OFFSHORE SEMI-SUBMERSIBLE PLATFORM FOR SUPPORTING A WIND TURBINE AND OFFSHORE ELECTRICAL ENERGY PRODUCTION FACILITY

An offshore semi-submersible platform includes: at least three stabilizing columns; a truss structure securing the at least three stabilizing columns to one another; for at least one of the stabilizing columns, a substantially horizontal perforated plate and a fastening arranged for fastening the substantially horizontal perforated plate to the stabilizing column at least in a working position below a bottom surface of the stabilizing column, creating a wave load attenuation chamber being defined between the substantially horizontal perforated plate in the working position and the bottom surface of the stabilizing column.

This application claims the benefit of European Patent Application No. EP 20315401.8 filed on Sep. 1, 2020, which is hereby incorporated by reference.

TECHNICAL FIELD

The present invention concerns offshore wind turbines supported by floating semi-submersible platforms.

BACKGROUND

U.S. Pat. No. 9,446,822 discloses a floating wind turbine platform with three stabilizing columns, each equipped with a planar water entrapment plate attached to the bottom portion of the column. The planar water entrapment plate allows damping the roll, pitch and heave motions of the floating platform.

However, said motions are only partially dampened.

As a consequence, a need exists for an attenuation system that would allow a more efficient reduction of the roll, pitch and heave motions of the floating platform.

SUMMARY

To that end, the invention is directed according to a first aspect to an offshore semi-submersible platform for supporting a wind turbine, the offshore semi-submersible platform comprising:at least three stabilizing columns;a truss structure securing the at least three stabilizing columns to one another;for at least one of the stabilizing columns, a substantially horizontal perforated plate and a fastening arranged for fastening the substantially horizontal perforated plate to the stabilizing column at least in a working position below a bottom surface of the stabilizing column, creating a wave load attenuation chamber being defined between the substantially horizontal perforated plate in said working position and the bottom surface of the stabilizing column.

The attenuation chambers below the stabilizing columns help reducing heave, roll and pitch movements through combination of added mass and damping, viscous damping through perforated plates and reduction of combined wave loads acting on columns bottom slabs and perforated bottom plates through time phase shifts of pressure loads. In some instances, it avoids using active ballast control.

The offshore semi-submersible platform according to the invention may comprise one or more of the following features, considered alone or according to any technically possible combinations:the stabilizing column has a defined column section taken perpendicularly to a center axis of said stabilizing column, said defined section presenting a maximum column width, the wave load attenuation chamber having a chamber height taken along said center axis, a ratio between said chamber height and said maximum column width being comprised between 0.2 and 2;the chamber height is comprised between 3 and 25 meters;the substantially horizontal perforated plate has a maximum plate width, the ratio between the maximum plate width and the maximum column width being comprised between 0.8 and 1.4;at least one additional perforated plate is arranged in the wave load attenuation chamber, providing additional damping and attenuation of the wave load;the at least one additional perforated plate is substantially horizontal, or substantially vertical, or extends in a plane inclined with respect to both the vertical direction and the horizontal direction;the fastening is arranged for fastening the substantially horizontal perforated plate to the stabilizing column in a retracted position closer to the bottom surface of the stabilizing column than the working position;the fastening comprises at least three beams, each beam being secured to the substantially horizontal perforated plate and being connected to a lateral surface of the stabilizing column;the at least three beams comprise at least three mains beams, the fastening comprising a sliding connection of each main beam to the lateral surface of the stabilizing column;one of the beam and the lateral surface of the stabilizing column comprises a fastener, the other of the beam and the lateral surface of the stabilizing column bearing an upper complementary fastener and a lower complementary fastener, cooperating with the fastener for fastening the substantially horizontal perforated plate in the working position and in the retracted position respectively;each stabilizing column has a circular or rectangular cross section;the substantially horizontal perforated plate bears a ballast;the at least three stabilizing columns comprise a main stabilizing column for bearing the wind turbine, said main stabilizing column having a larger size than the other stabilizing columns;the at least three stabilizing columns comprise exactly three stabilizing columns arranged at the apexes of a triangle;the at least three stabilizing columns comprise exactly four stabilizing columns, with three secondary stabilizing columns in addition to the main stabilizing column, the three secondary stabilizing columns being arranged at the apexes of a triangle and the main stabilizing column at a geometrical center of said triangle;the at least three stabilizing columns comprise exactly five stabilizing columns, with four secondary stabilizing columns in addition to the main stabilizing column, the four secondary stabilizing columns being arranged at the apexes of a square and the main stabilizing column at a geometrical center of said square.

The invention also relates to an offshore electrical energy production facility, comprising:a floating offshore semi-submersible platform having the features above;a wind turbine mounted on one of the at least three stabilizing columns.

DETAILED DESCRIPTION OF EMBODIMENTS

The facility comprises a wind turbine12and a floating offshore semi-submersible platform14.

The floating offshore semi-submersible platform14comprises:at least three stabilizing columns16,18;a truss structure20securing the at least three stabilizing columns16,18to one another.

The wind turbine12is mounted on one of the at least three stabilizing columns. The stabilizing column bearing the wind turbine12will be designated as the main stabilizing column16in the description below. The other stabilizing columns will be designated as the secondary stabilizing columns18in the description below.

The secondary stabilizing columns18are arranged for providing the optimum platform stability, both for quasi-static and dynamic conditions with minimum quantities (mass, size, etc).

The wind turbine12typically comprises a support mast21fastened to the upper part of the main stabilizing column16, a nacelle22positioned at the apex of the mast21, and a rotor24including blades26and fastened on a shaft rotating in bearings installed in the nacelle22.

Such a wind turbine12is known by those skilled in the art and will not be disclosed in more detail hereinafter.

The floating offshore semi-submersible platform14is for example suitable for being positioned in a zone where the water depth is greater than 50 m.

The floating platform14is anchored to the seabed by a mooring system, not shown.

The mooring system for example comprises at least three anchor lines coupling each column to an anchor positioned on the seabed.

Each column16,18respectively extends along a center axis A. When the column has a cylindrical shape, the center axis coincides with the axis of the cylinder.

The center axes A are for example parallel to one another.

Each center axis A in particular extends substantially parallel to the axis along which the mast20of the wind turbine12extends.

The center axis A of each column16,18is substantially vertical, but its incline relative to the horizontal varies as a function of weather conditions, such as the swell of the sea or the wind.

Each stabilizing column16,18is delimited by an outer surface30.

The outer surface30comprises a bottom surface32turned downward, and a top surface34turned upward.

The top and/or bottom surfaces are typically substantially perpendicular to the center axis A.

The outer surface30has a lateral surface36, extending from the bottom surface to the top surface. The lateral surface is cylindrical, and surrounds the center axis A.

As illustrated inFIGS. 1 and 2, each stabilizing column16,18comprises at least one shell38. The shell defines the outer surface30. It is at least partially empty, for providing buoyancy to the semi-submersible platform.

The lower part of each stabilizing column16,18is immersed in the water, while the upper part of each stabilizing column16,18extends above the sea level.

For at least one of the stabilizing columns16,18, the semi-submersible platform14comprises a substantially horizontal perforated plate40and a fastening42arranged for fastening the substantially horizontal perforated plate40to the stabilizing column16,18at least in a working position below the bottom surface32of the stabilizing column.

A wave load attenuation chamber44is defined between the substantially horizontal perforated plate40in said working position and the bottom surface38of the stabilizing column16,18.

Typically, the semi-submersible platform14comprises a substantially horizontal perforated plate40and a wave load attenuation chamber44for each secondary stabilizing columns18.

Preferentially, the semi-submersible platform14comprises such a substantially horizontal perforated plate40and a wave load attenuation chamber44for the primary stabilizing column16as well.

The wave load attenuation chambers44improve the hydrodynamic response of the platform to the wave load.

The perforated plate40is said to be substantially horizontal since it extends in a plane substantially perpendicular to the center axis A of the stabilizing column. The perforated plate40may form a small angle relative to the horizontal due to the swell of the sea or the wind. When the weather conditions are calm, the perforated plate40is substantially horizontal.

The perforated plate40is substantially parallel to the bottom surface32.

The perforated plate40comprises an upper surface and a lower surface at the opposite of the upper surface. The upper surface is oriented towards the wave load attenuation chamber44. The lower surface is oriented towards the bottom of the sea.

The perforated plate40forms a lower free end of the wave load attenuation chamber40facing directly the bottom of the sea.

The perforated plate40has an average porosity range of 10%-50%, preferentially 20%-40%. The porosity is defined here as the ratio between the free section of the perforations of the perforated plate, and the overall section of the perforated plate.

The perforated plate40has two large faces46,48, turned downward and upward respectively. As shown on theFIG. 9, the perforated plate40comprises a number of perforations50, opening in both large faces46,48. The seawater is able to circulate across the perforated plate40through the perforations50.

The perforations50have a regular or irregular section. For example, they have a circular section.

The perforations50all have the same section. They have the same shape and the same dimensions. Alternatively, the perforations50have different sections. For example, a number of perforations have a circular section and a number of perforations have non-circular sections. According to another example, a number of perforations have a relatively smaller size and a number of perforations have a relatively larger size.

The shape and the size of the perforations50as well as the thickness of the perforated plate40are optimized in order to both increase the viscous losses through the perforations and shift in time the peak pressure loads acting though the plates and column bottom slab, reducing therefore the overall concomitant wave load, given the average porosity range. They are optimized though CFD and basin tests.

The perforations50are distributed over at least 80% of the surface of the perforated plate40, preferentially over at least 90% of the surface of the perforated plate40, and if possible, over all the surface of the perforated plate40.

As shown on theFIG. 9, the only areas of the perforated plate40that do not bear perforations50are along the edge of the plate, and where the fastening42is secured to the plate40.

Preferentially, the perforations50are regularly distributed over the perforated plate40. In other words, the spacing between the perforations50is identical across all the plate40.

For example, the perforations50are arranged according to a grid having a square mesh, the perforations50being located at the intersections between the rows and the columns of the grid as shown on theFIG. 9.

The height of the attenuation chamber44, taken along the center axis A, is chosen in consideration of the section of the stabilizing column16,18. The chamber height corresponds to the spacing between the perforated plate40and the bottom surface32of the stabilizing column.

The stabilizing column16,18has a defined column section, taken perpendicularly to the center axis A of said stabilizing column. Said defined section presents a maximum column width. If the column section is circular, the maximum column width is the diameter of the circular section. If the column section is rectangular, the maximum column width is the diagonal of the rectangular section.

The chamber height is chosen such that a ratio between said chamber height and said maximum column width is comprised between 0.2 and 2, preferentially between 0.3 and 1.

The chamber height is comprised between 3 and 25 meters, preferentially between 5 and 15 meters.

The chamber height is usually the same for all the attenuation chambers44.

The perforated plates40thus extend in the same substantially horizontal plane.

When a stabilizing column is not equipped with a dissipation chamber, typically the main stabilizing column, the bottom plates of the attenuation chamber of the other columns are aligned horizontally with the draft of said main stabilizing column. In other words, the bottom surface32of said stabilizing column and the perforated plates40of the other columns extend substantially in the same horizontal plane.

The geometry of the attenuation chambers is optimized to maximize their efficiency for both the reduction of the peak vertical wave loads as well as the columns induced motions. This is achieved by appropriately choosing the combination of the distance between the perforated plate40and the bottom slab of the column above, the extent and perforations of the plate40(porosity index, arrangement of perforations). These parameters are calibrated though numerical fluid modelling (CFD) and basin tests.

The perforated plates40and the attenuation chambers44of the secondary stabilizing columns18are typically all identical.

The perforated plate40and the attenuation chamber44of the main stabilizing column16, is usually different from those of the secondary stabilizing columns18.

Advantageously, at least one additional perforated plate is arranged inside the attenuation chamber44(FIG. 3). It is arranged for providing additional viscous damping of the wave load and further improve hydrodynamic efficiency.

The at least one additional perforated plate is substantially horizontal, or substantially vertical, or extends in a plane inclined with respect to both the vertical direction and the horizontal direction.

For example, one or several additional horizontal perforated plate(s)52is/are arranged at intermediate level(s) between the bottom surface32of the stabilizing column and substantially the horizontal perforated plate40, to improve hydrodynamic efficiency.

Alternatively, or additionally, one or several vertical perforated plates54are arranged for reducing the first order wave induced horizontal movements though appropriate wave load reduction (damping and reduction of peak pressure loads), creating perforated sub-chambers at the base of the stabilizing columns.

The fastening42is arranged for fastening the substantially horizontal perforated plate40to the stabilizing column16,18in a retracted position closer to the bottom surface32of the stabilizing column than the working position.

In other words, the fastening42is adapted for allowing the substantially horizontal perforated plate40to be fastened to the stabilizing column16,18in two different positions: the working position and the retracted position.

The working position is depicted on theFIGS. 1, 3, 5, 7, and 10. The retracted position is depicted on theFIG. 11.

In the retracted position, the draft of the floating platform14is reduced. Therefore, the retracted position is particularly adapted during the floating construction and the assembly stages of the platform14and the production facility10. The working position provides optimum hydrodynamic efficiency and is particularly adapted during the active energy producing operation.

The fastening42comprises at least three vertical beams, each beam being secured to the substantially horizontal perforated plate40and being connected to the lateral surface36of the stabilizing column16,18.

The at least three beams comprise at least three mains beams56.

The fastening42comprises a sliding connection58of each main beam56to the lateral surface36of the stabilizing column.

The main beams56cooperate with the sliding connections58for guiding the perforated plate40when the perforated plate is moved between its retracted position and its working position.

The number of main beams56is chosen such that said guiding can be performed satisfactorily, without the plate becoming inclined with respect to the horizontal.

Typically, the at least three beams comprise exactly three mains beams56angularly shifted with respect to one another around the center axis A of the stabilizing column.

The at least three beams comprise as well several secondary beams60.

The number of secondary beams60is chosen such that the primary and secondary beams, together, securely fasten the perforated plate to the stabilizing column, in the working position whereas the primary beams secure the plate(s) in the retracted position.

Typically, the number of secondary beams60is chosen such that the total number of beams is comprised between 3 and 12, preferentially between 4 and 10, typically between 6 and 8.

For example, the fastening42comprises three primary beams56and five secondary beams60.

The beams56,60are regularly angularly shifted with respect to one another around the center axis A of the stabilizing column. If possible, they are arranged with the same number of secondary beams60between two primary beams56, plus or minus one beam.

In the example depicted on the figures, two secondary beams60are interposed between the first and the second primary beams56, two secondary beams60are interposed between the second and the third primary beams56, and one secondary beam60is interposed between the third and the first primary beams56.

The beams56,60are metallic beams, for example with a circular, 20 inches diameter (about 70 cm).

The beams56,60are substantially parallel to the center axis A of the stabilizing column, and are parallel to one another.

The lower end62of each beam is rigidly secured to the perforated plate40. The sliding connection58comprises a sleeve secured to the lateral surface36of the stabilizing column, in which the main column56is slidingly received.

One of the beam56,60and the lateral surface36of the stabilizing column comprises a fastener64, the other of the beam56,60and the lateral surface36of the stabilizing column bearing an upper complementary fastener66and a lower complementary fastener68, cooperating with the fastener64for fastening the substantially horizontal perforated plate40in the working position and in the retracted position respectively.

In the example depicted on theFIGS. 10 and 11, the lateral surface36of the stabilizing column bears one fastener64for each beam56,60.

Each beam56,60bears one upper complementary fastener66and one lower complementary fastener68.

The upper complementary fastener66is located above the lower complementary fastener68.

The fasteners and the complementary fasteners are of any adapted type. For example, the fastener comprises a flange and the complementary fastener comprises another flange adapted for being bolted to the flange of the fastener.

In a variant (not depicted on the figures), each beam56,60bears one fastener64. The lateral surface36of the stabilizing column bears one upper complementary fastener66and one lower complementary fastener68for each beam.

For moving the perforated plate40between the retracted position and the working position, the floating platform14comprises, for each main beam56, a winch69and a cable or a chain70connected to the upper end71of the main beam56(FIG. 11).

In the example depicted on theFIG. 11, the winch69is secured on the top surface34of the stabilizing column. The cable or chain70runs substantially horizontally from the winch69to a deflecting pulley72arranged at the edge of the top surface34, and vertically from the deflecting pulley72to the upper end71of the main beam56.

For lowering the perforated plate40from its retracted position to the working position, the winches69associated to all the main beams56are actuated, such that the cables or chains70are simultaneously unwinded.

For lifting the perforated plate40from its working position to the retracted position, the winches69associated to all the main beams56are actuated, such that the cables or chains70are winded.

The winches69are removably fastened to the stabilizing column. When the lowering/lifting operations are completed, the winches are removed.

The fastening42is designed for the external beams56,60to distribute evenly loads acting on the perforated plate(s) of the attenuation chamber44.

The main beams56are longer than the secondary beams60.

The length of the secondary beams60is such that the secondary beams60remain submersed below the sea level both in the working position and in the retracted position of the perforated plate40.

The length of the main beams56is such that the upper ends71of the main beams56remain above the sea level both in the working position and in the retracted position of the perforated plate40.

In the retracted position, the upper ends71of the main beams56remain below the top surface34, and do not protrude above the stabilizing column.

Typically, the stabilizing columns16,18have a circular cross section, taken perpendicularly to the center axis (FIGS. 2, 6, 8, 10). According to another variant, the stabilizing columns16,18have a rectangular cross section, possibly with rounded corners (FIG. 3). According to another variant, the stabilizing columns16,18have a cross section of any other shape. The shape of the cross section is determined based on a trade off between construction cost and hydrodynamic efficiency.

The substantially horizontal perforated plate40has a shape identical to the cross section of the stabilizing column16,18. If the cross section is circular, the perforated plate40is circular. If the cross section is rectangular, the perforated plate40is rectangular.

The ratio between the maximum plate width and the maximum column width is comprised between 0.8 and 1,4, preferably 1 and 1.2.

If the plate is circular, the maximum plate width is the diameter of the circular plate. If the plate is rectangular, the maximum plate width is the diagonal of the rectangular plate.

As a consequence, the perforated plate does not extend far beyond the outer perimeter of the stabilizing column, and the quay side mooring and lifting of masts and turbine with quay crane is facilitated.

As indicated above, the stabilizing columns16,18have their center axis A substantially vertical. Alternatively, the secondary stabilizing columns18have their center axis A inclined with respect to the vertical direction.

The main stabilizing column16is of larger volume than the secondary stabilizing columns18, to sustain the weight of the wind turbine12.

The truss structure20securing the at least three stabilizing columns16,18to one another is advantageously as defined in the French patent application filed under number FR1874136.

The truss structure20comprises beams74connected to the stabilizing columns though gussets and shear plates (FIG. 10).

The shell38of each stabilizing column comprises an upper slab and a lower slab, not apparent on the figures, defining the top and bottom surfaces34,36respectively.

The beams74are secured to the upper and lower slabs, as shown onFIGS. 3 and 5. Alternatively, the beams74are secured to the upper slabs and to the perforated plates40of dissipation chambers, as shown on theFIG. 1.

The truss structure20is designed to minimize wave loads acting on the truss structure while transferring restoring loads between stabilizing columns to ensure overall stability of the floating facility.

In the examples depicted on theFIGS. 1 to 4, 7 and 8, two given stabilizing columns are connected to one another by two main beams74. Each main beam is a tubular, cylindrical profile, having a large circular section.

Alternatively, the tubular main beams74are replaced by main beams made of a lattice of small sections profiles, as shown on theFIGS. 5 and 6.

Several examples of the overall arrangement of the stabilizing columns16,18in the offshore semi-submersible platform14are described below.

According to a first embodiment, the at least three stabilizing columns comprise exactly three stabilizing columns arranged at the apexes of a triangle.

It comprises one main stabilizing column16, and two secondary stabilizing column18.

The three stabilizing columns16,18are arranged for example as described in the French patent application filed under the number FR1874136.

The three stabilizing columns16,18have circular cross sections (FIGS. 1 and 2).

In the second embodiment, depicted on theFIGS. 3 and 4, the at least three stabilizing columns comprise exactly three stabilizing columns arranged at the apexes of a triangle. It comprises one main stabilizing column16, and two secondary stabilizing column18.

The three stabilizing columns16,18are arranged for example as described in the French patent application filed under the number FR1874136.

The three stabilizing columns16,18have rectangular cross sections with rounded corners (FIGS. 3 and 4).

According to a third embodiment, the at least three stabilizing columns comprise exactly four stabilizing columns, with three secondary stabilizing columns18in addition to main stabilizing column16.

The three secondary stabilizing columns18are arranged at the apexes of a triangle and the main stabilizing column16is located at the geometrical center of said triangle. The triangle is for example substantially equilateral.

According to a fourth embodiment, the at least three stabilizing columns comprise exactly five stabilizing columns, with four secondary stabilizing columns18in addition to main stabilizing column16.

The four secondary stabilizing columns18are arranged at the apexes of a square and the main stabilizing column16is located at the geometrical center of said square.

In all the embodiments, the stabilizing columns, especially their sizes, are designed to reduce the need for permanent ballasting.

Ballasting is achieved with water or with a high-density solid material such as concrete, orecrete or any other adapted material.

The ballast76is located at the base of the stabilizing column, inside the shell38(FIGS. 3 and 5).

Alternatively, the substantially horizontal perforated plate40bears the ballast76(FIGS. 1, 5, 7). In this case, the ballast is arranged at the top of the perforated plate40, or is the perforated plate40.

According to another alternative, a part of the ballast76is located inside the stabilizing column, and another part of the ballast76is born by the perforated plate.

The secondary stabilizing columns are usually loaded with the same quantity of ballast76. The primary stabilizing column is usually loaded with a different quantity of ballast76(FIGS. 1, 3 and 5).

According to an example depicted on theFIG. 5, the main stabilizing column is loaded with liquid ballast76, whereas the secondary stabilizing columns are loaded with solid ballast76.

The ballast76improves the overall stability though lowered metacentric height.

The stabilizing column16,18are made of reinforced steel pates (offshore standard), of structural prestressed reinforced concrete, with normal density concrete or light weight concrete. The truss structure20is made of steel or concrete.

The perforated plate40is made of steel, concrete or a combination thereof.

The material selections are made at the project design stage to optimize the fabrication and transportation costs with the project specifics, while achieving the project criteria related to maximum platform motions

According to a variant, the fastening42of the perforated plate40to the corresponding stabilizing column does not comprise beams connected to the side surface36of the stabilizing column. Instead, the fastening42comprises a truss structure connecting the perforated plate40to the bottom surface32of the stabilizing column.