Hollow subsea foundations

Abstract

A method of installing an upright elongate hollow subsea foundation that is higher than it is wide, such as a suction pile. The foundation is at least partially embedded in seabed soil. A partition layer is placed within the foundation, for example by injecting a grout, supported by a plug of soil that is surrounded by the foundation. The partition layer is placed on the plug of soil at a level that is spaced from the top of the foundation by at least 20% of the height of the foundation. Above the partition layer, the interior of the foundation may be filled with water and/or a rigid body, such as a solid mass or a hollow liquid-filled tank.

Claims

1. A method of installing an upright elongate hollow subsea foundation that is higher than it is wide, the method comprising: at least partially embedding the foundation in seabed soil; shortening a plug of the soil that is surrounded by a wall of the foundation by excavating soil from the plug; and after shortening the plug, placing a partition layer within a flooded interior of the foundation, supported by the plug; wherein the partition layer is placed on the plug of soil within the foundation at a level that is spaced from the top of the foundation by at least 20%, but no more than half, of the height of the foundation.

2. The method of claim 1, comprising engaging the partition layer with the surrounding wall of the foundation.

3. The method of claim 1, comprising excavating the soil from the plug to a level below that of the seabed surrounding the foundation.

4. The method of claim 1, comprising excavating the soil from the plug from within the foundation after embedding the foundation.

5. The method of claim 4, comprising excavating the soil from the plug while the foundation remains substantially stationary relative to the surrounding seabed.

6. The method of claim 1, comprising shortening the plug before embedding the foundation by: excavating a cavity in the seabed soil to below the level of the seabed surrounding the cavity; and embedding the foundation into the seabed soil within the cavity.

7. The method of claim 6, further comprising infilling the cavity around the embedded foundation.

8. The method of claim 1, comprising embedding the foundation under its self-weight.

9. The method of claim 1, comprising embedding the foundation by generating an underpressure within the foundation.

10. The method of claim 1, comprising placing the partition layer by introducing a flow of grout into the foundation and then curing the grout.

11. The method of claim 1, comprising placing the partition layer by lowering a slab into the foundation.

12. The method of claim 1, comprising placing the partition layer beneath a rigid body that occupies an upper portion of the interior of the foundation.

13. The method of claim 12, wherein the partition layer is in supporting contact with the rigid body.

14. The method of claim 12, wherein the rigid body is a solid mass or a hollow chamber.

15. The method of claim 12, comprising directing a flowable material through the rigid body to form the partition layer.

16. The method of claim 1, further comprising supporting a subsea structure on the foundation.

17. The method of claim 16, comprising resting the structure on the wall of the foundation.

18. The method of claim 17, comprising clamping the structure to the wall of the foundation.

19. The method of claim 16, comprising supporting the structure on the partition layer within the foundation.

20. The method of claim 19, comprising lowering at least one supporting leg of the structure into the foundation and into contact with the partition layer.

21. The method of claim 19, comprising interposing at least one supporting leg between the structure and the partition layer.

22. The method of claim 20, comprising adjusting the length of the or each supporting leg.

23. The method of claim 1, comprising placing the partition layer at a level that is spaced from the top of the foundation by at least one third of the height of the foundation.

24. A combination comprising: an elongate hollow foundation at least partially embedded in seabed soil in an upright orientation, the foundation being higher than it is wide, wherein the foundation contains a partition layer supported by a plug of soil that is surrounded by a wall of the foundation, the partition layer being at a level that is spaced from the top of the foundation by at least 20%, but no more than half, of the height of the foundation; and a subsea structure supported by the foundation, wherein the structure rests on a wall of the foundation and is supported on the partition layer within the foundation, and wherein at least one supporting leg of the structure extends into the foundation and into contact with the partition layer.

25. The combination of claim 24, wherein the structure is clamped to the wall of the foundation.

26. The combination of claim 24, comprising at least one supporting leg interposed between the structure and the partition layer.

27. The combination of claim 24, wherein the length of the or each supporting leg is adjustable.

28. The combination of claim 24, wherein the structure extends above the foundation.

29. The combination of claim 24, wherein the structure extends laterally or horizontally beyond an outer diameter of the foundation.

Description

(1) 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:

(2) FIGS. 1a to 1f are a sequence of schematic cross-sectional side views showing the installation of a suction pile in accordance with the invention;

(3) FIG. 2 is a schematic cross-sectional side view of another suction pile of the invention;

(4) FIGS. 3a and 3b correspond to the installation stages shown in FIGS. 1b and 1e but show another suction pile of the invention;

(5) FIGS. 4a to 4c are a sequence of schematic cross-sectional side views that show another technique for installing the suction pile shown in FIGS. 3a and 3b;

(6) FIGS. 5a to 5e are a sequence of schematic cross-sectional side views that show the installation of an open caisson in accordance with the invention; and

(7) FIG. 6 shows a variant of a support arrangement shown in FIG. 5e.

(8) Referring firstly to FIGS. 1a to 1e of the drawings, a subsea foundation of the invention is exemplified here by a suction pile 10. The pile 10 comprises a tubular skirt 12 that has an open bottom and a closed top. The skirt 12 is rotationally symmetrical about an upright central longitudinal axis. The top of the skirt 12 is closed by a substantially horizontal top plate 14.

(9) In this example, the top plate 14 is penetrated by three ports each dedicated to a respective function, namely a grout injection port 16, a pumping port 18 and an excavation port 20. In practice, one port could serve two or more of those functions, for example being used initially for excavation and subsequently for injecting grout. There could therefore be fewer than three ports.

(10) Any or all of the ports 16, 18, 20 may be fitted with valves or removable closures that close or seal the ports 16, 18, 20 when required and that can be opened to perform the function required. Such valves or closures have been omitted from these simplified views.

(11) FIG. 1a shows the pile 10 being lowered in deep water 22 toward a soft seabed 24 comprising marine sediments or clay soil. The pile 10 is suspended from a wire 26 of a crane or winch aboard an installation vessel at the surface, potentially hundreds of metres above the seabed 24. The interior of the pile 10 is flooded with water 22.

(12) FIG. 1b shows the skirt 12 of the pile 10 now partially embedded in the soil 28 of the seabed 24 under its self-weight and momentum. A plug 30 of soil 28 is trapped within the pile 10, surrounded by the skirt 12 as the bottom edge of the skirt 12 penetrates beneath the level of the seabed 24.

(13) The ports 16, 18, 20 are typically open during lowering as shown in FIG. 1a and during initial embedment of the pile 10 as shown in FIG. 1b. This stabilises the pile 10 and reduces resistance to insertion into the soil 28 as the plug 30 of soil 28 acts like an upwardly-moving piston against the water 22 trapped within the pile 10. The trapped water 22 can then escape through the open ports 16, 18, 20.

(14) In FIG. 1c, the wire 26 has been uncoupled from the pile 10. A suction pump 32 has been coupled to the pumping port 18 and activated to draw water 22 out from the interior of the pile 10, while the grout injection port 16 and the excavation port 20 are closed. This creates an underpressure within the pile 10 that draws the pile 10 deeper into the soil 28 of the seabed 24. The underpressure also lifts the top of the plug 30 of soil 28 above the level of the surrounding seabed 24.

(15) The suction pump 32 may be integrated into the pile 10 or may be external to the pile 10, for example being implemented onboard a skid attached to a remotely-operable vehicle (ROV).

(16) FIG. 1d shows an excavation tool 34 comprising an elongate wand 36 that is inserted into the pile 10 through the excavation port 20. In this example, the excavation tool 34 uses flows of water 22 to erode, fluidise, entrain and expel soil 28 from the top of the plug 30 trapped within the skirt 12 of the pile 10. An example of such an excavation tool 34 is also shown in FIG. 5c and will be described in more detail later. In principle, the suction and excavation operations of FIGS. 1c and 1d could be combined by using the excavation tool 34 also as a suction pump 32.

(17) The wand 36 of the excavation tool 34 directs one or more jets of water downwardly and laterally to erode and fluidise the soil 28 from the plug 30 within the pile 10. The fluidised soil 28 is entrained in an outflow of water that contra-flows back up the wand 36 and is then expelled into the surrounding water 22. Excavation in this way lowers the top of the plug 30 to beneath the level of the surrounding seabed 24, effectively lightening the pile 10 and applying less stress to the adjoining soil 28 of the seabed 24.

(18) When excavation is complete, FIG. 1e shows a grout injection tool 38 comprising an elongate injection tube 40 that is inserted into the pile 10 through the grout injection port 16. A thin horizontal plug or layer of grout 42, such as cement, is deposited on top of the lowered plug 30 of soil 28 within the pile 10. The grout 42 is denser than the water 22 but may be less dense than the soil 28. The grout 42 flows laterally and cures to form a partition layer that seals against and engages the full inner circumference of the skirt 12. This maintains a water-filled space, chamber or cavity 44 within the pile 10 between the top plate 14 and the layer of grout 42, and effectively restrains the pile 10 against excessive downward settlement relative to the seabed 24 under the loads of use.

(19) The bottom face of the layer of grout 42 corresponds with the top of the plug 30 and so is at a level beneath the level of the surrounding seabed 24. In this example, the top face of the layer of grout 42 is also beneath the level of the surrounding seabed 24. Also, in this example, the layer of grout 42 occupies a minor portion of the volume between the top of the plug 30 and the top plate 14 of the pile 10. Conversely, the water-filled space, chamber or cavity 44 between the top plate 14 and the layer of grout 42 occupies more than one-third of the internal volume of the pile 10.

(20) In other arrangements, the top face of the layer of grout 42 could instead be above the level of the surrounding seabed 24. Also, the layer of grout 42 could instead occupy a major portion, or indeed substantially all, of the volume between the top of the plug 30 and the top plate 14 of the pile 10. In that case, the layer of grout 42 itself could occupy more than one-third of the internal volume of the pile 10.

(21) FIG. 1f shows a subsea structure such as a template 46 subsequently lowered to the seabed 24 and seated on top of the pile 10, which then serves as a foundation for the structure. In this example, the template 46 is also supported by a mudmat 48 that spreads the weight of the template 46 across an enlarged area of the seabed 24.

(22) A structure such as a template 46 could instead be lowered with one or more piles 10 already integrated into the structure and hence may be lowered simultaneously with the or each supporting pile 10, which will then be subject to the installation operations described above and shown in FIGS. 1b to 1e.

(23) FIG. 2 shows a variant of the pile 10 shown in FIGS. 1a to 1f, at a stage of initial embedment corresponding to that shown in FIG. 1b. In this variant, a pre-installed mass 50 of lightweight concrete fills an upper portion of the interior of the pile 10, occupying more than one-third of the internal volume of the pile 10. In this example, the concrete mass 50 extends downwardly from the top plate 14 of the pile 10. In other examples, there could be a gap between the top plate 14 and the concrete mass 50. The concrete mass 50 could therefore be a disc that is positioned at an intermediate level along the length or height of the pile 10. The concrete of the mass 50 is denser than the water 22 but may be less dense than the soil 28 of the seabed 24.

(24) In FIG. 2, the concrete mass 50 surrounds a passageway 52 that extends downwardly from a multi-purpose port 54 that penetrates the top plate 14. The passageway 52 communicates with the port 54 and extends through the full height of the concrete mass 50.

(25) The multi-purpose port 54 can be used selectively to apply suction, as shown in FIG. 1c, to the flooded interior of the pile 10 below the concrete mass 50. The port 54 can also be used to excavate soil 28, as shown in FIG. 1d, from the plug 30 below the concrete mass 50. The port 54 can then be used to inject grout 42, as shown in FIG. 1e, between the top of the plug 30 and the bottom of the concrete mass 50. The result of injecting grout 42 will be akin to that shown in FIG. 3b, to be described below.

(26) FIGS. 3a and 3b show a further variant of the pile 10 shown in FIG. 2, Here, the concrete mass 50 is replaced by one or more chambers, cavities or tanks 56 for holding water 22. Like the concrete mass 50, the tanks 56 are rigid bodies with which the layer of grout 42 is in supporting contact. There may be two or more tanks 56 spaced angularly around the central longitudinal axis of the pile 10 or a single tank 56 that encircles the central longitudinal axis. The or each tank 56 has open ports 58 in fluid communication with the surrounding water 22 and so remains flooded.

(27) Again, a passageway 52 extends downwardly from a multi-purpose port 54 that penetrates the top plate 14 of the pile 10. The passageway 52 extends through, or between, the or each tank 56 to effect fluid communication between the port 54 and the hollow interior of the pile 10 below the tanks 56.

(28) Once the pile 10 has been partially embedded in the soil 28 as shown in FIG. 3a, the port 54 can be used selectively to apply suction, as shown in FIG. 1c, to the flooded interior of the pile 10 below the tanks 56 and to excavate soil 28, as shown in FIG. 1d, from the plug 30 below the tanks 56. Then, as shown in FIG. 3b, grout 42 is injected through the port 54 to flow laterally from the passageway 52 into a shallow space between the bottom of the tanks 56 and the top of the plug 30, The resulting layer of grout 42 cures to seal against and engage the full inner circumference of the skirt 12, in a manner similar to FIG. 1e.

(29) It is possible for any pile 10, caisson or other foundation of the invention to be installed in the base of a depression, hollow or cavity that is pre-excavated in the seabed 24. In effect, this allows the pile 10 to embed simply by self-penetration to a required depth relative to the seabed 24 that surrounds the cavity. This therefore obviates or reduces the need to apply suction or other assistance to embed the pile 10 to the required depth. Also, in effect, the top of the plug 30 is pre-excavated by digging the cavity, which reduces the height of the plug extending upwardly within the pile 10. This therefore obviates or reduces the need for excavation of the plug 30 within the pile 10.

(30) In this respect, reference is made to FIGS. 4a to 4c that show a downwardly-tapering cavity 60 extending below the level of the surrounding seabed 24. The cavity 60 is shallower than the length or height of the pile 10. However, the cavity 60 is wider than the outer diameter of the pile 10, extending down from the level of the surrounding seabed 24 to where the inclined wall of the cavity 60 intersects the outer surface of the skirt 12.

(31) FIG. 4a shows the cavity 60 in the seabed 24 immediately after excavation. FIG. 4b shows the pile 10 of FIGS. 3a and 3b lowered into the cavity 60 and fully embedded and grouted with grout 42 as shown in FIG. 3b.

(32) To the extent that the cavity 60 is wider than the outer diameter of the pile 10, a trench 62 encircles the embedded pile 10. The trench 62 extends downwardly from the level of the surrounding seabed 24 to the intersection of the wall of the cavity with the skirt 12 of the pile 10. The trench 62 is then backfilled as shown in FIG. 4c, for example using a dumped mass of soil or rocks 64 as shown.

(33) In the example shown in FIGS. 4b and 4c, the layer of grout 42 is approximately level with the bottom of the cavity 60. More specifically, the top of the plug 30 and hence the bottom face of the layer of grout 42 is substantially level with the bottom of the cavity 60. However, the bottom face of the layer of grout 42 could lie below or above the bottom of the cavity 60. Similarly, the top face of the layer of grout 42 could lie below or above the bottom of the cavity 60.

(34) The remaining drawings show a subsea foundation of the invention in the form of an open tubular caisson 66.

(35) FIG. 5a shows the caisson 66 initially embedded in the soil 28 of the seabed 24 after being lowered onto the seabed 24 suspended from a wire 26, corresponding to the status of the suction pile 10 shown in FIG. 1b.

(36) FIG. 5b shows a penetration driver 68 engaged with the exposed open top of the pile 10, which is now embedded to a sufficient extent in the soil 28 of the seabed 24. The penetration driver 68 could be a deadweight or could be arranged to vibrate or to hammer the pile 10 down into full engagement with the soil 28.

(37) In FIG. 5c, an excavation tool 34 is being used to excavate soil 28 from the top of the plug 30 surrounded by the tubular wall of the caisson 66. This excavation has lowered the top of the plug 30 to below the level of the surrounding seabed 24. In this example, as in FIG. 1d, the excavation tool 34 uses water contra-flowing along an elongate wand 36 to erode, fluidise, entrain and expel the soil 28. Specifically, the wand 36 contains concentric or parallel channels, one channel 70 for downward flow to emit a jet of water from the distal end of the wand 36, and the other channel 72 for upward flow to draw the fluidised soil 28, entrained in water, up the wand 36 and out of the caisson 66, The excavation tool 34 therefore has an inlet for drawing in surrounding water 22 and an outlet for expelling the fluidised soil 28 mixed with water.

(38) FIG. 5d shows a layer of grout 42 such as cement deposited on top of the excavated plug 30. Advantageously, the grout 42 can flow under gravity so that its top face is substantially level and horizontal even if its bottom face undulates to follow an uneven top surface of the excavated plug 30. Once cured to form a solid concrete disc, the grout 42 therefore provides a substantially flat and horizontal base for a subsequently-lowered subsea structure such as the template 46 shown in FIG. 5e.

(39) The layer of grout 42 forms the base of an open-topped flooded cavity 74 that is recessed into the top of the caisson 66 and is surrounded by the upstanding peripheral wall of the caisson 66. The cavity 74 may, for example, extend longitudinally to more than a third of the length or height of the caisson 66.

(40) In the example shown in FIG. 5e, the template 46 is supported by the caisson 66 in two ways. Either or both of those support arrangements may be adopted, preferably both.

(41) Firstly, an inverted U-section channel 76 on the underside of the template 46 matches the shape and circumference of the circular top edge of the caisson 66. This downwardly-opening channel 76 receives and engages the top edge of the caisson 66 when the template 46 is lowered onto the caisson 66. Clamping or fastening devices such as radially-movable bolts 78 act between the channel 76 and the caisson 66 to fix the template 46 to the top of the caisson 66. The channel 76 may be continuous or discontinuous, in the latter case with angularly-spaced gaps between separate downwardly-depending clamp formations that engage the top of the caisson 66.

(42) Secondly, the template 46 has one or more integral legs 80 that project downwardly below the base of the template 46, which is defined in this example by the mudmat 48. The or each leg 80 extends into the cavity 74 through its open top to rest on the layer of grout 42 at the base of the cavity 74. The legs 80 terminate at their lower ends in one or more enlarged feet, in this example defined by a horizontal bottom plate 82 that extends across most of the width of the layer of grout 42.

(43) Thus, by virtue of the preferred combination of support arrangements shown in FIG. 5e, weight loads of the template 46 are fed directly to the wall of the caisson 66 and also directly to the plug 30 of soil 28 within the caisson 66. The optional mudmat 48 provides a further load path between the template 46 and the seabed 24.

(44) Finally, FIG. 6 shows a variant of the support arrangements shown in FIG. 5e. In this example, the or each leg 80 is not integral with the template 46 but instead is part of a separate support structure that extends vertically from the layer of grout 42 at the base of the cavity 74 to the base of the template 46.

(45) The support structure shown in FIG. 6 comprises a horizontal upper plate 84 on the upper end of the or each leg 80, on which the template 46 rests. Thus, a load path extends from the template 46, through the upper plate 84, along the or each leg 80 and through the bottom plate 82 into the layer of grout 42 that surmounts the plug 30 of soil 28 within the caisson 66. As before, further load paths extend from the template 46 into the wall of the caisson 66 via the channel 76 and from the template 46 into the seabed 24 via the optional mudmat 48.

(46) FIG. 6 shows another feature that may also be applied to the embodiment shown in FIG. 5e, namely a provision for the length of the or each leg 80 to be adjusted. For example, the or each leg 80 may be telescopic as shown, may preferably be adjustable independently and could possibly be actuated hydraulically whenever required. This facility for height adjustment allows for load management, for levelling the template 46 and for compensating for any settlement of the foundation over time.

(47) In principle, the concrete disc cast in situ as a layer of grout 42 in FIGS. 5d, 5e and 6 could be replaced by a prefabricated concrete slab that is lowered through the open top of the caisson 66 and laid on top of the excavated plug 30. Such a slab could be lowered into the cavity 74 in one or more pieces or sections.

(48) Many other variations are possible within the inventive concept. For example, the piles and caissons shown in the drawings are shown as protruding slightly above the surrounding seabed when fully installed. However, it would be possible instead for the top of a pile or caisson to be substantially level with the surrounding seabed or even to be recessed slightly beneath the level of the surrounding seabed.

(49) In principle, the open top of a caisson precludes the use of suction to embed the caisson. However, the caisson could be installed as a suction pile with the temporary addition of a top plate to close its open top. In that case, once the caisson is fully embedded with the assistance of suction, the top plate can be removed to enable further operations. In particular, the top plate can be removed before or after excavating soil from the plug that is encircled by the tubular wall of the caisson, or before or after depositing a layer of grout on top of the plug.

(50) In other variants, the excavation tool could dig and lift the soil mechanically, for example using an auger screw. Again, such an excavation tool could be supported by an ROV.