A METHOD OF INSTALLING A FOUNDATION AND A FOUNDATION FOR A STRUCTURE

Abstract

Method of installing a foundation (1) having a toe (2) which is inserted into a soil (10) until a depth of the toe (2) reaches at least a minimum installation depth threshold (23). During insertion, fluid is jetted from a plurality of nozzles (7) provided at the toe (2) for directing fluid distally into the soil (10) ahead of the toe (2). The jetting of fluid from the plurality of nozzles (7) is controlled based on the depth of the toe (2), wherein the rate of jetting of fluid is reduced when the depth of the toe (2) reaches a stabilisation depth (22) ahead of the minimum installation depth threshold (23).

Claims

1. A method of installing a foundation (1) for a wind turbine comprising: inserting the foundation (1) having a toe (2) into a soil (10) and measuring installation resistance, wherein the foundation (1) is inserted until a depth of the toe (2) reaches at least a minimum installation depth threshold (23), and until the installation resistance reaches at least a minimum installation resistance threshold (16); during insertion, jetting fluid from a plurality of nozzles (7) provided at the toe (2) for directing fluid distally into the soil (10) ahead of the toe (2); and controlling the jetting of fluid from the plurality of nozzles (7) based on the depth of the toe (2), wherein the rate of jetting of fluid is reduced when the depth of the toe (2) reaches a stabilisation depth (22) ahead of the minimum installation depth threshold.

2. A method according to claim 1, wherein the stabilisation depth (22) is 0.5 meter or more shallower than the minimum installation depth threshold (23).

3. A method according to claim 1, wherein the stabilisation depth (22) is 10 meters or less shallower than the minimum installation depth threshold (23).

4. A method according to claim 1, wherein the step of controlling the jetting comprises reducing the rate of jetting of fluid to zero when the depth of the toe (2) reaches the stabilisation depth (22).

5. A method according to claim 1, wherein the reduction of the rate of jetting is controlled based on the measured installation resistance.

6. A method according to claim 1, further comprising forming a particle trap (12) at an insertion site where the foundation is inserted into the soil (10) for trapping fines particles, the step of forming the particle trap (12) comprising controlling the speed of inserting the foundation (1) during an initial phase and, during the initial phase, jetting fluid from the nozzles (7) to displace soil (10) to form a trench at the insertion site.

7. A method according to claim 6, wherein the step of jetting of fluid comprises increasing the rate of jetting during the initial phase.

8. A method according to claim 1, wherein the step of inserting further comprises applying ballast (13) to the foundation (1) for driving the toe (2) into the soil (10).

9. A method according to claim 8, wherein the step of inserting further comprises removing ballast (13) before the depth of the toe (2) reaches a maximum installation depth threshold (24).

10. A method according to claim 1, wherein the plurality of nozzles (7) provided at the toe (2) comprise a interior array of nozzles (4a) distributed around the interior circumference of the foundation (1) and an exterior array of nozzles (4b) distributed around the exterior circumference of the foundation (1), wherein the nozzles (7) are directed in a foundation insertion direction such that the step of jetting fluid through the nozzles (7) forms interior and exterior circumferential cuts in the soil (10) ahead of the toe (23).

11. A method according to claim 10, wherein the interior and exterior circumferential cuts are for separating soil (10) from interior and exterior lateral surfaces of the foundation (1), respectively.

12-13. (canceled)

14. A controller (32) for controlling the installation of a foundation (1) for a wind turbine using the method of claim 1, the foundation (1) comprising a toe (2) for insertion into a soil (10) until a depth of the toe (2) reaches at least a minimum installation depth threshold (23), and an installation resistance reaches at least a minimum installation resistance threshold (16), the controller comprising: a toe depth calculator for determining the current depth of the toe (2); and a jetting control for controlling the jetting of fluid from a plurality of nozzles (7) provided at the toe (2) for directing fluid distally into the soil (10) ahead of the toe (2), wherein the controller (32) measures the installation resistance and controls the jetting based on the depth of the toe (2), and the rate of jetting is reduced when the depth of the toe (2) reaches a stabilisation depth (22) ahead of the minimum installation depth threshold (23).

15. Software for operating the controller (32) of claim 12 for controlling the installation of a foundation (1) for a wind turbine, the foundation comprising a toe (2) for insertion into a soil (10) until a depth of the toe (2) reaches at least a minimum installation depth threshold (23), and an installation resistance reaches at least a minimum installation resistance threshold (16), the software comprising: instructions for measuring installation resistance; instructions for determining the current depth of the toe (2); and instructions for controlling the jetting of fluid from a plurality of nozzles (7) provided at the toe (2) for directing fluid distally into the soil (10) ahead of the toe (2), wherein jetting is controlled based on the depth of the toe (2) and the rate of jetting is reduced when the depth of the toe (2) reaches a stabilisation depth (22) ahead of the minimum installation depth threshold (23).

16. Software according to claim 15, wherein the instructions for controlling are configured to control the reduction of the rate of jetting based on the measured installation resistance.

17. A controller (32) according to claim 14, wherein the controller is configured to control the reduction of the rate of jetting based on the measured installation resistance.

Description

[0023] Illustrative embodiments of the present invention will now be described with reference to the accompanying drawings in which:

[0024] FIG. 1 shows a view of a monopile foundation according to a first embodiment of the invention;

[0025] FIG. 2 shows an isometric sectional view of the toe of the foundation shown in FIG. 1;

[0026] FIG. 3 shows an enlarged isometric sectional view of the toe of the foundation shown in FIG. 1;

[0027] FIG. 4 shows a bottom view of a section of the toe of the foundation shown in FIG. 1;

[0028] FIG. 5 shows a top view of a section of the toe of the foundation shown in FIG. 1;

[0029] FIG. 6 shows a cross-sectional view of the toe of the foundation shown in FIG. 1;

[0030] FIG. 7 shows a schematic bottom view of the toe of the foundation received within an upending tool;

[0031] FIGS. 8 to 10 show a sequence of schematic illustrations of the foundation being installed; and

[0032] FIG. 11 shows a graph plotting installation resistance against installation depth during two illustrative use case examples.

[0033] FIG. 1 shows a foundation 1 according to an embodiment of the invention. In this embodiment, the foundation 1 is a monopile for installation in an offshore location for supporting a wind turbine.

[0034] The foundation 1 comprises a hollow tubular body having an exterior lateral surface, and an interior lateral surface that defines an interior cavity in the form of a bore. In this example, the foundation 1 is provided with a conical section toward its proximal end. The distal part of the foundation 1 comprises a toe 2 for insertion into the soil at the sea bed.

[0035] As shown in FIGS. 2 to 6, in this embodiment, the toe 2 of the foundation is provided with a manifold 3 which forms a ring like conduit around the aperture defining the opening into the interior cavity of the foundation 1. In use, the manifold 3 is pressurised by fluid fed through the fluid feed pipe 30 provided on the exterior of the foundation body, as shown in FIG. 1. In other embodiments, one or more fluid feed pipes may be used, and these may be secured to the exterior or interior surface of the foundation 1, or through the body of the foundation itself. The fluid feed pipe 30 is fed by a fluid pump 31 under the control of controller 32 provided on the installation vessel, another vessel or any other appropriate place. The pressurisation of the manifold 3 by fluid during use may help to resist its compression if, for example, the toe 2 is driven into a rock in soil during installation.

[0036] The manifold 3 feeds two arrays of nozzle heads 4, with a first array 4a arranged around the interior edge of the toe 2 and the second array 4b arranged around the exterior edge of the toe 2. Each nozzle head 4 comprises two nozzles 7 for directing pressurised fluid jets 5 downwardly in a fanned configuration. The arrangement of nozzle heads 4a,4b is shown more clearly in the bottom and top views of FIGS. 4 and 5, respectively. As shown, the cutting action of the jets 5 from adjacent nozzles 7 in the same row of nozzle heads 4a,4b overlap to form circumferential cuts into the soil ahead of the toe 2. Accordingly, an interior circumferential cut is created by the first array of nozzle heads 4a, and an exterior circumferential cut is created by the second array of nozzle heads 4b.

[0037] Turning to FIG. 6, the nozzles 7 of the heads 4 are fed by their connectors 6, which extend into the interior conduit of the manifold 3, and contain an internal fluid channel between the nozzles 7 and the manifold 3. As such, fluid fed into the manifold 3 is jetted through the nozzles 7.

[0038] FIG. 7 shows a plan view of the exterior surface of the toe 2 of the foundation 1, when received within the base of an upending tool 8. A plurality of support plates 9 are fitted to the outer surface of the pile 1 between the gaps in the nozzle heads 4. In embodiments, the support plates 9 are 20-40 mm thick and sized so that their width may support the foundation 1 between the gaps in the nozzle heads 4, without interfering with the jets. In embodiments, the support plates 9 are welded to the exterior surface of the foundation 1 to axially reinforce the manifold 2. Support plates 9 may also be provided on the interior surface of the foundation 1.

[0039] During deployment, the foundation 1 is fitted into the upending tool 8 from a secured horizontal position on an installation vessel. The heads 4 are protected by the support plates 9 so that they are not damaged during the upending operation. The foundation 1 is then tilted up into a vertical orientation, pivoting about the upending tool 8, from where it can be lifted by a crane down to the seabed.

[0040] FIGS. 8 to 10 show a sequence of the foundation 1 being installed according to an embodiment of the invention. In FIG. 8, the foundation 1 is slowly lowered to the soil 10 on the seabed. The controller 32, as shown in FIG. 1, controls the pump 31 to generate fluid jets 5 through the nozzles 7 as the toe 2 approaches the seabed. This has the effect of forming a wide particle trap trench 12 around the circumference of the foundation 1.

[0041] Once the particle trap trench 12 is formed, the toe 2 of the foundation 11 is lowered to an insertion site at the base of the particle trap trench 12. The foundation is lowered further, causing its toe 2 to be driven downward into the soil 10 under the foundation's own weight. This insertion is further facilitated by the jetting 5 from the nozzles 7.

[0042] As shown in FIG. 9, the jets 5 act to cut into the soil 10 ahead of the toe as the foundation 1 continues down, thereby forming a deep trench 11 from the base of the particle trap trench region 12. The deep trench 11 extends from the jetted region ahead of the toe 2 up to the particle trap trench region 12 at the seabed. Depending on the installation resistance exhibited by the soil, ballast 13 may be applied to the top of the foundation 1 to keep the toe 2 advancing down. Toe depth and installation resistance are determined by the controller 32 based on how far the foundation 1 has been driven into the soil 10 and the rate of its advance.

[0043] Depending on the soil properties, the trench 11 may exhibit different characteristics. The left side of FIG. 9 shows a low resistance situation where the fluid suspension pressure acts to stabilise the trench wall. This allows a thin fluid channel to be maintained over the lateral surfaces of the foundation 1, thereby minimising installation resistance. Conversely, the right side of FIG. 9 shows a high resistance situation where the trench wall collapses back onto the lateral surfaces of the foundation 1, resulting in active earth pressure increasing the installation resistance. In practice, it will be understood that the soil characteristics may vary through the depth, and the foundation 1 may be subjected to both low and high resistance regions.

[0044] As the toe 2 advances, fines particles excavated from the soil region ahead of the toe are suspended in the fluid, and are conveyed up the fluid channels formed between the deep trench 11 and the lateral surfaces of the foundation 1. As the soil suspension reaches the particle trap trench region 12, the expansion of fluid volume as the trench widens acts to slow the flow of fluid. This acts to trap particles in the particle trap trench 12, thereby minimising their expulsion into the surrounding seawater.

[0045] Once the toe 2 reaches a predetermined depth before the installation depth, the controller 32 turns off or reduces the jetting 5. As such, the soil 10 ahead of the toe 2 is less fluidised, which results in an increase in installation resistance. The toe 2 is nevertheless driven further by the ballast 13, until it reaches a target installation depth, as shown in FIG. 10.

[0046] The predetermined depth defines the starting point of a stabilisation region 14 of soil 10 ahead of the target installation depth. Depending on the soil, the threshold for starting the stabilisation region 14 may be 1 to 5 metres ahead of the target installation depth. As such, the foundation 1 may be driven relatively quickly using jetting assistance through the main phase of installation, with the slower driving speeds associated using minimal jetting assistance or ballast only driving being limited to the final phase of installation.

[0047] While the toe 2 is advancing through the stabilisation region 14 during the final phase of installation, the controller 32 may use the measured installation resistance to more accurately estimate the ultimate axial load bearing capacity of the foundation. That is, as the stabilised region of soil around the distal end of the foundation is less fluidised, it may be used to validate the axial capacity of the foundation.

[0048] FIG. 11 shows a graph plotting installation resistance (x axis) against toe installation depth (y axis) during a low resistance scenario 20 and a high resistance scenario 18.

[0049] In both scenarios, the toe 2 progresses downward under the self-weight of the foundation 1 up until the self-weight threshold 15, albeit that in the low resistance scenario 20, the toe 2 will have reached a much deeper depth.

[0050] In the low resistance scenario 20, the controller 32 may apply a shallower depth stabilisation threshold 19, to provide a larger stabilisation region 14. As such, once the shallower depth stabilisation threshold 19 is reached, the jetting rate may be reduced to a lower rate. This results in an increased installation resistance, and a slower rate of descent. Ballast 13 may be applied until the toe of the foundation 1 exceeds the minimum installation depth threshold 23 and the measured installation resistance exceeds the minimum installation resistance threshold 16, to thereby validate the required axial capacity. This is shown by point 25 on the graph. Once reached, the ballast 13 is removed before the toe 2 exceeds the maximum installation depth threshold 24, which could otherwise cause the foundation 1 to be installed too deep, outside of its design tolerances.

[0051] In the high resistance scenario 18, the ballast 13 is applied at a relatively shallow installation depth in order to maintain advancement of the toe 2. In this case, the minimum installation resistance threshold 16 is exceeded well before the minimum installation depth threshold 23 is reached. Consequently, the main driving phase continues with jetted assisted insertion until the controller 32 determines that a deeper depth stabilisation threshold 22 has been reached. At this stage, the fluid jetting is turned off, resulting again in an increased installation resistance, and a slower rate of descent. Ballast 13 is applied until the foundation 1 achieves the minimum installation depth threshold 23. At the same time, the ballast 13 is removed before the installation resistance threshold 17 is exceeded. That is, installation will continue until a target penetration depth between the minimum and maximum installation depth thresholds is reached, or until the available down force from the ballast 13 and the foundation's self-weight is exhausted. Thereafter, the ballast 13 will be removed.

[0052] As will be understood, in both the low resistance and high resistance scenarios, the foundation is inserted into the soil until it reaches a desired target installation window 26 that exists between the minimum and maximum installation resistance thresholds 16,17 and the minimum and maximum installation depth thresholds 23,24. Furthermore, although the stabilisation region 14 in the high resistance scenario will be much shorter than the low resistance scenario, this is compensated by the inherently higher strength soil.

[0053] After installation, the surplus water will drain away from the trench 11, and soil particles will resettle. Over time, the resettled soil will compact through cyclic shake down effects, thereby restabilising the soil.

[0054] It will be appreciated that with the above methods and arrangements, a foundation may be installed into the soil more easily using the jetted assisted installation. This reduces cost and allows installation noise to be minimised. At the same time, a higher load bearing capacity may be achieved by maintaining the structural integrity of the soil beneath the stabilisation depth.

[0055] It will be understood that the embodiments illustrated above show applications of the invention only for the purposes of illustration. In practice the invention may be applied to many different configurations, the detailed embodiments being straightforward for those skilled in the art to implement.

[0056] For example, the above-described control method may be applied to other jetting arrangements. For instance, the control method may be applied to foundations having a single array of downward nozzles.

[0057] It will also be understood that additional mechanisms and systems may be used in combination with the fluid jetting system for further reducing driving resistance. For instance, the foundation may further incorporate electrodes for electro-osmosis. As such, the fluid jetting system may work synergistically with the electro-osmosis system. Furthermore, in other arrangements, further upward facing jets may be provided on the foundation body for reducing resistance and resisting trench wall collapse.