INSTALLATION AND REMOVAL OF SUBSEA FOUNDATIONS

Abstract

An underwater pile foundation comprises a pump in fluid communication with an internal chamber of the pile to pump water out of or into the chamber. This reduces or increases the pressure of water in the chamber relative to ambient pressure of water outside the chamber during installation or removal of the pile. While that pumping phase is ongoing, and potentially before or after that pumping phase, a pressure variator in fluid communication with the chamber imparts oscillations in the pressure of the water in the chamber. The resulting pressure waves in water within the chamber reduce resistance to movement of the pile relative to soil in which the pile is embedded.

Claims

1.-35. (canceled)

36. A method of reducing resistance to movement of a pile relative to soil during installation or removal of the pile underwater, the method comprising: pumping water out of or into an internal chamber of the pile, defined between a top plate of the pile, a skirt of the pile and the soil, to reduce or to increase a level of pressure of water in the chamber relative to ambient pressure of water outside the chamber; and while that pumping is ongoing, imparting additional oscillations in the pressure of the water in the chamber via a pressure variator pump, wherein the pressure variator pump is in fluid communication with the chamber.

37. The method of claim 36, comprising employing the oscillations to vibrate a wall of the pile in contact with the soil.

38. The method of claim 36, comprising employing the oscillations to drive oscillatory vertical movement of the pile relative to the soil.

39. The method of claim 38, comprising driving the oscillatory vertical movement by cyclically expanding and contracting the chamber in response to the oscillations.

40. The method of claim 36, comprising employing the oscillations to drive pressure waves through the water in the chamber to impact against soil in the chamber.

41. The method of claim 40, comprising directing the pressure waves downwardly within the chamber.

42. The method of claim 36, wherein the pressure variator pump imparting the oscillations is distinct from a pump that pumps the water.

43. The method of claim 42, comprising effecting fluid communication with the chamber through a common port shared by the pump and the pressure variator pump.

44. The method of claim 43, comprising effecting fluid communication with the chamber through the pump and the pressure variator pump in series.

45. The method of claim 43, comprising effecting fluid communication with the chamber through the pump and the pressure variator pump in parallel.

46. The method of claim 43, comprising directing a fluctuating output of the pressure variator pump into the pump as a motive fluid to drive oscillatory flow in the pump

47. The method of claim 42, comprising effecting fluid communication with the chamber through separate ports, at least one of those ports communicating with the pump and at least one other of those ports communicating with the pressure variator pump.

48. The method of claim 42, wherein the pressure variator pump and the pump are disposed outside the chamber.

49. The method of claim 42, comprising effecting fluid communication between the pressure variator pump and the water outside the chamber.

50. The method of claim 42, comprising enclosing the pressure variator pump within the chamber.

51. The method of claim 36, wherein the pressure variator pump pumps the water to reduce or to increase the level of pressure of water in the chamber and the method comprises imparting the oscillations by oscillating flow passing through the pressure variator pump.

52. The method of claim 36, performed when installing the pile, comprising maintaining pressure within the chamber continuously below the ambient pressure of the water outside the chamber.

53. The method of claim 36, performed when installing the pile, comprising employing the oscillations to generate a series of pressure pulses within the chamber, each pulse being above the ambient pressure of the water outside the chamber and the pulses being separated by a period in which pressure within the chamber is below that ambient pressure.

54. The method of claim 36, comprising imparting the oscillations with a frequency of from 5 Hz to 50 Hz.

55. The method of claim 36, wherein the oscillations follow a waveform in which pressure varies continuously.

56. The method of claim 36, wherein the oscillations follow a waveform with step-change transitions.

57. The method of claim 36, comprising varying any of the following parameters during installation or removal of the pile: frequency of the oscillations; amplitude of the oscillations; and/or an average level of water pressure in the chamber about which the water pressure oscillates.

58. The method of claim 57, comprising increasing any of said parameters in accordance with depth of penetration of the pile into the soil.

59. An underwater pile, comprising: a pump in fluid communication with an internal chamber of the pile defined between a top plate of the pile, a skirt of the pile and the soil, the pump being configured to pump water out of or into the chamber during installation or removal of the pile, thus reducing or increasing a level of pressure of water in the chamber relative to ambient pressure of water outside the chamber; and a pressure variator pump in fluid communication with the chamber for imparting oscillations in the pressure of the water in the chamber, wherein the pressure variator pump is configured to apply negative pressure to the water in the chamber.

60. The pile of claim 59, wherein the pump and the pressure variator pump are in fluid communication with the chamber through a common port.

61. The pile of claim 60, wherein the pump and the pressure variator pump are disposed in series.

62. The pile of claim 60, wherein the pump and the pressure variator pump are disposed in parallel.

63. The pile of claim 60, wherein the pump is a jet pump and an outlet of the pressure variator pump communicates with a motive fluid inlet of the pump.

64. The pile of claim 59, wherein the pump and the pressure variator pump are in fluid communication with the chamber through respective separate ports.

65. The pile of claim 60, wherein the or each port opens downwardly into the chamber in opposition to soil that closes a lower end of the chamber.

66. The pile of claim 59, wherein the pressure variator pump and the pump are disposed outside the chamber.

67. The pile of claim 59, wherein the pressure variator pump is in fluid communication with the water outside the chamber.

68. The pile of claim 59, wherein the pressure variator pump is enclosed within the chamber.

69. The pile of claim 59, wherein the pressure variator pump is a positive-displacement pump.

70. The pile of claim 69, wherein the pressure variator pump comprises a reciprocating element that is movable to draw water from the chamber and to expel water into the chamber in alternation.

Description

[0037] 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:

[0038] FIGS. 1 to 7 are schematic sectional side views of suction piles comprising various pumping arrangements of the invention;

[0039] FIGS. 8a to 8d are a sequence of schematic representations of a piston-based pressure variator for use in the invention;

[0040] FIGS. 9 to 12 are graphs of pressure within a suction chamber of a suction pile during installation or removal of the pile in accordance with the invention, plotted against time; and

[0041] FIGS. 13 and 14 are graphs of pressure within a suction chamber of a suction pile during installation or removal of the pile in accordance with the invention, plotted against depth of penetration of the pile into seabed soil.

[0042] FIGS. 1 to 7 show various suction piles 10 equipped with pumping arrangements of the invention. The piles 10 are shown during installation in a suction phase after initially being partially embedded by momentum and self-weight into the soil 12 of the seabed 14.

[0043] Each suction pile 10 comprises a tubular skirt 16 whose upper end is closed by a top plate 18, defining a suction chamber 20 in the space within the skirt 16 between the top plate 18 and the soil 12 surrounded by the skirt 16. Water occupies the suction chamber 20 and fills pores between particles or grains of the soil 12 within the skirt 16, in fluid communication with the suction chamber 20.

[0044] As is conventional, a wall of each suction pile 10 is penetrated by at least one suction port 22 through which water is pumped out of the suction chamber 20 during the suction phase of installation. For this purpose, the suction port 22 communicates with a suction pump 24 whose outlet 26 exhausts water into the surrounding sea. Typically, valves will close the suction port 22 when the suction phase is complete and will remain closed thereafter while the pile 10 remains in service but such valves have been omitted from the drawings for simplicity.

[0045] Conveniently, the suction port 22 is located in the top plate 18 of the suction pile 10, as shown in these examples, although that location is not essential. The suction pump 24 could be mounted permanently on a suction pile 10 or could be coupled to the pile 10 temporarily during the suction phase only, for example if hosted by an ROV.

[0046] In accordance with the invention, the pumping arrangements of FIGS. 1 to 7 are all capable of generating pressure fluctuations, pulses or oscillations that disturb, stir or agitate the body of water within the suction chamber 20. Those oscillations are expressed in the drawings as pressure waves 28 radiating downwardly toward the soil 12 within the skirt 16, although pressure waves 28 or other perturbations could propagate in various directions within the water in the suction chamber 20.

[0047] Oscillation of fluid pressure within the suction chamber 20 tends to expand and contract the suction chamber 20 cyclically, hence generating up-and-down oscillation of the skirt 16. The pressure waves 28 also impact against the surrounding skirt 16, hence giving rise to vibrations 30 in the wall of the skirt 16 as shown in FIG. 1. Oscillation or vibration of the skirt 16 reduces the effect of friction or cohesion of the soil 12 against both the inner and outer sides of the skirt 16, hence reducing resistance to downward movement of the suction pile 10.

[0048] It will also be noted that in view of its saturated, fluid consistency, the pressure waves 28 penetrate into and propagate through the soil 12 within the skirt 16 to some extent. This may disturb, agitate and liquefy or fluidise that soil 12, easing downward movement of the suction pile 10 that is driven primarily by the suction pump 24 evacuating water from the suction chamber 20.

[0049] The principle of the invention could also be applied to retrieval of a suction pile 10, in which case the suction pump 24 could be reversed to pump water into the suction chamber 20 through the suction port 22 as the pile 10 is lifted out of engagement with the seabed 14. Again, pressure waves 28 generated in the water within the suction chamber 20 oscillate and vibrate the skirt 16 and may disturb the soil 12 within the skirt 16, with the benefit of reducing resistance to upward movement of the pile 10.

[0050] In the variants of FIGS. 1 to 5, pressure fluctuations within the suction chamber 20 are generated by an auxiliary pump 32 serving as a pressure variator. In each of those examples, the auxiliary pump 32 is in fluid communication with ambient water through an external port 34 that enables the auxiliary pump 32 to expel and/or draw in water in successive pumping and/or suction phases of cyclical operation. However, it is not essential for a pressure variator such as an auxiliary pump 32 to communicate directly with ambient water, as some variants will make clear.

[0051] FIG. 1 shows an arrangement in which the suction pump 24 and the auxiliary pump 32 communicate separately with the suction chamber 20 on parallel fluid paths. The suction pump 24 communicates with the suction chamber 20 through the aforementioned suction port 22 whereas the auxiliary pump 32 communicates with the suction chamber 20 through an auxiliary port 36.

[0052] In FIG. 2, the suction pump 24 and the auxiliary pump 32 communicate with the suction chamber 20 on a common fluid path extending through the suction port 22. The common fluid path is defined by a manifold 38 that branches outwardly from the suction port 22 to communicate with the suction pump 24 and the auxiliary pump 32.

[0053] FIG. 3 shows that the auxiliary pump 32 need not necessarily communicate directly with the suction chamber 20 but could instead drive, or otherwise influence, the flow of water through the suction pump 24 to create pressure fluctuations within the suction chamber 20. In this example, the suction pump 24 is a jet pump and the auxiliary pump 32 draws in and exhausts ambient water, as a motive fluid, into a motive fluid nozzle 40 aligned with a diffuser throat 42 of the jet pump. The resulting pressure drop in the diffuser throat 42 draws water from the suction chamber 20 through the suction port 22 and into the suction pump 24.

[0054] Fluctuations in the flow rate of the water injected by the auxiliary pump 32 through the motive fluid nozzle 40 generate corresponding fluctuations in the flow rate of water through the suction pump 24. Those fluctuations, in turn, create pressure fluctuations in the suction chamber 20.

[0055] In FIGS. 4 and 5, the suction pump 24 and the auxiliary pump 32 are in series in a fluid path extending from the suction port 22 to the ambient water. In FIG. 4, the auxiliary pump 32 is upstream of the suction pump 24, hence being between the suction pump 24 and the suction port 22. Conversely, in FIG. 5, the suction pump 24 is upstream of the auxiliary pump 32, hence being between the auxiliary pump 32 and the suction port 22. Thus, in FIG. 4, the auxiliary pump 32 communicates with the suction chamber 20 directly whereas in FIG. 5, the auxiliary pump 32 communicates with the suction chamber 20 via the suction pump 24.

[0056] The suction pump 24 may be primarily responsible for evacuating water from the suction chamber 20 during the suction phase, hence having a greater aggregate outflow than the auxiliary pump 32. However, the auxiliary pump 32 may also contribute to evacuating water from the suction chamber 20.

[0057] FIG. 6 shows that a secondary pressure variator such as an auxiliary pump 32 can be omitted if the suction pump 24 itself is configured to operate cyclically, hence imparting the desired pressure fluctuations to the suction chamber 20 via the suction port 22. Thus, the suction pump 24 itself could serve as a pressure variator.

[0058] FIG. 7 shows that a pressure variator such as an auxiliary pump 32 need not communicate directly with the ambient water. Indeed, the auxiliary pump 32 could be placed within the suction chamber 20 as shown, with an outlet port 44 in direct fluid communication with the suction chamber 20. In this example, the outlet port 44 of the auxiliary pump 32 faces downwardly to project pressure waves 28 toward the seabed soil 12 within the skirt 16 of the suction pile 10. The auxiliary pump 32 may use the outlet port 44 both to draw water in from the suction chamber 20 and to pump water out into the suction chamber 20. Alternatively, the auxiliary pump 32 could draw water in from the suction chamber 20 through an inlet port, not shown, before exhausting the water through the outlet port 44.

[0059] FIGS. 8a to 8d exemplify a positive-displacement auxiliary pump 32 that could be used in the variant of FIG. 7, and potentially also in other variants of the invention. In this example, a reciprocating element such as a piston 46 driven via a crankshaft 48 draws in water from the suction chamber 20 through the outlet port 44 in a suction stroke shown in FIGS. 8a and 8b. The piston 46 then expels water through the outlet port 44 in a compression stroke shown in FIGS. 8c and 8d, before another suction stroke begins.

[0060] The graphs of FIGS. 9 to 12 plot the pressure of water in the suction chamber 20 on the vertical axis against time on the horizontal axis. They illustrate how a suction pump 24 and a pressure variator such as an auxiliary pump 32 can be used in combination to generate rapid oscillations in the pressure of water in the suction chamber 20 while the suction phase is ongoing. Pressure applied by the suction pump 24 and pressure applied by the auxiliary pump 32 are aggregated to result in the overall pressure 50 applied to water in the suction chamber 20.

[0061] In each of these simple examples, the output of the suction pump 24 is nominally constant whereas the output of the auxiliary pump 32 varies cyclically or is reversed periodically. With reference to FIG. 6, however, it should be noted that an auxiliary pump 32 is optional and that the output of a suction pump 24 could be varied cyclically, or even reversed periodically, to serve as a pressure variator.

[0062] In FIG. 9, the flow through the auxiliary pump 32 fluctuates continuously, in this example sinuously, while continuing to apply negative pressure to the suction chamber 20. The suction pump 24 also applies negative pressure, hence suction, to the suction chamber 20. The result is that overall pressure 50 oscillates continuously while remaining negative as suction is applied to the suction chamber 20 throughout.

[0063] In FIG. 10, the flow through the auxiliary pump 32 again fluctuates continuously but in this case alternates between positive and negative pressure. The amplitude of the waveform relating to the auxiliary pump 32 is greater relative to the corresponding waveform in FIG. 9 and relative to the plot of pressure applied by the suction pump 24 in FIG. 10. This causes the overall pressure 50 to enter the positive pressure domain briefly during each cycle. Consequently, the auxiliary pump 32 generates brief pulses of positive pressure that, transiently, reverse suction to drive a pressure wave 28 into the suction chamber 20. However, the negative pressure domain, hence suction, predominates so that water continues to be evacuated from the suction chamber 20 over time.

[0064] FIG. 11 shows that the frequency or wavelength of fluctuating overall pressure 50 can be varied over time. Here, this variation is driven by varying the frequency of oscillation of pressure applied to the suction chamber 20 by the auxiliary pump 32. The frequency could, for example, increase with increasing penetration of the suction pile 10 into the seabed 14.

[0065] FIG. 12 shows that pressure need not necessarily fluctuate constantly or smoothly. In this example, pressure applied to the suction chamber 20 by the auxiliary pump 32, and hence also the overall pressure 50, oscillates with an angular, near-square waveform. That waveform comprises periods of constant pressure 52 alternating with sudden transitions 54. The sudden transitions 54 lend a step-change character to the waveform, increasing the intensity of the pressure waves 28 within the suction chamber 20 and enhancing their shockwave effect.

[0066] Finally, the graphs of FIGS. 13 and 14 plot the pressure of water in the suction chamber 20 on the vertical axis against depth of penetration of the suction pile 10 on the horizontal axis. These graphs show that pressure in the suction chamber 20, and fluctuation of that pressure, may vary in accordance with the extent to which the skirt 16 is buried in the seabed soil 12.

[0067] In FIG. 13, negative pressure of water in the suction chamber 20, hence suction, is increased with increasing penetration of the suction pile 10. In this example, the increased suction is due to increasing suction applied by the suction pump 24.

[0068] Pressure applied to the suction chamber 20 by the auxiliary pump 32 oscillates but in this instance the amplitude of that oscillation remains constant with increasing penetration of the pile 10. The overall negative pressure 50 applied to the suction chamber 20 therefore follows an increasing but constantly fluctuating profile as the skirt 16 of the pile 10 is buried deeper in the seabed soil 12. More generally, the flow rate at which water is pumped out of or into the suction chamber 20 during installation or removal of the pile may be varied to change the average pressure over time.

[0069] FIG. 14 shows that the amplitude of fluctuating pressure applied by the auxiliary pump 32 can increase with increasing penetration of the suction pile 10. The overall negative pressure 50 applied to the suction chamber 20 therefore follows an increasingly fluctuating profile as the skirt 16 of the suction pile 10 is buried deeper in the seabed soil 12. Nevertheless, the average of the overall negative pressure 50 remains substantially constant with increasing penetration.

[0070] Many variations are possible within the inventive concept. For example, the approaches shown in FIGS. 13 and 14 could be combined, so that the overall negative pressure 50 applied to the suction chamber 20 follows not only an increasing profile but also an increasingly fluctuating profile with increasing penetration of the suction pile 10.

[0071] As noted previously, the inventive concept could also be applied to withdrawal or removal of a foundation pile from the seabed. In that case, it will be understood that the operation of a pump to evacuate water from the foundation could be reversed to pump water into the foundation, hence operating mainly or wholly in the positive pressure domain. It would also be possible to apply fluctuating fluid pressure within a foundation to reduce resistance to movement without necessarily applying negative or positive pressure on average over time. For example, fluctuating fluid pressure could be applied before or after a pumping phase, or in addition to or instead of a pumping phase.