with the respective wall element, even when the tops of the piles are at different levels. Distributing the weight of the wall element reduces the risks of having load concentrations in the foundation and in the wall element, thus reducing the risks of damage to foundation and wall element. The in situ fitting enables a more standardized manufacturing process of the wall elements, as customization of each wall element for the specific piles designated for connection with the respective wall element is obviated. Further, costs associated with determining the accurate level of the top of the specific pile designated for connection with the respective wail element or costs associated with performing a precision levelling of the tops of the piles designated for connection with the respective wall element are avoided. Thereby, a reduction in the overall costs of the reservoir is achieved. By 'bearing underside' is intended an underside of the wall element that is configured for carrying the weight of the wall element. The gap is a space extending primarily in the vertical direction. The gap typically varies in size due to differences in level of the tops of the piles, but it is typically less than 1 m, e.g. about 20 cm or 30 cm 40 cm. By 'first positioning means' is intended equipment capable of positioning the wall element in a correct position on top of the foundation, e.g. one or more cranes, cables or wires attached from floating vessels to the wall element, towboats, propellers or jets installed on the wall element, one or more jacks, jack up vessels, external floaters customized for the wall element, internal buoyancy within the wall element or any combination thereof.

By 'second positioning means' is intended equipment capable of bridging the gap and carrying a substantial portion of the load of the wall element, e.g. wedges, jacks, screw jacks, flexible bags containing a filler material, cables/wires or any combination thereof.

In an embodiment, said second positioning means comprises wedges pushed towards each other from opposite sides, such that one wedge is sliding up on top of the other. Thereby a simple and well known mechanical method is used to span the gap.

In an embodiment, said second positioning means comprises a screw jack. Thereby a simple and well known mechanical method is used to span the gap.

In an embodiment, the second positioning means are activated mechanically from above sea level, e.g. by starting a pump, tensioning a cable/wire, turning a shaft or pushing down a rod. In an embodiment, the second positioning means are left in the position bridging the gap and then embedded by a casting process. Thereby, there is no need for a method of retrieving the second positioning means, which enables a simple design of the wall element, e.g. where the second positioning means are located in a hollow compartment within the wall element. As the second positioning means are located at deep sea, retrieving them can be difficult and thus costly.

In an embodiment, a filler material with curing ability is placed within a flexible bag positioned between the bearing underside of a wall element and the top of a pile. The filler material is charged or filled into the flexible bag through a passage, such that the flexible bag expands to bridge a gap between the bearing underside of wall element and the top of the pile. Thereby, an in situ fitting between the wall element and the pile protruding from the in situ constructed foundation is performed. When the filler material is charged or filled into the flexible bag, the flexible bag adapts its size and shape to fit the gap between the wall element and the pile. The passage for charging or filling the filler material into the flexible bag is sealed subsequently to the positioning of the filler material. Thereby, the flexible bag is able to carry a load without the filler material escaping through the passage. By 'flexible bag' is intended a bag that expands when the filler material is charged or filled into the bag and where the filler material is contained within the flexible bag. By 'filler material' is intended a materia! filling to be filled inside a flexible bag, e.g. grout, cement, water, air, epoxy or poiyurethane.

The second positioning means can e.g. be multiple flexible bags. The step of filling the filler material into the bags can be a way of activating second positioning means in order to bridge a gap and establish contact between a top portion of the piles designated for connection with a respective wall element and the bearing underside of the wall element. in an embodiment, multiple flexible bags are filled with a filler material, such that they expand and bridge a gap between a bearing underside of a wall element and the top of multiple piles designated for connection with the respective wall element. The passages for placing the filler materia! in the flexible bags are interconnected, such that the filler material can flow from one flexible bag to another. Thereby it is ensured that the weight of the wall element is evenly distributed onto the piles, thus reducing the risks of load concentrations and damage to the bearing underside. In an embodiment, the filler material is a fluid that later cures, e.g. grout or epoxy; thereby transforming the flexible bags into a solid element when the filler material has cured, thus creating a rigid connection with a shape customized to fit in the gap between the wall element and the respective piles.

In an embodiment, the first positioning means are released in steps, and the filler material is a fluid that later cures, e.g. grout or epoxy. Thereby, the first step of releasing the first positioning means makes the filler material adapt to the shape of the gap between the wall element and the respective piles, without the flexible bag and the sealed passage experiencing the full load of the wall element, as the first positioning means are stilf carrying a portion of the weight of the wall element. When the filler material has cured to form a solid element, the first positioning means are fully released. As the filler material is now a solid material, the flexible bags and the sealed passage are no longer necessary to withstand the load from the wall element. Thus, the flexible bags and the sealed passage need not be able to carry the full load of the wall element, and are therefore less expensive to manufacture.

In an embodiment, a wall element has a bearing underside with a portion configured for deformation, and placing the wall element comprises: positioning a wall element such that the bearing underside is positioned on a top portion of the piles designated for connection with the respective wall element; and lowering the wall element such that the portion configured for deformation deforms and such that the weight of the wall element is distributed substantially evenly on the piles designated for carrying the wall element. This configuration enables the wall element to compensate for small variations in the actual level of the top piles compared to the planned level of the top of the piles. Thereby, load concentrations in the wall element in the piles and in the foundation arising from inaccuracies in the level of the tops of the piles are prevented. By 'deforming' is intended changing shape from its original shape, due to the weight of the wall element. This is a cost-efficient method of distributing the load of the wall element substantially evenly on the piles designated for carrying the wall element, as the load is distributed as a part of the process of lowering the wall element, without any additional offshore operations.

In an embodiment, the portion of the bearing underside configured for deformation comprises an elastic material, e.g. rubber. Thereby the flexible nature of the elastic material is utilized for the deformation, ensuring that the wall element is not damaged.

In an embodiment, the wall element is constructed from reinforced concrete, and the portion of the bearing underside configured for deformation is a layer of concrete, configured for crumbling if the load exceeds a certain value above the weight of the wall element distributed onto all the piles designated for connection with the respective wall element. Thereby manufacturing advantages are achieved as similar materials are used for the wall element and the portion of the bearing underside configured for deformation.

In an embodiment, the method of building at least a portion of an offshore reservoir for energy storage comprises: positioning a pile cap on top of multiple piles designated for connection with a respective wall element; interconnecting the pile cap and said piles, such that the pile cap is substantially levelled; and positioning the wall element such that a bearing underside, retracted from the bottom underside of the wall element, is resting on the pile cap, where the pile cap is carrying a significant amount of the load of the wall element. The pile cap is a strong and substantially levelled structure, e.g. a concrete box, which is interconnected to multiple piles, to carry the load of the wall element. As the pile cap is connected to multiple piles, the load of the wall element is distributed substantially evenly among these piles, thereby reducing load concentrations and risks of failure within the wall element or the piles or the foundation. Further, the manufacturing process of the wall element is easier, as the bearing underside of the wall element is resting on a substantially levelled pile cap, with a known height level. Thereby, each wall element can be customized during manufacturing to fit the pile cap at its designated position at the reservoir site.

In an embodiment, the pile cap is interconnected with the piles designated for connection with a respective wall element by a grouting process. Thereby, the flexibility of a grouted connection is utilized to ensure that the pile cap is substantially levelled.

In an embodiment, the pile cap is lowered down over a top portion of the piles designated for connection with a respective wall element, such that the top portion of the piles protrudes up into at least one hollow compartment within the pile cap, where the pile cap is subsequently interconnected with the piles by a casting process of said hollow compartment. Thereby, the flexibility of a grouted connection is utilized to connect the substantially levelled pile cap to piles protruding to various levels.

In an embodiment, the method of building at least a portion of an offshore reservoir for energy storage comprises: attaching a cable to the portion of a pile protruding from the foundation; attaching the cable to a wall element, by means of a motor-driven winch, before placing the wall element. The cable connects to the wall element at a point that is located above said pile, when the wall element is in its designated position. By applying controlled tension to the cable, the control during placing of the wall element is increased. This configuration enables the cable connection to guide the wail element and thus helps to position the wall element correctly. The cable connects to the wall element at a point which, when the wall element is at its designated position, is located above the pile that the cable is attached to. Thereby, applying controlled tension to the cable will pull the wall element towards its designated position and thus increase control during placing of the wall element, as the wall element cannot move freely. The tension is applied to the cable by the motor-driven winch. This winch can be installed at a level that remains above the sea surface, also when the wall element is in its designated position. This reduces the requirements to the winch and makes it easier to control the winch.

Moreover, further tensioning the cables when the wall element is at its designated position will hold the wall element in its designated position, e.g. while an interconnection formed by grout or cement between the wall element and the foundation is curing. By 'cable' is intended a flexible structure that can transfer mainly tension forces, such as a cable, a wire, a rope, a chain or a wire rope. By 'controlled tension' is intended that the level of the tension in the cable is controlled. In an embodiment, the wall element floats by internal or external buoyancy means or a combination. Thereby, applying tension to the cables to the effect that the wall element is pulled down into the water results in the buoyancy creating a restoring force on the wall element, which will steer the wall element towards a position directly above its designated position. Thereby, the requirements to the offshore equipment performing the lowering and placement of the wall element are reduced, thereby enabling the use of less expensive offshore installation equipment.

In an embodiment, multiple cables are connected to multiple respective piles, and tension is applied to said cables by multiple respective winches. Thereby, better control of the wa!l element is achieved as there are multiple cables to increase control during placing of the wall element. Each winch can be operated individually to counteract variations in the forces acting on the wall element during positioning in an offshore environment.

In an embodiment, a first and a second wali element are erected adjacently to each other with a gap between them. A first plate is positioned on the reservoir side of the wall elements and a second plate on the sea side of the wall elements. The plates have contact with both the first and the second wall element and form an enclosure between the wall elements. The plates are pulled together by use of tensioning means. This configuration is a simple and cost-efficient method for interconnecting wall elements to form at least a portion of a reservoir. As the plates are connecting to both wall elements, the plates utilize the strength of both wall elements for the connection. The plates can be e.g. straight or curved and made of e.g. steel. The tensioning means press the plates towards the first and second wall element, thereby sealing the connection and thus enabling the reservoir to dam up water. By 'tensioning means' is intended equipment capable of pulling the plates towards each other, e.g. cables/wires or a rod or bolts being tightened.

Further, the plates can be installed from a vessel located on one side of the wall elements. Thereby, the installation speed of the connection is increased, compared to installation of the plates from each side. The plate designated for being positioned on the side of the wall elements opposite that of the vessel, is moved through the gap on the narrow side. The plate is then turned and placed with contact to both the first and the second wall elements, such that the width of the plate covers the gap. Thereby, the vessel for installing the plates does not require lifting capabilities to lift the plate over the wall elements for installation on the opposite side of the wall. Further, the plates can thereby be immersed in the sea during positioning which reduces positioning complications, as lifting a plate over the wall elements makes it very exposed to wind, which may complicate the process of positioning it correctly.

In an embodiment, the first plate is hanging on the reservoir side of a first wall element and the second plate is hanging on the sea side of the first wall element when the first wall element is erected adjacently to a second wall element. Thereby the plates are already placed on the right sides of the first and second wall element, and moving the first and second plates to span the gap between the first and the second wall element Is the only operation needed to position the plates correctly. This reduces the time consumption of positioning the plates correctly as well as reduces costs, as the moving operation is simple and can be performed by e.g. a small crane or tracks on the first wall element. The plates can e.g. be made to hang on the wall element at the manufacturing site before ferrying the wail element to the erection site.

In an embodiment, the connection between the plates and the wall elements comprises sealing means, e.g. a gasket. Thereby, it is ensured that the connections between the plates and the wall elements are sealed.

In an embodiment, the same tensioning means are attached to the first plate and the second plate through the gap. Thereby, a fast installation time is enabled, as the same tensioning means are attached to both plates, such that the same tensioning means puil both plates towards each other, thus creating the enclosure between adjacently positioned wall elements. Thereby, only one set of tensioning means has to be activated to install both the first and the second plate.

In an embodiment, the same tensioning means attached to both plates through the gap comprise a cable or a wire. Thereby the plates are connected with flexible tensioning means which enable the plates to move more freely relative to each other during the installation. This makes the installation process easier. In an embodiment, the tensioning means are flexible, e.g. a cable/wire, and are attached to both plates before the plates are positioned on each side of the wall elements. Thereby the tensioning means can be attached to the plates in an on shore facility which is considerably easier compared to the offshore environment. Further this increases the offshore installation speed of the connection, as fever operations need to be performed at the offshore reservoir site.

In an embodiment, the enclosure between the first and the second wall element created by the plates is filled with a filler material, e.g. grout or cement. Thereby the strength and stiffness of the connection is increased, thus reducing the risks of a connection failure when the connection is subjected to large forces, e.g. when the reservoir is damming up water. In an embodiment, the plates are inserted into the seabed. Thereby, the plates are sealed along their bottom, which enables them to withhold grout or concrete.

In an embodiment, an intermediate piece is positioned between the ends of two neighbouring wall elements for wet and/or dry interconnection with the neighbouring wall elements. The intermediate piece may be positioned e.g. in front of a wall supporting element, on that side of the wall supporting element facing the surrounding sea. A backside (i.e. a side opposite that side facing the surrounding sea) of the intermediate element may abut that side of the wall supporting element facing the surrounding sea. Alternatively the intermediate piece is configured to leave a gap between the backside and the side of the wall supporting element facing the surrounding sea e.g. for being filled with a filler material. The intermediate piece may be constructed from a composition of materials similar to the wall elements. The intermediate piece may be constructed with a reinforcement structure similar to the wall elements. The intermediate piece may have a shape complementary to the shape of the ends of the wall elements. In an embodiment, the method of building at least a portion of an offshore reservoir for energy storage comprises: installing an intermittent renewable energy converter on the reservoir, where the renewable energy converter is coupled to a pump configured for draining water from the reservoir. This configuration utilizes the reservoir wall for the dual purpose of reservoir for storing energy and foundation for a renewable energy converter, thereby reducing the total cost of renewable energy converter and reservoir for storing energy. Further, the energy produced by the renewable energy converter is stored within the reservoir by using it for the pump configured for draining water from the reservoir. This increases the value of the renewable energy as it can be reproduced when it is in demand rather than at the time when the intermittent renewable energy resource is available. By 'installed on the reservoir' is intended installation on any element comprised by the reservoir, e.g. a wall supporting element or a wall element. In an embodiment, said intermittent renewable energy converter is a wind turbine. Thereby, the wind turbine benefits from the favourable offshore wind conditions, resulting in a high energy production. Further, the total costs are reduced compared to a reservoir and a separate offshore wind turbine, as the foundation for the offshore wind turbine is combined with the reservoir wall thus achieving structural benefits and material savings. In an embodiment, said intermittent renewable energy converter is an oscillating water column wave energy converter driven by the waves hitting the reservoir wall. Thereby the waves present in the offshore environment are utilized to produce energy, while the installation costs of the oscillating water column wave energy converter are reduced compared to a separate oscillating water column wave energy converter facility, as the reservoir wall is utilized as foundation.

In an embodiment, said intermittent renewable energy converter is a wave energy converter with multiple floats attached to the reservoir wail. Thereby the waves present in the offshore environment are utilized to produce energy, while the installation costs of the floats are reduced compared to a separate wave energy converter facility with multiple floats, as the reservoir wall is utilized as foundation.

In an embodiment, the method of building at least a portion of an offshore reservoir for energy storage comprises: trenching a substantially levelled trench at least the length of a wall element; and preparing the foundation in the trench. The substantially levelled trench ensures that the foundation constructed in the seabed within the trench is substantially levelled. Thereby an easy manufacturing process of the wall element is enabled, as it is designated for interconnection with a substantially levelled foundation and is thus manufactured with a substantially levelled bottom underside. The sealing between the substantially levelled foundation and the substantially levelled bottom underside of the wall element requires less customization than a sealed connection to a non-levelled foundation.

In an embodiment, the foundation is constructed in a substantially levelled seabed, and a substantially levelled plate is pressed down onto the top of the foundation while this is not fully cured. Thereby it is ensured that the top of the foundation is substantially plane and levelled and thus easy for the wall element to seal off against.

In an embodiment, the trench is constructed with different levels, where each levelled portion has the length of one or multiple wall elements. Thereby the amount of seabed material required to be trenched is minimized, thus keeping trenching costs to a minimum. The wall elements are customized to the level of the trench in which they are designated to be positioned, Further there is disclosed an offshore reservoir, where the reservoir encloses a volume and is configured to withstand water pressure from surrounding sea water. The offshore reservoir is configured with a pump for draining water from the reservoir by use of energy and/or a turbine for generating energy when filling surrounding seawater into the reservoir. The offshore reservoir comprises: a reservoir wall that comprises multiple precast wall elements, a coherent foundation comprising drilled interlocking columns, and multiple piles cast into the foundation. The interlocking columns comprise a mixture of seabed material and a cement material. The coherent foundation extends vertically from a level substantially corresponding to the seabed level and down into the seabed. The piles protrude from the foundation up into the reservoir wall and are interconnected to the reservoir wall. This configuration minimizes the costs of manufacturing and erecting a reservoir in offshore conditions. The reservoir may be configured with a pump and a turbine for storing power and can thus be denoted as a power plant or an offshore power plant. Alternatively a power plant can also be defined as a plant comprising a reservoir and a pump and a turbine. The reservoir wall is constructed from multiple wall elements, which enables onshore prefabrication of the wall elements, where series production and a more controlled environment, compared to offshore, reduces manufacturing costs and ensures a homogeneous quality. The offshore reservoir comprises a wall interconnection that connects adjacent wall elements in a watertight manner. Thereby, the reservoir wall constructed from multiple precast wall elements is able to dam up water and thus the reservoir is able to store power. By a 'wall interconnection' is intended means to connect two adjacent wall elements in a watertight manner, e.g. a direct connection between the wall elements, a buttress element connecting to both wall elements, plates connecting to both wall elements, a precast element configured for connection of the two wall elements or a combination thereof. Said wall interconnection is sealed, e.g. by an in situ casting process or by at least partially compressed gaskets or by flexible bags filled with a filler material.

The coherent foundation comprising drilled interlocking columns provides foundation for the reservoir wall while impeding flow of water underneath the reservoir wall. The succession of drilled interlocking columns extends along a theoretical line, where said line e.g. is curved or linear or piecewise linear. The piles cast into the foundation and protruding up into the reservoir wall ensure a good load transfer from the wall elements to the foundation as well as provide a strong connection between wall elements and reservoir foundation, thus enabling the reservoir wall to withstand the large forces from dammed-up water. The reservoir wall is interconnected with the piles protruding from the foundation and with the foundation in a watertight manner. In an embodiment, the offshore reservoir comprises a pump for draining water from the reservoir by use of energy and a turbine for generating energy when filling surrounding seawater into the reservoir, integrated into a single pump-turbine unit, thereby reducing the amount of material used for the pump and the turbine, as they are integrated into one single unit, thus reducing costs. Further, a single pump-turbine unit takes up less space than a separate pump and a separate turbine. In an embodiment, the offshore reservoir comprises a pump for draining water from the reservoir by use of energy, located at or below seabed level. Thereby the pressure provided by the water inside the reservoir is utilized to reduce problems with cavitation within the pump, which can lower performance and damage the pump. Further, the possible height difference between the surrounding sea and the water inside the offshore reservoir, and thus the energy storage capacity of the offshore reservoir, is maximized. In an embodiment, the offshore reservoir comprises a separate housing facility located inside the reservoir and communicating with the sea through a pipe system; where the housing facility comprises a pump, for draining water from the reservoir by use of energy, and a turbine, for generating energy when filling surrounding seawater into the reservoir. Thereby the pump and turbine are easily installed below seabed level, as the separate housing facility can be located and installed separately. Further a separate housing facility enables the pump and turbine to be pre-installed within the separate housing facility on shore. In an embodiment, an offshore reservoir comprises: a first coherent foundation comprising drilled interlocking columns, a second coherent foundation, comprising drilled columns, and a reservoir wall. The reservoir wall comprises means to convert at least a portion of the overturning moment acting on the sea side of the reservoir wail, when damming up water to a substantially downwards force on the second foundation. The columns comprise a mixture of seabed material and a cement material. The first and second coherent foundations extend vertically from a level substantially corresponding to the seabed level and down into the seabed. The second coherent foundation is located on the reservoir side of the first coherent foundation. This configuration increases the ability of the reservoir to resist the large overturning moments acting on it from dammed-up water. The second coherent foundation reduces the load on the first coherent foundation i two ways. Firstly it adds foundation capacity to carry the weight of the reservoir wall, but secondly it reduces load concentrations in the first coherent foundation and in the reservoir wail by providing an increased lever arm to counteract the overturning moments acting on the reservoir wall, as the reservoir converts at least a portion of these overturning moments to a downwards force on the second coherent foundation.

In an embodiment, a wall element comprises a substantially vertical seaside wall portion and a reservoir side wall portion. The reservoir wall portion extends from an upper portion of the wall element downwards towards the seabed inside the reservoir, at least for some length of the reservoir side wall portion. One or more hollow compartments are located between the seaside wall portion and the reservoir side wall portion. The seaside wall portion connects to the first coherent foundation and the reservoir side wall portion connects to the second coherent foundation. Thereby, the inclined reservoir side wall portion converts normal forces acting on the seaside wall portion to a substantially downwards force on the second coherent foundation. Further, the one or more hollow compartment(s) within the wall element add(s) buoyancy to the wall elements, thus making transportation of the wall element easier.

In an embodiment, a wall element comprises a substantially vertical wall portion and multiple buttress portions extending from an upper portion of the substantially vertical wall portion. The multiple buttress portions extend downwards towards the seabed on the reservoir side of the substantially vertical wall portion and connect to the second coherent foundation. Thereby, the multiple buttress portions of the wall element convert the overturning moments acting on the seaside face of the wall element to a substantially downwards force on the second coherent foundation, while keeping material consumption of the wall element at a minimum. In an embodiment, an offshore reservoir comprises: a wall element with a retracted underside and a bottom underside. The retracted underside is located above a top portion of the portion of the piles protruding from the foundation, and the bottom underside is located below the top of the portion of the piles protruding from the foundation. Further, the bottom underside is located on the sea side of the portion of the piles protruding from the foundation. The wall element is reinforced in a portion extending from the bottom underside to up above the retracted underside. This configuration ensures that the wall element has a portion extending beiow the top of the protruding piles, such that the large shear forces acting on the wall element can be transferred to the piles as normal compression forces. A connection transferring normal compression forces is favourable compared to a connection transferring shear or tension forces, as the surfaces in the connection transferring normal compression forces are pushed together instead of being pulled apart. This makes a connection transferring normal compression forces more durable, as the surfaces of the connection are pushed together and thus stay in place. The wall element is reinforced in the area that is transferring the forces to the piles, as the load concentrations in this portion of the wall element are increased from transferring the forces. The wall element is reinforced up above the retracted underside to ensure that the portion of the wall element transferring the forces to the piles is well connected to the rest of the wall element such that the wall element does not break near the retracted underside. Said reinforcement can be e.g.: stronger concrete, stronger steel reinforcement, more steel reinforcement compared to the rest of the wall element, or it can be reinforced with a stronger material than the rest of the wall element, e.g. glass fibre or carbon fibre.

In an embodiment, the bottom underside circumscribes the retracted underside when viewed in a substantially vertical direction when the wall element is in an upright position. Thereby, the wall element forms a hollow compartment with a downwards opening. This hollow compartment provides a simple interconnection between the wall element and the piles protruding from the foundation, as the hollow compartment can accommodate a portion of the piles protruding from the foundation.

In an embodiment, the bottom underside circumscribes the retracted underside when viewed in a substantially vertical direction when the wall element is in an upright position and the distance from the bottom underside to the retracted underside is shorter than the length of the portion of the piles protruding from the foundation. Thereby, the wall element is able to hang on the protruding portion of the piles by resting the retracted underside on a top portion of the piles. This makes the process of interconnecting the wall element and the foundation simple as the wall element can be placed in its final position and hang on the piles, while the rest of the interconnection is finished, e.g. by a sealed enclosure being established between the foundation and the wall element and this being filled with grout or cement.

In an embodiment, an offshore reservoir comprises; second positioning means enclosed within an in situ cast portion of the reservoir wall, where the second positioning means connects to a prefabricated portion of a wail element and to piles cast into the foundation. This configuration ensures that the load of the wail element is substantially evenly distributed to the piles that the second positioning means connects to and that the wall element is balanced while the in situ cast portion cures.

In an embodiment, an offshore reservoir comprises: a wall element comprising a bearing underside with a portion configured for deformation that is located on top of the portion of the piles protruding from the foundation; where the portion of the bearing underside configured for deformation is locally deformed. This configuration ensures that the load of the wall element is substantially evenly distributed to the piles connecting to the wall element, as the pi!es protruding to the highest level cause the portion configured for deformation to deform until the load of the wall element is carried by a sufficient number of piles. in an embodiment, an offshore reservoir comprises: a tensioned cable extending from a pile in a substantially vertical direction up into a wall element, where the cable is connected io the wall element by an in situ cast portion of the reservoir wall. This configuration strengthens the connection between the wall element and the pile protruding from the foundation. The substantially vertical and tensioned cable pulls the wall element down towards the foundation, thus fixating the wall element, while the cable works as a reinforcement of the connection between the wail element and the pile. Thus, the connection between the wall element and the pile is made stronger.

Brief description of the figures

fig. 1 shows a cross sectional view of the seabed, a foundation and a wall element;

fig. 2 shows parallel coherent foundations, wall supporting elements and wall elements;

fig. 3 shows a cross sectional view of a wall element connecting to a foundation;

fig. 4 shows a cross sectional view of a wall element connecting to a foundation;

fig. 5 shows a floating wall element and the foundation installed in a seabed; fig. 6 shows a top view of connections between wall elements;

fig. 7 shows an offshore reservoir for energy storage; and

fig. 8 shows a cross sectional view of a reservoir wall. Detailed description

Figure 1 shows a cross sectional view of a section of a seabed 101 and a coherent foundation 102 extending into the seabed. The coherent foundation 102 is a coherent foundation being prepared by multiple overlapping drills 103. The multiple overlapping drills 103 create multiple neighbouring interlocking columns 105 simultaneously. The drills 103 are shown with means to mix 104 the cement material with the seabed material. The foundation 102 comprises interlocking columns 105 filled with a mixture of seabed material and cement material 102. The coherent foundation 102 shown is straight. While the foundation 02 is not fully cured, piles 106 are inserted down into the foundation 102. The insertion of the piles 106 is ended such that the piles 106 protrude from an upper portion of the foundation 102. The insertion of the piles 106 is ended such that the top of the piles are in substantially the same level. The means for inserting the piles 106 into the foundation 02 are not shown. The piles 106 shown have an I-beam shape that has been cut through in the cross sectional view. The piles 106 are inserted into every column in the coherent foundation 102.

The wall element 107 is straight and is placed for interconnection with the foundation 102. The portions of the piles 106 protruding from the foundation 102 act as male parts, and the wall element 107 contains a hollow compartment 108 in the bottom surface acting as a female part of the interconnection between the wall element 107 and the foundation 102. The length of the coherent foundation 102 is at least the length of the wall element 107. A cable 109 is attached to a portion of a pile 111 protruding from the foundation 102 and to the wall element 107 by means of a motor- driven winch 110. Controlled tension is applied to the cable 109 to increase control during the placing of the wall element 107, The cable 109 connects to the wall element 107 at a point that is located above the pile 1 1 which the cable 109 is connected to, when the wall element 107 is in its designated position. The tensioned cable 109 extends from the pile 111 in a substantially vertical direction up into the wall element 107. The interlocking columns forming the foundation 102 and the piles 106 protruding from the foundation 102 are substantially vertical. Figure 2 shows a first coherent foundation 102 and multiple second coherent foundations 201 , where the second coherent foundations 201 comprise multiple interlocking columns and are substantially parallel to the first coherent foundation 102 and located on the reservoir side of the first coherent foundation 102. This is the upper left side of the figure. The second coherent foundations 201 are parallel with a portion of the first coherent foundation 102 of at least of similar length and located in a transverse position of the second coherent foundation 201. Piles 202 are protruding from the first coherent foundation 102 and from the multiple second coherent foundations 201. The piles 202 have a hollow pile shape. Two wall supporting elements 203 are placed for connection with piles 202 protruding from the first coherent foundation 102 and the second coherent foundation 201. One wall supporting element 203 is shown in a cross-sectional view 204 down through the middle. The first coherent foundation 102 is continuous and extends further along the seabed 101 than the wall supporting elements 203. Wall elements 205 are interconnected with piles protruding from the first coherent foundation 102 and the wall supporting elements 203. The wall elements 205 are configured for a water impermeable connection with upper wall elements 206 along a substantially horizontal division. The upper wall elements 206 are placed on top of the wall elements 205. Both the wall elements 205 and the upper wall elements 206 have a height that is lower than the depth of the sea at the designated position of the wall elements 205, such that the wall elements 205 and the upper wall elements 206 must be stacked to reach above the sea surface 207. The wall supporting elements 203 are converting a portion of the overturning moment acting on the sea side of the wall elements 205 and upper wall elements 206 when these are damming up water to a substantially downwards force on the second foundation 201. A portion of this substantially downwards force is transferred to the second foundation 201 through the piles 202. The wall elements 205 and the upper wall elements 206 are arched such that they bulge out towards the surrounding sea. The first coherent foundation 102 comprises sections with a curvature substantially equal to the wall elements 205.

Figure 3 shows a cross sectional view of a wall element 107 connecting to a coherent foundation 102 extending into the seabed 101 . The foundation 102 comprises piles 202 protruding from an upper portion of the foundation. The wail element 107 hangs on the portion of the piles 202 protruding from the upper portion of the foundation 102. The top surfaces of the piles 202 protruding from the upper portion of the foundation 102 are substantially levelled. The wall element 107 comprises a hollow compartment 108 with a downward opening and an upwardly extending channel 301. The hollow compartment 108 accommodates a portion of the piles 202 protruding from the foundation 02. The hollow compartment 108 and the upwardly extending channels 301 are filled with grout. The hollow compartment 108 filled with grout provides a simple interconnection between the wall element 107 and the piles 202 protruding from the foundation 102. A gasket 302 is placed between the underside of the compartment 108 and the top of the foundation 102 and is at least partially compressed by the bad of the wall element 107. The gasket 302 seals off along the underside of the wall element 07. The upwardly extending channel 301 has a low exit point, such that it can work as a tremie pipe when grout or concrete is poured into the sealed hollow compartment 108. The upwardly extending channel 301 is a pipe that is preinstailed within the wall element 107. Figure 4 shows a cross sectional view of a wall element 107 connecting to a coherent foundation 102 extending into a naturally varying seabed 101 where the level of the top of the foundation 102 substantially follows the level of the naturally varying seabed 101. The coherent foundation 102 comprises l- beam shaped piles 106 protruding from an upper portion of the foundation. The piles 106 comprise a top plate 401. The tops of the piles 401 are substantially in the same level, even though the foundation 102 substantially follows the naturally varying seabed 101. A rigid skirt 402 is attached to the wall element 107 and is inserted into the seabed 101. The rigid skirt 402 compensates for the naturally varying seabed 101 and forms at least a part of a sealed enclosure 408 established between the wall element 107 and the seabed 101. The wall element 107 has a substantially plane bottom underside 409. The wall element 107 comprises a hollow compartment 108 that forms a part of the sealed enclosure 408. The sealed enclosure 408 is fully encapsulating the portion of the piles 106 protruding from the foundation 102. The wall element 107 is configured with a retracted underside 403 that is configured for carrying the load of the wall element 107, thus making it a bearing underside 403. The bearing underside 403 forms the top of the hollow compartment 108. The wall element 107 is placed such that a bearing underside 403 of the wall element is positioned above a top portion of the piles 401 , such that a gap 404 is created between the bearing underside 403 and the top portion of the piles 401. The first positioning means are not shown. A flexible bag 411 is shown before it is activated to bridge the gap 404. The flexible bag 411 is positiored between the bearing underside 403 of a wall element 107 and the top of a pile 401. Another flexible bag 412 is shown after it has been activated and has expanded to establish a contact between the top portion of the piles 401 and the bearing underside 403. Passages 410 are connected to the flexible bags 411 ; 412 such that the filler material that causes the flexible bag to expand can be charged or filled through the passages 410. The flexible bags 4 1 ; 412 can be activated from above the sea surface, by the filler material being charged or filled into the flexible bags 411 ; 412 through the passages 410. The flexible bags 411 ; 412 are examples of a type of second positioning means. A set of wedges 405 is shown before the wedges are activated to bridge the gap 404. The set of wedges 405 is activated by them being pushed towards each other from opposite sides. The means for activating the sets of wedges 405; 406 are not shown, but could be e.g. by turning a threaded rod. Another set of wedges 406 is shown after they have been activated and have established a contact between the top portions of the piles 401 and the bearing underside 403. The sets of wedges 405; 406 are examples of a type of second positioning means. Different types of second positioning means 41 1 ; 412; 405; 406 are shown for illustration purposes. Ordinarily, only one type of second positioning means would be used for a wall element, but different types can be used. The wall element 107 is configured with a portion of the bearing underside configured for deformation 407. The portion configured for deformation 407 is shown in a deformed state due to the weight of the wall element 107. Typically, a wall element 107 is not configured with both second positioning means and a bearing underside with a portion configured for deformation 407. Typically, a wall element 107 configured with a bearing underside with a portion configured for deformation 407 is configured such that each pile 06 designated for connection with the respective wall element 107 connects to a portion of the bearing underside configured for deformation 407.

Figure 5 shows a wall element 107 floated by external buoyancy means 501. The external buoyancy means 501 are shown as a rigid shell structure. The sea surface 207 is shown. The external buoyancy means 501 are attached to the wall element 107 by use of cables 502, and the wall element 107 floats in an upright position. The wail element 107 is lowered by slackening of the cables 502. Rigid skirts 402 are attached to the wall element 107. The skirts 402 are attached to the wall element 107 in their final position. When the wall element 107 is lowered down to its designated position, the skirts 402 are inserted into the seabed 101 by the operation of lowering the wall element 107. The skirts 402 are inserted into the seabed 101 on both sides of the foundation 102, along the length of the wall element 107. Only a section of seabed 101 is shown. A pile cap 503 is interconnected to piles 202 designated for interconnection with the wall element 107. The pile cap 503 is shown in a cross-sectional view, revealing the connection between the pile cap 503 and the piles 202. The pile cap 503 is connected to the piles 202 by a grouting process. The flexibility of the grouted connection 504 is utilized to ensure that the pile cap 503 is substantially levelled, even though the tops of the piles 202 are at different levels. The wall element 107 comprises a bearing underside 403 retracted from the bottom underside 409 of the wall element 107. When the wall element 107 is positioned, the bearing underside 403 is located above the top of the protruding piles 202, and a bottom underside 409 is located below the top of the protruding piles 202, such that the wall element 107 between the retracted underside and the bottom underside forms a wall section 506 that extends below the top of the protruding piles 202. The piles 202 can have another cross sectional shape, e.g. such as it is shown in figure 1 and figure 4. The coherent foundation 102 is prepared in a substantially levelled trench 505 in the seabed 101.

Figure 6a-c shows a top view of examples of connections between adjacent wall elements 601 ; 602. Only an end portion of the wail elements 601 ; 602 is shown.

Figure 6a shows a first wall element 601 and a second wall element 602 erected adjacently with a gap between them. Plates 603 are positioned on both sides of the wall elements 601 ; 602 such that the plates 603 have contact with both wall elements 601 ; 602 and form an enclosure 604 between the wall elements 601 ; 602. The plates 603 are pulled together by tensioning means 605 attached to both plates 601 ; 602 through the gap. Figure 6b shows a direct connection between a first wall element 601 and a second wall element 602 that are overlapping. The first wall element 601 and the second wall element 602 are interconnected by means of a bolted connection 606. The connection between the neighbouring wail elements 601 and 602 comprises sealing means 607, e.g. gaskets, to ensure a sealed connection. The neighbouring wall elements 601 ; 602 are positioned adjacently and are interconnected directly to each other with a sealed interconnection.

In an embodiment, a direct interconnection between neighbouring wall elements is grouted to form a solid and sealed interconnection.

Figure 6c shows a wall supporting element 203 placed at a connection between two adjacent wail elements 601 ; 602, where the wall supporting element 203 is overlapping both wall elements 601 ; 602. A plate 603 seais off an enclosure 604 between the wall elements 601 ; 602 and the wall supporting element 203. The plate 603 is pulled towards the wall supporting element 203 by tensioning means 605, such that the plate 603 seals off against the first and second wall elements 601 ; 602, in an embodiment, this sealed enclosure 604 is grouted to form a sealed and rigid interconnection. Figure 7 shows an offshore reservoir 701 for energy storage. The reservoir 701 encloses a volume 702 and is configured to withstand water pressure from surrounding sea water 703. The reservoir 701 stores energy by pumping water from the lower reservoir 702 to the surrounding sea 703. The energy is reproduced by letting water from the surrounding sea 703 into the lower reservoir 702 through a turbine. The reservoir 701 is formed by multiple interconnected wall elements 107 that are connected directly to each other with a sealed interconnection. The reservoir 701 substantially forms a regular polygon when viewed from above. Intermittent renewable energy converters 704; 705 are installed on the reservoir 701. The shown intermittent renewable energy converters are wind turbines 704 and a wave energy converter 705 with multiple floats attached to the reservoir wall.

The wall elements 107 comprise skirts 402 inserted into the naturally varying seabed 101. The skirts 402 compensate for the variations in seabed height, such that the wall elements 107 can be manufactured with a plane bottom surface. The individual wall elements 107 are configured with a height that corresponds to the sea depth at its designated position. Thereby, the height of the skirt 402 is minimized, and thus also the size of the sealed enclosure which is grouted in situ.

The offshore reservoir 701 comprises a separate housing facility 706 located inside the reservoir. The separate housing facility 706 comprises a pump, for draining water from the reservoir 701 by use of energy, and a turbine, for generating energy when surrounding seawater 703 is filled into the reservoir 701. The separate housing facility 706 communicates with the sea 703 through a pipe system 707. The separate housing facility 706 is located at seabed level to reduce problems with cavitation within the pump. In an embodiment, the separate housing facility 706 accommodates a pump- turbine. The wall elements 107 have a height that makes them extend above the sea level, at the location of the reservoir, when it is placed at its designated position on the foundation.

Figure 8 shows a cross-sectional view of a reservoir wall 801. The reservoir wall 801 comprises a wall element 107 that connects to a first coherent foundation 102 and a second coherent foundation 201. Piles 106 are protruding from the first coherent foundation 02 and the second coherent foundation 201. The wall element 107 is connected to the piles 106 and the first coherent foundation 102 and second coherent foundation 201 with grouted connections 802. The reservoir wall 801 dams up the surrounding sea water 703. Water has been pumped out of the lower reservoir 702 such that energy is stored. Both the first coherent foundation 102 and the second coherent foundation 201 extend to a lower layer of the seabed 803 with a lower permeability than the top of the seabed, e.g. a layer with a hydraulic conductivity of 10^i_7) m/s or lower. The wall element 107 is interconnected with piles 106 protruding from the first coherent foundation 102 and with piles 106 protruding from the second coherent foundation 201. The piles 106 protrude from the foundation 02; 201 up into the wail element 107 to form a good interconnection between the wall element 107 and the foundation 102; 201. The piles 106 work as shear keys and transfer the large forces, acting on the wall element 107 from damming up water, to the foundation 102; 201. The wall element 107 comprises a substantially vertical sea side wail portion 804 that connects to the first coherent foundation 102 and a reservoir side wall portion 805 that extends from an upper portion of the wall element 07 downwards towards the seabed inside the reservoir, at least for some length of the reservoir side wall portion, and connects to the second coherent foundation 201. Thereby, the reservoir wall 801 comprises means to convert at least a portion of the overturning moment acting on the sea side of the reservoir, when damming up water, to a substantially downwards force on the second coherent foundation 201.

In an embodiment, the wall element 107 comprises transverse walls extending from the substantially vertical sea side wall portion 804 to the reservoir side wall portion 805, thereby making the wall element stronger and stiffer. The piles 106 can have another cross sectional shape, e.g. such as it is shown in figures 2, 3 and 5.

In an embodiment there is provided a method of building at least a portion of an offshore reservoir for energy storage, comprising: preparing, on an offshore location, a foundation that extends into the seabed; lowering a respective wall element and placing it for interconnection with the foundation; erecting plurality of said wall elements and interconnecting them to form at least a portion of a reservoir; wherein the foundation is prepared by a series of operations comprising: lowering a drill into the seabed to loosen seabed material within a first bore, retracting the drill upwards, and adding cement material to mix with the seabed material to thereby make a first column with a first volume; making a second column with a second volume such that the first column and the second column interlock in a coherent foundation structure when the material within the first and second volume cures; while the foundation is not fully cured, inserting piles down in the foundation and ending the insertion of a respective pile such that the pile protrudes from an upper portion of the foundation; and repeating the above such that multiple columns in the foundation interlock with a neighhouring column to form a coherent foundation; wherein the step of erecting multiple wall elements comprises securing the wall element to a portion of a pile that protrudes from the upper portion of the foundation.

Generally, other terms for a 'wall element' could include terms like 'panel' or 'wall panel' or similar terms.

Claims

1. A method of building at least a portion of an offshore reservoir (701 ) for energy storage, where the reservoir encloses a volume and is configured to withstand water pressure from surrounding sea water; where the offshore reservoir is configured with a pump for draining water from the reservoir by use of energy and a turbine for generating energy when filling surrounding seawater into the reservoir; said method comprising:

- preparing, on an offshore location, a foundation (102) that extends into the seabed;

- ferrying wall elements (107; 205) to the offshore location;

- lowering a respective wall element and placing it for interconnection with the foundation;

- erecting plurality of said wall elements and interconnecting them to form at least a portion of a reservoir; wherein the foundation is prepared by a series of operations comprising:

- lowering a drill (103) into the seabed to loosen seabed material within a first bore, retracting the drill upwards, and adding cement material to mix with the seabed material to thereby make a first column with a first volume;

- making a second column with a second volume such that the first column and the second column interlock in a coherent foundation structure when the material within the first and second volume cures;

- while the foundation is not fully cured, inserting piles (106; 202) down in the foundation and ending the insertion of a respective pile such that the pile protrudes from an upper portion of the foundation;

- repeating the above such that multiple columns in the foundation interlock with a neighbouring column to form a coherent foundation; wherein the step of erecting multiple wall elements comprises securing the wall element to a portion of a pile that protrudes from the upper portion of the foundation.

2. A method according to claim 1 , comprising:

- preparing a further coherent foundation (201 ) in the seabed, such that the reservoir has a first and a second coherent foundation, where the second coherent foundation is located on a reservoir side of the first coherent foundation; and wherein the reservoir wall comprises means to convert at. least a portion of the overturning moment, acting on a sea side of the reservoir when damming up water, to a substantially downwards force on the second coherent foundation.

3. A method according to claim 2, comprising:

- ferrying a wall supporting element (203) to the offshore location;

- placing the wall supporting element for interconnection with piles protruding from the first coherent foundation and with piles protruding from the second coherent foundation;

- ferrying a wall element to the offshore location; and

- placing the wall element for interconnection with piles protruding from the first coherent foundation and for interconnection with the wall supporting element.

4. A method according to any of claims 1-3, comprising:

- placing a wall element such that a retracted underside (403) of the wall element is located above a top portion of the protruding piles and a bottom underside (409) of the wall element is located below the top of the protruding piles; wherein the wall element between the retracted underside and the bottom underside forms a wall section (506) that extends below the top of the protruding piles, at least on the side of the piles facing the surrounding sea when the reservoir is formed; and wherein said wall section is reinforced to transfer forces, acting on the wall element, to the top portion of the piles.

5. A method according to any of claims 1-4, comprising:

- positioning a wall element such that it is hanging on the portion of the piles protruding from the upper portion of the foundation.

6. A method according to any of claims 1-5, where the wall element comprises a hollow compartment (108) with a downward opening and with an upwardly extending channel; and wherein the method comprises:

- placing the wall element such that the hollow compartment accommodates a portion of the piles protruding from the top of the foundation; where a gasket (302) is placed between the underside of the compartment and the top of the foundation, such that the gasket is at least partially compressed by the load of the wall element; and

- pouring grout or concrete into the hollow compartment through the upwardly extending channel; where the gasket is configured to withhold the grout or concrete.

7. A method according to any of claims 1-5, comprising:

- inserting rigid skirts (402), attached to the wall element, into the seabed on both sides of the coherent foundation, along the length of a wall element, where said rigid skirts forms a part of a sealed enclosure (408), established between the wall element and the seabed on both sides of the coherent foundation; and

- pouring grout or cement into the sealed enclosure through a pipe.

8. A method according to any of claims 1-7, wherein placing a wall element comprises:

- positioning a wall element by means of first positioning means, such that a bearing underside of the wall element is positioned above a top portion of the piles designated for connection with the respective wall element, such that a gap is created between the bearing underside of the wall element and the top portion of the piles;

- activating second positioning means (405; 412) in order to bridge the gap and establish a contact between the top portion of the piles and the bearing underside; where the second positioning means are configured to carry the load of the wall element; and

- releasing the first positioning means such that the second positioning means carry a substantial amount of the load of the wall element.

9. A method according to claim 8, comprising:

- placing a filler material with curing ability within a flexible bag (411 ; 412) positioned between the bearing underside of a wall element and a top portion of a pile, wherein the filler material is placed in the flexible bag through a passage, such that the flexible bag expands to bridge the gap between the bearing underside of wall element and the top of the pile.

10. A method according to any of claims 1-7, where a wall element has a bearing underside with a portion configured for deformation (407) and wherein placing the wall element comprises:

- positioning a wall element such that the bearing underside is positioned on a top portion of the piles designated for connection with the respective wall element; and

- lowering the wall element such that the portion configured for deformation deforms and such that the weight of the wall element is distributed substantially evenly on the piles designated for carrying the wall element.

11. A method according to any of claims 1 -9, comprising:

- positioning a pile cap (503) on top of multiple piles designated for connection with a respective wall element;

- interconnecting the pile cap and said piles, such that the pile cap is substantially levelled; and

- positioning the wall element such that a bearing underside, retracted from the bottom underside of the wall element, rests on the pile cap, where the pile cap is carrying a significant amount of the load of the wall element,

12. A method according to any of claims 1-11 , comprising:

- attaching a cable (109) to the portion of a pile protruding from the foundation;

- attaching the cable to a wall element, by means of a motor-driven winch, before placing the wall element, such that the cable connects to the wall element at a point that is located above said pile, when the wall element is in its designated position; and

- applying controlled tension to the cable to increase control during placing of the wall element.

13, A method according to any of claims 1-12, comprising:

- erecting a first and a second wall element adjacently to each other with a gap between them;

- positioning a first plate (603) on the reservoir side of the wall elements and a second plate on the sea side of the wall elements, such that said plates have contact with both the first and the second wall element and form an enclosure between the wall elements;

- pulling the plates together by use of tensioning means.

14. A method according to any of claims 1 -13, comprising:

- installing an intermittent renewable energy converter (704; 705) on the reservoir, where the renewable energy converter is coupled to a pump configured to drain water from the reservoir.

15. A method according to any of claims 1-14, comprising:

- trenching a substantially levelled trench (505) of at least the length of a wall element; and

- preparing the foundation in the trench.

16. An offshore reservoir, wherein the reservoir encloses a volume and is configured to withstand water pressure from surrounding sea water; wherein the offshore reservoir is configured with a pump for draining water from the reservoir by use of energy and/or a turbine for generating energy when filling surrounding seawater into the reservoir, said reservoir comprising:

- a reservoir wall that comprises multiple wall elements;

- a coherent foundation comprising drilled interlocking columns, said interlocking columns comprising a mixture of seabed material and a cement material, where the foundation extends vertically from a level substantially corresponding to the seabed level and down into the seabed; and

- multiple piles cast into the foundation and protruding from the foundation up into the reservoir wall, where the piles are interconnected to the reservoir wall.

17. An offshore reservoir according to claim 16, comprising:

- a first coherent foundation comprising drilled interlocking columns, said interlocking columns comprising a mixture of seabed material and a cement material, where the foundation extends vertically from a level substantially corresponding to the seabed level and down into the seabed;

- a second coherent foundation, comprising drilled columns, said columns comprising a mixture of seabed material and a cement material, where the foundation extends vertically from a level substantially corresponding to the seabed level and down into the seabed; wherein the second coherent foundation is located on the reservoir side of the first coherent foundation;

- a reservoir wall comprising means to convert at least a portion of the overturning moment acting on the seaside of the reservoir wall when damming up water to a substantially downwards force on the second foundation.

18. An offshore reservoir according to claim 7, comprising:

- a wail supporting element configured to be interconnected with piles protruding from the first coherent foundation and with piles protruding from the second coherent foundation;

- a wall element interconnected with piles protruding from the first foundation and with the wall supporting element.

19. An offshore reservoir according to any of claims 16-18, comprising:

- a wall element with a retracted underside and a bottom underside; where the retracted underside is located above a top portion of the portion of the piles protruding from the foundation, and the bottom underside is located below the top of the portion of the piles protruding from the foundation and on the sea side of the portion of the piles protruding from the foundation; and wherein the wall element is reinforced in a portion extending from the bottom underside to up above the retracted underside.

20. An offshore reservoir according to any of claims 16-19, comprising:

- a wall element comprising a compartment that accommodates a portion of the piles protruding from the foundation, where said compartment is filled with cured grout or cement; and

- a gasket located between the underside of the compartment and the top of the foundation, where the gasket is at least partially compressed.

21. An offshore reservoir according to any of claims 16-19, comprising:

- rigid skirts extending from the reservoir wall and into the seabed on both sides of a foundation, where the skirts are of at least the length of the wall element they are connected to; and

- an in situ cast portion connecting to the skirts, the foundation and the wall element the skirts are connected to.

22. An offshore reservoir according to any of claims 16-21 , comprising:

- second positioning means enclosed within an in situ cast portion of the reservoir wall, where the second positioning means connects to a prefabricated portion of a wall element and to piles cast into the foundation.

23. An offshore reservoir according to any of claims 6-2 , comprising:

- a wall element comprising a bearing underside with a portion configured for deformation that is located on top of the portion of the piles protruding from the foundation; where the portion of the bearing underside configured for deformation is locally deformed.

24. An offshore reservoir according to any of claims 16-22, comprising:

- a substantially levelled pile cap positioned on top of multiple piles protruding from the foundation;

- a wall element positioned with a retracted bearing underside on top of the substantially levelled pile cap.

25. An offshore reservoir according to claim 16-24, comprising:

- a tensioned cable extending from a pile in a substantially vertical direction up into a wall element, where the cable is connected to the wall element by an in situ cast portion of the reservoir wall.

26. An offshore reservoir according to any of claims 16-25, comprising:

- a first plate located on the reservoir side of the reservoir wall and connecting to two neighbouring wall elements;

- a second plate located on the sea side of the reservoir wall and connecting to the same two neighbouring wall elements;

- tensioning means attached to the first plate and tensioning means attached to the second plate;

where the first and second plates form a sealed connection between the two neighbouring wall elements.

27. An offshore reservoir according to any of claims 16-26, comprising:

- an intermittent renewable energy converter installed on the reservoir, where the renewable energy converter is coupled to a pump configured to drain water from the reservoir.

28. An offshore reservoir according to any of claims 16-27, comprising:

- a foundation prepared in a substantially levelled trench in the seabed.