Subsea Foundations

Abstract

A method of constructing a sliding subset foundation comprises embedding at least one mass of rocks in seabed soil and placing a sliding mudmat on the seabed over the or each mass of rocks. The rocks may be lowered into a cavity in the seabed by dumping the rocks into the cavity or when contained within a gabion that is inserted into the cavity. Alternatively, a gabion containing the rocks may penetrate the seabed soil, driven by self-weight or additionally by a deadweight bearing on the gabion. The gabion may be lowered toward tie seabed suspended from the deadweight. The deadweight may be removed and recovered after the gabion has been embedded in the seabed. The mass of rocks are embedded within an area of excursion of the mudmat to ensure that the mudmat will be directly above the mass of rocks at any position within the area of excursion.

Claims

1. A method of constructing a sliding subsea foundation, the method comprising: embedding at least one mass of rocks in seabed soil; and placing a sliding mudmat on the seabed over the or each mass of rocks.

2. The method of claim 1, comprising lowering the rocks into a cavity in the seabed soil.

3. The method of claim 2, comprising dumping the rocks into the cavity to form the mass of rocks.

4. The method of claim 2, comprising inserting the rocks into the cavity when contained within at least one gabion.

5. The method of claim 1, comprising penetrating the seabed soil with at least one gabion containing the rocks.

6. The method of claim 5, comprising driving penetration of the or each gabion by self-weight of the gabion.

7. The method of claim 6, comprising additionally driving penetration of the or each gabion by a deadweight bearing on the or each gabion.

8. The method of claim 7, comprising lowering the or each gabion toward the seabed suspended from at least one deadweight.

9. The method of claim 7 or claim 8, comprising removing the or each deadweight from the or each gabion after embedding the or each gabion in the seabed.

10. The method of any of claims 4 to 9, comprising embedding a plurality of gabions in the seabed in adjoining or stacked relation.

11. The method of any preceding claim, comprising: embedding first and second discrete masses of rocks in the seabed at locations mutually spaced across the seabed; and placing the sliding mudmat on the seabed to span the spacing between the first and second masses of rocks.

12. The method of any preceding claim, comprising: determining an area of excursion of the mudmat on the seabed; and embedding the or each mass of rocks in the seabed within the area of excursion at a position, or positions, that ensure that at least part of the mudmat will lie directly above at least part of at least one mass of rocks when the mudmat is at any position within the area of excursion.

13. A subsea foundation, comprising: at least one man-made mass of rocks embedded in seabed soil to form a support structure; and a sliding mudmat placed on the seabed over the support structure.

14. The foundation of claim 13, wherein the or each mass of rocks is columnar.

15. The foundation of claim 13 or claim 14, wherein the or each mass of rocks has a height in a vertical direction that is greater than a width dimension of the mass in a horizontal direction.

16. The foundation of any of claims 13 to 15, wherein the or each mass of rocks is elongated in a horizontal direction.

17. The foundation of any of claims 13 to 16, wherein the or each mass of rocks comprises at least one gabion.

18. The foundation of any of claims 13 to 17, wherein: the support structure comprises first and second discrete masses of rocks embedded in the seabed at locations mutually spaced across the seabed; and the mudmat spans the spacing between the first and second masses of rocks.

19. The foundation of any of claims 13 to 18, wherein a pipeline follows a path extending over the mudmat.

20. The foundation of claim 19, wherein the support structure comprises first and second discrete masses of rocks embedded in the seabed at locations mutually spaced along the path of the pipeline.

21. The foundation of claim 19 or claim 20, wherein the or each mass of rocks is elongated horizontally in a direction transverse to the path of the pipeline.

22. The foundation of any of claims 13 to 21, wherein: the mudmat is movable within an area of excursion on the seabed; and the or each mass of rocks is embedded in the seabed within the area of excursion at a position, or positions, that ensure that at least part of the mudmat will lie directly above at least part of at least one mass of rocks when the mudmat is at any position within the area of excursion.

Description

[0041] In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which:

[0042] FIGS. 1a to 1f are a sequence of schematic sectional side views that show the preparation of a support structure, in accordance with the invention, for a sliding mudmat foundation of an inline pipeline accessory;

[0043] FIG. 2 is a plan view of the support structure beneath the mudmat foundation and the accessory;

[0044] FIGS. 3a and 3b show a range of excursion of the sliding mudmat foundation with respect to the support structure in response to thermal cycling of the pipeline;

[0045] FIG. 4 is a schematic side view of a gabion for use in some embodiments of the invention;

[0046] FIGS. 5a to 5c are a sequence of schematic sectional side views that show an alternative technique for preparing a support structure in accordance with the invention, using gabions as shown in FIG. 4;

[0047] FIGS. 6a to 6c are a sequence of schematic sectional side views that show another technique for preparing a support structure in accordance with the invention, also using gabions as shown in FIG. 4; and

[0048] FIG. 7 is a schematic sectional side view that shows another technique for preparing a support structure in accordance with the invention, again using gabions as shown in FIG. 4.

[0049] Referring firstly to the sequence of views shown in FIGS. 1a to 1f of the drawings, a soft seabed 10 of silt or clay soil in deep water 12, as shown in FIG. 1a, is firstly excavated to form mutually-spaced, discrete cavities 14 as shown in FIGS. 1b and 1c.

[0050] Specifically, FIG. 1b shows a subsea excavator 16 that has already excavated a cavity 14 in the seabed 10 in the form of a first trench and is in the process of excavating another cavity 14 in the form of a second trench spaced across the seabed 10 from the first trench. The open-topped cavities 14 are shown completed in FIG. 1c.

[0051] Each cavity 14 has a depth greater than its width. In top plan view, each cavity 14 may be of similar width in mutually-orthogonal horizontal directions or, substantially, rotationally symmetrical. Alternatively, each cavity 14 may be elongate in a horizontal direction, hence having a length greater than its width and possibly also greater than its depth, for example with a generally rectangular shape in top plan view or being rotationally asymmetrical.

[0052] A drill or auger could be used to dig the cavities 14 instead of the excavator 16, in which case the cavities 14 could be circular-section bores. However, if desired, a series of overlapping conjoined bores could be drilled in the seabed 10 on parallel vertical axes to form a cavity 14 that is horizontally elongate.

[0053] In FIG. 1d, a rock dumper 18 suspended from a surface vessel is shown having filled the first cavity 14 with rocks 20 up to the level of the seabed 10 and is now in the process of filling the second cavity 14 with rocks 20. An ROV 22 is shown monitoring the rock-dumping operation. FIG. 1e shows the rock-dumping operation now completed, with both cavities 14 filled with rocks 20 to the level of the seabed 10. This creates a foundation support comprising mutually-spaced, discrete, man-made pillars or columns 24 of rock that are buried or embedded in the seabed 10. Each rock column 24 is taller than it is wide.

[0054] Optionally, the masses of dumped rocks 20 may be compacted, after dumping, to increase the density, integrity and compressive strength of each rock column 24. Also optionally, a layer of soil of the seabed 10 could be spread over the top of each rock column 24.

[0055] FIG. 1f and the corresponding plan view of FIG. 2 show a pipeline 26 now laid on the seabed 10 above the rock columns 24 of the foundation support. The pipeline 26 may have been laid by any conventional pipelaying technique such as S-lay, J-lay or reel-lay. The pipeline 26 bridges the gap between the rock columns 24 and extends across the seabed 10 beyond the rock columns 24. Thus, the rock columns 24 are spaced longitudinally with respect to the length of the pipeline 26.

[0056] The pipeline 26 comprises an accessory 28, in this example an in-line accessory 28 such as an ILT disposed between successive lengths of pipe. A sliding mudmat 30 disposed under the accessory 28 rests on the rock columns 24 and on the seabed 10 between and around the rock columns 24. The rock columns 24 therefore bear some of the weight of the accessory 28 and the mudmat 30 and dissipate those loads into the surrounding soil of the seabed 10 with which the rock columns 24 are engaged.

[0057] The weight-bearing contribution of the rock columns 24 allows the mudmat 30 to be significantly smaller than if the rock columns 24 were absent. Yet, there is no greater risk of such a compact mudmat 30 becoming embedded in, and hence being locked relative to, the soft soil of the seabed 10 under the weight of the accessory 28 supported above. The mudmat 30 can therefore slide freely with the supported accessory 28 in response to displacement of the pipeline 26, in particular thermal expansion and contraction causing the accessory 28 to slide longitudinally with respect to the path of the pipeline 26. Consequently, the pipeline 26 is not subjected to buckling stresses that could arise if movement of the accessory 28 was excessively constrained.

[0058] In this respect, FIG. 1f shows a range of longitudinal movement of the mudmat 30, characterising a horizontal area of excursion or box 32 in which the accessory 28 and hence the mudmat 30 may be expected to move horizontally in use of the pipeline 26.

[0059] FIG. 2 shows that the box 32 may also extend horizontally in lateral directions transverse to the path of the pipeline 26, for example if the path of the pipeline 26 across the seabed 10 is curved when viewed from above.

[0060] The plan view of FIG. 2 also shows that, in this example, the rock columns 24 are elongated horizontally in a direction transverse to the path of the pipeline 26 to form transverse foundation walls. This ensures that the rock columns 24 continue to support the mudmat 30 even if the mudmat 30 is displaced laterally across the seabed 10. Yet, the rock columns 24 remain compact and so are simple and inexpensive to install.

[0061] Turning next to FIGS. 3a and 3b, these drawings show the pipeline 26, the accessory 28 and hence the mudmat 30 slid across the seabed 10 to respective longitudinal extremities of the box 32 shown in FIG. 1f and in FIG. 2. It will be apparent that at least one of the rock columns 24 remains underneath the mudmat 30 at each extreme position. The positions, number and mutual spacing of the rock columns 24 are chosen specifically to ensure that the mudmat 30 will always be supported by at least one rock column 24 regardless of where the mudmat 30 may move within the predicted area of the box 32.

[0062] FIG. 4 shows another way of forming and delivering the rock columns 24 that support a sliding mudmat foundation in accordance with the invention. Here, the rock columns 24 are defined by one or more gabions 34 that hold a packed mass of rocks 20 within a surrounding basket or cage 36 of steel wire netting or mesh.

[0063] The gabion 34 exemplified in FIG. 4 and the remaining drawings is taller than it is wide and so is capable of forming a rock column 24 by itself. However, two or more gabions 34 could be stacked one on top of each other, laid beside each other or otherwise assembled or brought together to form a rock column 24, depending upon the shape and size of the gabions 34 and of the rock column 24 that is required.

[0064] Thus, a gabion 34 could have a different aspect ratio to that shown and may, for example, be substantially as tall as it is wide, or indeed may be shorter than its width. Also, the gabion 34 shown in FIG. 4 has a cuboidal shape but it could have a cylindrical shape, for example with a circular cross-section in a horizontal plane.

[0065] The invention contemplates various ways in which gabions 34 may be installed and buried or embedded in the soil of the seabed 10. In this respect, reference will now be made to FIGS. 5a to 5c, FIGS. 6a to 6c and FIG. 7.

[0066] FIGS. 5a to 5c show the use of deadweights 38, for example of concrete, mounted on top of each gabion 34 to force the gabions 34 down into the seabed 10. During installation, the gabions 34 are lowered to the seabed 10 hanging from a crane wire 40 under respective deadweights 38 and are attached to the deadweights 38 by subsea-operable release mechanisms 42.

[0067] FIG. 5a shows one of the gabions 34 already lowered to the seabed 10 and the other gabion 34 in the process of being lowered to the seabed 10. An ROV 22 is shown monitoring the lowering operation. Each gabion 34 is surmounted by a respective deadweight 38. A release mechanism 42 is interposed between each gabion 34 and the associated deadweight 38.

[0068] FIG. 5b shows the gabions 34 being forced into the soft soil of the seabed 10 under the downward load of their self-weight supplemented by the additional weight of the deadweights 38.

[0069] FIG. 5c shows the gabions 34 now fully embedded into the seabed 10. The ROV 22 has operated the release mechanism 42 between one of the gabions 34 and the associated deadweight 38, freeing that deadweight 38 and the release mechanism 42 to be lifted back to the surface. The deadweight 38 on the other gabion 34 can be released and lifted simultaneously, or subsequently.

[0070] FIGS. 6a to 6c show that gabions 34 could potentially sink and embed themselves into a soft seabed 10 under their self-weight. FIG. 6a shows one of the gabions 34 already lowered to the seabed 10 and the other gabion 34 in the process of being lowered to the seabed 10 on a crane wire 40. FIG. 6b shows the gabions 34 being forced into the soil of the seabed 10 under the downward load of their self-weight. Again, an ROV 22 can monitor the lowering operation and release the crane wire 40 from the gabions 34. FIG. 6c shows the gabions 34 now fully embedded into the seabed 10.

[0071] Finally, FIG. 7 shows the possibility of gabions 34 being lowered into pre-excavated cavities 14 in the seabed 10 like those shown in FIG. 1c. Here, one of the gabions 34 has already been lowered into a cavity 14 in the seabed 10 and the other gabion 34 is in the process of being lowered into another cavity 14 in the seabed 10. The horizontal cross-sectional shape and area of the cavity 14 suitably matches the horizontal cross-sectional shape and area of the gabion 34.

[0072] When the gabions 34 shown in FIGS. 5a to 5c, FIGS. 6a to 6c and FIG. 7 have been embedded fully into the seabed 10 to form rock columns 24, a pipeline 26 comprising an accessory 28 and a sliding mudmat 30 can be laid above them onto the seabed 10 as shown in FIG. 1f and FIG. 2.

[0073] 3D finite element modelling has been undertaken to simulate the effect of the invention, assuming: shear strength of soil su=2+1z; a foundation footprint of 10 m×6 m; and a pair of parallel rock columns 24 being 4 m long, 1 m wide and 2 m high, each rock column 24 comprising gabions 34 embedded in the soil. In the model, the foundation was loaded with self-weight of 400 kN and a horizontal load was applied in the longitudinal direction at a height of 2 m above the seabed to reflect a typical PLET-type structure. The analysis revealed an improvement of nearly 50% in displacement of the foundation under load.

[0074] Many variations are possible within the inventive concept. For example, rock columns 24 may be spaced not just longitudinally but also at various lateral positions with respect to the path of the pipeline 26.

[0075] Two or more gabions 34 could be suspended from, and pressed down by, a single deadweight 38 shared between those gabions 34.

[0076] Penetration of a gabion 34 into the seabed 10 could be accelerated by dropping the gabion 34 from a short distance above the seabed 10, hence impacting the seabed 10 with momentum and substantial kinetic energy. A gabion 34 could also be driven into seabed soil with the assistance of a penetration driver that vibrates the gabion 34 or that impacts the gabion 34.

[0077] A gabion 34 could be shaped to ease penetration into seabed soil, for example by having a downwardly-tapering shape, at least at a lower or leading end of the gabion 34.