Stabilising elongate subsea elements

Abstract

An elongate subsea element such as a cable is stabilized by coupling a length of the element to an array of interconnected tanks placed on a seabed. The coupled length of the elongate element is disposed between groups of tanks of the array, conveniently being coupled to connectors of the array that interconnect those groups of tanks. The array and the element can be coupled on the seabed, hence after landing the element on the seabed, or the array and the element can be coupled above the seabed and landed on the seabed together. The tanks of the array are in fluid communication via the connectors and may be ballasted, after landing, with a fluid of greater density than water.

Description

[0001] Stabilising elongate subsea elements

[0002] This invention relates to elongate subsea elements, such as cables, whose lightness and flexibility render them vulnerable to displacement, damage and failure under the influence of dynamic loads when in service offshore. The invention relates particularly to stabilisation of cables for use with submerged structures such as offshore wind turbines.

[0003] The marine environment is characterised by constant changes and unpredictable events such as strong currents, high waves and activity of marine life. The resulting sudden or fluctuating loads, and consequent exposure to fatigue, can pose a serious threat to the integrity of elongate elements such as cables, leading potentially to failure. Cable failures have been on the rise in recent times, leading to significant financial losses and disruption of services that rely on them.

[0004] In the context of offshore wind systems, the importance of protecting cables from failure is especially pronounced. Offshore wind farms rely heavily on subsea cables to transmit generated electrical power within the wind farm, namely array cables, and onwards to a grid, namely export cables. Those cables are often in water that is shallow enough to expose them to the influence of surface dynamics driven by winds and waves.

[0005] The offshore wind industry is facing a significant challenge arising from subsea cable failures. According to a DNV (Det Norske Veritas) report, around 83% of all offshore wind-related financial losses and insurance claims are attributed to power cable failures. With an average downtime of forty to sixty days for repairs, the average financial losses, per incident, arising from repair and loss of generation range between US$1.2m and US$12m for array cables and US$10m and US$30m for export cables.

[0006] Subsea power cables can fail for various reasons. Many of the publicly-known subsea cable failures in recent years are a result of poor interface design and management together with inadequate design for the site conditions. Given the high costs and downtime associated with such failures, stabilisation of cables and their protection systems by improved design and management is essential to minimise risks and to reduce downtime. An effective solution could save significant costs in the offshore wind industry. As wind farms are typically designed for a long operational lifetime, often exceeding twenty years, it is of paramount importance to maximise the lifespan of their subsea cables through effective stabilisation. This not only reduces the need for frequent cable replacement and maintenance but also contributes to the overall efficiency and costeffectiveness of offshore wind systems.

[0007] Various cable protection systems have been used or proposed to mitigate the risk of cable failure. Typically, such systems involve surrounding a cable with a sheath or tube that protects the cable and may also control bending and deflection of the cable. A sheath or tube can also add stabilising weight and improve frictional engagement with the seabed, where subsea cables are typically laid. In this way, a cable protection system can significantly extend the lifespan of a cable, reducing the frequency of replacement and maintenance and improving reliability.

[0008] Various motion stabilisers are known for resisting unwanted movement of a subsea cable laid upon the seabed. Typically, such motion stabilisers surround a cable and have longitudinally-extending fins that resist lateral movement of the cable across the seabed.

[0009] EP 2341592 discloses a method for protecting submarine cables and elongate tubes by installing flexible bags filled with crushed rock. The bags cover the cable to provide stabilisation and protection against scouring and other environmental factors. The flexibility of the bags allows them to adapt to the shape of the underlying seabed, ensuring stable placement. The weight of the stones or other objects within the bags adds stability, preventing the filter units from being displaced by water currents. Porosity of the bags allows water to flow through without applying sufficient hydrodynamic pressure to move the bags.

[0010] WO 2023 / 019228 discloses a stabilisation solution for power cables and control umbilicals, particularly in subsea and shoreline environments. An artificial cable trench stabilises a cable by constraining its movement due to hydrodynamic forces. The trench is surrounded by a body whose curved shape is optimised to reduce drag forces from water currents.

[0011] KR 100596640, KR 100705283, KR 102046937, KR102617305, KR102008877,

[0012] KR101825739 and KR101183080 provide cable protection and stabilisation using mattresses of pivotably-interconnected concrete blocks that are draped over a submarine cable. US 10527199 discloses a pipeline mattress in which pipe-clamping blocks have a recessed surface to together form a cavity for receiving a subsea pipeline. A bottom surface of each block rests on the seabed.

[0013] In KR 101341563, a submarine cable is protected from various hazards such as anchors, fishing gear, ocean currents and scouring. For this purpose, an arched protection unit extends along and over the cable. The protection unit is held in place by multi-part, mattress-like units on each side.

[0014] EP 3839311 discloses a method for protecting and stabilizing subsea pipelines using a rock berm of substantially trapezoidal shape. More generally, installation of rock is the most widely-used method for protecting cables in the offshore wind industry. In this respect, Figure 1 of the drawings shows a conventional subsea cable installation 10 in which a subsea array cable 12 extends outwardly from an offshore wind turbine 14. The wind turbine 14 is fixed to the seabed 16 in this instance.

[0015] Much of the length of the cable 12 is disposed within a cable protection system 18 that surrounds and protects the cable 12. For example, the cable protection system 18 may comprise an articulated armoured pipe or tube of serially-arranged tubular sections or segments. Such segments can interlock pivotably end-to-end so that the articulated tube can bend along its length, thereby to follow the curvature of the cable 12.

[0016] In this example, the cable 12 conveys electrical power generated by the wind turbine 14. Typically, such a cable 12 will terminate at its outer end at an offshore substation (not shown) at which power generated by multiple wind turbines 14 of a windfarm is gathered for export to a terrestrial grid.

[0017] The illustrated example shows the cable 12 originating and terminating at its inner end within a tubular monopile 20 that serves as the submerged foundation of the wind turbine 14. Other foundation arrangements such as jackets are possible and are also embraced by the invention, as are other cables 12 or other elongate buried elements such as flexible conduits that extend from other subsea structures or equipment, such as platforms or floating wind turbines 14. In service, the cable 12 may be subjected to lateral loads, uplift loads and pull-out loads. Lateral loads, typically caused by environmental influences such as waves or currents, are generally parallel to the plane of the seabed 16 and transverse to the upright plane of the cable 12, in either horizontal direction. Uplift loads, also typically caused by environmental influences, are generally orthogonal to the plane of the seabed 16. Pull-out loads, arising in consequence of lateral loads and uplift loads, are generally aligned with the longitudinal direction of the cable 12 and act toward the point of origin of the inner end of the cable 12, being the wind turbine 14 in this example.

[0018] For stability and for protection from threats such as storms, dropped equipment, anchor dragging or overtrawling, most of the length of the cable 12 is buried or embedded in the soil or rock of the seabed 16. For this purpose, the cable 12 can be positioned in a trench that is excavated in the seabed 16 before or after laying the cable 12, and then the trench can be covered over or filled in with soil, rock and / or other protective measures such as mats. Elements of the cable protection system 18 disposed along that buried length of the cable 12 may, similarly, be buried or embedded in the soil or rock of the seabed 16.

[0019] A bend stiffener 22 surrounds the innermost portion of the cable 12 within the hollow interior of the monopile 20. Outboard of the bend stiffener 22, the cable 12 extends outwardly through an aperture 24 in the wall of the monopile 20 at a level above the seabed 16, but still typically submerged. A foundation interface device 26 extending through the aperture 24 surrounds the cable 12 at the transition where the cable 12 exits the monopile 20 through the aperture 24.

[0020] A bend restrictor 28 surrounds the cable 12 outside the monopile 20, outboard of the foundation interface device 26. A negatively-buoyant weighted section 30, for example comprising a series of weighted modules, surrounds the cable 12 outboard of the bend restrictor 28.

[0021] The cable 12 crosses, and is supported by, a scour protection berm 32 comprising rock that is deposited on the seabed 16 around the monopile 20. Specifically, an inner suspended portion of the cable 12, including the portion surrounded by the bend restrictor 28, hangs as a catenary in the water column across the free span between the aperture 24 in the wall of the monopile 20 and a touch-down point 34 on an upper surface of the berm 32. The weighted section 30 pulls the cable 12 downwardly and rests upon the berm 32, hence frictionally engaging the berm 32 at the touch-down point 34.

[0022] An outer portion of the cable 12 extends from the upper surface of the berm 32 to a burial point 36 on the seabed 16, from where the cable 12 extends beneath the level of the seabed 16. The buried length of the cable 12 comprises a generally horizontal section, not shown in Figure 1 , disposed at a desired burial depth beneath the level of the seabed 16 and so extending generally parallel to the seabed 16. The buried length of the cable 12 further comprises an inclined transition section 38 that descends from the burial point 36 at the level of the seabed 16 to the burial depth of the horizontal section.

[0023] The dynamic portion of the cable 12 close to the level of the seabed 16 is most susceptible to the influence of lateral loads, uplift loads and pull-out loads. Consequently, to stabilise the dynamic portion, the scour protection berm 32 is enlarged with additional rock that extends the berm 32 radially across the outer portion of the cable 12 and the upper part of the transition section 38.

[0024] The use of additional rock is disadvantageous for various reasons, notably: the high costs of vessel time and rock procurement; potentially limited vessel availability; potentially limited rock availability in the installation region; and the impact of additional operations on the project schedule.

[0025] Against this background, the invention resides in a method of stabilising an elongate subsea element laid on a seabed, the method comprising coupling a length of the elongate element to an array of interconnected tanks and disposing the coupled length of the elongate element on the seabed between groups of tanks of the array, such as rows of tanks extending substantially parallel to the elongate element on respective sides of the elongate element. The tanks of the array may be in mutual fluid communication.

[0026] Elegantly, the elongate element may be coupled to connectors of the array that interconnect the groups of tanks. Coupling the elongate element to the array may be performed after lowering the array to the seabed, for example by lowering the array onto the elongate element after installing the elongate element on the seabed. Alternatively, the elongate element can be coupled to the array before lowering the array and the elongate element to the seabed together.

[0027] The tanks of the array can be engaged with the seabed, for example with engagement formations that protrude downwardly from the tanks. The tanks can also, or instead, be ballasted after landing the array on the seabed, for example by pumping a denser-than- seawater ballasting fluid into the tanks.

[0028] Correspondingly, the inventive concept embraces a subsea installation comprising an elongate element laid on a seabed and coupled along a coupled length to a stabilising array of interconnected tanks placed on the seabed, with the coupled length of the elongate element being disposed on the seabed between groups of tanks of the array. The tanks may, for example, be grouped into one or more matrices and may be in mutual fluid communication, preferably via pipes that effect structural interconnection between the tanks of the array.

[0029] Rows of tanks may extend substantially parallel to the elongate element on respective sides of the elongate element. Moreover, the tanks could be grouped into at least two rows on each side of the elongate element. Conveniently, the elongate element can extend along an elongate gap extending between and parallel to rows of the tanks. In that case, connectors extending between the rows of tanks separated by the gap suitably extend above, and bridge across, the elongate element disposed in the gap. Such connectors may support interfaces that couple the elongate element to the array.

[0030] The seabed can be engaged by engagement formations protruding downwardly from the tanks. The tanks may contain a ballasting fluid, which fluid may be denser than seawater.

[0031] The inventive concept also extends to a corresponding stabilising structure for stabilising an elongate subsea element laid on a seabed. The structure comprises an array of tanks that are interconnected by connectors extending between adjoining tanks of the array, such that the structure can be a discrete unitary structure with a self- supporting degree of rigidity across the array, allowing the tanks of the array to be landed together on the seabed as a unit. The tanks are arranged in at least two groups and the connectors extending between an adjoining pair of the groups support interfaces for coupling along a length of the elongate element disposed on the seabed.

[0032] Conveniently, the connectors can effect fluid communication between the tanks.

[0033] Where the tanks are grouped in at least two parallel rows, the interfaces may be supported by connectors extending between the rows. More generally, the connectors may comprise an end connector extending from at least one end of each tank and a side connector extending from at least one side of each tank. The end connector and the side connector may extend along mutually orthogonal axes.

[0034] Each tank may have a rounded cross-section on at least an upper surface of the tank. For example, each tank may be of generally elliptical or generally circular cross-section.

[0035] At least one tank of the array may comprise engagement formations protruding downwardly from a hollow body of the tank.

[0036] Aspects of the invention involve stabilisation of cables, cable protection systems and other elongate subsea elements such as pipelines using prefabricated tanks with vent openings. This solution aims to eliminate high costs involved with rock berms as exemplified in EP 3839311 and to provide an alternative to rock bags exemplified in EP 2341592 and to concrete mattresses exemplified in several of the other prior art disclosures noted above.

[0037] The invention provides a robust and cost-effective installation solution that eliminates the need for procurement and installation of rock. Thus, the invention proposes a new technique for stabilisation as an alternative to expensive rock berm installation. The invention provides robust and inexpensive stabilisation that mitigates risks of cable failures and consequent liability for financial losses while significantly reducing costs associated with rock procurement and rock installation offshore. The invention also simplifies scheduling of an installation project, which no longer depends on rock installation to meet project milestones.

[0038] Apparatus of the invention allows for easy assembly either on board a vessel or in a yard or factory and ensures low manufacturing costs. The apparatus can employ environmentally-friendly materials, this being a crucial consideration for the offshore wind industry. The invention employs anchor elements in the form of prefabricated tanks. The tanks may be moulded or fabricated of a recyclable polymer, glass reinforced polymer (GRP), steel or other suitable material, and may each have vent openings in at least two locations that can be opened during lowering to assist sinking in the water column. The tanks are connected to each other and to an elongate element such as a cable, with at least one row or other group of tanks being disposed on each side of the elongate element. Two or more rows or other groups of tanks may be disposed on each side of the elongate element to handle loads applied through the elongate element.

[0039] The tanks can be connected to each other and fitted either on land, for example at a fabrication yard or in a factory, or aboard an installation vessel. The tanks can be attached to the elongate element on the vessel and lowered with the elongate element to the seabed. Alternatively, the tanks can be lowered to the seabed separately from the elongate element. In other words, the elongate element can be lowered to the seabed first and then the tanks can be submerged, lowered and attached to the elongate element, for example by ROV intervention.

[0040] Once on the seabed, the tanks can be left filled with water or filled with a higher-density material, such as grout or barite, to make them heavier.

[0041] The resulting system of tanks provides resistance to loads acting on the elongate element, particularly horizontal and upward forces, and to hydrodynamic loads. Each tank has a tubular or cylindrical shape whose rounded cross-section helps the tank to disperse subsea currents and therefore causes those currents to apply lower hydrodynamic loads to the tank.

[0042] The bottom or underside of a tank can be fitted with skirts or legs for better engagement with the seabed to increase resistance to loads and to reduce scour of the seabed soil around the tank. Similarly, the underside of a tank can be finished with a roughened, textured, contoured or ridged surface such as a sawtooth profile to improve contact and engagement with the seabed and to reduce scour.

[0043] In summary, an elongate subsea element such as a cable is stabilised in accordance with the invention by coupling a length of the element to an array of interconnected tanks placed on a seabed. The coupled length of the elongate element may be disposed between groups of tanks of the array, conveniently being coupled to connectors of the array that interconnect those groups of tanks.

[0044] The array and the element can be coupled on the seabed, hence after landing the element on the seabed, or the array and the element can be coupled above the seabed and then landed on the seabed together. The tanks of the array may be in fluid communication via the connectors and may be ballasted, after landing, with a fluid of greater density than water.

[0045] To put the invention into context, reference has already been made to Figure 1 of the drawings, which is a schematic side view of a conventional subsea cable installation for conveying electrical power from a bottom-fixed offshore wind turbine.

[0046] In order that the invention may be more readily understood, reference will now be made, by way of example, to the remainder of the accompanying drawings in which:

[0047] Figure 2 is a schematic perspective view of an installation like that of Figure 1, apart from the provision of an array of tanks that serve as anchor elements of the invention in place of additional dumped rock;

[0048] Figure 3 is a schematic plan view of an array of tanks of the invention;

[0049] Figure 4 is a schematic perspective view of two tanks of the invention, coupled together end-to-end;

[0050] Figure 5 corresponds to Figure 2 but shows a variant of the array of tanks;

[0051] Figure 6 corresponds to Figure 4 but shows a variant of the tanks;

[0052] Figure 7 is a schematic perspective view showing a further variant of a tank of the invention;

[0053] Figure 8 is a schematic side view of an array of the tanks of Figure 7;

[0054] Figures 9a to 9c are schematic side views showing an installation sequence of the array of tanks of Figure 8; Figures 10a to 10c are schematic side views showing further details of the installation sequence shown in Figures 9a to 9c; and

[0055] Figures 11a to 11c are schematic side views showing another installation sequence of the array of tanks of Figure 8.

[0056] To recap the conventional subsea cable 12 installation shown in Figure 1 , the dynamic portion of the cable 12 close to the level of the seabed 16 is most susceptible to displacement under loads experienced during offshore operation. Consequently, the scour protection berm 32 is typically enlarged with additional rock to stabilise the dynamic portion of the cable 12.

[0057] In Figure 2, however, the outer portion of the cable 12 extending from the upper surface of the berm 32 to the burial point 36 is instead engaged and stabilised by a modular array 40 of tanks 42 of the invention. The tanks 42 may be regarded as anchor elements or stabilising elements of the array 40. The tanks 42 are hollow but are ballasted to have negative buoyancy, hence lying upon and engaging the seabed 16 around the cable 12 to resist lateral loads and uplift loads that generate pull-out loads acting on the cable 12.

[0058] Referring now also to Figures 3 and 4, each tank 42 comprises a hollow elongate body 44 whose tubular side wall 46 has a rounded or elliptical cross-section. In this example, the side wall 46 is substantially rotationally symmetrical about a central longitudinal axis 48 and so has a circular cross-section. Domed or hemispherical end caps 50 close the opposed ends of the tubular side wall 46, whereby the tank 42 has a stadium shape in longitudinal section. The side wall 46 and the end caps 50 surround and define an internal chamber 52 within the hollow interior of the tank 42.

[0059] End connectors 54 extend longitudinally from one or both ends of each tank 42, in alignment with the central longitudinal axis 48. At least one side connector 56 extends laterally from a central location on the side wall 46 of each tank 42, on an axis orthogonal to the central longitudinal axis 48. The end and side connectors 54, 56 are suitably pipes or tubes. Conveniently, therefore, the end and side connectors 54, 56 may be in fluid communication with the internal chamber 52. A tubular upper vent 58 in fluid communication with the internal chamber 52 extends upwardly from a central location on the side wall 46 of each tank 42, on an axis orthogonal to the central longitudinal axis 48.

[0060] Any or all of the end and side connectors 54, 56 and the upper vent 58, or other such conduits, may include valves to control passage of water, air or other fluids into or out of the internal chamber 52 during installation.

[0061] In this example, the tanks 42 are arranged in serial relation in rows 60 extending along and parallel to the cable 12 and to each other. The tanks 42 of each row 60 are connected end-to-end by the end connectors 54, which thereby effect mechanical coupling of the serially-connected tanks 42 to each other. Conveniently, the end connectors 54 may also provide for fluid communication between internal chambers 52 of those tanks 42.

[0062] The tanks 42 of each row 60 are paired with corresponding tanks 42 of the other row 60. The paired tanks 42 are connected side-to-side by the side connectors 56, which thereby effect mechanical coupling of the paired tanks 42 to each other. Thus, the array 40 is a matrix of tanks 42, in this case a 3 x 2 matrix. Again, conveniently, the side connectors 56 may also provide for fluid communication between the internal chambers 52 of those tanks 42.

[0063] By virtue of being coupled to each other via the end connectors 54 and side connectors 56, the tanks 42 of the array 40 work together as a single, unitary mechanical system or structure to provide resistance against loads exerted by the cable 12 and the environment.

[0064] The side connectors 56 serve as cross-connectors that bridge the longitudinally- extending gap 62 between the parallel rows 60 and so extend across, and over, a cable 12 disposed in that gap 62. The array 40 straddles the cable 12 in the gap 62. The array 40 may be fixed to the cable 12 by interfaces 64 such as collars, clamps or saddles that lie upon or embrace the cable 12. Conveniently, the interfaces 64 can be mounted on the side connectors 56 as shown.

[0065] When installed, as best appreciated in Figure 4, the tanks 42 each present a rounded bottom profile to the seabed 16 that promotes effective frictional engagement with soil or rock of the seabed 16. Conversely, the rounded upper profile of each tank 42 reduces hydrodynamic drag, and hence induced loads, of water flows acting on the tank 42 after installation.

[0066] Figure 5 shows a variant of the array 40 in which two rows 60 of tanks 42 are disposed on each side of the cable 12, namely an inner row 60 adjacent to the cable 12 and an outer row 60 outboard of the inner row 60. Thus, side connectors 56 link tanks 42 of the outer rows 60 to corresponding tanks 42 of the associated inner rows 60, side-to-side.

[0067] In this example, the array 40 is a 3 x 4 matrix of tanks 42, illustrating how its modularity allows the array 40 to be tailored in size simply by choosing an appropriate number of tanks 42 and coupling those tanks 42 in any desired arrangement.

[0068] Tanks 42 of each inner row 60 are paired, side-to-side, with corresponding tanks 42 of the other inner row 60. Further side connectors 56 connect those paired tanks 42 to each other. Again, those side connectors 56 effect mechanical coupling of the paired tanks 42 to each other and may also provide for fluid communication between those tanks 42.

[0069] Figures 6, 7 and 8 show that the underside of each tank 42 can be shaped or provided with downwardly-extending or protruding engagement formations to improve engagement with the seabed 16.

[0070] In Figure 6, the engagement formations take the form of skirts or legs 66 that extend downwardly from the underside of the tanks 42 to embed in and to engage with soil of the seabed 16. The skirts or legs 66 can also to minimise scour effects caused by water flowing across the soil of the seabed 16.

[0071] In Figure 7, the engagement formations take the form of a longitudinal series of circumferentially-extending sharp-edged ridges 68 that extend outwardly to varying extents from the underside of the tank 42. The ridges 68 lie in parallel planes that are orthogonal to the central longitudinal axis 48 of the tank 42. Thus, the ridges 68 lend a sawtooth profile to the underside of the tank 42 in side view. A similar arrangement is shown schematically in the row 60 of three interconnected tanks 42 shown in Figure 8, forming part of an array 40 of such elements like the arrays 40 shown in Figures 2 and 5. Figures 9a to 10c show an installation sequence in which an array 40 of tanks 42 like those shown in Figure 8 is lowered onto a cable 12 pre-installed on the seabed 16 by a cable-laying vessel 70 floating at the surface 72 of a body of water. Whilst shown in these drawings as being landed over the cable 12 on the seabed 16 for simplicity, at least part of the array 40 could be landed over the cable 12 on an edge of a scour protection berm 32 as shown in Figures 2 and 5.

[0072] Figures 9a and 10a show the array 40 floating at the surface 72, with sufficient air held within the internal chambers 52 of the tanks 42 to confer positive buoyancy on the array 40.

[0073] In Figure 10b, the internal chambers 52 of the tanks 42 are at least partially flooded with water 74, or otherwise ballasted, to confer negative buoyancy that causes the array 40 to sink in the water column. Water 74 enters and flows between the internal chambers 52 of the tanks 42 via the end and / or side connectors 54, 56. Air thereby displaced from within the internal chambers 52 can exit through the upper vents 58. The position and orientation of the array 40 can be controlled during sinking by guide wires, winches, cranes and / or underwater vehicles such as ROVs.

[0074] The array 40 sinks until it lands on the seabed 16 as shown in Figure 9b and 10c, straddling the pre-installed cable 12 that then lies between rows 60 of the tanks 42. At this stage, the self-weight of the array 40 presses the ridges 68 of the tanks 42 into initially shallow engagement with soil of the seabed 16. ROVs or remote operation may be employed to fix the array 40 to the cable 12 after the array 40 lands on the seabed 16 over the cable 12.

[0075] Figure 9c shows a denser-than-water ballast material 76, such as grout or barite, being pumped into the tanks 42 of the array 40 landed on the seabed 16. Again, the ballast material 76 can enter and flow between the internal chambers 52 of the tanks 42 via the end and side connectors 54, 56. As a result, the weight of the array 40 increases, forcing the ridges 68 of the tanks 42 into deeper engagement with the soil of the seabed 16. Eventually, the underside of the tanks 42 may also bear against the seabed 16, further improving engagement between the tanks 42 and the seabed 16.

[0076] By virtue of being in fluid communication with each other, for example via tubular end connectors 54 and side connectors 56, the tanks 42 of the array 40 also work together as a singe, unitary fluid system in terms of enabling fluids to flow into, out of and through the array 40.

[0077] By way of example, a tank 42 may be 0.3m in diameter and 1m in length, hence defining a volume of about 0.07m3if approximated to a plain cylinder. The weight of that volume when filled with seawater would be about 69kg, or about 168kg if filled with grout, or about 313kg if filled with barite. Thus, in that example, two tanks 42 filled with grout would weigh about 336kg or four tanks 42 filled with barite would weigh about 1252kg, in addition to the weight of the tanks when empty. The latter would be sufficient to provide 7kN of lateral resistance against loads exerted by the cable 12.

[0078] Finally, Figures 11 a to 11 c show a variant of the installation method in which an array 40 of tanks 42 is installed with the cable 12 as the cable 12 is being laid from a cablelaying vessel 70. Figure 11a shows the array 40 assembled on a deck of the vessel 70 and united with the cable 12 aboard the vessel 70 before the cable 12 and the array 40 are launched together from the vessel 70 and into the water. The cable 12 and the array 40 are then lowered together through the water column as shown in Figure 11b before being landed together on the seabed 16. as shown in Figure 11c.

[0079] In an alternative approach, an array 40 of tanks 42 could be united with the cable 12 in the water, for example at the surface or in the water column between the surface and the seabed 16, before the cable 12 and the array 40 are then lowered together to the seabed 16. The array 40 of tanks 42 need not necessarily be assembled aboard the cable-laying vessel 70 but could instead be assembled elsewhere, such as on another vessel on in a yard of factory on land.

[0080] Many other variations are possible within the inventive concept. For example, the connectors that link the tanks of an array could be rigid, flexible, hinged or articulated. If provided, fluid communication between linked tanks may be effected by a conduit that is separate from and parallel to a mechanical connection between the tanks.

[0081] The rows or other groups of tanks on each side of a cable or other elongate element need not necessarily be coupled or connected directly to rows or other groups of tanks on the other side of the elongate element. For example, each group of tanks could be coupled to an elongate element that serves to couple the groups indirectly.

Claims

Claims1. A method of stabilising an elongate subsea element laid on a seabed, the method comprising coupling a length of the elongate element to an array of interconnected tanks, and disposing the coupled length of the elongate element on the seabed between groups of tanks of the array.

2. The method of Claim 1, comprising coupling the elongate element to connectors of the array that interconnect the groups of tanks.

3. The method of Claim 1 or Claim 2, comprising coupling the elongate element to the array after lowering the array to the seabed.

4. The method of Claim 3, comprising lowering the array onto the elongate element after installing the elongate element on the seabed.

5. The method of Claim 1 or Claim 2, comprising coupling the elongate element to the array before lowering the array and the elongate element to the seabed together.

6. The method of any preceding claim, comprising engaging the seabed with the tanks of the array.

7. The method of Claim 6, comprising engaging the seabed with engagement formations protruding downwardly from the tanks.

8. The method of Claim 6 or Claim 7, comprising ballasting the tanks after landing the array on the seabed.

9. The method of Claim 8, comprising pumping a ballasting fluid into the tanks, which fluid is denser than seawater.

10. The method of any preceding claim, wherein the tanks of the array are in mutual fluid communication.

11. The method of any preceding claim, wherein the groups of tanks are rows of tanks extending substantially parallel to the elongate element on respective sides of the elongate element.

12. A subsea installation comprising an elongate element laid on a seabed and coupled along a coupled length to a stabilising array of interconnected tanks placed on the seabed, with the coupled length of the elongate element being disposed on the seabed between groups of tanks of the array.

13. The installation of Claim 12, wherein the groups of tanks are rows of tanks extending substantially parallel to the elongate element on respective sides of the elongate element.

14. The installation of Claim 13, wherein the tanks are grouped into one or more matrices.

15. The installation of Claim 13 or Claim 14, wherein the tanks are grouped into at least two rows on each side of the elongate element.

16. The installation of any of Claims 13 to 15, wherein the elongate element extends along an elongate gap between rows of the tanks.

17. The installation of Claim 16, wherein connectors extending between the rows of tanks extend above, and bridge across, the elongate element.

18. The installation of Claim 17, wherein the connectors support interfaces that couple the elongate element to the array.

19. The installation of any of Claims 12 to 18, wherein the tanks of the array are in mutual fluid communication.

20. The installation of Claim 19, wherein said fluid communication is effected via pipes that effect structural interconnection between the tanks of the array.

21. The installation of any of Claims 12 to 20, wherein the seabed is engaged by engagement formations protruding downwardly from the tanks.

22. The installation of any of Claims 12 to 21 , wherein the tanks contain a ballasting fluid.

23. The installation of Claim 22, wherein the ballasting fluid is denser than seawater.

24. A stabilising structure for stabilising an elongate subsea element laid on a seabed, the structure comprising an array of tanks interconnected by connectors extending between adjoining tanks of the array, wherein the tanks are arranged in at least two groups and the connectors extending between an adjoining pair of the groups support interfaces for coupling along a length of the elongate element disposed on the seabed.

25. The structure of Claim 24, wherein the tanks are grouped in at least two parallel rows and the interfaces are supported by connectors extending between the rows.

26. The structure of Claim 24 or Claim 25, wherein the connectors comprise an end connector extending from at least one end of each tank and a side connector extending from at least one side of each tank.

27. The structure of Claim 26, wherein the end connector and the side connector extend along mutually orthogonal axes.

28. The structure of any of Claims 24 to 27, wherein the connectors effect fluid communication between the tanks.

29. The structure of any of Claims 24 to 28, wherein each tank has a rounded crosssection on at least an upper surface of the tank.

30. The structure of Claim 29, wherein each tank is of generally elliptical cross-section.

31. The structure of Claim 30, wherein each tank is of generally circular cross-section.

32. The structure of any of Claims 23 to 30, wherein each tank comprises engagement formations protruding downwardly from a hollow body of the tank.

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