GB2538329A

GB2538329A - Platform and assembly solution for a floating offshore device

Info

Publication number: GB2538329A
Application number: GB1511857.3A
Authority: GB
Inventors: Mackie Graeme
Original assignee: OCEANFLOW DEV Ltd
Current assignee: OCEANFLOW DEV Ltd
Priority date: 2015-05-05
Filing date: 2015-07-07
Publication date: 2016-11-16
Anticipated expiration: 2035-07-07
Also published as: GB201507710D0; GB201507661D0; GB2538329B; GB201511857D0

Abstract

Apparatus for supporting a device in a body of water comprises surface piercing buoyancy elements (3, Fig 7) radially spaced apart from a spar 7 in a fixed geometric relationship. A lower end of the spar element extends below the surface piercing buoyancy elements while an upper end supports the device that may be a wind turbine tower. The apparatus which may have an open ended centre column (4, Fig 7) may be constructed, launched and used as a floating platform for the afloat assembly of a centre spar and wind turbine. The spar may be lowered into the centre column and fixed by bolting and grouting (11, Fig 7). The spar may then be ballasted to achieve the required level of stability for the floating body such that it can safely support a wind turbine that is bolted to the ballasted spar.

Description

Platform and Assembly Solution for a Floating Offshore Device Field of the Invention The present invention relates to a system and method for assembling a floating offshore wind turbine.

Background of the Invention

The deployment of offshore wind turbines in deeper water depths requires the adoption of floating foundations. A number of solutions have been proposed which generally fall under the categories of: (a) surface floating platforms; (b) semi-submerged floating platforms; (c) mono-spar floating platforms; and (d) tension tethered plafform.

Surface floating platforms suffer from the disadvantage of having adverse motion responses in waves which would make them unacceptable for use as a floating wind turbine support plafform in any sea areas with an energetic wave environment.

Semi-submerged platforms can have acceptably low motions in waves but require widely separated surface piercing buoyancy elements with sufficient freeboard or height of element above the static waterline to achieve the required transfer of buoyancy as the platform heels in order to generate the necessary stabilising or righting moment. This righting moment is necessary to resist the heeling moment or overturning moment generated by the wind and waves. Achieving stability through the transfer of buoyancy as the device heels is known as metacentric stability and sufficient metacentric stability can be achieved even when the centre of gravity of the structure is above the centre of buoyancy provided that the surface piercing buoyancy elements are sufficiently widely spaced and project sufficiently far above the waterline.

These widely spaced buoyancy elements and their associated cross bracing structures are costly to build and maintain.

Tension tethered platforms are positively buoyant structures that are anchored to the seabed by tethers that are tensioned by the excess buoyancy of the submerged structure. Any change in environmental loading from wind and waves is taken up by a change in tension of the tethers which in turn transmit their loads into the seabed anchor system. Anchor systems for the tension tethers to resist the excess buoyancy of the submerged platform are expensive to deploy. In addition a tension tethered platform may not be inherently stable until constrained by its tethers thus making transportation and installation more complex and costly.

Mono-spar structures achieve their stability to resist the turbine wind heeling moment by having the centre of gravity sufficiently far below the centre of buoyancy to achieve the required righting moment. Stability achieved by having the centre of gravity below the centre of buoyancy is known as pendulum type stability and this type of floating body stability does not rely on the transference of buoyancy as with the semi-submersible or surface floating platforms. Arranging for the centre of gravity to be below the centre of buoyancy is achieved by building a spar structure with a very deep draft and filling the lower portion with some heavy ballast medium in order to lower the overall centre of gravity such that it is sufficiently below the centre of buoyancy to achieve the desired stability.

The solution proposed in this patent combines a spar structure with a semi-submerged low freeboard buoyant multi-column flotation collar such that the buoyant multi-column floats raise the overall centre of buoyancy. This structure is henceforth referred to as a multifloat-spar. By so doing it is possible to achieve the required stability with an overall geometry that has a shallower draft than the mono-spar solution. As well as raising the vertical position of the centre of buoyancy the low freeboard buoyant collar contributes some additional stabilising moment through the transfer of buoyancy elements as the platform heels contributing some metacentric stability. However the overall freeboard is such that large waves overtop the buoyancy collar thus preventing excessive buoyancy response forces that could cause large excitations in waves.

Summary of the Invention

According to a first aspect of the invention there is provided an apparatus for supporting a device in a body of water, the apparatus comprising surface piercing buoyancy elements radially spaced apart from a spar element having two ends, wherein one end of the spar element extends below the surface piercing buoyancy elements, the other end supporting the device, the spar element and the surface piercing buoyancy elements being in a fixed geometric relationship.

Preferably, the centre spar element is ballasted such that the overall centre of gravity of the complete assembly including the supported device and the overall centre of buoyancy of the complete assembly are in such proximity that the apparatus is stable when upright and has a range of stability through which environmental forces on the complete assembly are counteracted.

The actual arrangement of the centres of gravity and buoyancy will depend on the environmental forces that the device is to be subjected to. A wind turbine will experience very significant heeling moments due to wind pressure for example. The apparatus needs to be stable, that is self-righting, over a range of heeling angles that the apparatus can be expected to experience in the environment where the device is to be located.

Preferably, the overall centre of gravity of the complete assembly is below the overall centre of buoyancy of the complete assembly or wherein the overall centre of gravity of the complete assembly is above the overall centre of buoyancy of the complete assembly or wherein the overall centre of gravity of the complete assembly and the overall centre of buoyancy of the complete assembly are substantially coincident.

Advantageously, the apparatus comprises a spar housing, the surface piercing buoyancy elements attached to the housing and the housing configured to receive the spar.

The apparatus may comprise attachment means configured for attachment of the spar to the spar housing.

Advantageously, the attachment means includes a bolting flange connected to the housing.

Preferably, the spar housing has two ends and each end is open, the spar housing providing a through passageway for receiving the spar.

The apparatus may further comprise seal means configured for mounting between the spar and the spar housing.

The seal means may comprise an inflatable collar.

Preferably, the apparatus comprises a grout material for introduction between the spar and the spar housing to make a structural connection between the spar and the spar housing, such grout being retained by the seal until it has set.

Preferably, the attachment means includes a bolting flange connected to the spar.

The spar may be configured for receiving ballast.

The spar may include openings therein to permit free flooding of the spar.

The spar may include a valve or valves to control flow through the free flood openings.

The spar may include at least one compartment selectively sealable against ingress or egress of fluid therefrom.

Preferably, the lower end of the spar is provided with at least one guide member for guiding the spar into the housing.

The device may include a tower and the tower is attached to the spar.

Advantageously, the apparatus is configured such that when assembled, ballasted and moored, the freeboard of the surface piercing buoyancy elements above the static waterline is such that larger waves may overtop the buoyancy elements.

The device may be a wind turbine, or a transformer station.

According to a second aspect of the invention there is provided a method for assembling an apparatus for supporting a device in a body of water comprising the steps of: i. launching an apparatus of the first aspect of the invention including the apparatus comprising a spar housing, the surface piercing buoyancy elements attached to the housing and the housing configured to receive the spar into a body of water; ii. lowering a free flooding spar into the spar housing and seating and bolting the spar on the spar housing; introducing ballast into the spar; iv. sealing the space between the spar and the spar housing and structurally connecting the spar to the spar housing.

The step of introducing ballast into the spar may comprise introducing flowable material into the spar.

The flowable material may comprise concrete or sand or drilling mud or other pumpable fluid for ballasting purposes which is introduced into the spar through the top opening.

The step of introducing ballast into the spar may comprise lowering solid or bagged ballast material into the spar through the top opening.

The step of sealing the space between the spar and the spar housing may comprise inflating an inflatable collar in the said space.

The step of making a structural connection between the spar and the spar housing may comprise the step of filling the said space with structural grout.

The method may comprise the further step of attaching a device to the spar.

Preferably, the method comprises the further step of towing the assembled wind turbine to a site in a body of water.

The invention combines the metacentric stability derived from the distribution of buoyant surface piercing members of a semi-submerged multi column floating platform with the low centre of gravity of a mono-spar structure to obtain the best features from both solutions. The laterally spaced waterplane area of the semi-submerged multi-column buoyancy elements combined with the low centre of gravity of the ballast filled centre tube provides the same level of stability as a single spar structure but at a shallower draft. Likewise by combining a ballasted centre spar with the semi-submerged multi-column platform it is possible to reduce the dimensions and radial offset of the semi-submerged multi-column buoyancy elements. The shallower draft of a combined semi-submersible floating platform plus a mono-spar structure, referred to as a multifloat-spar, allows the complete unit to be assembled in shallower inshore waters thus reducing installation costs and improving overall economics. It also allows the unit to be deployed in shallower seas which increases the number of potential deployment sea areas compared to a conventional deeper draft mono-spar solution.

The facility to assemble the complete turbine in shallower waters requires an innovative structural configuration and assembly procedure which is described below.

Preferred features of the multifloat-spar for inshore assembly are: A multi-element semi-submerged flotation collar consisting of three or more radially spaced surface piercing buoyancy elements supporting a centre spar housing column that is open at both ends is launched into the sea and towed into deeper water where it is moored adjacent to a jack-up crane barge. In very sheltered waters the jack-up crane barge can be replaced by a floating crane barge with moorings or spud legs.

A centre spar tube that is lowered into the open ended centre spar housing column. The centre spar tube has a bolted flange connection to the top of the centre column. The centre spar tube is lifted and lowered into the centre column without ballast fitted. The centre spar tube has free flooding hole(s) or valve(s) in its base to allow seawater to flood into the tube as it is lowered into the centre column.

The free flood hole(s) or valve(s) are closed off and the centre spar tube is then ballasted with some fluid medium (e.g. sand, concrete, drilling mud, etc.) that is pumped into the centre tube from an adjacent barge or by direct suction off the seabed. Alternatively the centre spar tube is ballasted with solid or bagged ballast material that is lowered into the spar tube from the top opening.

The centre tube may be subdivided by watertight flats that inhibit the flooding of adjacent watertight spaces so as to prevent excessive sinkage of the assembly should any space accidentally be flooded. The flats have openings that are closed off and made watertight after the ballast medium has been placed into the lower compartment within the subdivided centre tube.

An inflatable collar is inflated inside the annulus between the centre spar housing column and the centre spar tube. The void space above the collar is filled with grout in order to fix the tube centrally in the column and to transmit transverse loads between the tube and the column.

Once the grout has set the wind turbine tower is lowered onto an upper flange face on the top of the centre spar tube. The wind turbine tower is bolted up to the centre tube. The nacelle and blades are then fitted to complete the assembly of the wind turbine. Commissioning of the complete system can be carried out at the assembly site before towing the unit to its offshore wind farm site where it is hooked up to its moorings and power export umbilical.

The freeboard of the buoyant columns of the flotation collar are sufficiently low when the assembly is fully fitted out and the mooring lines are connected such that larger waves can overtop the columns thus limiting the buoyancy restoring forces in high sea states. The limitation in freeboard of the buoyancy columns is derived from simulations of the motion responses of the complete moored assembly in an environment representative of the deployment sea area.

Brief Description of the Drawings

In the drawings, in which Figures 1 to 4 illustrate the current state of the art and Figures 5 to 9 illustrate preferred embodiments of the present invention; Figure 1 is a schematic representation of a surface floating platform supporting a wind turbine; Figure 2 is a schematic representation of a semi-submerged floating platform supporting a wind turbine; Figure 3 is a schematic representation of a tension tethered buoyant platform supporting a wind turbine; Figure 4 is a schematic representation of a mono-spar floating platform supporting a wind turbine; Figure 5 is a schematic representation of a multifloat-spar floating platform supporting a wind turbine; Figure 5a is a schematic representation of another embodiment of a multifloatspar floating platform supporting a wind turbine; Figure 6 is a schematic representation of a typical configuration of the centre column supported by radially mounted surface piercing floats; Figure 7 is a schematic representation of the centre spar tube fitted into the centre column.

Figure 8 is a schematic representation of the complete assembly fitted with wind turbine tower, nacelle and blades.

Figure 9 is a schematic representation of the complete assembly fitted with a transformer station.

Current State of the Art Figure 1 shows a surface floating turbine support platform where the centre of gravity (1) is vertically above the centre of buoyancy (2). Such a platform achieves the necessary stability to support a wind turbine by metacentric stability.

Figure 2 shows a semi-submerged floating platform where the centre of gravity (1) is vertically above the centre of buoyancy (2). Such a platform achieves the necessary stability to support a wind turbine by metacentric stability.

Figure 3 shows a tension tethered buoyant platform where the centre of gravity (1) is vertically above the centre of buoyancy (2). Such a platform achieves the necessary stability to support a wind turbine by the tension in the tethers.

Figure 4 shows a mono-spar floating platform where the centre of gravity (1) is vertically below the centre of buoyancy (2). Such a platform achieves the necessary stability to support a wind turbine by pendulum stability.

Detailed Description of the Preferred Embodiments

Figure 5 shows the preferred embodiment of a multifloat-spar floating platform. Such a platform achieves the necessary stability to support a wind turbine by generating a restoring moment as the device heels under the action of environmental loads on the floating platform and wind turbine by a combination of pendulum stability from the overall centre of gravity (1) being below the overall centre of buoyancy (2) augmented by metacentric stability due to the lateral movement of the centre of buoyancy caused by the immersion of the surface piercing flotation members (3) as the unit heels.

The multifloat-spar floating platform illustrated in Figure 5a differs from that shown in Figure 5 only insofar as in the embodiment illustrated in Figure 5a the overall centre of gravity (1) is above the overall centre of buoyancy (2). This arrangement generates a restoring moment as the device heels through the action of metacentric stability from the lateral movement of the centre of buoyancy of the flotation collar. The restoring moment due to the lateral movement of the centre of buoyancy is made more effective by the centre spar lowering the position of the overall centre of gravity though not necessarily below the position of the centre of buoyancy.

Figure 6 shows an open ended centre column (4) supported by the buoyancy of radially positioned buoyancy elements (3), in this case four such elements, the minimum number required being three. The centre column is attached to the buoyancy elements by support struts (5). The complete assembly will freely float at a waterline (6). As wind turbine components are added to the multifloat-spar the immersion will increase but the dimensions of the buoyancy elements is sized such that they prevent total immersion.

Figure 7 shows how the tubular member (7) is positioned inside the centre column (4) such that its external flange (8) abuts with the flange (4a) on the centre column. The tubular member can optionally be fitted with a stabbing guide (9) to aid alignment with the centre column (4) when it is being lowered into place. The gap between the centre column (4) and the tubular member (7) is sealed off by an inflatable collar (10) such that grout (11) injected into the annulus is retained while it sets. The tubular member has horizontal watertight flat or flats (12) which after the tubular member is filled with a ballasting medium (13) are sealed off by watertight cover plate or plates (14). A closable opening or valve (15) is installed low down in the tubular member to allow the tube to free flood when being immersed as it is lowered through the centre column.

The complete assembly of radially positioned floats supported off a centre column by support struts and fitted with a ballast filled tubular spar member is referred to as the multifloat-spar.

Figure 8 shows the configuration of the assembled wind turbine on the multifloat-spar. The turbine support tower (16) is bolted to the top flange of the tubular member (7). The multifloat-spar is now more deeply immersed due the added weight of the tubular member (7), ballast medium (13), turbine support tower (16), turbine nacelle (17) and turbine blades (18) as is represented by the new position of the waterline (6). The assembled unit is then towed to its deep water deployment site where it is hooked up to its mooring system (19).

Figure 9 shows the configuration wherein a transformer station for the export of electricity from a wind farm is supported by the multifloat-spar. The transformer station (20) is mounted on a structure that is bolted to the top flange of the tubular member (7)

Claims

Claims 1. Apparatus for supporting a device in a body of water, the apparatus comprising surface piercing buoyancy elements radially spaced apart from a spar element having two ends, wherein one end of the spar element extends below the surface piercing buoyancy elements, the other end supporting the device, the spar element and the surface piercing buoyancy elements being in a fixed geometric relationship.
2. Apparatus for supporting a device according to Claim 1, wherein the centre spar element is ballasted such that the overall centre of gravity of the complete assembly including the supported device and the overall centre of buoyancy of the complete assembly are in such proximity that the apparatus is stable when upright and has a range of stability through which environmental forces on the complete assembly are counteracted.
3. Apparatus for supporting a device according to Claim 1 or 2, wherein the overall centre of gravity of the complete assembly is below the overall centre of buoyancy of the complete assembly or wherein the overall centre of gravity of the complete assembly is above the overall centre of buoyancy of the complete assembly or wherein the overall centre of gravity of the complete assembly and the overall centre of buoyancy of the complete assembly are substantially coincident.
4. Apparatus for supporting a device according to any preceding claim, comprising a spar housing, the surface piercing buoyancy elements attached to the housing and the housing configured to receive the spar.
5. Apparatus according to Claim 4, comprising attachment means configured for attachment of the spar to the spar housing.
6. Apparatus according to Claim 5, wherein the attachment means includes a bolting flange connected to the housing.
7. Apparatus according to any of Claims 4 to 6, wherein the spar housing has two ends and each end is open, the spar housing providing a through passageway for receiving the spar.
8. Apparatus according to any of Claims 4 to 7, further comprising seal means configured for mounting between the spar and the spar housing.
9. Apparatus according to Claim 8, wherein the seal means comprises an inflatable collar.
10. Apparatus according to Claim 8 or 9, comprising a grout material for introduction between the spar and the spar housing to make a structural connection between the spar and the spar housing.
11. Apparatus according to any preceding claim, wherein the attachment means includes a bolting flange connected to the spar.
12. Apparatus according to any preceding claim, wherein the spar is configured for receiving ballast.
13. Apparatus according to any preceding claim, wherein the spar includes openings therein to permit free flooding of the spar.
14. Apparatus according to Claim 13, wherein the spar includes a valve or valves to control flow through the free flood openings.
15. Apparatus according to any of Claims 12 to 14, wherein the spar includes at least one compartment selectively sealable against ingress or egress of fluid therefrom.
16. Apparatus according to any of Claims 4 to 15, wherein the lower end of the spar is provided with at least one guide member for guiding the spar into the housing.
17. Apparatus according to any preceding claim, wherein the device includes a tower and the tower is attached to the spar.
18. Apparatus according to any Claim 17, wherein the apparatus is configured such that when assembled, ballasted and moored, the free board of the surface piercing buoyancy elements above the static waterline is such that larger waves may overtop the buoyancy elements.
19. Apparatus according to any preceding claim, wherein the device is a wind turbine, or a transformer station.
20. A method for assembling an apparatus for supporting a device in a body of water comprising the steps of: i. launching an apparatus according to any preceding claim when dependent on Claim 4 into a body of water; ii. lowering a free flooding spar into the spar housing and seating and bolting the spar on the spar housing; iii. introducing ballast into the spar; iv. sealing the space between the spar and the spar housing and structurally connecting the spar to the spar housing.
21. A method according to Claim 20, wherein the step of introducing ballast into the spar comprises introducing flowable material into the spar 22. A method according to Claim 20, wherein the flowable material comprises concrete or sand or drilling mud or other pumpable fluid for ballasting purposes which is introduced into the spar through the top opening.23. A method according to Claim 20, wherein the step of introducing ballast into the spar comprises lowering solid or bagged ballast material into the spar through the top opening.24. A method according to any of Claims 20 to 23, wherein the step of sealing the space between the spar and the spar housing comprises inflating an inflatable collar in the said space.25. A method according to any of Claims 20 to 24, wherein the step of making a structural connection between the spar and the spar housing comprises the step of filling the said space with structural grout.26. A method according to any of Claims 20 to 25, comprising the further step of attaching a device to the spar 27. A method according to Claim 25, comprising the further step of towing the assembled wind turbine to a site in a body of water.28. An apparatus substantially as shown in, and as described with reference to Figures 5 to 8 of the Drawings.29. An apparatus substantially as shown in, and as described with references to Figures 5 to 8 of the Drawings, where the wind turbine is replaced with a transformer station.Claims 1. Apparatus for supporting a device in a body of water, the apparatus comprising surface piercing buoyancy elements radially spaced apart from a spar element having two ends, wherein one end of the spar element extends below the surface piercing buoyancy elements, the other end being adapted to support the device, the spar element and the surface piercing buoyancy elements being in a fixed geometric relationship, and comprising a spar housing, the surface piercing buoyancy elements being attached to the housing and the housing configured to receive the spar, wherein the spar housing has two ends and each end is open, the spar housing providing a through passageway for receiving the spar, the apparatus further comprising seal means configured for mounting between the spar and the spar housing.Apparatus for supporting a device according to Claim 1, wherein the C\I spar element is ballasted such that the overall centre of gravity of the complete C\I assembly including the supported device and the overall centre of buoyancy of the CD complete assembly are in such proximity that the apparatus is stable when upright and has a range of stability through which environmental forces on the complete assembly are counteracted.3. Apparatus for supporting a device according to Claim 1 or 2, wherein the overall centre of gravity of the complete assembly is below the overall centre of buoyancy of the complete assembly or wherein the overall centre of gravity of the complete assembly is above the overall centre of buoyancy of the complete assembly or wherein the overall centre of gravity of the complete assembly and the overall centre of buoyancy of the complete assembly are substantially coincident.4. Apparatus according to Claim 1, comprising attachment means configured for attachment of the spar to the spar housing.5. Apparatus according to Claim 4, wherein the attachment means includes a bolting flange connected to the spar housing.6. Apparatus according to Claim 1, wherein the seal means comprises an inflatable collar.7. Apparatus according to Claim 1 or 6, comprising a grout material for introduction between the spar and the spar housing to make a structural connection between the spar and the spar housing.8. Apparatus according to Claims 4 to 7, wherein the attachment means includes a bolting flange connected to the spar.9. Apparatus according to any preceding claim, wherein the spar is configured for receiving ballast.10. Apparatus according to any preceding claim, wherein the spar C\I includes openings therein to permit free flooding of the spar.O11. Apparatus according to Claim 10, wherein the spar includes a valve or valves to control flow through the free flood openings.12. Apparatus according to any of Claims 9 to 11, wherein the spar includes at least one compartment selectively sealable against ingress or egress of fluid therefrom.13. Apparatus according to any of Claims 1 and 4 to 12, wherein the lower end of the spar is provided with at least one guide member for guiding the spar into the spar housing.14. Apparatus according to any preceding claim and comprising a device supported by the apparatus, wherein the device includes a tower and the tower is attached to the spar.15. Apparatus according to Claim 14, wherein the apparatus is configured such that when assembled, ballasted and moored, the free board of the surface piercing buoyancy elements above the static waterline is sufficiently low that larger waves can overtop the buoyancy elements limiting the buoyancy restoring forces in high sea states, wherein the limitation in freeboard of the surface piercing buoyancy elements is derived from simulations of the motion responses of the complete moored assembly in an environment representative of a sea area where the apparatus is to be deployed.16. Apparatus according to any preceding claim, wherein the device is a wind turbine, or a transformer station.17. A method for assembling an apparatus for supporting a device in a body of water comprising the steps of: C\I launching an apparatus according to any preceding claim when dependent on Claim 1 into a body of water; CD lowering a free flooding spar into the spar housing and seating and bolting the spar on the spar housing; introducing ballast into the spar; iv. sealing the space between the spar and the spar housing and structurally connecting the spar to the spar housing.18. A method according to Claim 17, wherein the step of introducing ballast into the spar comprises introducing flowable material into the spar.19. A method according to Claim 18, wherein the flowable material comprises concrete or sand or drilling mud or other pumpable fluid for ballasting purposes which is introduced into the spar through a top opening.20. A method according to Claim 17, wherein the step of introducing ballast into the spar comprises lowering solid or bagged ballast material into the spar through a top opening.21. A method according to any of Claims 17 to 20, wherein the step of sealing the space between the spar and the spar housing comprises inflating an inflatable collar in the said space.
22. A method according to any of Claims 17 to 21, wherein the step of making a structural connection between the spar and the spar housing comprises the step of filling the said space with structural grout.
23. A method according to any of Claims 17 to 22, comprising the further step of attaching a device to the spar.
24. A method according to Claim 23, wherein the device is a wind turbine.C\I
25. A method according to Claim 24, comprising the further step of towing CD the assembled wind turbine to a site in a body of water.
26. An apparatus substantially as shown in, and as described with reference to Figures 5 to 8 of the Drawings.
27. An apparatus substantially as shown in, and as described with references to Figures 5 to 8 of the Drawings, where the wind turbine is replaced with a transformer station.