Protecting subsea structures

Abstract

An elongate subsea element, such as a pipeline, is covered by a series of cover units that each comprise a rigid shell arranged to bridge over the element and inner mudmats that are movable relative to side walls of the shell. When one of the cover units is suspended in water or in air, the inner mudmats are moved relative to the shell from a stowed configuration into a deployed configuration in which the inner mudmats extend inwardly toward each other from the side walls. The cover unit is then landed on the seabed over the element with the element disposed between the side walls and the inner mudmats.

Description

[0001] Protecting subsea structures

[0002] This invention relates to the mechanical protection of subsea structures, in particular elongate elements laid on the seabed. Examples of such elements are rigid or flexible pipelines, spools, umbilicals and cables as used in the subsea oil and gas industry and in the offshore renewable energy industry.

[0003] When laid on the seabed, especially in shallow seas, elongate subsea elements such as pipelines, spools, umbilicals and cables may require protection against damage from dropped objects or overtrawling. In shallow seas, lightweight elongate elements are also susceptible to movement across the seabed due to sea movement, particularly during exceptional storms or under rapid tidal flow.

[0004] In the context of the invention, 'shallow' means that the seabed is close enough to the surface as to be influenced, at least occasionally, by near-surface sea motions such as wind-driven movement. For illustration but without limitation, depths of up to, say, 90 to 100 metres may therefore be considered shallow in some sea areas. The North Sea is an example of shallow water of particular relevance to the subsea oil and gas industry, whose seabed is also characterised by very soft clay.

[0005] Conventional subsea protection solutions include burying an elongate element in a trench or placing concrete mattresses or dumping a berm of crushed graded rock fragments over an element previously laid on the seabed. These solutions anchor the element to the seabed and particularly resist lateral movement of the element, hence stabilising the element in addition to protecting it.

[0006] Whilst it is desirable to limit or control movement of an elongate element across the seabed, a lack of compliance is disadvantageous where the element needs some freedom of movement. A prime example is where the element carries hot or cold fluids in use and so has to handle the expansion and contraction of thermal cycling. In these circumstances, a rigidly-constrained element would impart enormous loads to tie-in support structures and could itself buckle under such loads. Also, sliding movement of an elongate element relative to the rocks of a surrounding static berm could damage a protective or insulative sleeve or coating of the element. This could lead to corrosion of the element or undermine thermal insulation of hot or cold fluids that the element carries in use. Where an elongate element is likely to move considerably in service due to thermal fluctuations or sea movement, it is preferred to protect the element with a cover system that maintains a clearance gap around the element. The cover system bears against the seabed on opposite sides of the element and bridges over the element to protect it from damage. The clearance gap allows the element to move relative to the cover system and the seabed, and to do so without suffering damage from contact with a rock berm. Yet, to stabilise the element, lateral movement of the element is limited by the sides of the cover system so that excessive lateral movement is prevented.

[0007] Typical pipeline cover systems comprise a rigid upper shell that sits on the seabed to cover the top and surround the sides of a pipeline, for example as shown in KR IQ- 1444171. Such cover systems usually comprise multiple cover units that can be connected to each other in series as an assembly to define a cover system of any desired length. The cover units may be made of steel, concrete or, most commonly, of a reinforced polymer composite such as GRP (glass-reinforced plastics) or other fibre- reinforced plastics. Where cover units are of a lightweight polymeric material such as GRP, rock dumping may be used after their installation on the seabed to stabilise them against sliding or overturning and to add further protection.

[0008] To increase the capacity of the seabed soil to support a cover system including any dumped rock, rock gravel may be pre-laid on the seabed around the pipeline path before installing the cover units on top of the gravel-reinforced soil. Also, for use on soft soils, wing-like mudmats may be added outside the shell of a cover unit, potentially with other structural features such as those shown in US 8702347, NO 341517 or WO 2016 / 188923. In another approach disclosed in WO 2022 / 221422, guillotine-like blades can be deployed downwardly from peripheral skirts of mudmats to engage the seabed soil. More conventionally, especially where cover units are of lightweight materials such as GRP, an advantage of external mudmats is the possibility to dump rock on top of the mudmats to increase the stability of the cover system.

[0009] Further to explain the prior art, reference is now made to Figures 1 to 4 of the accompanying drawings. Figure 1 shows a cover system 10 comprising two cover units 12 of moulded GRP that are used together to protect an elongate subsea element, exemplified here by a subsea pipeline 14 shown lying on the seabed 16 after pre-installation.

[0010] Each cover unit 12 comprises an open-ended arched shell 18 that defines a hollow interior dimensioned to provide clearance all around the pipeline 14 within. In this example, the shell 18 is defined by upwardly-convergent side walls 20 joined by a top wall 22. In other examples, the shell 18 could have a continuously-curved arch section. The shell 18 has lifting points 24 mounted on each side wall 20.

[0011] The cover units 12 are identical so as to fit together with adjoining cover units 12 in successively overlapping relation to form a longitudinal series along the path of the pipeline 14. Thus, once the cover units 12 are installed to form the cover system 10, the hollow interiors of their successive bodies 18 conjoin as a long open-ended tube to form a continuous elongate part-cylindrical tunnel extending across the seabed 16 around and along the pipeline 14.

[0012] The shell 18 of each cover unit 12 is flanked to both sides by integral outer mudmats 26 that extend outwardly from the base of respective side walls 20. The outer mudmats 26 are flat, flange-like lateral projections that extend in a common horizontal plane to lie flat upon the seabed 16, in the manner of feet, upon installation of the cover units 12. Ballast weights 28 are disposed on top of the outer mudmats 26.

[0013] The outer mudmats 26 of the cover units 12 serve as footings to support the cover system 10 on a soft seabed 16. They also facilitate stabilisation of the cover system 10 by rock-dumping. In this respect, Figure 2 shows berms of crushed rock 30 dumped along both sides of the cover system 10. Consequently, the outer mudmats 26 extending outwardly from the bodies 20 of the units 12 are sandwiched between the rock 30 and the seabed 16. The weight of the rock 30 therefore presses the outer mudmats 26 against the seabed 16 and so stabilises the cover system 10 against movement relative to the seabed 16.

[0014] For ease of illustration, the pipeline 14 and hence the cover system 18 are shown as being straight in Figures 1 and 2 but they could instead be curved in plan view, that curvature being accommodated by articulation between adjoining overlapping cover units 12. Figure 3 shows a stack 32 of the cover units 12 to exemplify how each cover unit 12 is stackable with like cover units 12 for compactness of storage and for ease of transportation together. For this purpose, the upwardly-convex outer side of the shell 18 of a lower unit 12 of the stack 32 can be received as a complementary fit within the downwardly-concave inner side of the shell 18 of an upper unit 12 of the stack 32. Indeed, conveniently, a stack 32 of the cover units 12 can be lowered to the seabed 16 before being lifted one-by-one from the stack 32 as the cover system 10 is assembled underwater from those cover units 12 over a pre-installed pipeline 14 or other elongate element.

[0015] Where cover units 12 are used on very soft soils, the size of their outer mudmats 26 may have to be increased to the extent that the cover units 12 become difficult to transport and handle offshore. The alternative of dumping more rock 30 is expensive and could even be counterproductive if the resulting weight bearing down on the outer mudmats 26 exceeds the bearing capacity of the soil of the seabed 16 beneath. There is a need to improve the stability of a pipeline cover system 10 without over-reliance upon dumping rock 30.

[0016] Thus, Figure 4 shows a variant of the cover system 10 shown in Figures 1 and 2 in which the cover units 12 further include integral inner mudmats 34 that extend inwardly from the base of respective side walls 20, parallel to or coplanar with the outer mudmats 26. Like the outer mudmats 26, the inner mudmats 34 are flat, flange-like projections that extend in a common horizontal plane to lie flat upon the seabed 16 upon installation of the cover units 12.

[0017] Inner mudmats 34 may particularly be required where the seabed 16 comprises very soft clay. In that respect, the critical design issue is that stabilising the system 10 against overtrawling could require so much rock 30 to be dumped on the outer mudmats 26 that there is a risk of bearing failure due to the weight of the rock 30. Increasing the size and hence the bearing area of the outer mudmats 26 will not help because the weight of rock 30 on top of the outer mudmats 26 will then increase commensurately.

[0018] In contrast, the inner mudmats 34 increase the bearing area without a commensurate increase in weight as they do not support any rock 30. In this respect, it will be noted that as the inner mudmats 34 are disposed within the shell 18 of the unit 12, they are separated by the side walls 20 from the rocks 30 dumped on top of the outer mudmats 26.

[0019] The presence of inner mudmats 34 creates a problem, however, which is that they preclude compact nested stacking of cover units 12 in the manner shown in Figure 3. Specifically, the inner mudmats 34 prevent the downwardly-concave inner side of the shell 18 receiving the upwardly-convex outer side of the shell 18 of the underlying cover unit 12 in a stack. The inability to stack them increases the cost and reduces the practicality of storing, transporting and installing the cover units 12. For example, there is no possibility of lowering a stack of the cover units 12 to the seabed 16 to be lifted from the stack in turn as a cover system 10 is assembled underwater.

[0020] Against this background, the invention resides in a cover unit for covering an elongate subsea element. The unit comprises a rigid shell having side walls in mutual opposition and inner mudmats that are movable, for example pivotable, relative to the side walls from a stowed configuration into a deployed configuration. In the deployed configuration, the inner mudmats extend inwardly from the side walls. Conversely, in the stowed configuration, the inner mudmats lie outside a chamber defined between the side walls and may splay apart downwardly.

[0021] The unit may further comprise outer mudmats that extend outwardly from the side walls. In that case, when in the deployed configuration, the inner mudmats can be coplanar with or parallel to the outer mudmats.

[0022] The outer mudmats may be movable with the inner mudmats with respect to the side walls. For example, the inner and outer mudmats may be pivotable in mutual opposition about pivot axes that lie between them and that extend parallel to the respective side walls. In that case, when the inner mudmats are in the stowed configuration, the outer mudmats can lie against the side walls. Latches can then act between the outer mudmats and the side walls to hold the inner mudmats in the stowed configuration.

[0023] Each inner mudmat may be part of a plate that further comprises at least one wing that is opposed to the inner mudmat and that extends through the shell. When the inner mudmats are in the stowed configuration, the wings can lie against the side walls and can be latched to the side walls to hold the inner mudmats in the stowed configuration. Conversely, the wings can move away from the side walls in response to movement of the inner mudmats from the stowed configuration into the deployed configuration. For example, when the inner mudmats are in the deployed configuration, the wings can lie on top of outer mudmats that are fixed integrally to the side walls.

[0024] Conveniently, when the inner mudmats are in the stowed configuration, the shell of the unit can be nested with a shell of a neighbouring similar unit disposed above or below in a stack. For this purpose, when in the stowed configuration, the inner mudmats may lie wholly outside a width of the shell as measured between bottom edges of the side walls. Conversely, when in the deployed configuration, spacing between the inner mudmats can be less than the width of the shell as measured between the bottom edges of the side walls. It may also be the case that when in the stowed configuration, the inner mudmats lie outside a width of a top wall of the shell that joins the side walls. Conversely, when in the deployed configuration, spacing between the inner mudmats can be less than the width of the top wall of the shell.

[0025] When the unit is installed on a seabed, the inner mudmats lie parallel to the seabed when they are in the deployed configuration. Ballast such as rock may then be deposited on outer mudmats, for example to sandwich wings attached to the inner mudmats between the ballast and the outer mudmats.

[0026] The inventive concept embraces a corresponding method of covering an elongate subsea element. The method comprises: suspending a cover unit that comprises a rigid shell and inner mudmats that are movable relative to side walls of the shell; moving the inner mudmats relative to the shell from a stowed configuration into a deployed configuration in which the inner mudmats extend inwardly from the side walls; and landing the cover unit over the element with the element disposed between the side walls of the shell.

[0027] Subsequently, ballast may be placed onto outward extensions of the inner mudmats that extend outwardly beyond the shell and that are movable with the inner mudmats relative to the side walls.

[0028] Where the outward extensions are wings, they may be sandwiched between the ballast and outer mudmats that extend outwardly from the side walls and may be fixed to the side walls. Conversely, the outward extensions may be outer mudmats that extend outwardly from the side walls when the inner mudmats are in the deployed configuration.

[0029] The outward extensions can be latched to the side walls to hold the inner mudmats in the stowed configuration and can be unlatched from the side walls to release the inner mudmats for movement into the deployed configuration.

[0030] The inner mudmats can be moved from the stowed configuration into the deployed configuration when the cover unit is suspended above a body of water or when the cover unit is submerged in or being lowered through a body of water.

[0031] A stack of the cover units can be lowered to a submerged location with their shells in nested relation. In that case, the inner mudmats can be moved from the stowed configuration into the deployed configuration after separating each cover unit from the stack.

[0032] The invention embodies the concept of making inner mudmats deployable or unfoldable, hence being movable relative to the rigid shell of a cover unit from a stowed or folded position into a deployed or unfolded position. The cover units may conveniently be stackable in closely nested relation when the inner mudmats are in the stowed position.

[0033] In exemplary embodiments, each inner mudmat comprises a plate that is distinct from the rigid shell of a cover unit. Stabiliser beams, tabs or wings projecting from the plate are received in slots or holes provided in walls of the cover unit. A joint is formed so that the inner mudmats can pivot relative to the remainder of the cover unit. Once rock is installed on top of the wings, the inner mudmats are fixed permanently in the deployed position due to the weight of the rock.

[0034] The inner mudmats may be tied or latched to the structure of the cover unit in the stowed position to be untied, released or unlatched beneath or above the surface, hence releasing the inner mudmats to a substantially horizontal deployed position ready for landing on the seabed. For example, the inner mudmats could be released on the deck of an installation vessel and then tied down to the footing defined by an outer mudmat, or the inner mudmats could be released subsea. The inner mudmats could be flipped down by an ROV subsea or could be biased into the deployed position, for example by a weight mounted at an elevated position or by a spring.

[0035] Embodiments of the invention provide a pipeline cover comprising: a cover shell to cover the top and the sides of the pipeline; at least one pivoting mudmat, distinct from the cover shell; and hinges connecting the cover shell and the pivoting mudmat, to pivot the mudmat between a storage and transport configuration along a side of the cover shell and a deployed configuration on the seabed. The hinges may be located at some distance from edges of the mudmat.

[0036] The cover shell may also comprise integral outer mudmats on each side, and holes for the pivoting mudmats. Each pivoting mudmat may, for example, be an inner mudmat that comprises outer wings.

[0037] At least part of the pivoting mudmat may be inside the cover shell. The pivoting mudmat may be partially inside the cover shell and partially outside the cover shell, for example laying directly on the seabed or on an outer mudmat when in the deployed configuration. In the deployed configuration, the mudmat may be substantially horizontal.

[0038] Embodiments of the invention also implement a method to install a pipeline cover, the method comprising: transporting and installing the cover on the seabed in a first configuration wherein mudmats of the cover are upwardly inclined and attached to sides of a cover shell; and pivoting the mudmats to a second, deployed and preferably horizontal configuration, in which at least part of each mudmat is located inside the cover shell.

[0039] In summary, an elongate subsea element, such as a pipeline, is covered in accordance with the invention by a series of cover units that each comprise a rigid shell arranged to bridge over the element and inner mudmats that are movable relative to side walls of the shell. When one of the cover units is suspended in water or in air, the inner mudmats are moved relative to the shell from a stowed configuration into a deployed configuration in which the inner mudmats extend inwardly toward each other from the side walls. The cover unit is then landed on the seabed over the element with the element disposed between the side walls and the inner mudmats. To put the invention into context, reference has already been made to Figures 1 to 4 of the accompanying drawings, in which:

[0040] Figure 1 is a schematic perspective view of a prior art cover system comprising a series of cover units that together protect a subsea pipeline lying on the seabed;

[0041] Figure 2 corresponds to Figure 1 but shows the cover system stabilised by berms of crushed rocks dumped over outwardly-extending outer mudmats of the cover units;

[0042] Figure 3 is a schematic end view of a stack of the cover units shown in Figure 1 ; and

[0043] Figure 4 is a schematic end view of a variant of the cover system of Figures 1 and 2 in use when protecting a subsea pipeline, the cover units of this variant including inwardly-extending inner mudmats.

[0044] In order that the invention may be more readily understood, reference will now be made, by way of example, to the remainder of the drawings in which:

[0045] Figure 5 is a schematic exploded perspective view of a cover unit of the invention in a storage and transport configuration;

[0046] Figure 6 corresponds to Figure 5 but shows the cover unit assembled and in a deployed configuration;

[0047] Figures 7a to 7d are a sequence of schematic end views showing the cover unit of Figures 5 and 6 being installed over a subsea pipeline;

[0048] Figures 8a to 8d are a sequence of schematic end views showing another cover unit of the invention being installed over a subsea pipeline;

[0049] Figure 9 is a schematic end view of a stack of cover units of Figures 5 to 7d in a storage and transport configuration; and Figure 10 is a schematic end view of a stack of cover units of Figures 8a to 8d in a storage and transport configuration.

[0050] Referring next, then, to Figures 5 and 6, these drawings show a cover unit 12 of the invention. Like numerals are used for parts that correspond to those of the prior art cover units 12 of Figures 1 to 4.

[0051] Again, the cover unit 12 comprises an open-ended arched shell 18 defined by upwardly-convergent side walls 20 joined by a top wall 22. The side walls 20 are in mutual opposition about a central longitudinal open-bottomed, open-ended chamber that can accommodate an elongate subsea element such as the pipeline 14. The shell 18 is flanked to both sides by integral outer mudmats 26 that extend outwardly from the base of respective side walls 20.

[0052] In this example, the cover unit 12 has a series of longitudinally-spaced, longitudinally- extending slots 36 disposed at the junction between each side wall 20 and the associated outer mudmat 26. Each slot 36 extends upwardly into the associated side wall 20 to a short distance above the upper side of the outer mudmat 26.

[0053] The cover unit 12 further comprises a pivoting planar panel 38 that comprises a series of longitudinally-spaced wings 40 extending integrally from an elongate inner mudmat 34. The wings 40 are received within respective ones of the slots 36 to protrude above the outer mudmat 26 while the inner mudmat 34 lies beneath the outer mudmat 26. Two series of hinges 42, shown in Figure 6, hold the wings 40 of the panels 38 in the slots 36 and define longitudinal pivot axes 44 about which the panels 38 can pivot relative to the remainder of the cover unit 12. The wings 40 serve as outward extensions of the inner mudmats 34.

[0054] As before, at least the shell 18 of the cover unit 12 is apt to be moulded from GRP. The panels 38 comprising the inner mudmats 34 can also be moulded from GRP. However, the shell 18 and / or the panels 38 could instead be made of other materials such as steel.

[0055] In a stowed configuration for storage and transport as shown to the left of Figure 5, the panels 38, including their inner mudmats 34, adopt the upwardly-convergent inclination of the side walls 20. Thus, the wings 40 of the panels 38 lie against the outer faces of the side walls 20 and may be latched to the side walls 20 to hold the panels 38 with that mutually-opposed inclination. The inner mudmats 34 then splay apart downwardly.

[0056] Conversely, in a deployed configuration shown in Figure 6, the panels 38 are turned about the respective pivot axes 44 of the hinges 42 to adopt a substantially horizontal orientation. In doing so, the inner mudmats 34 turn inwardly across the open bottom of the shell 18 and the wings 40 pivot down to lie on top of the outer mudmats 26.

[0057] Figures 7a to 7c show the cover unit 12 suspended from lifting wires 46 attached to the lifting points 24 on the side walls 20. While suspended in this way, the panels 38 can pivot from the stowed configuration shown in Figure 7a to the deployed configuration shown in Figure 7c while transitioning through the intermediate position shown in Figure 7b.

[0058] In the deployed configuration shown in Figure 7c, the inner mudmats 34 lie in a plane parallel to the outer mudmats 26, albeit not necessarily in fully coplanar alignment with the outer mudmats 26. Thus, when the cover unit 12 is then landed on the seabed 16 over a pipeline 14 as shown in Figure 7d, the outer and inner mudmats 26, 34 lie beside each other horizontally on the seabed 16. This increases the contact area between the cover unit 12 and the seabed 16, hence improving the stability and bearing capacity of the cover unit 12. The inner mudmats 34 remain spaced apart sufficiently to accommodate the pipeline 14 between them.

[0059] After the cover unit 12 has been landed on the seabed 16, ballast such as rock 30 can be dumped onto the outer mudmats 26 as shown in Figure 7d. The weight of the rock 30 also holds the wings 40 of the panels 38 against the outer mudmats 26. This locks the panels 38 so that the inner mudmats 34 form a rigid structure with the reminder of the cover unit 12.

[0060] Figure 7a also shows latches 48 that temporarily hold the wings 40 of the panels 38 against the side walls 20 so that the inclination of the panels 38 corresponds to that of the side walls 20. As will be appreciated, the inner mudmats 34 of the panels 38 are thereby held clear of the downwardly-concave underside of the shell 18 to facilitate nesting of cover units 12 when forming a stack 32 as shown in Figure 9. The latches 48 can be released, for example by being pivoted away from the wings 40, or can be removed, cut or broken to free the panels 38 for pivotal movement about the hinges 42. In the variant shown individually in Figures 8a to 8d and as part of a stack 32 in Figure 10, a cover unit 12 of the invention comprises pivoting planar panels 38 that each define outer and inner mudmats 26, 34. For this purpose, the panels 38 are mounted to the lower edges of the side walls 20 by hinges 42 defining respective pivot axes 44 disposed between inner and outer longitudinal edges of the panels 38. In this way, on pivoting from the stowed configuration shown in Figure 8a through the intermediate position shown in Figure 8b to the deployed configuration shown in Figure 8c, the panels 38 swing around the lower edges of the side walls 20.

[0061] In the deployed configuration shown in Figure 8c, the outer mudmats 26 and the inner mudmats 34 lie in a common, substantially horizontal plane. Thus, when the cover unit 12 is then landed on the seabed 16 over a pipeline 14 as shown in Figure 8d, the outer and inner mudmats 26, 34 lie beside each other on the seabed 16 to increase the contact area between the cover unit 12 and the seabed 16.

[0062] Interaction between the panels 38 and the seabed 16 locks the panels 38 so that the outer and inner mudmats 26, 34 form a rigid structure with the reminder of the cover unit 12. Again, however, ballast such as rock 30 can be dumped onto the outer mudmats 26 to stabilise the cover unit 12 as shown in Figure 8d. Thus, like the wings 40 of the previous example, the outer mudmats 26 serve as outward extensions of the inner mudmats 34.

[0063] As before, Figure 8a shows latches 48 that temporarily hold the outer mudmats 26 of the panels 38 against the side walls 20 so that the inclination of the panels 38 corresponds to that of the side walls 20. Consequently, the inner mudmats 34 of the panels 38 splay apart downwardly, holding them clear of the downwardly-concave underside of the shell 18 to facilitate nesting of cover units 12 when forming a stack 32 as shown in Figure 10. Again, the latches 48 can be released, removed, cut or broken to free the panels 38 for pivotal movement about the hinges 42.

[0064] For simplicity of illustration, lifting points 24 and lifting wires 46 are omitted from the cover units 12 of the stacks 32 shown in Figures 9 and 10.

[0065] In each of the examples shown in Figure 7a to 10, nesting of the cover units 12 is assured because when in the stowed configuration, the inner mudmats 34 lie outside the width of the shell 18 as measured at the bottom of the side walls 20. More generally, when in the stowed configuration, the inner mudmats 34 lie outside the width of the top wall 22 of the shell 18. Conversely, when in the deployed configuration, the spacing between the inner mudmats 34 is less than the width of the top wall 22 of the shell 18 or, more generally, is less than the spacing between the bottom of the side walls 20.

[0066] The panels 38 can be pivoted from the stowed configuration shown in Figures 7a and 8a to the deployed configuration shown in Figures 7c and 8c at any time before landing on the seabed 16 over a pipeline 14 as shown in Figures 7d and 8d. For example, the panels 38 can be released from the latches 48 at an above-surface location such as the working deck of an installation vessel or at a below-surface location in the water column between the surface and the seabed 16. For the latter purpose, an ROV or a diver can release, remove, cut or break the latches 48. The panels 38 can therefore pivot from the stowed configuration to the deployed configuration while the cover unit 12 is being lowered toward the seabed 16.

Claims

Claims1. A cover unit for covering an elongate subsea element, the unit comprising: a rigid shell having side walls in mutual opposition; and inner mudmats that are movable relative to the side walls from a stowed configuration into a deployed configuration in which the inner mudmats extend inwardly from the side walls.

2. The unit of Claim 1 wherein, in the stowed configuration, the inner mudmats lie outside a chamber defined between the side walls.

3. The unit of Claim 1 or Claim 2 wherein, in the stowed configuration, the inner mudmats splay apart downwardly.

4. The unit of any preceding claim, wherein the inner mudmats are pivotable with respect to the side walls.

5. The unit of any preceding claim, further comprising outer mudmats that extend outwardly from the side walls.

6. The unit of Claim 5, wherein in the deployed configuration, the inner mudmats are coplanar with or parallel to the outer mudmats.

7. The unit of Claim 5 or Claim 6, wherein the outer mudmats are movable with the inner mudmats with respect to the side walls.

8. The unit of Claim 8, wherein the inner and outer mudmats are pivotable in mutual opposition about pivot axes that lie between them and that extend parallel to the respective side walls.

9. The unit of Claim 8, wherein when the inner mudmats are in the stowed configuration, the outer mudmats lie against the side walls.

10. The unit of Claim 9, further comprising latches acting between the outer mudmats and the side walls to hold the inner mudmats in the stowed configuration.

11. The unit of any of Claims 1 to 6, wherein each inner mudmat is part of a plate that further comprises at least one wing that is opposed to the inner mudmat and that extends through the shell.

12. The unit of Claim 11 , wherein when the inner mudmats are in the stowed configuration, the wings lie against the side walls.

13. The unit of Claim 12, further comprising latches acting between the wings and the side walls to hold the inner mudmats in the stowed configuration.

14. The unit of any of Claims 11 to 13, wherein the wings are movable away from the side walls in response to movement of the inner mudmats from the stowed configuration into the deployed configuration.

15. The unit of Claim 14, wherein when the inner mudmats are in the deployed configuration, the wings lie on top of outer mudmats that are fixed integrally to the side walls.

16. The unit of any preceding claim, wherein when the inner mudmats are in the stowed configuration, the shell of the unit is nestable with a shell of a like unit disposed above or below in a stack.

17. The unit of Claim 16, wherein when in the stowed configuration, the inner mudmats lie outside a width of the shell as measured between bottom edges of the side walls.

18. The unit of Claim 17, wherein when in the deployed configuration, spacing between the inner mudmats is less than the width of the shell as measured between the bottom edges of the side walls.

19. The unit of any of Claims 16 to 18, wherein when in the stowed configuration, the inner mudmats lie outside a width of a top wall of the shell that joins the side walls.

20. The unit of Claim 19, wherein when in the deployed configuration, spacing between the inner mudmats is less than the width of the top wall of the shell.

21. The unit of any preceding claim when installed on a seabed with the inner mudmats in the deployed configuration lying parallel to the seabed.

22. The unit of Claim 21 when dependent on Claim 5 or Claim 6, further comprising ballast deposited on the outer mudmats.

23. The unit of Claim 21 when dependent on Claim 15, further comprising ballast deposited on the wings and sandwiching the wings between the ballast and the outer mudmats.

24. A stack of the units of any of Claims 1 to 20 with their shells in nested relation.

25. A method of covering an elongate subsea element, the method comprising: suspending a cover unit that comprises a rigid shell and inner mudmats that are movable relative to side walls of the shell; moving the inner mudmats relative to the shell from a stowed configuration into a deployed configuration in which the inner mudmats extend inwardly from the side walls; and landing the cover unit over the element with the element disposed between the side walls of the shell.

26. The method of Claim 25, comprising subsequently placing ballast onto outward extensions of the inner mudmats that extend outwardly beyond the shell and are movable with the inner mudmats relative to the side walls.

27. The method of Claim 26, comprising sandwiching the outward extensions between the ballast and outer mudmats that extend outwardly from the side walls.

28. The method of Claim 26, wherein the outward extensions are outer mudmats that extend outwardly from the side walls when the inner mudmats are in the deployed configuration.

29. The method of any of Claims 26 to 28, comprising latching the outward extensions to the side walls to hold the inner mudmats in the stowed configuration.

30. The method of Claim 29, comprising unlatching the outward extensions from the side walls to release the inner mudmats for movement into the deployed configuration.

31. The method of any of Claims 25 to 30, comprising moving the inner mudmats from the stowed configuration into the deployed configuration when the cover unit is suspended above a body of water.

32. The method of any of Claims 25 to 30, comprising moving the inner mudmats from the stowed configuration into the deployed configuration when the cover unit is submerged in a body of water.

33. The method of Claim 32, comprising lowering a stack of the cover units to a submerged location with their shells in nested relation, and moving the inner mudmats from the stowed configuration into the deployed configuration after separating each cover unit from the stack.

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