NO348102B1

NO348102B1 - Device, system and method for removing a subsea filter layer

Info

Publication number: NO348102B1
Application number: NO20201101A
Authority: NO
Inventors: Arild Ariansen
Original assignee: Scanmudring As
Priority date: 2020-10-12
Filing date: 2020-10-12
Publication date: 2024-08-19
Also published as: WO2022081019A1; NO20201101A1

Description

DEVICE, SYSTEM AND METHOD FOR REMOVING A SUBSEA FILTER LAYER

The present invention relates to a device for removing a subsea filter layer. More particularly the invention relates to a device comprising a mass removal unit for moving granulated material from the filter layer and a power unit for powering the mass removal unit. The invention also relates to a method for installing a jacket of an offshore wind turbine.

Layers of granulated material are often used to protect subsea installations and to avoid winnowing of underlying sand and soil particles. So-called filter layers are used to prevent migration of underlaying sand and soil particles through the relatively large voids of an overlying, cruder amour layer. Without the filter layer, the underlying sand would be lost, and larger stones in the armour layer would sink into the void. In addition, a filter layer may be beneficial for the distribution of weight over the underlying base material to provide more uniform settlement. Finally, a filter layer may also be beneficial for reducing hydrodynamic loads acting on subsea installations.

More recently the size of subsea installations, including fundaments for offshore wind turbines, have been increasing. The increased size requires larger foundation piles and thereby also thicker filter layers consisting essentially of larger rocks. Installation of such foundation piles usually involves the use of pre-piling templates to ensure correct and precise positioning and inclination of the piles. With the increased size of the piles, such as with a diameter of 2.5 m or more, and increased thickness of the filter layer, the present applicant has found that piles may be damaged by forcing/piling them through the filter layer. As such, there may be a need to remove at least a portion of the filter layer in the areas where the piles are to be installed. With larger subsea installations also the size and height of the pre-piling templates are increasing. Recently the present applicant has also found that with the current height of modern pre-piling templates, which in extreme cases can be more than 20 meters, is so large that no pumps or ejectors are commercially available that is adapted to lift granulates of the filter layer out of the pre-piling holes/sleeves. The alternative has been to use a dredge to remove a small portion of the filter layer at the time. Dredging is time-consumable, particularly inside templates where the amount of available space is limited.

Patent document JPH01219220A discloses enabling sediment to be easily collected by carrying the sediment with a robotic vehicle moving freely on the seabed.

Patent document KR20120067072A discloses a submarine mineral collecting method to primarily store submarine mineral in a storage container by installing the storage container in the seawater and to load the collected submarine mineral on a transferring barge by floating the container when the storing container is full so that the submarine mineral can be rapidly moved out of the water.

Patent document JPS50122402A discloses a capsule placed in water that communicates with water and a force transport pipe and is configured to separate the solid-liquid mixed flow nigiri air sent through the inter-pipe.

Patent document CN105714863A discloses an unmanned dredge for processing industrial sludge. The unmanned dredge comprises an outer frame composed of a circular-ringshaped bottom plate and a connecting beam.

Patent document WO2019/013646A1 discloses a device for removal of sediment from inside piles being at least partly immersed in water, comprising an outer guiding unit arranged to be temporarily positioned on the top of a pile. The outer guiding unit envelopes at least one inner dredging unit being arranged to be lowered from within the outer guiding unit, the inner dredging unit at its lower end exhibiting movable jet nozzles arranged to loosen sediment. The inner dredging unit furthermore comprises a central passage which is connected to a discharge hose arranged to transport loosened sediment therefrom.

Patent document EP2492401A discloses a device for manufacturing a foundation for a mass located at height, such as the jacket of a wind turbine or a jetty, wherein the foundation comprises a quantity of piles driven into an underwater bottom in a geometric pattern. The device comprises a positioning framework of a number of mutually connected guide tubes arranged in a geometric pattern and adapted to receive and guide a pile to be driven into the underwater bottom, wherein the guide tubes comprise measuring means adapted to determine the height of a pile present in the guide tubes.

Patent document EP3483347A1 discloses a dredging system for a pre-paved gravel foundation bed surface in open sea deep water including a dredging mechanism.

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art.

The object is achieved through features, which are specified in the description below and in the claims that follow.

The invention is defined by the independent patent claims. The dependent claims define advantageous embodiments of the invention.

In a first aspect, the invention relates to a device for removing a subsea filter layer, the device comprising:

- a mass removal unit for moving granulated material from the filter layer; and

- a power unit for powering the mass removal unit, wherein the device further comprises a storage unit for receiving and temporarily storing the granulated material from the mass removal unit wherein the storage unit comprises two or more storage tanks symmetrically arranged about a vertical axis of the device.

By providing the device with a storage unit, the required lifting height of granulates/rocks of the filter layer may be reduced. Instead of lifting the granulates from the seabed to a height above a subsea template or another subsea installation, such as a pile, the granulates are lifted to a storage unit connected to and/or integrated with the device according to the first aspect of the invention. Preferably, the storage unit is of a size and construction which makes it possible to collect and temporarily store as much as possible of the material that needs to be removed at a certain location, such as within a hole/sleeve of a pre-piling template, without having to make several runs to/from this location, as will be explained below. When the filter layer has been removed, at least to the required extent, and/or the storage unit has reached its storage capacity, then the device according to the invention may be lifted away from the location at which it was operating for dumping the granulates at a position outside this location where no piling is to take place.

Such an arrangement of the storage tanks may allow the one or more two inlets of the mass removal unit, such as one or more ejector inlets, to be provided between the two or more storage tanks, and wherein two outlets of the mass removal unit, one for each storage tank, may be directed into one of the storage tanks. This provides for a simple, yet flexible and robust solution.

In one embodiment the mass removal unit may include one or more ejectors for moving the granulated material from the filter layer to the storage unit. It may be advantageous to use ejectors subsea as ejectors do not have any moving parts and therefore require little maintenance. The one or more ejectors may be movable in the horizontal plane to (together) cover an area inside the boundaries of the device according to the invention. This may be useful to cover a larger area of filter layer without having to translate the device itself. In one embodiment the one or more ejectors may be linearly moveable back and forth on a guide beam, such as by means of linear actuators / travel screws. In an alternative embodiment, the mass removal unit may comprise one or more pumps for pumping the granulated material from the filter layer to the storage unit. The one or more pumps may be provided as airlift pumps.

In one embodiment, the storage unit may have a storage capacity the range of 5 m<3 >to 50 m<3>, preferably in the range of 10 m<3 >to 20 m<3>, and even more preferably around 17 m<3>, where the latter preferred volume represents that of an amount of filter layer that the applicant has found necessary to remove in a specific development project before installing foundation piles into the seabed at the position of the removed filter layer.

Preferably, the inlet for each of the one or more mass removal units may be telescopically arranged so that the inlet is vertically movable substantially through the thickness of the filter layer. This implies that the device itself may remain vertically fixed through the removal of the filter layer. This may be particularly beneficial if the inlet(s) of the mass removal unit is/are also movable in the horizontal plane, as discussed above, whereby the whole device itself may remain fixed and potentially latched to and/or supported by a subsea structure, such as a pre-piling template, during removal of the subsea filter layer.

In one embodiment, each storage tank may be provided with a discharge hatch at its lower end. This may be useful for discharging the removed and stored granulates of the filter layer outside the area from which it was removed. As an alternative to a discharge hatch, the storage tanks may be tiltable to be emptied.

In one embodiment, the device may be rotatable around its vertical axis, such as by means of a rotary drive suspension. This may be useful to cover a large area of subsea ground by the inlet of the mass removal unit without having to translate the device as such. In combination with one or more linearly movable ejectors, this may cover substantially the whole area within the envelope of the device according to the invention.

It should be noted that the power unit as included in the device according to the invention may be any unit adapted to generate and/or distribute power to the device, including to the mass removal unit with pumps and/or ejectors and any mechanism, such as a hatch, to dispense with the granulates from the filter layer. It may be electric power as generated locally subsea or provided via an umbilical from topside which is treated and distributed via the power unit. Alternatively, or in addition, the power unit may be able to receive and distribute hydraulic power from topside via an umbilical by means of subsea hydraulic valves. Hydraulic power may alternatively or in addition be generated subsea.

In a second aspect, the invention relates to a system for removing a subsea filter layer, the system comprising a device according first aspect of the invention and the system further comprising:

- a device deployment system for deploying the device from a surface vessel.

The deployment system may be any combination of components enabling lowering of the device from the surface to the seabed and bringing it back up. In one embodiment, the deployment system may further include a topside power unit and a topside control unit for powering and controlling, respectively, the device when deployed subsea. As an alternative or addition, the device itself may be adapted to generate electric and/or hydraulic power and/or the device may be provided with batteries and/or hydraulic accumulators and optionally a control unit adapted to control the device in closed loop configuration when subsea.

In one embodiment, the deployment system may further comprise an

umbilical winch and an umbilical for connecting the device to topside and for providing power to and communication with the device.

In one embodiment, the system may further comprise a pre-piling template including one or more openings into which a pile is to be positioned. Pre-piling templates are used to ensure correct positioning and inclination of piles before the piles are used to support and found subsea structures such as platforms and jackets or monopiles for offshore wind turbines.

In one embodiment, the device according to the first aspect of the invention may be adapted to be provided/fit inside the one or more openings in the pre-piling template to remove the filter layer from the inside of the one or more openings. As such, it may be useful if the device is formed with a circular or substantially circular outer shape in the horizontal plane. Preferably, the device is adapted to latch onto and/or be supported by the pre-piling template during a subsea filter layer removal operation to ensure stability and correct positioning of the device.

The system according to the second aspect of the invention will, during operation, include a filter layer of granulated material. In certain embodiments, the filter layer may essentially comprise rocks with a size of 125-180 mm, as this has been shown to offer particularly good filter layer protection while at the same time making it possible of remove the rocks efficiently with a pump or ejector. However, the system according to the invention may also include filter layers with rocks of smaller and/or larger sizes.

In certain embodiments the one or more openings in the pre-piling template may have a diameter of 2.5 m or more, preferably 4 m or more and even more preferably around 4.7 m or even more. The one or more openings may be provided as sleeves with a certain height, such as above 10 m and more, and in some embodiments around 20 m.

In a third aspect the invention relates to a method for installing a jacket of an offshore wind turbine, the method including the steps of:

- - providing a subsea filter layer on the seabed;

-providing a device according to the first aspect of the invention or a system according to the second aspect of the invention;

- placing a pre-piling template on the seabed at the position of the subsea filter layer, the pre-piling template including one or more openings into which a pile is to be positioned; - removing, within the one or more openings, at least a portion of the subsea filter layer;

-temporary storing granulates from the removed filter layer in a storage unit of the device;

- inserting a pile in each of the one or more openings in the pre-piling template; and - installing a jacket of an offshore wind turbine on or in the one or more piles.

The method may preferably be performed by means of a system according to the second aspect of the invention.

It should also be noted that the device and system according to the first and second aspects of the invention may also be used in methods to install other subsea structures, such as a monopile for an offshore wind turbine, which may or may not require the use of a pre-piling template for its installation, and a jacket for a oil/gas platform.

It may be of great advantage to be able to temporarily store the removed granulated material of the filter layer in a storage unit, whereby the granulated material of the filter layer may not need to be lifted all the way out of a pre-piling template or similar by means of pumps or ejectors.

After having collected and stored the filter layer, the whole device with the storage unit may be lifted out of the pre-piling template (if present) whereby the removed filter layer may be dumped without complicating or endangering subsequent piling.

In one embodiment, the step of providing a subsea filter layer may include the step of providing a subsea filter layer with a thickness of 0.3 m to 2.5 m, or preferably 0.7 m to 1.1 m, which has been shown to be optimal for protecting and supporting large foundation piles on the seabed.

In the following is described an example of a preferred embodiment illustrated in the accompanying drawings, wherein:

Fig.1 shows a device according to the invention in a perspective view;

Fig.2 shows the device from Fig.1 in a first side view;

Fig.3 shows the device from Fig.1 in a second side view;

Fig.4 shows the device from Fig.4 in a top view;

Fig.5 shows a system according to the invention; and

Fig.6 shows a portion of a subsea structure installed after use of the system from Fig.5

In the following the reference numeral 1 will be used to indicate a device according to the first aspect of the invention, while reference numeral 10 will be used to indicate a system according to the second aspect of the invention. Identical reference numerals are used to identify identical or similar features in the drawings. The drawings are drawn schematically and simplified, and various features therein are not necessarily drawn to scale.

Reference is first made to Figs.1-4 which show an embodiment of a device 1 for removing a subsea filter layer according to the invention. The device 1 comprises a power unit 2 and a mass removal unit 4, where the latter is best seen in Figs.2-3. The power unit 2 receives, treats and distributes electric and/or hydraulic power to the rest of the device 1.. The power unit 2 is adapted to connect to a topside vessel 100 via an umbilical 20 as can be seen in Fig.5. Workers 5 are shown in Figs.1-4 to give an indication of the size of the device 1 according to the shown exemplary embodiment. However, it should be noted that the device may vary greatly in size depending on its specific use. In the shown embodiment, the mass removal unit 4 includes a pair of ejectors 6, each with an inlet 8 near the bottom 12 of the device 1. In the shown embodiment, the storage unit of the device 1 includes two storage tanks 16. An outlet 18 of each ejector 6 is directed into a respective storage tank 16. The two storage tanks 16, which are formed with an outer shape as a circle sectors, are arranged inside a substantially circular frame 17, provided with a distance/gap 22 between them and arranged symmetrically around a central, vertical axis A of the device 1, as indicated in Figs.1 and 2. In this gap 22 a guide beam 24 is provided on which each of the ejectors 6 is linearly movable back and forth substantially along the full diameter of circular frame 17. In the shown embodiment, the device 1 is provided with a rotary drive suspension 26 at the top of the power unit 2 enabling controlled rotation of the whole device 1. The rotary drive suspension 26 is, in the shown embodiment, connectable to a not shown three-legged lifting yoke further connecting the device 1 to a lifting wire rope 42 as shown in Fig.5. Alternatively, the rotary suspension 26 may be provided at a height below the power unit 2 above the storage and filter layer removal units. Together with the linear movement of the ejectors 6, the rotary suspension drive 26 makes it possible for the ejector inlets 8 to cover more or less the whole x-y plane within the envelope of the device 1 without having to translate the device 1. In alternative embodiments, movement in the x-y plane may also be made possible by linear movement in two directions, potentially without a rotary joint. Movement in the vertical direction / z plane is enabled by the inlet portion 8 of each ejector being vertically extendable / telescopic to extend to a position below the storage unit 16, thereby reducing the need to lower/lift the device 1 from topside during collection of granulates from the filter layer 40. Preferably the inlet portion 8 is vertically movable to cover the full thickness of the filter layer 40. The ejectors 6 and storage tanks 16 may also optionally be provided with monitoring means such a camera and one or more sensors, such as weight and/or position sensors, to monitor the position of the ejectors as well as the status/fullness of the storage tanks 16.

The two storage tanks 16 in the shown embodiment have a combined storage capacity of about 17 m<3>, leading to an estimated underwater storage weight of approximately 16Te. Each of the storage tanks is provided with a discharge hatch 28 hydraulically or electrically operable by means of a pair of hatch cylinders 30, only one of each pair being shown in Figs. 2 and 3. The discharge hatches 28 enable controlled emptying of the storage tanks 16.

A system 10 according to the second aspect of the invention is shown highly schematically in Fig.5. On the seabed 32, a pre-piling template 34 is provided. Two openings/sleeves 36 are shown into which piles 38, as shown in Fig.6, are to be installed. Around and inside the pre-piling template 34 a filter layer 40 of granulated material is provided. In the shown embodiment, the filter layer has a thickness of about 1 m. Before installing the piles 38, the filter layer 40 needs to be, at least partially, removed from inside the holes/sleeve 36 to avoid damaging the piles 38 during installation (piling). In the shown embodiment, the openings 36 have a diameter of approximately 4.7 meters, while the filter layer essentially consist of rocks with a size of 125-180 mm. A system for deploying the device 1 subsea is generally referred to by reference numeral 60. A device 1 according to the invention is suspended from a lifting wire rope 42 lowered from a lifting winch 44 on the vessel 100. The lifting winch 44 may normally be included in a not shown crane. An umbilical winch 46 connects a topside power unit 48 and control unit 50 to the power unit 2 of the device 1 via the umbilical 20.

Fig.6 shows the same area of the seabed 32 as Fig.5 after the piles 38 have been installed and the pre-piling template 34 removed. The piles 38 have been used a fundament for a large subsea structure, here shown as a jacket 52 of a not shown offshore wind turbine, where the legs of the jacket 52 are placed and grouted inside the piles 38. Alternatively, the legs of the jacket 52 may be placed outside/around the piles 38. The piles 38 could equally well be used to found and support fundaments for oil/gas platforms or other offshore structures. An armour layer 54 of larger rocks has been added on top of the filter layer 40 for reasons explained above. In the shown embodiment, the filter and armour layers 40, 54 together define the scour protection for the jacket 52.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

C l a i m s

1. Device (1) for removing a subsea filter layer (40), the device (1) comprising:

- a mass removal unit for moving granulated material from the filter layer (40); - a power unit (2) for powering the mass removal unit (4); and

– a storage unit (16) for receiving and temporarily storing the granulated material from the mass removal unit (4) , c h a r a c t e r i s e d i n that the storage unit (16) comprises two or more storage tanks (16) symmetrically arranged about a vertical axis (A) of the device (1).

2. Device (1) according to claim 1, wherein the mass removal unit (4) includes one or more ejectors (6) for moving the granulated material from the filter layer (40) to the storage unit (16).

3. Device (1) according to claim 2, where in the ejector (6) is/are movable in the horizontal plane to cover an area inside the boundaries of the device (1).

4. Device (1) according to any one of the preceding claims, where in the storage unit (16) has a storage capacity in the range of 5 m<3 >to 50 m<3>, preferably in the range of 10 m<3 >to 20 m<3>, and even more preferably around 17 m<3>.

5. Device (1) according to any one of the preceding claims, wherein two inlets (8) of the mass removal unit (4) is provided between the two or more storage tanks (16), and wherein two outlets (18) of the mass removal unit (4), one for each storage tank (16), is directed into one of the storage tanks (16).

6. Device (1) according to claim 5, wherein each storage tank (16) is provided with a discharge hatch (28) at its lower end.

7. Device (1) according to any one of the preceding claims, wherein the device (1) is provided with a rotary drive suspension (26) enabling rotation of the device (1) around its vertical axis.

8. System (10) for removing a subsea filter layer (40), the system (10) comprising a device (1) according to any one of the preceding claims, the system (10) further comprising:

- a device deployment system (60) for deploying the device (1) from a surface vessel (1).

9. System (10) according to claim 8, wherein the deployment system (60) further includes a topside power unit (48) and a topside control unit (50).

10. System (10) according to claim 9, wherein the deployment system (60) further includes an umbilical winch (46) an umbilical (20) for connecting the device (1) to topside.

11. System (10) according to any one of the claims 8-10, wherein the system (10) further comprises a pre-piling template (34) including one or more openings (36) into which a pile (38) is to be positioned.

12. System (10) according to claim 11, wherein the device (1) is adapted fit inside the one or more openings (36) in the pre-piling template (34) to remove the filter layer from.

13. System (10) according to any one of the claims 10-12, wherein the system further comprises a filter layer (40) of granulated material mainly including rocks with a size of 125-180 mm.

14. System (10) according to any one of the claims 11-13, wherein the one or more openings (36) in the pre-piling template has a diameter of 2.5 m or more, preferably 4 m or more, and even more preferably around 4.7 m.

15. Method for installing a jacket (52) of an offshore wind turbine, the method including the steps of:

- providing a subsea filter layer (40) on the seabed (32);

-providing a device (1) according to any of claims 1 to 7 or a system (10) according to any of claims 8 to 14;

-placing a pre-piling template (34) on the seabed (32) at the position of the subsea filter layer (40), the pre-piling template (34) including one or more openings (36) into which a pile (38) is to be positioned;

- removing, within the one or more openings (36), at least a portion of the subsea filter layer (40);

-temporarily storing granulates from the removed filter layer (40) in the storage unit (16);

- inserting a pile (38) in each of the one or more openings (36) in the pre-piling template (34); and

- installing the jacket (52) of the offshore wind turbine on or in the one or more piles (38).

16. Method according to claim 15, wherein the method further comprises the step of: - dumping the removed granulates on the seabed (32) at a position outside the one or more holes (36) in the pre-piling template (34).

17. Method according to claim 15 or 16, wherein the step of providing a subsea filter layer (40) includes the step of providing a subsea filter layer with a thickness of 0.3 m to 2.5 m and preferably 0.7 m to 1.1 m.