Sliding Subsea Foundations

Abstract

A sliding subsea foundation comprises a polymeric shoe layer on the underside of a mudmat or subsea structure. The shoe layer defines a soil-engaging face that comprises an array of parallel grooves. The grooves are shallower than the thickness of the shoe layer such that each groove has a closed top, defined by and integral with the shoe layer, that spans the groove. Where a subsea structure is supported on the foundation with a subsea pipeline attached to the structure, the grooves are substantially parallel to a longitudinal axis of the pipeline.

Claims

1. A sliding subsea foundation comprises a polymeric shoe layer, wherein the shoe layer defines a soil-engaging face that comprises an array of substantially parallel elongate grooves.

2. The foundation of claim 1, wherein grooves of the array are separated by at least one land.

3. The foundation of claim 2, wherein two or more of the lands are integral with the shoe layer and with each other.

4. The foundation of claim 2, wherein the or each land has a width that is at least one third of the width of each groove, in a direction orthogonal to a length direction of the grooves.

5. The foundation of claim 2, wherein each groove has a depth, in a direction orthogonal to the soil-engaging face, of no more than twice the width of the or each land.

6. The foundation of claim 1, wherein the or each groove is shallower than a thickness dimension of the shoe layer in a direction orthogonal to the soil-engaging face.

7. The foundation of claim 6, wherein the or each groove has a closed top that spans the groove.

8. The foundation of claim 7, wherein the closed top is defined by the shoe layer.

9. The foundation of claim 7, wherein the closed top is defined by a mudmat of the foundation.

10. The foundation of claim 1, wherein the or each groove is of part-elliptical cross-section.

11. The foundation of claim 1, wherein the or each groove is of polygonal cross-section.

12. The foundation of claim 1, further comprising a subsea structure supported on the foundation.

13. The foundation of claim 12, further comprising a subsea pipeline attached to, and in fluid communication with, the subsea structure.

14. The foundation of claim 13, wherein the or each groove is aligned with a longitudinal axis of the pipeline where the pipeline is attached to the subsea structure.

15. The foundation of claim 1, wherein the shoe layer is of high density polyethylene.

16. The foundation of claim 1 when in situ on seabed soil, wherein a friction angle at an interface between the foundation and the seabed soil is lower in a longitudinal direction parallel to the length of the or each groove than in a transverse direction orthogonal to the length of the or each groove.

17. The foundation of claim 16, wherein in the transverse direction, the interface friction angle is substantially equal to the effective internal friction angle of the seabed soil.

18. The foundation of claim 16, wherein in the longitudinal direction, the interface friction angle is between 3° and 7° less than the effective internal friction angle of the seabed soil.

19. A method of supporting a subsea structure on the seabed, the method comprising: supporting the structure on a sliding subsea foundation that comprises a polymeric shoe layer; and engaging soil of the seabed with a soil-engaging face of the shoe layer, which face comprises at least one elongate groove.

20. The method of claim 19, comprising: promoting sliding of the foundation across the seabed in a direction parallel to the or each groove; and resisting sliding of the foundation across the seabed in a direction orthogonal to the or each groove.

21. The method of claim 20, comprising sliding the foundation parallel to the or each groove in response to thermal expansion or contraction of a pipeline that is attached to the structure and that extends parallel to the or each groove.

22. The method of claim 19, comprising the preliminary step of attaching the shoe layer to the foundation or to the structure as a shoe plate that comprises an array of two or more grooves.

23. The method of claim 19, comprising the preliminary steps of: lowering the foundation or the structure to the seabed with the or each groove of the shoe layer exposed on an underside of the foundation or the structure; and sandwiching the shoe layer between the seabed and the foundation or the structure.

Description

[0047] 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:

[0048] FIG. 1 is a schematic partial end view of a polymeric shoe plate for a mudmat in accordance with the invention;

[0049] FIG. 2 is a partial bottom plan view of the shoe plate shown in FIG. 1;

[0050] FIG. 3 is a schematic partial end view of a variant of the shoe plate shown in FIGS. 1 and 2;

[0051] FIG. 4 is a schematic part-sectional end view of a mudmat fitted with the shoe plate of FIGS. 1 and 2, forming a seabed foundation for a subsea structure exemplified here by an accessory at an end of a pipeline; and

[0052] FIG. 5 is a schematic plan view of the mudmat, accessory and pipeline shown in FIG. 4, partially cut away to show the shoe plate sandwiched between the mudmat and the seabed.

[0053] Referring firstly to FIGS. 1 and 2 of the drawings, a shoe plate 10 is moulded as a homogenous solid unitary body from a polymer material such as HDPE. The cross-sectional shape of the shoe plate 10 may be imparted by the moulding process or after moulding, for example by milling or otherwise machining a surface of a moulded panel or block.

[0054] The shoe plate 10 comprises an upper face 12 and a lower face 14 that extend in substantially parallel planes and are spaced apart by the overall thickness of the shoe plate 10, measured on an axis orthogonal to those planes.

[0055] The soil-engaging lower face 14 of the shoe plate 10 has a grooved texture that comprises an array of straight grooves 18, being trough-like elongate recesses. The grooves 16 extend part-way through the thickness of the shoe plate 10. Thus, the top of each groove 18 is closed by the remaining, unperforated thickness of the shoe plate 10.

[0056] In this example, the grooves 16 are substantially parallel. Thus, the array is substantially unidirectional. Also, the grooves 16 are substantially equi-spaced in this example. Thus, the grooves 18 of the array are distributed substantially uniformly across the width of the shoe plate 10.

[0057] In the preferred embodiments shown, the grooves 18 are open-ended and are uniform in cross-section, continuous and uninterrupted along their full length.

[0058] Adjacent grooves 16 of the array are bounded and spaced apart by elongate ridges or lands 18 that alternate with the grooves 16 across the width of the shoe plate 10. Collectively, the lands 18 are aligned with each other in a common plane that defines the plane of the lower face 14. Thus, collectively, the lands 18 are substantially co-planar. Individually, the lands 18 are each substantially planar, save for radiused edges at the interfaces between the lands 18 and the grooves 18 that adjoin them.

[0059] At the mid-point level of the radiused transitions between the grooves 18 and the lands 18, each land 18 has a width WL between an adjacent pair of grooves 16 of at least one third of the width WG of each groove 16. In other words, the grooves 18 are mutually spaced apart by at least one third of their width WG in a direction orthogonal to their length, parallel to the plane of the lower face 14 of the shoe plate 10.

[0060] Each groove 16 has a depth DG in an upward direction orthogonal to the lower face 14. The depth DG of each groove 18 is substantially equal to the width WL of each land 18 in this example. DG may be more or less than WL but is preferably no more than twice WL.

[0061] In FIGS. 1 and 2, each groove 16 is part-cylindrical and has a part-circular cross-section, in this example substantially semi-circular. Other curved or part-elliptical cross-sectional shapes are possible.

[0062] FIG. 3 shows a variant of the shoe plate 10 in which like numerals are used for like features. Here, each groove 18 has a substantially triangular cross-section whose general shape is akin to an isosceles or equilateral triangle. However, the apex of the triangular cross-section defining the base of the groove 16 is suitably radiused as shown. Other polygonal cross-sectional shapes are possible.

[0063] The shape of the planar upper face 12 of the shoe plate 10 is chosen to form an interface that complements the shape of the underside of a mudmat 20 to which the shoe plate 10 is to be attached as shown in FIGS. 4 and 5. Thus, the upper face 12 is not necessarily textured and is suitably plain.

[0064] FIGS. 4 and 5 show the shoe plate 10 of FIGS. 1 and 2 mounted to the flat underside of a sliding mudmat 20. The mudmat 20 is typically of steel. The shoe plate 10 extends continuously across the underside of the mudmat 20 and may be bonded or otherwise attached to the mudmat 20 onshore or offshore.

[0065] The mudmat 20 is shown here installed on the seabed 22. Thus, the shoe plate 10 is sandwiched between the mudmat 20 and the seabed 22 with its grooved lower face 14 engaged with the sandy or silty soil of the seabed 22.

[0066] The mudmat 20 is shown supporting a subsea structure 24 in the form of a pipeline accessory such as a PLET. A subsea pipeline 26 is shown joined to the structure 24 for fluid communication between them. The pipeline 26 will tend to extend and contract along its central longitudinal axis 28 in response to changes in the temperature of the fluid within the pipeline 26. Thus, it is desirable that the mudmat 20 can slide across the seabed 22 in directions parallel to the central longitudinal axis 28 of the pipeline 26.

[0067] For this reason, the grooves 16 formed in the lower face 14 of the shoe plate 10 extend substantially parallel to the central longitudinal axis 28 of the pipeline 26. In that longitudinal direction LD parallel to the length of the grooves 18, the interface friction angle is maintained at around ′−5° to allow limited freedom of longitudinal movement that facilitates thermal expansion of the pipeline 26. Conversely, in a transverse direction TD orthogonal to the length of the grooves 16, there is an increase of the interface friction angle to around ′. Beneficially, this resists unintended displacement of the mudmat 20 under lateral loads in use, transverse to the central longitudinal axis 28 of the pipeline 26, which could otherwise result in the structure 24 and the pipeline 28 ‘walking’ across the seabed 22.

[0068] Some variations have been described above; other variations are possible within the inventive concept. For example, the polymer material of the shoe plate may be reinforced by strands or fibres.

[0069] The grooves in the lower surface of the shoe plate have radiused edges and bases in the examples shown but they could have sharp edges and bases. The lands between the grooves need not be planar but could comprise sharp or radiused edges, peaks or ridges.

[0070] The shoe plate preferably extends across a majority of the plan area of the mudmat but need not extend across all of the plan area of the mudmat. The shoe plate could be discontinuous or more than one shoe plate could be attached to the mudmat. It would also be possible to attach a shoe plate directly to a subsea structure without an intervening mudmat.

[0071] In principle, it would be possible to assemble a shoe plate or shoe layer from an array of mutually-spaced parallel polymer strips that are affixed to the underside of a mudmat or directly to a subsea structure. Such strips would define the grooves in the spaces between them and would each define a respective land between adjacent grooves. In this case, the grooves could extend as slots through the full thickness of the shoe plate or shoe layer.