GB2611090A - Method of installing or remediating suction bucket structures for wind turbines - Google Patents

Abstract

A method of installing the suction bucket structure 1a, or remediating the previously installed suction bucket structure, comprises providing a suction bucket structure having a skirt 2 embedded in the seabed 120, wherein the skirt extends from a base of the suction bucket structure and wherein a suction volume is defined by the skirt and the base 3; and subsequently densifying soil located beneath the base of the suction bucket structure. Densifying the soil located beneath the base comprises generating a suction pressure within the volume; and cycling the suction pressure (figure 3). The structure may be a foundation for an offshore wind turbine, and may comprise a pump 8, standpipe, dividers 7, and filters 4.

Description

Method of Installing or Remediating Suction Bucket Structures for Wind Turbines The present invention relates to a method of installing a suction bucket structure in a seabed, a method of remediating a previously installed suction bucket structure in a seabed, and a method of installing a wind turbine in a seabed. Suction bucket structures are large caisson structures which can serve as the foundations, or anchors, of offshore structures such as platforms or wind turbines. Suction bucket structures generally comprise a base, along with a skirt extending generally perpendicularly from the base. Together, the base and the skirt define a cavity, or caisson, of the suction bucket structure.

The weight of the offshore structure, along with the suction bucket structure, is usually more than sufficient to stabilise the offshore structure longitudinally, or vertically. Instead, suction bucket structures are employed to anchor the offshore structure against lateral, or horizontal, forces acting globally on the offshore structure. These forces may generally be the result of wave action or wind action upon the offshore structure, and may also result in up-lift, or vertical, forces attempting to pull the offshore structure out of the seabed.

To install the suction bucket structure the suction bucket structure is placed on the surface of the seabed, with the skirt contacting the seabed. The skirt and the base of the bucket, along with the surface of the seabed, enclose the cavity or volume from which water is discharged. The weight of the suction bucket structure naturally causes the skirt to penetrate the seabed to an extent, or at least sufficiently create a seal between the soil of the seabed and the suction bucket structure. A pump is normally used to create a pressure differential across the volume, such that water is discharged from therein. As the water is evacuated, a hydrostatic pressure acting upon the suction bucket structure causes the skirt of the suction bucket structure to further penetrate the seabed, such that the suction bucket structure is anchored in the seabed via the embedding of the skirt in the seabed. Once inserted into the seabed, the skirt of the suction bucket structure provides resistance to lateral forces and/or stresses which the offshore structure may experience, such as from waves or wind, due to the interaction of the skirt with the soil of the seabed within the volume and surrounding the skirt.

The title 'suction bucket structure' is, to an extent, a misnomer. Suction is usually applied only during the installation of the suction bucket structure; that is -2 -once the skirt is fully inserted into the seabed, the application of suction is removed. The vertical extent of the skirt of the suction bucket provides the required resistance to lateral forces, such that once installed there is no need to maintain suction.

To increase the resistance of a suction bucket structure to lateral forces, it may be desirable to modify the properties of the soil in and around the skirt, once it is installed. For example, the weight of the structure may consolidate the soil within the volume (and directly below the volume) of an inserted suction bucket. The cyclical action of waves and the lateral forces acting on a suction bucket may also cause the soil to consolidate. However, it may be desirable to accelerate this process. Thus suction may be applied following insertion of the suction bucket structure into the seabed, such that the density of the soil within the volume increases. This process may be referred to as 'densification', with the desired endpoint of this process being referred to as 'consolidation', that is, the act of densifying the soil within the volume consolidates the soil. Consolidated soil may increase the resistance of the suction bucket structures to lateral forces, and thus better anchor offshore structures which utilise suction bucket structures as part of their foundation Watson, P.G. and Humpheson, C.: "Foundation Design and Installation of the Yolla A Platform" (2007) discloses the application of suction preloading, following insertion of a gravity based suction (GBS) foundation in the seabed. A constant suction preloading is applied to induce compression in the silt layers beneath the base of the GBS foundation. Suction preloading is applied immediately after installation of the foundation to reduce the magnitude of any movement experienced by the GBS foundation structure during its lifetime due to its own weight and environmental loading.

Eide, 0. et al: "Reduction of Pore Water Pressure Beneath Concrete Gravity Platforms" (1982) discloses the application of an underpressure (i.e. suction) to accelerate consolidation of the soil within a volume of a GBS foundation. An underpressure is applied continuously over a period of approximately 4-5 years following installation of the GBS foundation.

It is desirable to provide a suction bucket structure which is able to better withstand the lateral forces experienced during the operational lifetime of the suction bucket structure.

According to a first aspect of the present invention there is provided a method of providing a suction bucket structure (e.g. by installing a suction bucket -3 -structure or remediating a previously installed suction bucket structure) for an offshore wind turbine in a seabed, the method comprising: providing suction bucket structure having a skirt embedded in the seabed, wherein the skirt extends from a base of the suction bucket structure and wherein a suction volume is defined by the skirt and the base; and subsequently densifying soil located beneath the base of the suction bucket structure; wherein densifying the soil comprises: generating a suction pressure within the volume; and cycling the suction pressure.

Densifying the soil beneath the base of the suction bucket structure may increase the resistance of the suction bucket structure to lateral load events, and hence increases the stability of the suction bucket structure. This may be performed immediately after installation of the suction bucket structure in the seabed (i.e. during installation), or after a period of operation of the suction bucket structure such that the suction bucket structure may be remediated e.g. following gradual or sudden destabilisation. Cycling the suction pressure, rather than simply applying a continuous suction pressure however, accelerates the rate of densification such that a higher degree of densification may be achieved. The cycling of the suction pressure may be regarded as cyclic loading, due to the cycled effective stress across the volume of the suction bucket structure generated by the cycled suction pressure. Compared to the continuous application of a suction pressure, cyclic loading may reach a higher degree of consolidation of soil beneath a suction bucket structure, or may reach a similar degree of consolidation of soil beneath a suction bucket structure in a shorter fimeframe. The use of cyclic loading may mimic storm loading events, and hence the accelerated rate/higher degree of densification achieved by using cyclic loading may increase the resistance of the suction bucket structure to lateral forces/loads.

Whilst a suction pressure may be generated within the volume, it will be understood that the generated suction pressure will apply an effective stress in the soil of the seabed not just within the volume, but also extending below the base of the volume and at least partially around an edge of the skirt. The region of densified soil may comprise a prism extending from the base of the suction bucket structure and into the seabed. Thus, the resulting densification of soil may not comprise solely the densification of soil within the volume, but may also comprise the densification of soil below the volume, and hence may generally comprise the densification of soil beneath the base of the suction bucket structure. -4 -

Densifying the soil may comprise densifying the soil within the volume of the suction bucket structure. The soil below the volume, and in the immediate area around the edge of the skirt and extending radially outwards, may also be densified due to the cycled suction pressure. However, the highest degree of densification may be achieved within the volume.

The base is typically the main supporting structure of the suction bucket structure onto which the wind turbine tower is mounted (directly or indirectly). It is preferably configured to lie generally adjacent and/or parallel to the seabed when the skirt is in the seabed. The base may be generally planar in form, though it will commonly further comprise reinforcements, mounting points, etc., and may define the cross-sectional area and/or footprint of the suction bucket structure.

The skirt may extend from a perimeter, circumference, or outermost boundary of the base downwardly towards the seabed. Thus, it extends towards/ into the seabed when the suction bucket structure is in use/ in the seabed. For reasons of strength, the skirt may be a generally cylindrical structure depending from an at least generally circular base.

The underside of the base may define an upper boundary or ceiling of the volume, and the interior of the skirt may define a circumferential boundary or walls of the volume with the lower end of the volume being open. The skirt may thus define the radial extent of the volume. The volume may be configured to confine a volume of soil of the seabed when the skirt is inserted into the seabed. As such, the suction bucket structure may be regarded as defining a caisson structure. The volume/caisson structure may also be regarded as a cavity, wherein the suction pressure is generated within the cavity such that seawater may be discharged during installation, or such that a suction pressure is applied to the soil of the seabed upon insertion of the skirt such that the soil may be subsequently densified. In the context of the present disclosure, the bottom, or lower end, of the suction bucket structure should be understood to be the end of the suction bucket structure which is located towards, in, or proximal to, the seabed, when the suction bucket structure is being installed and when installed in the seabed. The top, or upper end, of the suction bucket structure (comprising the base) will be understood to be the end of the suction bucket structure which is located away from, or distal to, the seabed and towards and/or extending out of the sea, when the suction bucket structure is installed in the seabed. -5 -

The suction bucket structure may be provided with a pump in flow communication with the volume, the pump configured to generate the suction pressure.

The pump may be configured to create a pressure differential across the suction bucket structure which motivates water from within the volume through the suction bucket structure and to be discharged outside of the suction bucket structure. The suction bucket structure may comprise a standpipe which itself may generally be in fluid communication with the volume. The pump is generally configured to pump water from the within the volume and through the standpipe, before it is discharged preferably above sea level.

In one example, the pump may be a centrifugal pump suitable for submerged applications, and may be configured to work at a power level of between 175 kW and 325 kW; between 200 kW and 300 kW; or between 225 kW and 275 kW. The pump may be configured to operate at an alternating voltage of between 380 VAC and 420 VAC; between 390 VAC and 410 VAC; or about 400 VAC. The centrifugal pump may have a flow capacity of between 400 m3/hour and 1100 m3/hour; between 500 m3/hour and 1000 m3/hour; between 600 m3/hour and 900 m3/hour; or between 700 m3/hour and 800 m3/hour.

The centrifugal pump may be configured to lower a fluid level within the standpipe, such that the fluid level is below sea level. The differing fluid levels between water in the standpipe and sea level may generated a pressure differential across the suction bucket structure, such that water is motivated within the volume and into the standpipe.

A discharge pipe may be in fluid communication with the centrifugal pump, and may be configured to discharge water pumped by the pump out of the suction bucket structure. That is, the discharge pipe may provide a port through which water may be discharged by the centrifugal pump such that the fluid level in the standpipe may lower. The discharge pipe may generally have a diameter smaller than that of the standpipe, and thus water may be pumped out of the standpipe whilst the fluid level within the standpipe lowers. The discharge pipe may only be in flow communication with the standpipe via the centrifugal pump.

In another example the pump may be an electrical submersible pump. The electrical submersible pump may be characterised by multiple impeller stages, and may be capable of producing a larger lifting height of water within a pipe when -6 -compared to a centrifugal pump. Preferably, the electrical submersible pump may be a cavern pump.

The electrical submersible pump may comprise a sealing seat, and the sealing seat may be configured to create a watertight seal within the standpipe.

This is in contrast to a centrifugal pump, which may be simply submerged in the standpipe and in flow communication with a discharge pipe. The electrical submersible pump, and equivalently the standpipe, may have a diameter of between 16 inches and 28 inches; between 18 inches and 28 inches; between 20 inches and 26 inches; or between 22 inches and 24 inches. Preferably, the electrical submersible pump and/or the standpipe may have a diameter of about 18 inches; or about 28 inches.

The standpipe may comprise a discharge port located above sea level. The electrical submersible pump may be configured to generate a pressure differential across the suction bucket structure which motivates water through the standpipe and out of the suction bucket structure via the discharge port.

The electrical submersible pump may have a flow capacity of between 400 m3/hour and 1100 m3/hour; between 500 m3/hour and 1000 m3/hour; between 600 m3/hour and 900 m3/hour; or between 700 m3/hour and 800 m3/hour. The electrical submersible pump may have be configured to operate at an alternating voltage of between 380 VAC and 420 VAC; between 390 VAC and 410 VAC; or about 400 VAC.

Cycling the suction pressure may comprise switching the pump between: a pumping mode wherein a suction pressure is applied to the volume; and a standby mode wherein a suction pressure is not applied to the volume.

Each cycle may comprise two stages: in a first stage the pump may be in the pumping mode; and in the second stage the pump may be in the standby mode. Accordingly cycling the suction pressure may comprise alternatively operating the pump in the pumping mode and in the standby mode.

In the pumping mode, the pump may be configured to apply a suction pressure to the volume. That is, the pump may be configured to generate a pressure differential across the suction bucket structure, e.g. by switching the pump on. In the pumping mode, the suction pressure may be applied continuously or intermittently, e.g. in pulses. -7 -

In the standby mode, the pump may be configured to not apply a suction pressure to the volume. That is, the pump may be switched off or deactivated such that a pressure differential is not applied across the suction bucket structure.

The cycles may be drained or undrained cycles. During a first stage/a pumping mode of a drained cycle, the pressure differential applied across the suction bucket structure may be maintained, such that water is motivated into the suction bucket structure and out of the volume. During a first stage/a pumping mode of an undrained cycle, the pressure differential applied across the suction bucket structure may be rapidly applied and removed during, such that water is motivated into the volume but is not motivated into the suction bucket structure and an excess pore water pressure is generated within the volume.

Thus, if the cycles are drained cycles, during the pumping mode the suction pressure may be applied continuously to the volume. During the pumping mode the continuous application of a suction pressure may densify the soil located beneath the base of the suction bucket structure. During the standby mode, the suction pressure may be removed such that the soil located beneath the base of the suction bucket structure loosens (hence mimicking storm loading events). Multiple cycles may be applied until the desired degree of densification is achieved.

Alternatively, if the cycles are undrained cycles, the pumping mode may comprise: a first phase wherein the suction pressure is rapidly applied and removed such that an excess power water pressure is generated within the volume; and a second phase wherein the suction pressure is applied continuously once the excess power water pressure generated exceeds a first threshold.

That is, during the first phase the pump may be switched on and off rapidly such that the excess pore water pressure is generated within the volume, and during the second phase the pump may be configured to drain the volume following the generation of the excess pore water pressure. Pore water pressure will be understood to refer to the pressure of water held within the gaps and/or spaces between soil material. Thus an excess pore water pressure refers to the loading of the soil with water, which may displace and/or loosen the soil due to the displacement of the soil by excess water. The pump may be switched on and left on during the second phase, such that the continuous application of a suction pressure within the volume drains the volume. As the volume is drained during the second phase, the evacuation of water from the volume causes a volume change.

Voids or spaces between the soil material, left behind due to the removed water, -8 -may become occupied by soil as the suction pressure is continuously applied during the second phase, such that the soil densifies.

The first threshold may be: at least 20 kPa; at least 25 kPa; at least 30 kPa; at least 35 kPa; or at least 40 kPa.

Undrained cycles may be more effective at densifying clay-based soils, as clay-based soils may be capable of generating a higher excess pore water pressure. Drained cycles may be more effective at densifying sand-based soils, as sand-based soils may not generally be capable of generating a substantial excess pore water pressure. It may therefore be advantageous to apply a mixture of drained and undrained cycles to achieve a desired degree of densificafion of the soil.

Densifying the soil may comprise applying a package of drained cycles and applying a package of undrained cycles, to achieve the desired degree of densification. This may be particularly advantageous in soils comprising many different soil layers which may respond differently to the application of drained and/or undrained cycles. Applying a package of drained cycles may comprise switching the pump between: a first pumping mode, wherein a suction pressure is applied continuously to the volume; and a first standby mode, wherein the suction pressure is not applied to the volume. Applying a package of undrained cycles may comprise switching the pump between: a second pumping mode, wherein the first pumping mode comprises: a first phase, wherein the suction pressure is rapidly applied and removed such that an excess power water pressure is generated within the volume; and a second phase, wherein the suction pressure is applied continuously once the excess power water pressure generated exceeds a first threshold; and a second standby mode, wherein the suction pressure is not applied to the volume.

The method may comprise cycling the suction pressure: at least 10 times; at least 20 times; at least 30 times; at least 50 times; at least 100 times; at least 150 times; at least 200 times; or at least 250 times. Equivalently, the method may comprise applying: at least 10 cycles; at least 20 cycles; at least 30 cycles; at least cycles; at least 50 cycles; at least 100 cycles; at least 150 cycles; at least 200 cycles; or at least 250 cycles.

A period of each cycle may be: less than 10 minutes; less than 20 minutes; less than 30 minutes; less than 40 minutes; less than 50 minutes; less than 60 minutes; or generally of the order of about one hour. A period of a cycle may -9 -comprise the first stage (i.e. operating the pump in the pumping mode), and the second stage (i.e. operating the pump in the standby mode).

The suction bucket structure may comprise a plurality of openings located at the base and in flow communication with the volume, and may further comprise a standpipe in flow communication with the plurality of openings. The standpipe may extend through the suction bucket structure, and may extend from the base towards an upper end of the suction bucket structure.

The plurality of openings may be located such that when the skirt is provided in the soil each of the plurality of openings is adjacent to, or at least facing, the seabed. The standpipe may extend along a longitudinal axis of the suction bucket structure, and may divide, bifurcate or branch into a plurality of pipe lengths each extending along a radial extent of the base and to a respective opening of the plurality of openings.

The suction bucket structure may comprise a plurality of filters, and each filter may be located at a respective opening of the plurality of openings. Each filter of the plurality of filters may be configured to prevent or reduce soil ingress within the standpipe. This may prevent damage to the pump, as well as preventing the removal of soil from the volume which may otherwise decrease a relative density of the soil located beneath the base of the suction bucket structure.

The suction bucket structure may comprise a dividing structure within the volume, the dividing structure separating the volume into a plurality of volumes. Each of the plurality of volumes may be in flow communication with at least one respective opening of the plurality of openings. The dividing structure may be configured to divide the volume into: thirds; quadrants; fifths; sextants; octants or the like. The dividing structure may be a cylindrical element and the volume may comprise an annular cross section, wherein the dividing structure may define an inner annulus of the volume and the skirt may define an outer annulus of the volume.

The standpipe is preferably configured to receive the pump, and hence providing the pump may comprise lowering the pump into the standpipe. The pump may generate the suction pressure which facilitates the insertion of the skirt into the seabed, as well as the suction pressure which densifies the soil located beneath the base of the suction bucket structure. The method may additionally or alternatively comprise, following densification of the soil, removing the pump from the standpipe.

-10 -It is not necessary to apply a suction pressure, or to apply further cycles, once the desired degree of densification of soil below the base of the suction bucket structure has been achieved. Thus removing the pump may allow the pump to be used during future densification operations (e.g. installation or remediation) for other suction bucket structures, for example.

The suction bucket structure may comprise a mounting surface. The mounting surface may be located on an upper surface of the suction bucket structure, and may be configured to receive a wind turbine. The wind turbine may be bolted, welded or otherwise joined to the mounting surface.

The method may comprise mounting the wind turbine to the suction bucket structure. The wind turbine may only be mounted to the suction bucket structure once the desired degree of densification has been achieved. That is, mounting of the wind turbine to the suction bucket structure may subsequently follow the step of densifying the soil located beneath the base of the suction base structure.

A suction pressure may not be applied during operation of the wind turbine.

That is, once the desired degree of densification has been achieved, no more cycles of suction pressure may be applied to the volume.

The suction bucket structure may comprise a suction bucket and a mounting structure. The suction bucket may comprise the base and the skirt, (i.e. the caisson structure). The mounting structure may be attached to the base, and may extend from the base distally from the suction bucket, i.e. towards an upper end of the suction bucket structure. The mounting structure may comprise the standpipe, and may also comprise the mounting surface to which the wind turbine may be mounted. The cross-sectional area of the mounting structure may generally taper, or narrow, towards an upper surface of the suction bucket structure. The mounting structure may be configured to be attached to a single suction bucket (i.e. such that the suction bucket structure is a single suction bucket structure), or may be configured to be attached to a plurality of suction buckets (i.e. such that the suction bucket structure is a multiple suction bucket structure).

The suction bucket structure may be a gravity based suction bucket structure (i.e. a GBS foundation). The GBS foundation may comprise a single suction bucket in combination with the mounting structure, and hence may be understood to be a single suction bucket structure. The mounting structure may be a solid structure extending from the base and narrowing towards an upper surface of the suction bucket structure.

Alternatively the suction bucket structure may be a jacket foundation. The jacket foundation may comprise one or more suction buckets in combination with a jacket structure forming the mounting structure (and hence be understood to be a multiple suction bucket structure). The jacket structure may be a latticework structure or other suitable structure for mounting an offshore wind turbine. If the jacket foundation comprises a plurality of suction buckets, each suction bucket may be joined, mounted or attached to a respective leg of the jacket structure. The step of densifying the soil may be applied to one or more or all of the suction buckets provided in combination with the jacket, as appropriate.

Viewed from a second aspect of the present invention, there is provided a method of installing a suction bucket structure for an offshore wind turbine into a seabed. The method comprises: inserting a skirt of the suction bucket structure into the seabed, wherein the skirt extends from a base of the suction bucket structure and wherein a suction volume is defined by the skirt and the base; and subsequently densifying soil located beneath the base of the suction bucket structure; wherein densifying the soil comprises: generating a suction pressure within the volume; and cycling the suction pressure.

In other words, the method of the second aspect comprises providing a suction bucket structure for an offshore wind turbine in a seabed according to the method of the first aspect, wherein the step of providing a suction bucket structure having a skirt embedded in the seabed comprises inserting a skirt of the suction bucket structure into the seabed, where the step of subsequently densifying the soil may accelerate the consolidation process such that the installed suction bucket structure is more stable when installed in the seabed. Thus, the above description of the method of the first aspect, including but not limited to all technical advantages and alternative embodiments, may be equally applicable to the method of the second aspect.

The method may comprise lowering the suction bucket structure towards the seabed, wherein the lowering may be performed using a crane and a winch system from a floating vessel or the like. Once the skirt of the suction bucket structure contacts the seabed, the skirt of the suction bucket structure may be inserted into the seabed. Inserting the skirt of the suction bucket structure into the seabed may comprise applying a suction pressure within the volume, such that the skirt is motivated into the seabed.

-12 -The skirt may be inserted into the seabed until at least part of the base of the suction bucket structure contacts the seabed. That is, the process of densifying the soil may not begin until the base of the suction bucket structure may at least partially contact the seabed. Alternatively, the skirt may be inserted into the seabed until at least: 85% of the length of the skirt; 90% of the length of the skirt, or 95% of the length of the skirt, is inserted and/or embedded in the soil. Again, the process of densifying the soil may not begin until the aforementioned length of the skirt is inserted into and/or embedded in the seabed.

Viewed from a third aspect of the present invention, there is provided a method of remediating a previously installed (i.e. pre-installed and/or pre-existing) suction bucket structure for an offshore wind turbine in a seabed. The method comprises: providing a suction bucket structure having a skirt embedded in the seabed, wherein the skirt extends from a base of the suction bucket structure and wherein a suction volume is defined by the skirt and the base; and subsequently densifying soil located beneath the base of the suction bucket structure; wherein densifying the soil comprises: generating a suction pressure within the volume; and cycling the suction pressure.

In other words, the method of the third aspect comprises providing a suction bucket structure for an offshore wind turbine in a seabed according to the method of the first aspect, wherein the step of densifying the provided suction bucket structure stabilises, or reverses loosening of the soil beneath and within the volume of, a suction bucket structure which may have become destabilised during operation. Thus, the above description of the method of the first aspect, including but not limited to all technical advantages and alternative embodiments, may be equally applicable to the method of the third aspect.

In operation, the suction bucket structure may become destabilised due to one or more phenomena. These may include unprecedented storm events (e.g. greater than 100-year storms), collisions or disturbance of marine vessels, or due to geological events such as earthquakes and the like. Additionally, remediation may be required where the profile of the soil beneath the suction bucket structure does not behave as anticipated. By being able to densify the soil located beneath the volume by cycling the suction pressure generated within the volume, the suction bucket structure may be stabilised in an efficient manner. Accordingly long-term detriment or damage to the suction bucket structure may be reversed, or -13 -decommissioning of the suction bucket structure due to such damage may be avoided.

The method may comprise retrofitting the suction bucket structure with at least one of the following components: a/the pump; a/the standpipe; a/the plurality of openings; or a/the plurality of filters. Accordingly, the step of densifying the soil located beneath the base of the suction bucket structure may be able to be applied to suction bucket structures which are already commissioned/operating. Thus existing suction bucket structures may be remediated/stabilised, such that their operational lifetime is extended and/or improved.

According to a fourth aspect of the present invention, there is provided a method of installing a wind turbine in a seabed. The method of the fourth aspect comprises providing/installing/remediating a suction bucket structure according to the method of any of the previous aspects of the invention, and further comprises the step of mounting the wind turbine to the suction bucket structure. Thus, the above description of the method of the previous aspects, including but not limited to all technical advantages and alternative embodiments, may be equally applicable to the method of the fourth aspect.

Certain preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a cross-sectional schematic view of a first suction bucket structure used by the present invention; Figure 2 shows a perspective view of a second suction bucket structure used by the present invention; and Figure 3 shows a graph schematically representing the process of densification, according to an embodiment of the present invention.

Figure 1 shows a cross-sectional plan view of a suction bucket structure la installed in a seabed 120, the structure la extending upwardly from the seabed 120 to above the surface of the sea 110. The suction bucket structure 1a is a gravity based suction (GBS) foundation comprising a suction bucket portion 10 and a mounting structure portion 20a. The suction bucket portion 10 comprises a suction bucket, or caisson structure, comprising a skirt 2, a base 3 and a plurality of filters 4 covering a plurality of openings in flow communication with a suction volume, or cavity. In flow communication with the suction bucket, and extending from the suction bucket portion 10 and through the mounting structure portion 20a is a -14 -standpipe 5, which is in flow communication with a discharge port 6 located in the mounting structure portion 20a. The filters 4 are placed at openings of the standpipe 5 located towards the seabed 120, and the discharge port 6 is arranged such that it will be just above sea level when the suction bucket structure is is resting on or installed in the seabed 120.

The skirt 2 and the base 3 define a suction volume, or cavity, located underneath the suction bucket structure la. When the suction bucket structure la initially contacts the seabed 120, this cavity defines a closed volume comprising seawater, the cavity defined by the skirt 2, the base 3 and the seabed 120.

The cavity is a cylindrical volume, although in various embodiments the cavity may be a cuboid volume. As shown herein, the suction bucket structure comprises a divider 7. The divider 7 of the present embodiment divides the volume of the cavity into halves.

A pump 8 is shown in the standpipe 5, located towards the base 3 of the suction bucket structure la. The pump 8 is configured to pump water through the standpipe 5 by creating a pressure differential between the openings of the standpipe 5 in contact with the filters 4, and the discharge port 6 where water will be discharged out of the suction bucket structure la. In the present embodiment the pump 8 is not attached or joined to the suction bucket structure la and is therefore lowered into and withdrawn from the suction bucket structure 1 as required, via the standpipe 5.

Towards an upper end of the mounting portion 20a is a mounting portion 23a, which is suitable for mounting an offshore wind turbine to the suction bucket structure la.

Figure 2 shows a perspective view of an alternative suction bucket structure lb. The alternative suction bucket structure lb is a jacket foundation 2b. The suction bucket structure lb comprises three suction buckets in the suction bucket structure 10b, with each suction bucket mounted to a respective leg 22 of a jacket structure 20b. The jacket structure 20b forms the mounting structure portion 20b of the suction bucket structure lb, and is a latticework frame or jacket extending from the suction bucket portion 10a and terminating at a mounting surface 23b. The mounting surface 23b is suitable for mounting an offshore wind turbine to the suction bucket structure lb. Each of the suction buckets of the jacket foundation lb comprise similar structure to those of the GBS foundation la. Each suction bucket comprises a skirt -15- 2 and a base 3, with the skirt 2 and the base 3 together defining a suction volume, caisson or cavity. Whilst not shown, each suction bucket additionally comprises a plurality of openings of a standpipe in flow communication with the cavity, a plurality of filters covering the plurality of openings, and a discharge port located such that it will be above sea level when the suction bucket structure lb is resting on or installed in the seabed 120. Additionally a pump can be located towards the plurality of openings of each suction bucket, or can lowered in/out of the standpipe. The standpipes for each suction bucket are located in the jacket structure 20b, extending from the base of each suction bucket through the respective leg 22. In various embodiments however, the standpipes may be temporary structures located external to the suction bucket structure lb. To install the suction bucket structure 1 (i.e. either the GBS foundation la or the jacket foundation lb) in the seabed 120, the suction bucket structure 1 is lowered into the sea 110 using a crane and a winch system from a floating vessel.

The suction bucket structure 1 is lowered until the skirt 2 contacts the seabed 120.

Upon contact with the seabed 110, owing to the weight of the suction bucket structure 1 the skirt 2 will naturally penetrate the seabed 120. The skirt 2 will create a rough seal with the seabed 120, defining a closed volume from which water can be discharged from the volume.

The pump 8 is then lowered through the standpipe 5, where it is used to create a pressure differential across the suction bucket structure 1; the pressure differential created by the pump 8 causing water to be discharged from the volume. Water is motivated through and out of the standpipe 5, passing through the openings of the standpipe 5 where the filters 4 are located, through the pump 8 and out of the standpipe 5 via the discharge port 6.

The reduced volume of water within the suction volume results in the weight of the seawater above the suction bucket structure 1 forcing the suction bucket structure 1 into the seabed 120. That is, the skirt 2 is actively inserted into the seabed 120 due to the hydrostatic pressure acting upon the upper surface of the suction bucket structure 1 exceeding the generated underpressure within the volume of the suction bucket structure 1. The resulting penetration of the skirt 2 into the seabed 120 may be perceived as being caused by the application of suction, and the term 'suction' will generally be understood to be the creation of an underpressure by the pump 8.

-16 -If the penetration of the skirt 2 into the seabed 120 is resisted by the seabed 120 due to frictional forces or the like, it may be necessary to apply the underpressure within the volume in short bursts. Short bursts of underpressure can help the suction bucket structure 1 overcome the resistive forces experiences by the skirt 2, such that the skirt 2 is able to fully penetrate the seabed 120.

The suction bucket structure 1 is generally installed, or inserted, into the seabed 120 when the skirt 2 of the structure 1 is fully embedded in the soil of the seabed. At this point, the base 3 of the suction bucket structure 1 is at least in partial contact with the surface of the seabed 120, and more preferable is in substantial, or complete, contact with the surface of the seabed 120.

The suction bucket structure 1 of the present embodiment, when installed, will be part of a foundation structure of an offshore wind turbine (not shown). As will be appreciated, during its lifetime the suction bucket structure 1 will therefore experience large horizontal/lateral loads due to wind and/or wave action acting on the overall offshore wind turbine structure. These loads may be at their greatest during storm events.

To increase the resistance of the suction bucket structure 1 to lateral/horizontal loads, the soil within the volume (often referred to as the 'soil plug') of the inserted skirt 2 is densified. The soil is densified once the skirt 2 has been fully inserted into the seabed 120. This generally corresponds to the point at which at least a part of the base 3 of the suction bucket structure 1 is in contact with the seabed 120.

Once the skirt 2 is fully inserted in the seabed 120, the pump 8 is used to further generate a suction pressure which is applied to the volume. In the present embodiment, the suction pressure applied is around 200 kPa. However, in various embodiments the suction pressure applied can be anywhere in between 100 kPa and 400 kPa.

The application of a suction pressure within the volume of the suction bucket structure 1, once the skirt 2 is fully inserted in the seabed 120, causes a number of phenomena. The suction pressure generated by the pump 8 creates a differential water head across the base 3 of the suction bucket structure 1, such that a compressive force is applied to the soil of the seabed 120 within the volume. This compressive force generally increases an effective stress within the soil plug. At the same time, water flow into the suction base structure 1 owing to the applied suction pressure induces an upwards hydraulic gradient, which decreases the -17 -effective stress within the soil plug, relative to the volume of water flowing through the soil plug. However, the net result of the compressive force and the upwards hydraulic gradient is an increase in the effective stress of the soil plug. The increased effective stress induced in the soil plug generally induces the densification of soil within the soil plug.

In accordance with an embodiment of the present invention, the pumps is cycled such that the suction pressure applied to the volume is also cycled. This is in contrast to a maintained, or continuous, application of suction pressure to the volume. That is, the pump 8 is switched on and off such that a suction pressure generated by the pump 8 is also switched 'on' and 'off', or 'cycled' to create cyclic load conditions within the volume. The cyclic load conditions densify the soil of the seabed 120 within the skirt 2 of the suction bucket structure 1. In the present embodiment, the period of each cycle is less than 60 minutes, and at least ten (10) cycles are applied to achieve the desired densification of the soil plug. However, in various embodiments at least two-hundred (200) cycles are applied to achieve the required densification. In the present embodiment, the relative density of the soil plug can be increased from around 60% to around 80%.

The repeated application and removal of the suction pressure generally causes the effective stress induced within the soil plug to cycle. A first stage of a cycle involves the application of the suction pressure (i.e. using the pump 8) such that water is motivated into the soil plug and densification occurs. A second stage of the cycle then involves the removal of the application of the suction pressure (i.e. by switching off the pump 8) such that soil within the volume of the suction bucket structure 1 at least partially loosens, or settles in an equilibrium state. Whilst the loosening of the soil within the soil plug may reduce a local density of the soil during each cycle, the net effect per cycle still results in the densification of the soil plug. However, beneficially, the repeated cycling of applying the suction pressure during the first stage to cause densification and removal of the suction pressure during the second stage to cause the partial loosening of the soil generally mimics the effect of storm loading on the suction bucket structure 1 such that an overall rate of densification within the soil plug increases. This results in a higher degree of consolidation within the soil plug over a shorter period of time.

The applied cycles of suction pressure can either be 'drained' or 'undrained' cycles. As will be readily understood by those skilled in the art, 'drained' and -18 -tundrained' refers to the absence and presence of excess water (i.e. an excess pore water pressure), respectively, in the soil plug.

In one embodiment, the cycles are drained. That is, during the first stage of each cycle the suction pressure generated by the pump 8 is continuously applied to the volume. As the suction pressure is applied within the volume of the suction bucket structure 1, water motivated into the volume is discharged from the volume using the pump 8 and hence the soil plug is drained. Drainage and densification occur generally continuously during the first stage of each cycle, during drained cycles. The pump 8 is switched off during the second stage of each cycle.

In an alternative embodiment, the cycles are undrained. During undrained cycles, in the first stage of each cycle a suction pressure is first applied within the volume such that water is motivated into the volume, but water is not discharged from the volume using the pump 8. This is achieved by rapidly switching the pump 8 on and off rather than applying a constant suction pressure during the first stage.

Thus, during the first stage of undrained cycles, in a first phase the suction pressure is rapidly applied and removed within the volume. By rapidly switching the pump 8 on and off during the first phase, an excess pore water pressure can be generated within the volume. Once a desired excess pore water pressure is generated, which in the present embodiment is at least 25 kPa and preferably around 30 kPa, the rapid switching of the pump 8 is paused and the pump 8 is left on such that drainage can occur, which results in densification of the soil plug. When the cycles are undrained, drainage and densification therefore occurs during a second phase, towards the end of the first stage of each cycle. The pump 8 is again switched off during the second stage of each cycle.

Figure 3 shows a graph schematically representing the process of densification within the soil plug, in accordance with the present invention. The graph shows how the void ratio may vary with respect to the logarithmic value of the effective stress induced in the soil plug during each stage of the cycle. Two cycles are schematically represented, where each of the first stages of the cycles are undrained.

First, the pump 8 is switched on and off rapidly to generate an excess pore water pressure within the soil plug due to the application of a suction pressure without the soil plug draining. In the present embodiment, the generated excess pore water pressure is around 30 kPa. As seen in figure 3, arrow (1) represents the first phase of the first stage during undrained cycles. Following the generation of -19 -the excess pore water pressure, the volume is drained by leaving the pump 8 on (i.e. by pausing the rapid switching of the pump 8) which results in the densification of the soil plug. This is represented by the void ratio decreasing (see arrow (2)) during the second phase of the first stage of the undrained cycle. Arrows (1) and (2) hence represent both phases of the first stage of a single undrained cycle of the suction pressure. The process is then repeated during another cycle (see arrows (3) and (4)) such that the soil plug is further densified. With a generated excess pore water pressure of 30 kPa, around 200 cycles are generally applied to reach the desired densification.

As described above, the application of a cycled suction pressure can follow the insertion of the suction bucket structure 1 into the seabed 110. The subsequent densification thus forms part of the installation process of the suction bucket structure 1 into the seabed 110.

In accordance with another embodiment of the present invention, the application of a cycled suction pressure as described above is used to densify the soil located below a suction bucket structure 1 already installed in the seabed 110 (i.e. a preinstalled suction bucket structure 1). Applying a cycled suction pressure (drained or undrained) post-installation can remediate a suction bucket structure 1 that has become destabilised during operation, by densifying the soil located beneath the suction bucket structure 1 and thus restabilising the suction bucket structure 1. The cycling of the suction pressure follows an identical methodology as described above for densifying the soil beneath a suction bucket structure 1 during installation and therefore will not be repeated.

Claims (18)

1. -20 -Claims 1. A method of installing a suction bucket structure or remediafing a previously installed suction bucket structure for an offshore wind turbine in a seabed, the method comprising: providing a suction bucket structure having a skirt embedded in the seabed, wherein the skirt extends from a base of the suction bucket structure and wherein a suction volume is defined by the skirt and the base; and subsequently densifying soil located beneath the base of the suction bucket structure; wherein densifying the soil comprises: generating a suction pressure within the volume; and cycling the suction pressure.
2. 2. A method as claimed in claim 1, the method comprising: providing a pump in flow communication with the volume, the pump configured to generate the suction pressure.
3. 3. A method as claimed in claim 2, wherein cycling the suction pressure comprises switching the pump between: a pumping mode wherein a suction pressure is applied to the volume; and a standby mode wherein a suction pressure is not applied to the volume.
4. 4. A method as claimed in claim 3, wherein during the pumping mode the suction pressure is applied continuously to the volume.
5. 5. A method as claimed in claim 3, wherein the pumping mode comprises: a first phase wherein the suction pressure is rapidly applied and removed such that an excess pore water pressure is generated within the volume; and a second phase wherein the suction pressure is applied continuously once the excess pore water pressure generated exceeds a first threshold.
6. 6. A method as claimed in any preceding claim, wherein the suction pressure is cycled at least 10 times.
7. -21 - 7. A method as claimed in any preceding claim, wherein a period of each cycle is less than 60 minutes.
8. 8. A method of installing a suction bucket structure for an offshore wind turbine into a seabed, the method comprising: a method as claimed in any preceding claim; wherein the step of providing a suction bucket structure having a skirt embedded in the seabed comprises inserting a skirt of the suction bucket structure into the seabed.
9. 9. A method as claimed in claim 8, wherein the skirt is inserted into the seabed until at least part of the base of the suction bucket structure contacts the seabed.
10. 10. A method of remediafing a suction bucket structure for an offshore wind turbine in a seabed, the method comprising: a method as claimed in any of claims 1 to 7 applied to a previously installed suction bucket structure in need of remediation.
11. 11. A method as claimed in claim 10, the method comprising retrofitting the suction bucket structure with at least one of: a pump, a standpipe, a plurality of openings, or a plurality of filters.
12. 12. A method as claimed in any preceding claim, wherein the suction bucket structure is provided with: a plurality of openings located at the base and in flow communication with the volume; and a standpipe in flow communication with the plurality of openings, the standpipe extending through the suction bucket structure.
13. 13. A method as claimed in claim 12, wherein the suction bucket structure is provided with: a plurality of filters, each filter located at a respective opening of the plurality of openings.
14. -22 - 14. A method as claimed in claim 13, wherein each filter of the plurality of filters is configured to prevent soil ingress within the standpipe.
15. 15. A method as claimed in claim 12, 13 or 14, wherein the standpipe is configured to receive a pump; and wherein the method further comprises: following densificafion of the soil, removing the pump from the standpipe.
16. 16. A method as claimed in any of claims 12 to 15, wherein the suction bucket structure comprises a dividing structure within the volume, the dividing structure separating the volume into a plurality of volumes; and wherein each of the plurality of volumes is in flow communication with at least one respective opening of the plurality of openings.
17. 17. A method of installing a wind turbine in a seabed, the method comprising: providing a suction bucket structure in the seabed according to the method of any preceding claim; and mounting the wind turbine to the suction bucket structure.
18. 18. A method as claimed in claim 17, wherein a suction pressure is not applied within the volume during operation of the wind turbine.