WO2010018401A1

WO2010018401A1 - Installation tube

Info

Publication number: WO2010018401A1
Application number: PCT/GB2009/051001
Authority: WO
Inventors: Peter Miles Roberts
Original assignee: Verderg Limited
Priority date: 2008-08-15
Filing date: 2009-08-11
Publication date: 2010-02-18
Also published as: GB0814889D0; GB2462638A

Abstract

An apparatus for installing work pieces (12) on the sea bed comprising: a conduit (1) through which a work piece can be lowered and in use having a lower end open to the sea; and a plurality of pressure cups (14) for carrying the work pieces down the conduit, the cups being movable axially along the conduit, wherein in use the outer edges of each cup are engagable with the inner surface of the conduit for defining a chamber between two cups in which a volume of fluid can be contained in the conduit.

Description

. . INSTALLATION TUBE

Description

Technical field

[0001] This invention relates to an apparatus and method for installing a series of workpieces on the seabed.

Background art

[0002] Offshore oil and gas field development results in various forms of production hardware being placed on the sea bed generally above the location of the hydrocarbons reservoir, and interconnected with pipes and cables of various sorts. Frequently, and increasingly commonly in the very deep water now being met, floating production facilities are moored by chains and/or cables to the sea bed above the field and connected by cables and risers into the network of pipes and cables on the sea floor.

[0003] The initial development project for an oil or gas field offshore is to drill the wells and commission the field facilities that permit production and export of the best defined or most accessible part of the hydrocarbon reserves. It is commonly the case that significant uncertainty will still exist, even when production starts, about the geographical extent of the reservoir, the total amount of hydrocarbons in place and about the proportion of that volume of hydrocarbon that can be recovered before the operating costs rise and/or the production rate falls to a point where the field is no longer economic.

[0004] As the price of hydrocarbons has risen in recent years, and technology has advanced, various technologies have emerged that economically permit the proportion of reservoirs that can be usefully recovered to increase. Characteristically, if 50% of the hydrocarbons in a given field could have been economically recovered 25 years ago, it might now be possible to economically recover, say, 60% of the hydrocarbons in place. If a field contains 100 million barrels and each barrel is worth $100, then every one percent recovery improvement is worth $100 million. A 10% improvement in recovery, as in this hypothetical case, would be worth $1 billion, aside from the environmental benefits of getting more energy from a fixed infrastructure investment. There is thus a significant financial incentive to maximise reservoir recovery, and in response, there is a growing set of enabling technologies to achieve it.

[0005] As this trend continues, driven by the continuing high cost of energy and advancing technology, the complexity and sophistication of the hardware required to be placed on the sea floor advances. The congestion of the hardware increases and it becomes more difficult to install economically without damage, particularly in the very deep water increasingly being met, which may be up to 3000m.

[0006] Typically there is a need to place on or under the sea bed an array of regularly spaced assemblies forming a grid of similar or identical devices, possibly linked together with a network of pipes or cables. One example of such a regular grid is where hydrophones are placed in a regular pattern over a reservoir, on or just below the sea bed and linked together by electrical or fibre optic cables to monitor over time the hydrocarbon-water interface in the depleting reservoir. Survey vessels may emit seismic charges; the reflected sound waves being detected by the hydrophones. The installation of an array of such hydrophones permits the through-life monitoring of the depletion of an oil or gas reservoir by seismic mapping.

[0007] It is also possible that "shallow gas" or methane hydrate deposits may be recovered using a regularly spaced array of shallow wells, possibly in conjunction with heating and/or local de-pressurization facilities that require manifolding of several services from well to well. Further future embodiments are expected to be developed which have requirements for regular arrays of seabed assemblies.

[0008] The deployment of hydrophone or other networked hardware arrays into deep water present various installation challenges that this invention seeks to overcome. Traditionally hydrophones or other networked strings of hardware are deployed by reeling the chain of hydrophones or other hardware workpieces around an annular drum; the hydrophones or other hardware workpieces are lowered from the deployment vessel by unreeling. This method for lowering any load onto the sea bed presents various difficulties: marine currents can move a lowered object a long way off target where vertical lowering under gravity is proposed. Furthermore currents in opposing directions can be met at different depths in the water column. The movement of any such lowered load can become dynamically amplified and the stresses induced in the lowering cable can become excessive. The weight of the lowering cable itself can become a significant or even dominant part of the load being lowered. Such deployment methods also offer limited buoyancy and velocity control when lowering the assembly and the workpieces may be subjected to substantial frictional forces once placed on the seabed. As a result the assemblies themselves or the cables and/or pipes connecting them together are vulnerable to damage during installation. A further challenge exists as each hydrophone needs to be placed into intimate contact with the sea floor to ensure acoustic coupling. Typically this is by burial to approximately two metres depth.

[0009] Further difficulties exist where the regular grid of linked assemblies being installed are superimposed over existing field facilities with production hardware linked by cables and pipes. There will be circumstances where one network is to be installed over another network with multiple crossing points, where clash damage can occur. Damage to the first network may also be caused by the installation equipment used to install the second network. Furthermore, as mentioned above, the underlying production network may need to be continually updated, re-configured and extended, necessitating regular temporary removal of sections of the overlying new network. The use of sea bed crawler machines that move on powered tracks to install complex sea bed service networks are known. They are disadvantaged, however, where the sea bed they need to crawl across is already obstructed by existing production equipment, that may either be damaged when crossed or which precludes access by the crawler to the required target location.

[0010] Difficulties for installation at target locations also exist where the locations in which any such assemblies are to be installed lie underneath an obstruction that limits or prohibits the use of vertical lifting from a work boat directly overhead the location. Examples of such obstructions include but are not limited to floating ice and the mooring cables or catenary risers of floating production vessels. [0011] Therefore the invention provides an apparatus and method that can safely install equipment on the seabed, so as to avoid damage to existing structure and/or locate equipment on the seabed where it previously would not have been possible. Disclosure of the invention [0012] A first aspect of the invention comprises an apparatus for installing workpieces on the sea bed comprising: a conduit through which a workpiece can be lowered and in use having a lower end open to the sea; and a plurality of pressure cups for carrying the workpieces down the conduit, the cups being movable axially along the conduit, wherein in use the outer edges of each cup are engagable with the inner surface of the conduit for defining a chamber between two cups in which a volume of fluid can be contained in the conduit. [0013] The apparatus can comprise a locating device attached to the outer surface of the conduit for positioning the lower end of the conduit at a target position. The locating device may be ducted thrusters attached to the lower end of the conduit. [0014] In one embodiment the apparatus comprises a pressure chamber attached to the upper end of the conduit, the pressure chamber having a first opening through which the workpiece can be inserted into the chamber and a second opening connecting the chamber to the conduit. In this embodiment the pressure chamber may include a fluid inlet port and/or an air bleed valve. [0015] The pressure chamber may comprise a plug for attaching to a cable of the workpiece. [0016] In one embodiment the pressure cups comprise a workpiece attachment device. In another embodiment the pressure cups may be formed integral with the workpiece. [0017] The workpiece can comprise a tubular element, such as a cable or pipe.

The pressure cups may be attached to the tubular element of the workpiece. The tubular elements may be a assembly of concentric tubular elements. [0018] In one embodiment each workpiece may be a discrete element attached to one pressure cup. The workpiece may be a hydrophone assembly.

Alternatively the workpiece may be a continuous pipe string, for forming a deepwater pipeline. [0019] The workpiece may comprise may further comprise a drilling device for facilitating the installation of the workpiece into the seabed. [0020] The apparatus may comprise a plough assembly, drilling unit and/or tools typically associated with a ROV, such as manipulating arms, wrenches, sensors, camera and/or lights, attached to the lower end of the conduit.

The apparatus may also comprise a basket attached to the lower end of the conduit. [0021] The apparatus may comprise a roller device for engaging with a tubular element attached to the workpiece. A roller device may be located at the upper and/or lower end of the conduit. [0022] The apparatus can comprise at least one buoyancy control device. The buoyancy control devices may be drag chains attached to the lower end of the conduit, a sleeve attached to the outer surface of the conduit, blocks or rings of buoyancy material, and/or pressure-resisting tanks into which fluid can be pumped and evacuated. [0023] In some ebodiments the conduit is attached to a deployment frame on a work vessel. [0024] A pump can be attached to the conduit for maintaining the fluid level in the space between the pressure cups.

[0025] The conduit may be rotatable relative to the workpiece. [0026] A second aspect of the invention comprises a method for installing a workpiece on the seabed at a target location using an apparatus comprising a conduit having a at least two moveable pressure cups, the method comprising: inserting a first pressure cup into a first end of the conduit. inserting a workpiece into the first end of the conduit; inserting a second pressure cup into the first end of the conduit to define a chamber in which the workpiece is located; inserting a fluid into the chamber to form a volume a fluid between two pressure cups in which the workpiece is located; and creating a pressure difference across the first and second pressure cups; such that the workpiece and pressure cups move through the conduit and exits at a second end of the conduit at the target location.

[0027] The method further comprises guiding an end of the conduit to the target location.

[0028] In one embodiment inserting the workpiece and a second pressure cup into the conduit comprises inserting the workpiece and pressure cup into a pressure chamber attached to the first end of the conduit; such that the pressure cup is located in a orifice of the pressure chamber; and the fluid is inserted in the pressure chamber.

[0029] A cable attached to the workpiece is deployed from a spool located in the pressure chamber as the first pressure cup moves through the conduit. The fluid is inserted into the pressure chamber via a valve and the method may further comprise expelling air from the pressure chamber via an air bleed valve before inserting fluid into the chamber. The method can further comprise releasing the pressure from the pressure chamber when the first pressure cup has moved a predetermine distance through the conduit.

[0030] Controlling the delivery of the workpiece through the conduit can comprise lifting the top end of the conduit above sea level, injecting further fluid into the top of the conduit.

[0031] The workpiece can be pulled through the conduit using a roller device.

[0032] In one embodiment the method is for locating the workpiece at a target location that is under an obstruction. The obstruction may be a mooring cable and/or floating ice.

[0033] The method is also for locating the workpiece at a target location that is in deep water. The method can be used for laying pipelines or steel catenary risers in deep water. For example water having a depth of 3500m.

[0034] Where the conduit comprises a drilling unit at one end of the conduit, the method can further comprising drilling an oil or gas well in the seabed. The drilling may occur under an obstruction such as permanent floating ice.

[0035] A pump may be located on the conduit, and the method can then further comprise generating power from the fluid flow through the pump into the conduit.

[0036] Preferably the method comprises using an apparatus as described above. Brief description of the drawings

[0037] Figure 1 shows the deployment of a workpiece to the seabed.

Figure 2-5 show cross sectional views of workpiece assemblies that can be used with the invention;

Figure 6 shows a roller assembly for use in the invention;

Figure 7 shows pressure chamber for use with the conduit;

Figure 8 shows a pressure cup assembly according to the invention;

Figure 9 shows one use of the apparatus according to the invention

Figure 10 shows one use of the apparatus according to the invention;

Figure 11 shows one use of the apparatus according of the invention; and

Figure 12 shows a cross-sectional view across line A-A of Figure 11.

Mode(s) for carrying out the invention

[0038] The invention provides an apparatus and method for conveying a series of workpieces to their target location on the seabed. The apparatus offers protection to the workpiece and the connection between the tubes or cables running between them as they are being deployed. The workpieces are conveyed to the seabed by an installation conduit. The invention provides a method and apparatus which allows work vessel from which the conduit and workpiece are deployed to stand off away from the target location, which is particularly useful when installing a workpiece or workpiece string underneath an obstruction that lies directly above the target location.

[0039] Figure 1 shows a system according to the invention for deploying a workpiece down to the seabed. The system comprises a conduit 1 attached to a floating vessel 2 and pressure cups 3 which are attached to the workpiece 6 to be deployed and slide down inside the conduit. The conduit 1 may be formed of pipe segments joined together with welds or mechanical clamps and laid as a catenary from the vessel. A workpiece string 4 comprising a tubular element such as a cable 5 or pipe having workpiece 6 attached along its length is fed into the conduit 1.

[0040] Each workpiece 6 is attached to a pressure cup 3 and the pressure cups slide down the conduit 1 carrying the workpiece 6 to the end section 7 of the conduit and its target location. The workpieces 6 are guided to the sea bed 8 by sliding down the inside of the conduit flooded with water from the top of conduit with the lower end 7 of conduit having an open end to the sea. The containment of water volume between the sliding cups attached to each workpiece 6 provides a load path through the water and allows the cable or pipes between the workpieces 6 inside the conduit 1 to be slack, taking no tension load. The lower end 7 of the conduit may be guided to its desired position with respect to its target location by azimuthing ducted thrusters 9 fitted at suitable positions down the conduit, along with buoyancy control devices 10 where appropriate. The internal surface of the conduit may be coated to assist in the performance of the pressure cups, so that the cups slide more easily down the conduit. One coating that may be used is an epoxy resin coating.

[0041] The workpiece can be any apparatus that needs to be deployed to the sea bed and includes such tools as a continuous pipe or cable, discrete tools, a workpiece string, or hybrid workpiece string.

[0042] As shown in Figure 2 a continuous pipe(s) or cable(s) 11 to be deployed through the conduit 1 may pass through and be moulded into the pressure cups 3. The cable 11 or flexible pipe can hang slack between the pressure cups 3 and can therefore be of a smaller diameter and/or lower strength than could be tolerated if installed simply by unreeling it off the work vessel, where the cable or pipe coming off the reel would have to support the entire suspended length down to the sea floor, as well as accommodating the influence of waves and currents, and vessel motions.

[0043] With reference to Figure 3 discrete workpieces 12 may be deployed through the conduit. Each workpiece 12 is separated from each other and may be either reversibly attached by an attachment device such as a pin 13 to the bearer pressure cup 14 so that the workpiece 12 can be attached and deattached to the pressure cup or the workpiece 12 may be encastred permanently with its pressure cup 15. Such discrete workpieces may additionally carry an optional inboard terminal assembly 16 permitting the workpieces to be reversibly interconnected with pipes and cables as may be required after installation on the seabed. Preferred applications of such discrete workpieces that can be reversibly networked subsequently by pipes or cables include but are not limited to assemblies of concentric pipes and subsea tools such as hydrophones. [0044] Each discrete workpiece may be an assembly of concentric pipes inserted at regular centres into the seabed and used to recover methane from shallow deposits of methane hydrates. In this situation facilities for depressuhsing and for heating such deposits to release the methane from its clathrate can be provided if necessary. Networked manifolding of the necessary services between adjacent workpieces with cable and pipe jumpers that can be connected and disconnected to and from the workpieces as required during field expansion and maintenance. Such services may include but not be limited to electrical and hydraulic power, steam, hot water or solvent injection and formation water offtake. The methane produced from each workpiece is anticipated to be typically gathered through another network of pipes and taken to a common processing and export facility.

[0045] In a further embodiment each discrete workpiece may be an assembly carrying a hydrophone on a suitable carrier that facilitates its being drilled into the seabed or lowered in a pre-dhlled hole in the sea bed to a depth of 1 to 2 metres, typically, to achieve intimate contact. Such hydrophones may be used for reservoir seismic condition monitoring by measuring the seismic reflections from the hydrocarbon-water interface in the reservoir of a sonic signal actuated in the water column, every so often. The electrical or fibre optic cables that network each workpiece into a data collection grid can subsequently be attached to and disconnected from the workpieces as required using suitable connectors mounted on a suitable combination of inboard and outboard assemblies that facilitate diverless connection and disconnection. The discrete workpiece for seismic monitoring can be manufactured in such a way that a drill bit or auger to aid its installation into the sea bed is an integral part of the workpiece.

[0046] As shown in Figure 4 the workpiece string 17 may comprise continuous pipe or pipes and/or cable or cables 18, with a workpiece 19 carried on each pressure cup 3. The workpiece 19 can have any one of a wide variety of potential functions. One preferred application is where the workpiece is a hydrophone. Such hydrophones may be to be used for reservoir seismic condition monitoring by measuring the seismic reflections from the hydrocarbon-water interface in the reservoir of a sonic signal actuated in the water column, every so often. The cables joining the hydrophone can have electrical conductive cores or be fibre optic.

[0047] Another form of workpiece includes a hybrid workpiece string 20 as shown in Figure 5. The workpiece string 20 comprises a continuous cable 21 which is attached to the pressure cups 3 such that the workpieces 22 can be installed into a pre-dhlled vertical hole as described above for the discrete workpiece.

[0048] The workpieces are delivered through the conduit with the help of water between pressure cups in the conduit. The pressure cups are kept a fixed distance apart by the volume of sea water trapped between successive cups, which can slide up and down inside the conduit in response to applied pressure differentials. The distance between pressure cups will remain fixed unless there is flow by-pass around the pressure cups occurs. Flow by-pass may be limited by the design of the pressure cups. The water between the cups takes away the load from the cable.

[0049] The discrete volumes of water located between sequential pressure cups act as a load path which transmits pressure from one volume to the next through the material of the pressure cup, by-passing any workpiece carried by the cup.

[0050] When the friction of the cups in the complete string is overcome by a net applied pressure at one end and the whole string of cups separated by fixed volumes of water starts to move, the pressure transmitted from one volume to the next is reduced by the need to equal the friction force of the cup separating the two volumes plus the contribution made from the submerged weight or buoyancy of any workpieces carried by that pressure cup.

[0051 ] Control of the delivery of the workpiece through the conduit out of the open lower end section of the conduit can be achieved by various means. For example over-pressure or under-pressure at the top of the tube will control the progress of the workpiece down the conduit. The pressure can be controlled by extending the seawater filled conduit a sufficient distance above sea level, or by maintaining the water level in the conduit at a sufficient distance below sea level such that the driving head of water plus the submerged weight of the workpiece string just exceeds the accumulated friction of the pressure cups. Back-tension in the workpiece string can then be used to control the rate of payout. Over-pressure can also be applied by injecting sea water into the top of the conduit.

[0052] The workpiece string moves under control down inside the conduit under the application of a sufficient net pressure difference above ambient across the ends of the conduit. This net pressure increase produces an axially downwards force which causes the workpiece string to move when it exceeds the net aggregate of the accumulated friction force from every pressure cup less the submerged weight of the workpiece string inside the conduit.

[0053] Where the cumulative friction of the pressure cups exceeds the submerged weight of the workpiece string, a positive pressure has to be applied at the top of the conduit just sufficient to push the workpiece string out of the end section of the conduit. This critical threshold-of-movement pressure head "h" may be achieved simply by raising the top end level of the conduit above sea level and keeping it topped up with sea water. Another method uses a roller device installed at the lower end of the conduit. The rollers help pull the workpiece string down the conduit and control the movement of the workpiece in the conduit.

[0054] Where the workpiece string has sufficient stiffness, i.e. for a small diameter pipe, the workpiece string can be pushed down the conduit or restrained under controlled payout with rollers at the top of the conduit. With reference to Figure 6 the workpiece string 23 carries a semi-rigid small-diameter pipe the pipe can be pushed down the conduit by retractable roller pair 24, partially helped by hydrostatic driving head H where H is smaller than h. Where H is greater than h, the workpiece string will move itself under gravity and the roller pair 24 can be used to restrain and control the rate of deployment. A second pair of retractable rollers 25 may be installed so that the pressure cups 3 can be passed through the roller assembly with one pair of rollers always engaged about the conduit. As the workpiece string is lowered down the conduit 1 a first pair of rollers 24 engages the workpiece string while the second pair 25 are retracted away from the workpiece string 23. When the string is lowered sufficiently the second pair of rollers can then engage the conduit before the first pair of rollers is released, this allows the workpiece string and attached pressure cup to pass into the conduit in a controlled manner.

[0055] If the submerged weight of the workpiece string exceeds the cumulative pressure cup friction, the string will sink through the conduit under its own weight, taking the water level down with it until the submerged weight of the workpiece string inside the conduit is balanced by the cumulative friction of the pressure cups and excess hydrostatic head of the sea over the depressed level inside the installation conduit when equilibrium is reached. Equilibrium occurs when the ambient hydrostatic head at the open lower end of the end section of the conduit equals the weight of the water plus the weight of the workpiece string inside the conduit, divided by the cross-sectional area of the conduit. The deployment of the workpiece string can be controlled in this case by, for example, adding sea water to the conduit at or just above the depressed water level and either holding back the cable (up to its safe tension limit) with, for example, roller pairs 24, 25 or by careful control of the rate at which sea water is introduced into the installation conduit.

[0056] As each pressure cup moves down inside the conduit, sea water has to be introduced behind it at the desired head above or below sea level, as required, such that the next pressure cup can be introduced into the conduit without residual entrained air, trapping a fixed volume of water and leaving any interconnecting cable slack and unloaded between the pressure cups. This introduced sea water is vented back into the sea at the exit from the end section of the conduit, having fulfilled its function of safely supporting the workpiece string on its journey down through the sea surface to the sea bed.

[0057] Movement of the workpiece string in the reverse direction is also possible in the conduit by controlling the applied pressure in the opposite sense. All processes are reversible. This is useful when correcting some positioning error or when decommissioning a system at the end of its useful life.

[0058] In one preferred embodiment with reference to Figure 7, the workpiece string to be installed comprises a light, delicate, small-diameter flexible cable typically 300m to 500m long between the pressure cup/workpiece encastred units. In this situation the top extension of the conduit "h" might be inconveniently large to overcome the accumulated pressure cup friction because of the low submerged weight of the workpiece string. Therefore as an alternative source of the driving head to induce workpiece string deployment a pressure-retaining chamber 26 is mounted at the top end of the installation conduit. The pressure retaining chamber 26 is of a significantly larger diameter than the conduit 1. The chamber has a mechanical closure 30 at its top end. The closure is a flanged and splittable mechanical closure with a gasket seal 34 at and a cylindrical orifice 29 of equal internal diameter to the installation conduit 1.

[0059] The complete workpiece string may be up to approximately 6km in length, and is factory stowed by a multi-layer spiral winding inside a series of centre-feed formers 35, each former containing the 300m to 500m of lightweight cable between two consecutive pressure cups. The formers may be made from disposable lightweight material such as a cardboard or plastic, and preferably with a full length longitudinal cut held closed with one or more circumferential temporary straps.

[0060] In order to deploy the workpiece string down the conduit the first former is loaded into the pressure-retaining chamber 26 and the first pressure cup A 27 is engaged in the top end of the installation conduit 1 with the pressure- retaining chamber part full of sea water.

[0061] The second pressure cup B 28 is engaged in the orifice 29 of the pressure cylinder closure 30 and a keep bar(s) 31 is engaged to stop it blowing out of place when sea water is introduced under modest pressure into the pressure-retaining cylinder through an access pipe and inlet valve 32. Air bleed valve 33 in the high-point of the pressure-retaining cylinder is opened initially to expel all air from the chamber. When the bleed valve 33 is closed, the water pressure rises quickly as water is injected into the chamber through water inlet valve 32. When the force of the water pressure developed over the surface area of pressure cup A 27 reaches equilibrium with the residual friction of the workpiece string 36, the whole workpiece string 36 and pressure cup A 27 moves through the conduit 1 and the cable 37 inside the spool-former 35 is deployed, whilst the vessel moves ahead by the appropriate distance. The sea water injection through pipe and valve 32 is stopped when a sufficient volume has been pumped in to fully deploy the lightweight cable 37.

[0062] The apparatus may comprise a safety device to avoid putting the cable into tension. For example the pressure chamber 26 may comprises a tag line 38 that is attached to the cable 37 at one end a plug 39 in the cylinder wall at the other. The tag line 38 goes taught as the last part of the cable 37 deploys and pulls the plug 39 from the cylinder wall, releasing the internal pressure. Another safety device as shown in Figure 8 comprises the pressure cups 41 having a one-way connection to the cable 40, allowing the cable to safely pull out of pressure cup 41 if over-tensioned, again releasing the driving pressure.

[0063] The process completes by opening, splitting and removing the pressure cylinder closure30, and cutting off the disposable split former, or if pre- split, un-strapping and unclipping it from the cable. Pressure cup B 28 can then be relocated into the top of the conduit 1 , with a second former being loaded into the cylinder 26. The pressure cylinder closure 30 is then refitted with the next pressure cup along the workpiece string fitted into the closure orifice 29 and the process of deploying the workpiece string is repeated whilst the vessel moves ahead again.

[0064] Where the workpieces are discrete units, the process is similar but simpler. The pressure cup is engaged in the top of the installation conduit as above, but an alternate pressure cylinder closure may be used without a split or penetration. An appropriate volume of sea water is pumped in through pipe and valve to move the workpiece string the desired distance down the conduit.

[0065] In one embodiment the installation conduit may be deployed into a body of water from a floating vessel 2 such that the conduit is curved into a catenary path between the seabed 8 and the vessel 2. The lower portion of the conduit stretching down to just above the seabed 8 and the upper portion of the conduit preferably extending above the body of water. A workpiece string 4 is conveyed down the conduit 1 to the seabed to the target location. The apparatus allows the target location to be directly under some obstruction 45, such as floating ice or mooring cables from a floating production system. The conduit 1 is held to the work vessel 2 by a deployment frame 46 which may be mounted on a trunion (not shown) to permit the angle made by the conduit to the vessel to vary under a range of hold back tensions between the vessel and the conduit, so that the conduit can maintain its profile in a catenary shape.

[0066] The deployment frame may also be used as a tool for installing the conduit in a direct analogy to the use of a conventional pipe-lay tower on a J-Lay barge for installing a steel catenary riser (SCR). The conduit may be formed by welding pipe lengths together or by the use of mechanical connectors to join sections together. The use of mechanical connectors facilitates recovery of the conduit into short lengths and subsequent re-use of the conduit.

[0067] The diameter of the conduit will depend on the application and the size of the workpiece being conveyed through the conduit. Where only a smaller diameter conduit is required, the conduit may alternatively be deployed via a large reel on the vessel.

[0068] Other methods to deploy the conduit to the sea bed include fabricating the conduit on-shore, complete with its end section, thrusters, and all necessary umbilicals, cables and/or pipes. Temporary closures are attached to both ends of the conduit to keep it air filled. As each new section is added to the conduit onshore, it is towed out, top end first, further into protected shallow water and moored. The design of the conduit will determine whether the conduit is positively buoyant or not however the conduit will have a lower submerged weight than when it is flooded. Once finished, the complete installation conduit and end section can be towed by its top end to location by the vessel or by a tug(s). If buoyant, the conduit will remain on the surface during the tow and will be lowered into position by pumping a flooding pig slowly down the conduit from the end section whilst sufficient back-tension is maintained using the thrusters to prevent overstressing. If the conduit is not buoyant when air-filled, it will assume a mid-depth catenary shape during the tow and will sink naturally to the sea bed when the tow vessel stops, with the lowering of the end section may be controlled by the thrusters, prior to controlled flooding as in the buoyant case. [0069] Buoyancy devices may be located at the end of the conduit to ensure that the conduit is kept above the sea bed. The amount and type of buoyancy needed will depend on the design of the conduit, for example supplementary buoyancy may be required on larger diameter conduits.

[0070] The end section of the conduit may have an asymmetric bellmouth profile 47 to facilitate the exit of the workpiece from the end section 7. The end section of the conduit has three main functions:

- To hold the exit from the conduit above the target location against current, weight and buoyancy forces such that the workpiece is safely delivered to the correct place without damage to pre-existing facilities in the vicinity.

- To counteract the forward thrust of the vessel necessary to keep the conduit under tension to preserve its profile in a suitable catenary.

- To act as a tool carrier for supplementary operations including but not limited to drilling, workpiece manipulation and trenching.

[0071] As exemplified in figures 1 , 9 and 10 to allow the end section to perform these functions the conduit may comprise one of more of a buoyancy control device, drag chains, ducted thrusters, and or monitoring devices.

[0072] A buoyancy control device 9, such as a sleeve of collar may be located along the length of the conduit and/or pressure tanks may be mounted on the end section of the conduit. Water can be actively pumped into and out of the tanks to vary the submerged weights of the end section of the conduit.

[0073] Drag chains 47may be attached to the conduit 1 to help the end section 7 of the conduit "hover" over the target location. The chains may be preinstalled on the conduit and are suitable where damage to other structures on the seabed will not be caused. The drag chains increase the submerged weight of the end section as it rises above equilibrium elevation and reduce the submerged weight as it falls below equilibrium elevation.

[0074] Ducted thrusters 9 may be attached to the conduit, particularly at the end section. The thrusters have two degrees of freedom and are able to rotate. The rotational capabilities of the thrusters enable the end section of the conduit to be maintained in the correct position and orientation independently from movement of the vessel. The thrusters may be attached as groups of 2 or 3, enabling the symmetry of their reaction forces to be preserved and to provide redundancy. Thrusters may also be installed at other locations along the conduit to enable any unacceptable large deflections or unstable excursions in response to environmental and other imposed loadings to be controlled. Thrusters can be powered by a jet of sea water pumped coaxially through a short duct which acts as a mixing tube. The high velocity, small diameter water flow coming from the jet nozzle is thereby converted to a larger diameter lower-velocity flow out of the duct which thereby increases its thrust in accordance with Rankine's Actuator Disc Theory. The thrusters are also sized to counteract the horizontal tension imposed on the conduit by the vessel to maintain the conduit in a suitable catenary profile.

[0075] The thrusters can be powered by a number of ways including by electrical power generated on the vessel driving electrical motors in the hub of the ducted thrusters. Closed circuit hydraulic power from a hydraulic pump on the vessel may drive a hydraulic motor in the hub of each ducted thruster with the low pressure hydraulic fluid returning to the pump. An open circuit single pipe pumped sea water system may drive either a motor in the hub of each ducted thruster or directly developing thrust when jetted coaxially down a mixing tube as previously described. One or more umbilicals 48 and/or cables and pipes may be attached to the side of the conduit as it is laid from the deployment frame to supply the power to the pumps. These umbilicals, cables and pipes may be deployed from reels on the vessel.

[0076] A monitoring device is able to monitor features of the operation that are occurring for example, plan position, stresses in, orientation and/or elevation of the end section, workpiece string status, environmental parameters, and visual and acoustic monitoring of the target location.

[0077] Tools that may be attached to the end section include for example, subsea plough assemblies, drilling units, ROV tools and instruments, and basket.

[0078] A subsea plough assembly 50 attached to the end section of the conduit 1 by a plough attachment device 51 may be towed by the conduit 1 using the bollard pull capability of the vessel as augmented or moderated by the thrusters. The plough may also optionally have buoyancy devices 52 to facilitate its operation in a wide range of soil types and as a deployment/recovery aid and support sledges 53 (or tracks or rollers) to provide fine control over the trench depth. Where necessarily a support track 54 for the workpiece string to guide it safely trough the plough may also be fitted. A supplementary trailing plough share paired arrangement may be included to push the soil heaps on each side of the trench back into the trench on top of the workpiece string.

[0079] A drilling unit 60 may be attached to the end section of the conduit 1. In one embodiment the drill unit may comprise buoyancy control device 61 configured such that a shallow hole can be drilled by an auger 62 or other tool for excavating the seabed at the target location depending on the soil suitability. The workpiece 6 being delivered down the conduit 1 can then be lowered into the hole 63. Suitable manipulators to assist in the lowering of the workpiece into the hole may also be provided on the conduit or drilling unit. Alternatively where ground conditions are suitable two reamers can be mounted to cut a trench into which a workpiece string can be delivered down the conduit. In one preferred embodiment the drilling unit is provided with support feet 64 to provide vertical reaction against the auger or other tool, whereby it pulls the end section of the conduit down against its buoyancy during the drilling operation.

[0080] In another embodiment the drilling unit may be of a large size and capable of drilling an oil or gas well remotely subsea. The end section of the conduit may have an open top, forming a loading shoe such that drill-pipes or tubulars can be lifted and rotated in elevation from the section up into the drilling derrick tower, bringing the drill pipe or tubulars into place vertically over the well being drilled. One or more conduits of different sizes may feed the same drilling unit with different discrete workpieces such as drill pipe, casing and with drilling fluids and power. This is particular suitable for drilling under permanent floating ice. Where the surface drilling supply facility is a long distance from the drilling area, for example above 100km away when drilling under permanent ice, the conduit may be laid like a pipeline along the sea bed for most of its length. This limits the horizontal tension that needs to be continuously applied to the conduit as the vessel can be permanently moored with the installation conduit forming one of several catenary mooring lines, and with the drilling unit being set onto the sea bed during the drilling operation. The conduit may then be able to be used as a production riser when drilling of the wells is completed. In some situations the installation conduit may run from an accessible shore line whereby the vessel functions are performed onshore. Sufficient free movement to achieve the desired well pattern where the conduit is set onto the sea bed can be achieved by either:

- Mounting the derrick on a skid frame in the conventional manner of a surface rig, and designing sufficiently capable workpiece handling facilities to accommodate the varying relative positions of the drilling unit with respect to the fixed end section(s) of the conduit(s); or

- Laying a sufficient length of part of the conduit adjacent the end section into long-radius plan bends fitted with external buoyancy such that the drilling unit can relocate within a defined zone without overstressing the conduit, which is then free to assume a new plan geometry within acceptable elastic bending limits as the drilling unit de-ballasts and relocates.

[0081] Other methods may also be contemplated to ensure the drilling unit has sufficient free movement about the sea bed.

[0082] Tools and instruments typically found on a remote operating vehicle (ROV) may also be fitted to the end section of the conduit. Such tools and instruments include manipulator arms, torque wrenches, positioning sensors, cameras and lights. Attaching such tools to the conduit avoids the need to deploy a separate ROV down to the seabed.

[0083] A basket can be attached to the end section of the conduit to collect pressure cups that have been removed from workpieces before their installation. The basket may return to the surface with the pressure cups so that the pressure cups can be reused.

[0084] A further application of the invention is for the installation of full-sized marine pipelines into very deep water. The primary challenge when installing pipelines in very deep water is to accommodate the weight of the workpiece. A pipeline for very deep water has a heavy wall thickness to protect against buckling under hydrostatic loading. The pipe is hung nearly vertically in a very steep catenary from the lay vessel during installation, with just sufficient lateral thrust from the lay vessel's ducted thrusters to maintain the stress levels in the sag bend within acceptable limits. The lay vessel therefore has to support the full vertical submerged weight of the heavy wall pipe and the deeper the water the greater the load. In practice, the vertical hold-back capacity of lay vessels operating in this "J-Lay" mode can be exceeded when water depths in the 2000m to 3500m range are met. The frequently specified need to have a dual skinned pipe (Pipe- In-Pipe) with insulation between the two concentric tubes can make this weight penalty even more severe. A further complication is that the tensile strength of the workpiece string may be exceeded when the string is suspending its own self-weight into extreme depths; a design constraint can be met whereby increasing the wall thickness to provide additional tension capacity takes the self-weight of the string above the hold-back capacity of the lay barge.

[0085] The apparatus and method of the invention are able to laying marine pipelines and steel catenary risers without the need of a convention lay- barge. With reference to Figures 11 and 12 the invention can be used to deploy such pipelines in deep water by having a straight conduit with an internal cross-sectional area Ai which can be used to solve these problems encountered by providing supplemental buoyancy down through the upper part of the water column.

[0086] A reversible electric or hydraulic pump 70 is located at the chosen depth "x" below sea level at which the internal water level is to be maintained within an acceptable tolerance "y". A workpiece string 71 of pipe with an external cross-sectional area Of A₂ and fitted with pressure cups 3 passes down through the conduit 1. In the event that water leakage flow enters into the upper part of the conduit, by-passing the pressure cups, then the pump can be used to carry the leakage water away maintaining the internal water level within tolerance range "y". As the workpiece string passes down through the conduit taking the entrained water inside the conduit with it, sea water is allowed into the conduit in reverse through the pump to maintain the internal water level. The sea water pump, when acting in reverse to flood the conduit can be linked to a generator or hydraulic pump, and extract electrical or hydraulic power from the incoming flow, if desired. The power thus gained for useful work is ultimately derived from the potential energy released by transferring the mass of the workpiece from sea level to sea floor. Alternatively, the pump can itself have a sea water primary circuit used, for example, to directly power the ducted thrusters, either through sea water motors or by direct reaction from a sea-water jet nozzle co-axial with a mixing tube. The workpiece string will experience supplementary buoyancy equal to the hydrostatic pressure "p" at depth "x" acting over the net cross-section of a pressure cup less the pipe. To move down through the conduit, the workpiece string must also overcome the accumulated friction of all the pressure cups "f". The pipe will lower itself towards the sea bed under its open-water submerged self weight "W", therefore, where: W>p(Ai-A₂)+f

[0087] The conduit is supported from the vessel used to fabricate and position the vertical workpiece string, which need no longer be a specialist lay-barge as the conduit provides the hold-back tension required. The support load for the conduit will be the submerged weight of the conduit plus the pressure cup skin friction "f". Some residual hold-back tension may be left for the vessel to support, and to provide control over the pipeline installation process.

[0088] The conduit may also be fitted with thrusters. The ducted thrusters on the conduit will keep the pipe aligned correctly down the water column. Crosscurrents, such as the "Loop Currents" in the Gulf of Mexico can delay or prevent conventional pipe-lay operations by imposing excessive lateral defections and/or bending stresses on the workpiece string. The conduit can rotate freely around the workpiece string subject only to friction between the inside surface of the conduit and the pressure cups. This rotational freedom permits the lay vessel to align itself with surface currents, such as the "Loop Currents" in the Gulf of Mexico, which can force the vessel off-station unless it adopts a preferred head-on orientation

[0089] Various changes within the scope of the invention can also be made.

Claims

1. An apparatus for installing workpieces on the sea bed comprising: a conduit through which the workpiece can be lowered and in use having a lower end open to the sea; and a plurality of pressure cups for carrying the workpieces down the conduit, the cups being movable axially along the conduit, wherein in use the outer edges of each cup are engagable with the inner surface of the conduit for defining a chamber between two cups in which a volume of fluid can be contained in the conduit.

2. An apparatus according to claim 1 comprising a locating device attached to the outer surface of the conduit for positioning the lower end of the conduit at a target position.

3. An apparatus according to claim 2 wherein the locating device is a ducted thruster attached to the lower end of the conduit.

4. An apparatus according to claim 1 , 2 or 3 comprising a pressure chamber attached to the upper end of the conduit, the pressure chamber having a first opening through which the workpiece can be inserted into the chamber and a second opening connecting the chamber to the conduit.

5. An apparatus according to claim 4 wherein the pressure chamber comprises a fluid inlet port.

6. An apparatus according to claim 4 or 5 wherein the pressure chamber comprises an air bleed valve.

7. An apparatus according to claim 4, 5 or 6 wherein the pressure chamber comprises a plug for attaching to a cable of the workpiece.

8. An apparatus according to any preceding claim wherein the pressure cups comprise a workpiece attachment device.

9. An apparatus according to any of claims 1 to 7 wherein the pressure cups are formed integral with the workpiece.

10. An apparatus according to any preceding claim wherein the workpiece comprises a tubular element

11. An apparatus according to any preceding claim wherein the pressure cups are attached to a tubular element of the workpiece.

12. An apparatus according to any preceding claim wherein the workpiece is an assembly of concentric tubular elements.

13. An apparatus according to any of claims 1 to 8 wherein each workpiece is a discrete element attached to one pressure cup.

14. An apparatus according to any preceding claim wherein each workpiece is a hydrophone assembly.

15. An apparatus according to any of claims 1 to 13 wherein the workpiece is a continuous pipe string for forming a deepwater pipeline.

16. An apparatus according to any preceding claim wherein each workpiece comprises a drill device to facilitate the installation of the workpiece into the sea bed.

17. An apparatus according to any preceding claim further comprising a plough assembly attached to the lower end of the conduit.

18. An apparatus according to any preceding claim further comprising a drilling unit attached to the lower end of the conduit.

19. An apparatus according to any preceding claim comprising a tool attached to the lower end of the conduit, the tool comprising a manipulating arm, a wrench, a sensor, a camera and/or a light.

20. An apparatus according to any preceding claim further comprising a basket attached to the lower end of the conduit.

21. An apparatus according to any preceding claim further comprising a roller device for engaging with a tubular element attached to the workpiece.

22. An apparatus according to claim 21 wherein the roller device is located at the lower end of the conduit.

23. An apparatus according to claim 22 or 23 wherein a roller device is located at the upper end of the conduit.

24. An apparatus according to any preceding claim comprising at least one buoyancy control device.

25. An apparatus according to claim 24 when the buoyancy control device is drag chains attached to the lower end of the conduit.

26. An apparatus according to claims 24 or 25 when the buoyancy control device comprises a sleeve attached to the outer surface of the conduit.

27. An apparatus according to any preceding claim further comprising a pump attached to the conduit for maintaining the fluid level in the space between pressure cups.

28. An apparatus according to any preceding claim wherein the conduit is attached to a deployment frame on a work vessel.

29. An apparatus according to any preceding claim wherein the conduit is rotatable relative to the workpiece.

30. A method for installing a workpiece on the seabed at a target location using an apparatus comprising a conduit having at least two moveable pressure cups, the method comprising: inserting a first pressure cup into a first end of the conduit. inserting a workpiece into the first end of the conduit; inserting a second pressure cup into the first end of the conduit to define a chamber in which the workpiece is located; inserting a fluid into the chamber to form a volume of fluid between two pressure cups in which the workpiece is located; and creating a pressure difference across the first and second pressure cups; such that the workpiece and pressure cups move through the conduit and exits at a second end of the conduit at the target location.

31. A method according to claim 30 further comprising guiding an end of the conduit to the target location.

32. A method according to claims 30 or 31 wherein inserting the workpiece and a second pressure cup into the conduit comprises inserting the workpiece and pressure cup into a pressure chamber attached to the first end of the conduit; such that the pressure cup is located in a orifice of the pressure chamber; and the fluid is inserted in the pressure chamber.

33. A method according to claim 32, comprising deploying a cable of the workpiece from a spool located in the pressure chamber as the first pressure cup moves through the conduit.

34. A method according to claim 32 or 33 wherein the fluid is inserted into the pressure chamber via a valve.

35. A method according to claim 32, 33 or 34 further comprising expelling air from the pressure chamber via a air bleed valve before inserting fluid into the chamber.

36. A method according to claims 32 to 35 further comprising releasing the pressure from the pressure chamber when the first pressure cup has moved a predetermine distance through the conduit.

37. A method according to claim 30 or 31 wherein controlling the delivery of the workpiece through the conduit comprises lifting the top end of the conduit above sea level.

38. A method according to claim 30 or 31 wherein controlling the delivery of the workpiece through the conduit comprises injecting further fluid into the top of the conduit.

39. A method according to any of claims 30 to 38 further comprising pulling the workpiece through the conduit using a roller device.

40. A method according to any of claims 30 to 39 wherein the target location is located under an obstruction.

41. A method according to claim 40 wherein the obstruction is floating ice and/or mooring cables.

42. A method according to any of claims 30 to 41 wherein the target location is in deep water.

43. A method according to claim 42 wherein the method is for laying pipelines or steel catenary risers in a target location.

44. A method according to any of claims 30 to 42 wherein the conduit comprises a drilling unit located at the end of the conduit, and the method further comprises drilling an oil or gas well.

45. A method according to claim 44 wherein the drilling occurs under permanent floating ice.

46. A method according to any of claims 30 to 42 wherein the method is for laying an array of workpieces on the seabed.

47. A method according to any of claims 30 to 42 wherein the conduit comprise a pump and the method further comprises generating power from the fluid flowing through the pump into the conduit.

48. A method according to any of claims 30 to 47 comprising using the apparatus according to any of claim 1 to 29.