NL2031010B1

NL2031010B1 - A method and system of installing a floating foundation, assembly of floating foundation and ballasting frame, and ballasting frame

Info

Publication number: NL2031010B1
Application number: NL2031010A
Authority: NL
Inventors: Antonius Alphonsus Geene Paul; Poppelaars Benno; Franciscus Hubertus Bögels Erik
Original assignee: Heerema Marine Contractors Nl
Priority date: 2022-02-18
Filing date: 2022-02-18
Publication date: 2023-09-05

Abstract

The disclosure provides a method for installing a floating foundation in a body of water at a predetermined depth, the method comprising the steps of: providing a construction 5 vessel (2) comprising a hoisting system (4); providing a floating foundation (62) having a positive buoyancy; providing a ballasting frame (60) having a weight, using the hoisting system (4) to lift the ballasting frame (60) and move it to cover the floating foundation; connecting the ballasting frame to the floating foundation; using the hoisting system to lift the assembly of the ballasting frame (60) and the floating foundation (62) and move the 10 combination to an installation location; while the floating foundation (62) is unconnected to a sea floor (40), lowering the assembly into the water using the weight of the ballasting frame (60) to submerge the floating foundation (62) until the floating foundation reaches an installation depth; connecting the submerged floating foundation (62) to subsea anchors using one or more mooring lines (44), allowing the assembly of the frame (60) and the foundation 15 (62) to rise to an operating depth, wherein the foundation is submerged and the one or more mooring lines are under tension; and disconnecting the ballasting frame (60) from the floating foundation.

Description

P35511NLOO/LGO

A METHOD AND SYSTEM OF INSTALLING A FLOATING FOUNDATION, ASSEMBLY

OF FLOATING FOUNDATION AND BALLASTING FRAME, AND BALLASTING FRAME

FIELD OF THE INVENTION

The present invention relates to a method and system of installing a floating foundation, to an assembly of a floating foundation and a ballasting frame, and to a ballasting frame for installing a floating foundation. The foundation may be a floating foundation for a wind turbine.

BACKGROUND OF THE INVENTION

Traditionally, offshore wind turbines are installed on bottom-mounted foundations in relatively shallow water. A water depth of 40 to 50 m is normally considered the limit for such bottom-mounted foundations.

In many areas of the world, sufficient suitable offshore areas with water depths of 50 m or less are not available for deployment of offshore wind power to the desired extent. Here, floating foundations for wind turbines will be required.

A variety of different floating foundation concepts are possible for use with offshore wind turbines. The three primary concepts are spar buoys, semisubmersibles and TLPs (Tension

Leg Platforms). Each of these primary concepts has its advantages and limitations.

A spar buoy maintains stability from a deep draft combined with ballast. It is the simplest floating foundation concept, typically consisting of a simple air-filled, floating tube which is kept vertical in the water by ballasting at the bottom. Suitably dimensioned, a spar buoy can support the weight and loads from a large wind turbine while maintaining a near- vertical position. Typically, the function of the mooring lines is only to maintain position and preventing drifting. Some spar buoy designs seek to achieve additional benefits from taut mooring lines; these designs have not yet been tested in practice. The simplicity of the spar buoy concept makes it inherently attractive. However, the draft poses major challenges during the installation and transportation phases. Due to the motion of the sea it may be difficult to install wind turbines on floating foundations under ocean conditions at their final location.

Therefore, floating wind turbines may be installed at quayside using land-based cranes, or in sheltered waters using floating cranes. Spar buoys generally have drafts larger than 50 m, some designs even have drafts larger than 100 m, and this effectively prevents quayside wind turbine installation using land-based cranes. Therefore, wind turbines may be installed on spar buoy floating foundations in sheltered waters, such as deep fjords, using floating cranes.

While it is fairly easy in a few countries, e.g. Norway to find sheltered waters with sufficient depth to permit wind turbine installation using a floating crane, in many parts of the world such sheltered waters with sufficient depth are not available. Furthermore, even where such sheltered waters with sufficient depth are actually available in a region, the presence of ridges or shoals in the transportation corridor between the point of installation and the desired offshore locations will often effectively prevent the utilization of such sheltered waters for turbine installation.

These limitations caused by the deep draft of a spar pose a significant problem for the spar buoy concept.

An alternative is to install heavy ballast offshore to lower the centre of gravity, when the payload (for example a wind turbine) is installed offshore. Alternatively the WTG can be installed inshore.

WO2017157399 discloses a floating wind turbine comprising a hull, a wind turbine mounted on top of the hull and a counterweight suspended below the hull by means of counterweight suspension means, characterized in that - The counterweight comprises one or more counterweight buoyancy tanks; - The counterweight buoyancy tanks have dimensions such that when the internal volume is filled with air or another gas, the total buoyancy of the counterweight is close to or greater than its weight, making it capable of floating in a towing / maintenance position with moderate or no support in the vertical direction from the hull or other vessels; - When the counterweight buoyancy tanks are partly or completely flooded with water, the counterweight will sink to an installed position at a level determined by the counterweight suspension means; and - The counterweight suspension means are separately or jointly capable of transferring both forces and moments to the hull, thereby enabling the counterweight to stabilize the hull when the counterweight is in its installed position.

Drawbacks of the turbine disclosed in WO2017157399 relate for instance to the counterweight suspension means, which are relatively prone to unforeseen breakdown due to waves and currents. Offshore, this may result in prohibitive operating costs.

A semisubmersible floating foundation obtains stability from a large waterplane area at a moderate draft, in combination with ballast which ensures a relatively low centre of gravity.

The semisubmersible concept is not as simple as the spar buoy concept, but it has the advantage of shallow draft. The shallow draft allows turbine installation at quayside using land-based cranes, and it also poses few challenges during towing to the desired offshore location. As for the spar buoy concept, the function of the mooring lines is only to maintain position and preventing drifting. The relative simplicity of the semisubmersible concept makes it inherently attractive. However, the stability concept, which is based on differential buoyancy arising as a consequence of heel, leads to considerable heeling angles during turbine operation due to the overturning moment created by the large lateral forces acting on the turbine rotor.

WO2009/131826 discloses an arrangement whereby the heeling angle during turbine operation can be reduced with a ballast control system. The floating foundation is fitted with a set of pumps and valves that is used to redistribute water ballast between the three main columns comprising the stabilizing body of the foundation. Through redistribution of water ballast the overturning moment created by the large lateral forces acting on the turbine rotor can be offset by an overturning moment in the opposite direction created by the moveable ballast. The arrangement disclosed in WO2009/131826 has obvious disadvantages. Firstly, through the introduction of active sensor and pumping systems a new level of complexity is introduced, inherently violating the fundamental principle that due to the challenges in accessibility unmanned offshore structures should have as few active systems as possible.

Secondly, since the masses that need to be redistributed are significant, measured in hundreds or thousands of tons, the balancing system will be semi-static, typically with time constants on the order of minutes even when very large pumps are used. Consequently, transient changes in the overturning moment created by the large lateral forces acting on the turbine rotor cannot be balanced.

US 8118538 discloses an alternative way of reducing the heeling angle during turbine operation due to the overturning moment created by the large lateral forces acting on the turbine rotor. A counterweight is mounted some way below the floating platform, and it essentially acts as a keel. In further embodiments the counterweight is connected to adjustable anchor lines and also serves to tighten these lines. While the arrangement disclosed in US 8118538 serves the purpose of reducing the heeling angle, the mounting method outlined is complicated. Offshore operations include connecting the counterweight cables on a winch to the counterweight, following which the weight is lowered under the platform to take up the slack in the cables. Following this, assemblers will release the winch stops and complete lowering the weight below the platform to complete the installation. This sequence of events requires considerable efforts offshore, and it requires the platform to be fitted with winches having sufficient capacity to lower the counterweight in a safe manner. The complexity of the operations and the cost of the winches make this arrangement unattractive.

A TLP (Tension Leg Platform) obtains stability through a balance between excess buoyancy and mooring line tension. The TLP concept is not as simple as the spar buoy or the semisubmersible, since the installation method involves the submersion of the main buoyant part of the platform a certain distance below the surface prior to attachment to the tethers connected to the anchors. This submersion process may typically cause the platform to become unstable, since the waterplane area will not be of sufficient dimension and distribution to ensure stability.

The document "How to install a TLP Substructure for offshore Wind? TLPWIND® Case

Study", presented at the EWEA Offshore 2015 Conference by Iberdrola Ingeneria y

Construction provides a good overview of the state of the art in installation of TLPs. Three alternatives are presented.

Solution A comprises an ad-hoc installation barge with a footprint on the bottom that matches with the platform shape. Fixation between the platform and the barge is achieved through hydrostatic forces. Completion of the installation on site requires considerable technical arrangements, such as sliding guides and winches.

Solution B comprises temporary buoyancy modules mounted on the platform. While fixed to the platform, these buoyancy modules transform the platform into a semisubmersible.

Maintenance of a waterplane area during towing and submersion ensures the necessary stability, and after attachment to the tethers the temporary buoyancy tanks can be removed and re-used. This solution has the disadvantage that the towing resistance is significantly increased, reducing the weather window for installation. Furthermore, the handling and release of the temporary buoyancy elements requires considerable offshore operations.

Solution C comprises a U-shaped semisubmersible barge supporting the TLP during towing. At installation the barge is submerged with the TLP, maintaining stability through a large waterplane area created with fixed structures on the semisubmersible barge. This solution has the benefit of easy towing and safe, simple offshore operations, but it has the very substantial drawback that the semisubmersible barge is a special vessel of considerable dimensions, which inherently leads to higher costs.

The Iberdrola document itself summarizes the challenges: "TLP designers have to face some demanding challenges to assure stability during T&l phases, especially when self- installing these kind of platforms."

CN 103925172 discloses a solution comprising a variant of temporary buoyancy tanks.

Prismatic tanks are fitted to the structure of the floater, and through connection with both the radial and the vertical braces the temporary buoyancy tanks can obtain good structural connection to the body of the floater. The problem remains, however, that the removal of the temporary tanks after the connection of the TLP to the tethers is a complicated offshore operation with significant risks of damage to the floater and/or the temporary tanks during the operation. Furthermore, additional costs are associated with the purpose-built temporary tanks.

The Glosten Associates Inc. are involved in a floating system marketed as PelaStar™.

Related patent US9914505 discloses a method for installing a water-submersible platform.

The method includes lowering the platform in water from a vessel positioned above the platform while spuds connecting the vessel to the platform stabilize the platform during lowering. An assembly of the vessel and platform, and a vessel that is used to connect to the platform is also disclosed. The installation process is facilitated by the use of a spud system where the spuds (elongated steel boxes) can be moved vertically by a winch and cable system. The bottom end of each spud contains a tip that locks into a recess in the tendon arms. Once locked, the fully assembled floating turbine and barge act as one stable unit, able to withstand the rough waters and high winds of offshore transits and installation sites. Once positioned over the installation site, the spuds push the floating turbine to its installation draft for tendon hookup. This arrangement has the advantage that towing and installation can be carried out in an efficient and safe manner, but as for Solutions A and C in the Iberdrola document, the solution has the very substantial drawback that the installation barge is a special vessel of considerable dimensions, which inherently leads to higher costs.

US10774813 discloses a floating wind turbine comprising a floating base. In this construction a counterweight is used which is hollow and which may be filled with air or may be flooded. In this construction the installation of the TLP is difficult as the stability during lowering is unsecure. After the installation, the counterweight is not used for influencing dynamic response of the foundation.

Patent application US2008017093 discloses a floating platform includes a hull and a deck mounted thereon. The platform is anchored to the seabed by tendons connected to the hull at the upper ends thereof and secured to the seabed at the lower ends thereof. The platform includes a removable drawdown system for lowering the platform to the lock-off draft without utilizing an installation vessel. The drawdown system utilizes six strand jacks 28, one for each tendon 20. Each strand jack includes a bundle of strand cables 44 that passes through a strand guide or umbrella 46 and is connected to a lifting block 48. The strand jacks 28 are individually controlled by controllers 42 which are linked for equalizing the load among them. Disadvantages of the TLP system of US2008017093 relate to the removable drawdown system. The drawdown system requires active control and related strand jacks for each cable. As multiple control systems and active jacking equipment is required, the system is relatively expensive to procure. In addition, the system is delicate to operate, as forces between respective control systems and tendons need to be balanced during installation of the floating platform, while dealing with offshore conditions such as waves, weather and current.

The present invention aims to provide an alternative floating base for, for instance, a wind turbine or substation, improving on at least one or more of the disadvantages of the prior art.

SUMMARY OF THE INVENTION

Aspects of the present invention are set out in the accompanying claims.

The disclosure provides a method for installing a floating foundation in a body of water at a predetermined depth, the method comprising the steps of: providing a construction vessel comprising a hoisting system;

providing a floating foundation having a positive buoyancy; providing a ballasting frame having a weight, using the hoisting system to lift the ballasting frame and move it to cover the floating foundation; connecting the ballasting frame to the floating foundation; using the hoisting system to lift the assembly of the ballasting frame and the floating foundation and move the combination to an installation location; while the floating foundation is unconnected to a sea floor, lowering the assembly into the water using the weight of the ballasting frame to submerge the floating foundation until the floating foundation reaches an installation depth; connecting the submerged floating foundation to subsea anchors using one or more mooring lines, allowing the assembly of the frame and the foundation to rise to an operating depth, wherein the foundation is submerged and the one or more mooring lines are under tension; disconnecting the ballasting frame from the floating foundation.

In an embodiment, the one or more mooring lines comprise one or mere of tendons, steel wires, synthetic wires, hybrid steel and synthetic wires, chains, or combinations thereof.

In an embodiment, the floating foundation is a tension leg platform (TLP).

In an embodiment, the floating foundation and the ballasting frame are connected via compression, friction, a pinned connection, a clamped connection or combinations thereof.

In an embodiment, the hoisting system comprises at least two lifting cranes, the step of using the hoisting system to lift the combination of the ballasting frame and the floating foundation comprising lifting the combination with the at least two cranes working in conjunction.

In an embodiment, the frame comprises at least two separate parts connected to each other.

In an embodiment, the method comprises the step of providing the ballasting frame with one or more weights.

In an embodiment, the weights compensate for at least 50% of the positive buoyancy of the floating foundation, while the combined weight of the frame and the weights exceeds the positive buoyancy of the foundation.

In an embodiment, the frame weight is in the order of 500 to 1000 metric ton, and the clump weights having a weight in the order of 3000 metric ton or more.

In an embodiment, the method comprises the step of pumping a heavy fluid, for example drilling mud, into the ballasting frame to increase the weight of the frame.

In an embodiment, the floating foundation has a top view base shape, and the ballasting frame has a top view frame shape substantially matching the top view base shape.

In an embodiment, the top view base shape and/or the top view frame shape are substantially triangular.

In an embodiment, the floating foundation comprises at least ane ballasting element, the method comprising the step of adjusting the positive buoyancy of the foundation within a predetermined range to assist in submerging the floating foundation.

In an embodiment, the ballasting element is provided with an opening allowing water to freely enter or exit the ballasting element.

In an embodiment, the method comprises the step of using the hoisting system to arrange a wind turbine generator on the submerged floating foundation.

In an embodiment, a wind turbine generator is pre-installed on the submerged floating foundation.

In an embodiment, the hoisting system controls the position and/or orientation of the floating foundation.

In an embodiment, the hoisting system enhances the stability of the floating foundation during the installation phase.

In an embodiment, the method comprises the step of fixing the ballast frame and/or the floating foundation to the construction vessel during transport using the construction vessel.

In an embodiment, the step of fixing comprises using a rotatable subframe connected to the construction vessel.

According to another aspect, the disclosure provides a system for mooring a floating foundation to a plurality of mooring lines, the system comprising: a construction vessel comprising a hoisting system; a floating foundation having a positive buoyancy; and a ballasting frame for lifting and ballasting the floating foundation, the frame having a weight sufficient to submerge the floating foundation to a predetermined depth, the frame comprising a first connection releasably connectable to the floating foundation and a second connection releasably connectable to the hoisting system allowing the hoisting system to lift the assembly of the frame and the floating foundation.

In an embodiment, the hoisting system comprises at least two lifting cranes, the two cranes being adapted to lift the combination of the ballasting frame and the floating foundation working in conjunction.

In an embodiment, the ballasting frame comprises at least two parts connected to each other.

In an embodiment, the first connection for connecting the ballasting frame and the floating foundation comprises a connecter based on compression, friction, a pinned connection, a clamped connection or combinations thereof.

In an embodiment, the floating foundation is a tension leg platform (TLP).

In an embodiment, the ballasting frame has a truss structure, a box structure, a V- shaped structure, or a combination thereof.

In an embodiment, the system comprises one or more mooring lines, the mooring lines having one end adapted to be moored to the sea floor and an opposite end connectable to the floating foundation, the mooring lines being adapted to keep the floating foundation submerged at a predetermined depth.

In an embodiment, the floating foundation comprises at least one ballasting element, allowing to adjust the positive buoyancy of the base within a predetermined range.

In an embodiment, the ballasting element comprises an opening without valves, allowing water to enter and exit the ballasting element.

In an embodiment, the ballasting frame has one or more ballasting compartments capable of receiving a fluid, preferably having a density greater than water for example drilling mud, to increase the weight of the ballasting frame.

In an embodiment, the ballasting frame is provided with one or more weights to provide a combined weight of ballasting frame and weights at least exceeding the positive buoyancy of the floating foundation, wherein the weight of the weights at least exceeds 50% of the positive buoyancy of the floating foundation.

In an embodiment, the one or more weights are adapted to lower the centre of gravity of the frame.

In an embodiment, the one or more weights are provided by either a solid material or a fluid or a combination thereof.

According to another aspect, the disclosure provides an assembly comprising: a floating foundation having a positive buoyancy; and a ballasting frame having a weight sufficient to submerge the assembly of the ballasting frame and the floating foundation to a predetermined depth.

In an embodiment, the ballasting frame comprises a first connection releasably connectable to the floating foundation and a second connection releasably connectable to a hoisting system.

In an embodiment, the weight of the ballasting frame, including potentially added weights (94), exceeds 1500 metric tons.

In an embodiment, the ballasting frame comprises an opening allowing a foundation pile of the floating foundation to extend therethrough.

In an embodiment, the assembly is adapted for the system or the method as described above.

According to another aspect, the disclosure provides a ballasting frame for lifting and submerging a floating foundation to a predetermined depth.

In an embodiment, the ballasting frame is adapted for the system or the method as described above.

In an embodiment, the ballasting frame has a truss construction.

In an embodiment, the ballasting frame has a substantially triangular shape or a V- shape in top view.

In an embodiment, the ballasting frame comprises an opening for a foundation pile of the floating foundation.

In an embodiment, the frame is provided with first connectors to connect to frame to corresponding connectors of the floating foundation.

In an embodiment, the first connectors are provided on the bottom of three extremities of the ballasting frame.

In an embodiment, the ballasting frame is provided with second connectors to connect to frame to a hoisting system.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made to the figures on the accompanying drawings. The figures are schematic in nature and may not necessarily be drawn to scale. Similar reference numerals denote similar parts. On the attached drawing sheets:

Figure 1 shows a perspective view of an embodiment of a lifting vessel and a first transport barge preparing for a first step of a method of the disclosure;

Figure 2 shows a perspective view of a second step of an embodiment of a method of the disclosure;

Figure 3 shows a perspective view of a third step of an embodiment of a method of the disclosure;

Figure 4 shows a perspective view of an embodiment of a lifting vessel and a second transport barge preparing for a fourth step of a method of the disclosure;

Figure 5 shows another perspective view of the lifting vessel and the second transport barge of Fig. 4;

Figure 6 shows a perspective view of an embodiment of a lifting frame and the second transport barge;

Figure 7 shows a perspective view of an embodiment of the lifting frame lifting an embodiment of a floating base from the second transport barge;

Figure 8 shows a perspective view of an embodiment of the lifting frame submerging the floating base;

Figure 9 shows a perspective view of the floating base in a submerged position;

Figure 10 shows a perspective view of a step of attaching cables to the submerged floating base;

Figure 11 shows another perspective view of a step of attaching cables to the submerged floating base;

Figure 12 shows a perspective view of the floating base in submerged position with cables attached;

Figure 13 shows a perspective view of a step of releasing the lifting frame from the floating base;

Figure 14 shows a perspective view of a step of lifting an assembled wind turbine;

Figure 15 shows a perspective view of a step of arranging the wind turbine on the floating base;

Figure 16 shows a perspective view of a number of wind turbines arranged on respective floating bases in accordance with the present disclosure;

Figures 17 and 18 show a perspective view of another embodiment of a system of the disclosure;

Figure 19 shows a perspective view of yet another embodiment of a system of the disclosure;

Figure 20 shows a perspective view of another embodiment of the disclosure;

Figure 21 shows a perspective view of a detail of an embodiment of the disclosure in an engaged position;

Figure 22 shows the detail of the embodiment of Figure 21 in a disengaged position; and

Figures 23 to 27 show schematic side views of respective steps of an embodiment of a method according to the disclosure.

DETAILED DESCRIPTION

Herein below, reference may be made to the following terms and phrases:

Wind turbine or wind turbine generator: A wind turbine is a device that converts the wind's kinetic energy into electrical energy. Hundreds of thousands of large turbines, in installations known as wind farms, now generate over 650 gigawatts of power, with 60 GW expected to be added each year. They are an increasingly important source of intermittent renewable energy, and are used in many countries to lower energy costs and reduce reliance on fossil fuels.

A wind turbine may comprise a mast, nacelle, and blades. The wind turbine is typically arrange on a base or foundation. The foundation may extend into the ground, may be gravity based or the foundation may float.

A nacelle is a cover housing that houses all of the generating components in a wind turbine, typically including a generator, gearbox, drive train, and brake assembly. The mast is a column-like structure extending from the foundation to the nacelle. The wind turbine may comprise any number of blades, although turbines generally comprise two or three blades.

The blades are connected to and rotatable with respect to the nacelle, and typically drive the generator.

A platform or vessel is a waterborne body used for offshore use and can include single/ mono-hull vessels, catamaran vessels or semi-submersible vessels.

A semi-submersible vessel is a specialised marine vessel used in offshore roles including as offshore drilling rigs, safety vessels, oil production platforms, and heavy lift cranes. They have good ship stability and seakeeping, typically better than drillships. The semi-submersible vessel is typically self-propelled. When equipped with a hoisting system for heavy lifting, the vessel may be referred to as a semi-submersible crane vessel (SSCV).

A barge may refer to a relatively long vessel having a relatively flat deck for carrying freight, either under its own power or towed by another vessel.

Seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) refers to the bottom of the sea, ocean, or any other body of water suitable for the system and method of the present disclosure.

A tension leg platform (TLP) or extended tension leg platform (ETLP) is a mooring line stabilized floating structure. A TLP may be suited for, for instance, water depths greater than 100 metres (about 300 ft.) and less than 1500 metres (about 4900 ft.}. The platform is typically moored by means of tethers, tendons or anchor wires under substantial tension, typically grouped at each of the corners of a TLP platform. A group of tethers may be referred to as a tension leg. A feature of the design of the tethers or tendons is that they typically have relatively high axial stiffness (low elasticity), such that virtually all vertical motion of the platform is eliminated. The tethers or tendons are secured to anchors in the seabed. The tendons may be tubular-steel tethers or cables. The upward pull of the platform, as it tries to float, stretches the tethers, placing them in tension. The substantially taut tethers now serve as legs to locate the platform and maintain its position. The tethers also stabilize the platform.

Mooring wires can also be used for tension legs if they are suitably tensioned. Mooring lines can either be vertical or under an angle. Angled mooring lines are for instance used in SBM’s inclined TLP for wind, as disclosed for the Grande Large project off the coast of France.

Herein below, phrases such as 'tendons' or 'mooring lines’ may relate to any of the options for anchoring a TLP as referenced above.

Generally referring to Figure 1, a system 1 of the disclosure may comprise a first vessel 2 provided with a hoisting system 4. The hoisting system 4 may comprise any elements able to hoist, lift and/or move items. For instance, the hoisting system 4 may comprise one or two cranes 6, 8. The cranes may be able to rotate with respect to a deck 10 of the vessel 2.

Alternatively or in addition, the hoisting system 4 may comprise one or more of, for instance, a lift, a rig, a jack, forklift, etc. (not shown). The vessel 2 may be arranged in a body of water 12, typically a sea or ocean. The vessel 2 may be a semi-submersible vessel.

The system may comprise one or more barges 20 having a deck 22. The transport barge 20 and the barge deck 22 may be adapted to transport elements 24 of an offshore structures from one location to another. The one location may typically be a yard, harbour of near-shore storage or assembly location. The other location may typically be an operation location of the structure. The structure may be a wind turbine generator (WTG). The elements may be piles 26 for mooring or anchoring a floating platform to the sea floor.

In a first step of a method for arranging an offshore structure, such as a WTG, a number of mooring piles or monopiles 26 may be provided. The piles 26 may be transported on deck of the first barge 20. The hoisting system 4 may be used to lift one mooring pile 26, move it to a location of choice, and lower the pile to the sea floor.

In a second step, see Figure 2, the mooring pile 26 may be anchored in the seabed 40.

Anchoring may be done using any suitable system 42. Suitable systems include, but are not limited to, Applicant's DP Ill system or a HydroHammer system by IQIP [NL]. As an alternative, Applicant has received Statements of Feasibility for two of its “silent foundations” concepts, on which the company has been working together with the University of Dundee.

The concepts include helical or screw piles and so-called push-in piles, which could be installed without loud hammering that can be harmful to marine life and requires noise mitigation systems that produce a considerable CO2 footprint.

Referring to Figure 3, any number of mooring piles 26 may be anchored in the seabed 40. When the piles 26 have been anchored, one or more mooring lines 44 are connected to the anchor pile 26. The mooring lines 44 may be connected using a connector 46, for instance comprising a double-flanged structure 48 connectable to the anchor pile 26 using a pen 50. Herein, the pen 50 may be placed using a remotely operated vehicle (ROV) 52. The pen 50 may extend through an opening in both the double flanged structure 48 and a wall of the pile 26. For instance three mooring piles 26 may be installed for each floating platform or foundation, with for instance two mooring lines 44 per anchor connection as explained below.

The steps for mooring the anchor piles 26 as described above may be combined and executed for all anchor piles in one operations. Alternatively, anchoring the piles 26 to the seabed may be mixed with the steps for arranging a floating foundation, as described herein below.

Generally referring to Figures 4 and 5, the system 1 of the disclosure is provided with a frame 60. The system and method are suitable to install floating foundations 62. One or more floating foundations 62 may be provided, for instance on the deck of the barge 20, see Fig. 5.

The foundation 62 may comprise one or more float elements or buoyancy chambers 64.

The float elements 64 may be connected via structural members 66 to a central column 68.

The floating foundation 62 may have any suitable form, shape, weight and buoyancy. In top view, the shape may be, for instance, round, triangular, square, or hexagonal. In a practical embodiment, the floating foundation is triangular in top view. See for instance Figure 5. The triangular shape provides advantages in view of static determination and stability. The triangular shape in top view may be an equilateral triangle. Respective sides of the triangle may be arranged at an angle of about 120 degrees with respect to each other. In perspective view, the foundation may have the shape of a tripod, or a truncated pyramid.

Alternatively, the foundations 62 may have another shape in top view. The foundations may be provided with three protrusions extending from the central column 68 in different radial directions. Edges of corners of the foundation may be provided with the buoyancy chambers or float elements 64. However, the float elements may be provided at any suitable location in or on the foundation 62.

In a practical embodiment, the float elements 64 may be closed, i.e. lacking any vents, valves or openings. The latter allows a fully closed structure, which limits the chance of corrosion and increases the lifetime of the floating foundations.

In a practical embodiment, the lifting and ballasting frame 60 may have a shape matching the shape of the floating foundation 62. The ballasting frame may have a shape of, for instance, a triangle, a part-open triangle, a truncated pyramid, have a triangular shape in top view, or have a tripod shape. See the figures for examples.

The frame 60 may have a truss construction. Truss herein refers to two or more structural members, for instance fabricated from straight pieces of steel pipe, to form a series of triangles 70, 72 lying in parallel planes. The triangle shape provides advantages in withstanding stress. Sloping vertical pieces of pipe 74 can connect respective triangles, collectively referred to as the web of the truss. The frame 60 may comprise at least two layers 70, 72 of a triangular shaped section interconnected by sloping pieces of pipe 74.

In the middle, in top view, the frame 60 may comprise an opening 80 (Fig. 7) to allow passage of, or enclosing, a vertical segment of the foundation, such as the column 68. The opening 80 provides a passage for the center column of the TLP. The column 68 can extend through the opening 80.

The frame 60 may be provided with first connection elements 82. See for instance

Figure 8. The first connection element are adapted to connect the frame to a respective foundation 62. The first connection elements may be provided on, at least, extremities of the frame. The first connection elements 82 may be provided on a lower end of the frame 60. The connection elements may be arranged on one or more edges of the frame. The frame 60 may be provided with at least three first connection elements 82. The first connection elements may comprise any suitable shape or form. The first frame connection elements 82 may comprise, for instance, an opening and a locking pen.

The floating foundation may be provided with a first foundation connection element 84.

The first foundation connection element is adapted to connect to the first frame connection element 82. The first foundation connection elements 84 may be provided on, at least, upper sides of each float element 64. Each foundation may be provided with, at least, three first connection elements 84. First foundation connection elements 84 may be provided on upper surfaces of extremities of the foundation, for instance edges of the triangular base shape thereof.

The frame may be provided with multiple connection elements 82, 86 of different types.

The type may depend on which features could be beneficial during operations, where these systems may have different functionality. In an example of a connection system, the frame can be equipped with a pinned connection. A pinned connection may include a pad eye on one element (i.e. on the frame or on the foundation) and a receiving element with corresponding opening(s) on the other, which can be locked by inserting a pin. The pinned connection can accommodate the pulling load, for example during lifting.

In addition, optionally another connection element may be added for compression only, for instance a support pressing down on the floating foundation. Herein, during the situation that the compression only connection elements are activated, the pulling-only connection elements may be without loading. One inherent benefit would be that during the hook-on of the mooring system, the tension-only connection elements can be disconnected as during the hook-on phase of the mooring system, there is no need for pulling capacity. The added benefit is that, following the disconnection step of the tension-only connection elements, confirmation can be received that these connection elements are actually disconnected.

When the floating foundation is raised by lifting the frame upwards, and the mooring system is put under tension, the load in the compression-only connection elements is gradually reduced until the load becomes zero after which the frame is lifted clear without any risk for obstruction. When disconnection is not separate and disconnection would be performed during the raising step, this needs to be done when there is little load in the system.

Environmental loading, by wave motion for instance, will add dynamic loading as well.

Preferably this step would then be as short as possible making it less favorable for confirming disconnection.

The ballasting frame 60 may have any suitable shape or form. The ballasting frame may have a shape suitable to link to foundations having a certain range of one or more of, for instance: shapes, sizes, weight and buoyancy.

The floating foundations for a certain project for installation of wind turbines may be designed to have first foundation connection elements 84 located at a position suitable for the available ballasting frame 60.

Please note that the shape and size of the foundation 62 may vary within a range, suitable for the same lifting frame 60. Herein, the foundation connection elements 84 can be position to fit with the frame connection elements 82, while the shape and size of the foundation can differ. The shape of the foundation may be referred to as the hull of the foundation 62. The hull may differ within a range of, for instance +/- 50% with respect to the size or shape of the frame 60.

An upper end of the frame 60 may be provided with one or more second frame connection elements 88. See, for instance, Figure 6. The second connection elements 86 may provide a mortise connection. The connection elements 86 may be plate-shaped structures provided with one or more openings to receive an attachment element for a cable or other hoisting line 88. For instance, the frame 60 may be provided with at least three second connection elements 86. Each element 86 may be connectable to respective cables 88. The cables 88 may be connected to the hoisting system 4 via a line, cable or chain 90, for instance via a pulley 92.

Optionally, the frame 6 may be provided with additional weight. Herein, the frame may be constructed using structural steel, as described above. One or more weights 94 may be added. The weights may for instance be added to sloping sides of the frame. See Figures 6 and 7. Alternatively or in addition, weights 94 may be suspended from lower ends of the frame. See, for instance, Figures 17 and 18. In an embodiment, a heavy fluid, such as drilling mud or cement, may be inserted in the tubulars of the frame 60 to increase the overall weight of the frame.

Adding weights may assist in lowering the center of gravity. The weights 94 allow to adjust the total weight of the ballasting and lifting frame 60 to the buoyancy of the respective floating foundation 62. Also, adding weights 94 to the frame is a relatively cost effective manner to limit the capital expenditure of the frame, as capital expenditure for weights may be a factor 2 to 5 more cost effective than costs per kilo for the steel and construction of the frame itself.

In another embodiment, see for instance Figure 17, the frame 60 may be lifted with two cranes 6, 8 in conjunction. The first crane 6 is connected to one or more second frame connection elements 86. Each element 86 may be connectable to respective cables 88. The cables 88 may be connected to the first crane 6 of the hoisting system 4 via one or more lines or cables 90, for instance via a pulley 92. The second crane 8 may be connected to a number of other second frame connection elements 86, for instance via respective second cables or lines 94, second pulley 96, and/or second cables 98.

Generally referring to Figure 19, the frame 60 may comprise a first beam 100 and a second beam 102. Although referenced as a beam, the first and second beams 100, 102 may comprise any suitable structural element, such as a tube, H-beam, conduit, pipe, etc. A connection 104 between the first beam 100 and the second beam 102 may be a hinged connection. The first and second beams 100, 102 may be provided with weights. For instance, the beams 100, 102 may be tubular, and weights may be inserted in the tubes. For instance, ends of the tubulars 100, 102 may be provided with weights.

Cables 108, 108 may connect the frame 60 to the respective pulleys 92, 96. The weight distribution of the frame may be such that the frame is in balance when suspended from the hoisting system 4. For instance, the moment of inertia exerted on the cables 106, 108 by respective ends of the beams 100, 102 may be balanced by the shape and weight distribution of the frame and by the horizontal distance of attachments 82 with respect to the center of gravity. The center of gravity may be substantially located in line with the center column 64. If the foundation 62 has a triangular or tripod shape and/or the frame 60 has a triangular shape, as exemplified in Fig. 19, the horizontal distance of the center column 64 to the attachment of two cables 108 is about twice the horizontal distance of the center column 64 to the attachment of two cables 106. As there are two corners of the triangular frame connected to respective first cables 106, the moment of inertia is compensated by the longer horizontal distance to the connection of the second cables 108 to the third corner of the triangle.

Thus, the frame 60 may have a triangular shape open on one side. The open side may typically be the side of the frame facing the vessel 2. The open side enables the frame to lift a floating foundation 62 with a structural element attached thereto. For instance, see Figure 20, a wind turbine 120 can be attached to the floating base 62. The frame 60 may be connected to the floating base or foundation 62. The vessel 2 may lift the assembly of the frame 60, and the foundation 62 and wind turbine 120 connected thereto. The embodiment of Figure 20 allows the wind turbine to be connected to the foundation 62 onshore or near shore, while allowing the construction vessel 2 to pick up the assembly and move the assembly of the foundation and wind turbine to an installation location. Herein, the construction vessel 2 is typically self-propelled, i.e. the vessel 2 is provided with propulsion means such as engines and thrusters. This embodiment may be economically advantageous for installation locations relatively near shore. Relatively near shore herein may relate to transport times for the vessel 2 up to 10 hours, for instance in the order of two to six hours.

Generally referring to Figures 21 and 22, the vessel 2 may be provided with a fixation mechanism 130 to fixate the frame, the foundation or the assembly of the frame 60 and the foundation 62 with respect to the construction vessel 2. The fixation mechanism 130 may comprise a subframe 132 comprising a number of structural elements 134, 138, 138. The structural elements may be beams, pipes, tubulars, etc. The subframe 132 may be releasably connected to the vessel 2. For instance, the subframe may be connected to the vessel via a hinged connection 140. The mechanism may comprise one or more locking elements. The locking element allows to lock and unlock the mechanism with respect to the assembly of frame 60 and foundation 62. The locking mechanism may comprise, for instance, a pen 142 cooperating with an opening 144 in a pad eye 146. The pen may be connected to the subframe 132. The pad eye may be connected to the foundation 62. For instance, the pad eye 146 may be connected to a float element 64 of the foundation. The subframe 132 may be provided with a bumper or other pad element 148. The bumper element 148 allows the rotatable frame 132 to engage the vessel 2 while obviating or at least limiting wear and tear.

The pen 142 and pad eye 146 may be fixated (Fig. 21) or unlocked (Fig. 22) by rotating the subframe 132 with respect to the vessel 2 and with respect to the pad eye 148. The locked position, see Figure 21, enables the vessel 2 to stabilize the assembly of foundation, frame and optionally the wind turbine 120. Stabilizing the assembly is advantageous during, for instance, transport of the assembly, as exemplified in Figure 20.

Generally referring to Figures 23 and 24, in an embodiment, the floating foundation 62 may be provided with one or more ballasting chambers 150. Referring to detail 152, the ballasting chamber 150 may be provided with one or more openings or passages 154 to allows water 156 to flow into and out of the ballasting chamber. In a practical embodiment, the opening 154 may be open, i.e. free from valves or other closure elements. The opening 154 may be provided at or near a lower end of the ballasting chamber 150. Obviating valves and closures simplifies the construction, limits capital expenditure, and obviates the risk of malfunction and corrosion generally related to the use of valves in an offshore environment.

The opening 154 allows the corresponding ballasting tank 150 to fill automatically when the opening 154 is submerged (Fig. 24, 25), and to empty when the ballasting tank is lifted out of the water (Fig. 26).

Referring to Figure 4, in a method of the present disclosure, in a first step the ballasting frame 60 is provided on the deck of the construction vessel. In a first step, the ballasting and lifting frame 60 is connected to the hoisting system 4.

In a second step, see Fig. 5, one or more floating foundations may be provided. The floating foundations may float and be towed towards the construction vessel (not shown, but generally referred to as 'wet towing’). Or, one or more floating foundations may be provided on the deck of a transport barge 20.

The hoisting system 4 may lift the frame 60 off the deck of the vessel 2 and move the frame on top of one of the foundations 62 provided. See Figure 6. The frame 60 is connected to the respective foundation 62. The connection may be established using a releasable coupling such as the first connection elements 82, 84.

With reference to Figures 8 and 7, in a next step the hoisting system 4 lifts the assembly of the frame 60 and the foundation 62 and moves the assembly to a location of choice. Said location is the location of assembly, typically above one or more anchors 26 and associated anchor lines 44, as exemplified in Figure 3.

Referring to Figure 8, when the assembly has reached the predetermined location, the hoisting system 4 lowers the assembly. The assembly will sink into the water.

In a practical embodiment, the negative buoyancy due to the weight of the frame (including any weights attached to the frame) exceeds the positive buoyancy of the floating foundation 62. Thus, the assembly will sink due to the total weight of the frame 60. Total weight herein refers to the weight of the structural elements of the frame 60 in combination with the weights 94 attached thereto.

The hoisting system 4 will lower the assembly in a controlled manner, until the foundation has been submerged to a predetermined installation depth. See Figure 9.

When the floating foundation has been submerged to the installation depth, upper ends of respective tendons or anchor cables 44 are attached to anchor connections 62 of the foundation 62. See Figure 10. The anchor cables 44 may typically be connected to the foundations using a remote controlled vehicle (ROV) 52, obviating manual labor and all the associated safety risks. Typically, each corner or extremity of the foundation may be connected to at least two anchor lines 44, providing redundancy in case of failure for instance due to wear and tear or metal fatigue.

When all anchor lines 44 have been connected to the submerged foundation 62, the hoisting system 4 lifts the assembly up until the anchor lines 44 are tensioned to a predetermined level. See Figure 11. Herein, the submerged floating foundation 62 may be at or near an operating depth, which is shallower than the installation depth.

Referring to Figures 12 and 13, in a subsequent step the frame 60 is disconnected from the submerged foundation 62. The hoisting system 4 lifts the frame while the foundation remains submerged at the operating depth. Herein typically the center column 68 of the foundation 82 will extend above the water surface, while all float elements 64 are submerged.

The submerged float elements 64 provide positive buoyancy and upward pull. The tendons 44 provide stability and limit movement of the foundation. The submerged floating foundation 62 as shown in Figure 13 is, in fact, similar to a tension leg platform (TLP).

In a next step, the construction vessel 2 may lift an assembly wind turbine 120 and move the wind turbine onto the center column 68. See Figures 14 and 15. The wind turbine may have been preassembled, or may be assembled on board of the construction vessel 2.

Lifting the wind turbine may involve the hoisting system 4. Moving the wind turbine onto the column 68 may involve the hoisting system as well as engines of the vessel 2 to move the vessel 2.

The wind turbine is connected to the column 68. See Figure 16. Connecting the wind turbine to the center column 68 may involve any suitable connection. Typically, the connection involves a flanged connection. The flange connection may, for instance, include a flanged connection which is L-shaped in cross section. The flanges of column 68 and the mast of the wind turbine may be connected using a bolt-nut connection. Alternatively, a fork- shaped connection disclosed in, for instance, US10995463 or US11236726, may be used to connect the wind turbine 120 to the column 68 of the foundation 62.

Referring to Figure 16, the system and method of the disclosure enable to construct a park comprising any number of floating wind turbines in a relatively fast, efficient, and cost effective manner.

Generally referring to Figure 17, in an embodiment the frame 60 may be lifted using two or more cranes 6, 8 of the hoisting system 4. Using two cranes instead of one removes a degree of freedom of the frame 60 and all elements attached thereto, i.e. it obviates rotation in the plane of the connections to the two cranes.

Optionally, the frame or the foundation may be connected to the vessel 2 during transport. For instance by using the connection mechanism 130, a few more degrees of freedom of movement of the assembly of frame and foundation are obviated. The connection mechanism 130 prevents horizontal translation.

For operations relatively near shore, typically for geographies where water depths increase relatively fast, a pick-up inshore or near shore by the construction vessel 2 can be beneficial. A typical example of such a location would be Japan, but may also include the west coasts of North and South America. Herein, the sea floor, which is relatively shallow near shore, drops off steeply. The continental plateau ends relatively close to shore (for instance within about 2 to 100 km, for instance within 5 to 50 km from shore). Pick-up herein refers to the construction vessel 2 lifting and transporting the combination of floating foundation 62 and the wind turbine 120. See Figure 20.

Herein, the heavy lift vessel 2 will come into port or to a location near shore for a pick- up of the floating body 62. The foundation 62 may be already in the water, on a barge or on the quayside. The wind turbine 120 can be connected to the floating foundation before or after pick-up by the vessel. Typically, the vessel 2 will pick-up the assembly of foundation 62 and wind turbine 120. For pickup, the hoisting system 4 is connected to the frame 60. The frame 80 is connected to the foundation or base 62. When the foundation is connected to the lifting elements 8, 8 of the hoisting system via the frame 60, the heavy lift vessel 2 can sail to a predetermined location. Then, the lifting frame 60 can be used to submerge and install the foundation 62 as described herein above.

When sailing, the ballasting frame or floating body can be connected to the heavy lift vessel 2 to counter relative horizontal motions between the two bodies. The payload may be connected to the vessel using the mechanism 130. When the payload includes a wind turbine 120, an additional horizontal restraint 150 can be added. See Figure 20 for an example.

The horizontal restraint 150 can be used to prevent the assembly from swinging and generally control movement of the assembly when sailing, with the assembly suspended in one crane or both cranes 8, 8. A typical existing use is for sailing with a Jacket in cranes towards an installation location. The vessel 2 can sail with the assembly suspended from the hoisting system 4 either with the full assembly suspended in air. Alternatively, at least part of the suspended assembly may be submerged during transport. The latter reduces the load on the hoisting system. Keeping the floating body 62 at least partly in the water during transport increases the maximum of potential payload, allowing installation of larger TLPs and/or larger wind turbines. A downside may be that ballasting may be required in-shore or near-shore.

The benefit over offshore ballasting is that quayside or near-shore installed ballasting systems can be used which can be used for many TLPs. Another benefit would be that at the quayside, fresh water can be used rather than salt water removing the requirement to clean ballast tanks following ballasting operations to prevent or reduce corrosion as a result of salt.

Sailing can be either with the payload (partly) pre-installed or the payload post installed after hook up offshore. The benefit of (partly) pre-installing is that the payload reduces the required weight of the ballast lift frame 60, as the positive buoyancy is reduced by the total weigh of the added pre-installed payload. When used in combination with a fully pre-installed

WTG, this would leave a complete installed WTG installation in one phase.

One proposed procedure would be that the TLP is ballasted onto the ground at the quayside, typically referred to as grounding. When grounded, the floating body 62 would be completely still allowing installation of all wind turbine elements. WTG elements may be installed directly from the quayside. When the heavy lift vessel 2 comes into port, the remaining ballast, preferably being above the waterline as this could then be released by gravity draining, can then be removed after placing the ballast frame 60 onto the floating body 62. For the full combined WTG installation a stabilizing frame 150 may be added in the rigging to minimize swinging in the hoisting system 4. See Figure 20. The stabilizing frame 130 and lifting jacket 150 add transverse stiffness in the rigging, thus increasing the stabilizing moment. The same may be accomplished with a higher stabilizing frame 60.

Generally referring to Figures 23 to 27, in another embodiment, the floating foundation 62 may be provided with one or more ballasting chambers 150. Initial steps of providing the foundation 62, connecting the foundation to the lift frame 60, and lifting and moving the assembly of frame 60 and base 62 to a location of choice are the same as described above.

To submerge the base 62, the hoisting system 4 lowers the assembly into the water 12.

See Figure 23. Initially, only the weight of the frame 60 counteracts the positive buoyancy provided by the float elements of the foundation 62. The at least one ballasting chamber 150 is filled with air.

At a predetermined depth, the opening 154 of the ballasting chamber 150 will be level with the surface of the water, allowing water 156 to enter to ballasting chamber. Due to the added weight in the at least one ballasting chamber 150, the assembly will continue to sink.

See Figure 24.

When the foundation 62 has reaches the anchoring depth, see Figure 25, the hoisting system 4 maintains the foundation in position. The anchor lines 44 can be attached to the foundation.

In a subsequent step, see Figure 26, the hoisting system lifts the assembly of frame and foundation upward. Water 156 will exit the one or more ballasting chambers 150 via the opening 154 under the influence of gravity. This once again increases the positive buoyancy of the foundation 62.

When the foundation 62 has reached the operating depth, see Figures 26 and 27, the frame 60 is disconnected from the foundation and removed. A wind turbine or similar structure can be arranged on the submerged foundation as described herein above.

In general, during the installation of a TLP, the relatively small waterplane area (cross section) combined with a relatively high center of gravity may result in hydrostatic instability of the TLP. In this situation, a rotation (which may be referred to as a list’) of the TLP cannot be counteracted by the buoyancy. If so, the TLP may rotate further and topple. The stability is dependent on the TLP design. Stability can be improved by increasing the waterplane area, but this is directly associated to higher capital expenditure. Increasing waterplane area requires a bigger hull of the foundation at (almost) every level, whereas this increase in size of the hull (and the associated material, typically steel) is only needed for the installation of the floating platform. Also, the increased hull size and changed shape has a downside as this adds horizontal loading on the mooring system. As a result, wave induced loading on the hull is increased. Conventionally, and as described in the introduction, limited stability or instability was counteracted by actively ballasting a hull of the floating foundation. This however comes at the cost of time and investment on the basis that equipment needs to be installed on the hull (either permanently or temporary). If equipment is temporarily installed on the hull of the floating foundation, this inherently means that costs and effort will be required to remove the equipment from the hull after installation of the floating platform.

The method of the present disclosure uses a hoisting system. The hoisting system may include, for instance, one or more of a crane, a lift, a derrick . When using a crane, potential instability of the floating platform 62 during installation thereof can be overcome by adding weight, and/or by lowering the center of gravity. A temporary instability can be counteracted by the crane. List will create a lateral load on the crane at an elevation, for instance at the connection of the platform to the pulley. Herein, the crane creates a counter moment. This will limit the amount of list and will create a stable situation. More will be described herein below.

The crane may also minimize the rotation by restoring moment through the connection to the frame (see Fig. 24 for instance).

The relatively heavy lift frame 60 of the present disclosure is beneficial as lowering of the floating platform can be continuous and relatively fast. Herein, the method provides an advantage over conventional methods. The initial investment in a generic construction vessel 2 and a relatively heavy, large and expensive frame 60 provides the advantage of a relatively fast and stable installation (submerging) of the TLP foundations. Expensive herein is however relatively speaking, as the frame is still relatively simple, lacks moving parts of valves, and can therefore merely be a steel structure. By adding weights, which are even more cost effective than the steel frame, the overall capital expenditure for the frame can still be relatively limited. As an example, in a practical embodiment, constructing the frame may involve capital expenditure for material (steel) in the order of USD 4 to 8 per kg. The weights 94 may involve capital expenditure in the order of 1 to 3 USD per kg. This leads to overall capital expenditure for the frame which is still relatively limited compared to conventional active ballasting systems. The frame can be reused on multiple projects. Lacking moving parts such as valves, the frame is relatively robust, and has a relatively long lifespan. In addition, the same construction vessel and potentially also the frame may be used to install wind turbines on the TLP foundations. The method and system of the disclosure are suitable to install wind turbines of any size and shape. The latter includes the largest wind turbines currently envisaged.

Although the method of the disclosure may be combined with some ballasting of the floating platform during installation, for instance for relatively large TLPs, ballasting can be minimized and/or can be done by natural or gravity filling (See Figures 23-27). Thus, the method of the disclosure obviates the requirement for an active ballasting system on the TLP.

As can be seen from, for instance, Figures 17 to 20, when lifting with two cranes (dual crane), rotation of the TLP 62 about one axis is immediately counteracted by an increase in tension in one of the crane wires, without side-lead on the crane sheaves. This means that rotation about this axis is limited. For restoring moments against rotation about a perpendicular axis, off-lead remains present. Crane sheaves typically tend to withstand off- lead better than side-lead, especially as this off-lead will be present when a majority of the weight is already compensated by a large amount of buoyancy, thus resulting in a greatly reduced crane load. Therefore, dual crane TLP installation is deemed to provide additional advantage over single crane TLP installation in terms of side-lead and off-lead.

Using a dual crane system allows to design the foundation 62 such that the waterplane area of the TLP 62 may be increased in one direction, i.e. perpendicular to the plane of the two cranes. In other words, the foundation 62 may be designed to be asymmetrical in cross- sectional top view, having a longer cross section perpendicular to the plane of the two cranes, and a shorter cross section in the plane of the two cranes. This may increase stability in the direction perpendicular to said plane, resulting in crane off-lead only while limiting material and associated costs for the foundation.

The method of the disclosure may transport floating foundations 62 to the installation site either by wet towing (i.e. towing of the foundation while it is floating in the water) or by a barge (i.e. one or more foundations on a transport barge). Transporting one or more foundations on a barge has a few advantages with respect to wet tow, for instance: - Lower cost of the floater and reduced floater weight. In the absence of wet tow, the foundation can be optimized for in place conditions only. As an estimate, this may save 10 to 20% on steel for the construction of the foundation. Savings may be in the order of USD 3 to 6 million per floating foundation (2022 USD). - The floating foundation can be relatively low-tech, i.e. simple. By using the ballast installation frame 60 of the present disclosure, which can be reused for installation of each foundation and can be re-used for subsequent projects, there is no, or at least a significantly reduced or simplified, requirement for ballasting equipment and other installation aids that would otherwise be required. - Lower transport and marshalling cost. - Multiple floating foundations can be transported directly to site. This enables more efficient use of resources. Use of deck space on the construction vessel 2 can be optimized. - No in-port assembly of the floater and WTG. Reduced requirement for space in port, less draught, less quayside capacity. The generic SSCV of the disclosure allows safe and fast assembly offshore. In addition, as the SSCV is generic, i.e. not specifically designed for the installation of wind turbines, the SSCV vessel can be used for projects other than installation of wind turbines and associated foundations, increasing options to recoup investment. - Higher and more efficient throughput. - Omitting a time consuming and highly weather sensitive wet tow. The weather sensitive window is reduced to only the window of lifting and moving the floating foundation (Fig. 7), and/or the lifting and moving of the wind turbine (Fig. 14, 15). The latter window is relatively short, for instance in the order of 30 minutes, limiting dependence on weather with respect to conventional systems depending on wet tow of an assembled system of wind turbine and (floating) foundation. Thus, a higher throughput can be achieved further optimizing economics.

- Decoupling the floater and WTG installation results in a more efficient process. In other words, the method of the disclosure allows to install anchors, floating foundations, and wind turbines, in separate steps. The latter allows to optimize each step. Also, the time window sensitive to adverse weather can be minimized.

The system and method of the present disclosure defy conventional thinking in providing an economically beneficial system and method for placing relatively large wind turbine generators on floating (TLP) platforms using a relatively large lifting vessel 2. The lifting vessel may be used in combination with one or more smaller transport vessels 20.

Using the smaller transport vessels allows to minimize transport of the large construction vessel.

While the construction vessel 2, typically a relatively large semi-submersible lifting vessel provided with a hoisting system comprising one or more heavy lifting cranes, may have a relatively high day-rate (operating costs), the vessel also allows lifting a large variety of objects. Said objects include wind turbines ranging in size from small to the largest turbines currently envisaged. The objects also include the floating base and the lifting frame to submerge the floating base as described herein above. The concept of the disclosure contravenes conventional approach to placing wind turbines by using a relatively large and heavy lifting frame. The large and stable construction vessel however allows to use a lifting frame having a size and weight able to submerge a wide range of floating bases in a controlled and stable manner.

The floating base 62 typically may range in size from small to the largest currently envisaged, able to provide a foundation for large wind turbine generators. The lifting frame is so large and so heavy, that the floating foundations can be submerged relatively quickly and in a controlled manner, saving time and costs. As the frame 60 can be re-used, the upfront costs of the lifting vessel and lifting frame can be offset by the relatively fast and predictable arrangement of a number of submerged floating foundations 62. Also, corresponding wind turbine generators can be arranged on the submerged foundations relatively quickly. Thus, the system and method of the present disclosure are uniquely positioned for the near-future challenge of placing a large number of very large wind turbine generators in relatively deep offshore water.

Capital expenditure of the system is relatively limited, as the frame 60 is relatively simple and as the construction vessel 2 can be generic. Although the frame is relatively large, there are no moving parts. Also, costs for material for the frame can be optimized by using added weights 94 to increase the total weight of the frame. The frame can be re-used for different projects, and allows a range of sizes for respective floating foundations. The vessel 2 is generic and can be used for offshore projects in general, in addition to arranging wind turbines and the associated floating foundations, allowing its operator more options to recoup costs and to optimize economics.

Large wind turbine generators herein may relate to wind turbines having, for instance, a mast height in the order of 50 to 150 m, and/or a mast diameter at the base in the order of 10 to 40 m, and/or a blade length in the order of 40 to 120 m, and/or a power indication in the order of 10 to 20 MW or more. The system and method of the present disclosure are suitable for any water depth suitable for TLP platforms, for instance water depths in the range of 50 to 2500 m.

The floating foundation may have any suitable positive buoyancy. In a practical embodiment, wind turbine generators (WTGs) are increasing in size, as described above.

Floating foundation therefore will provide a positive buoyancy sufficient to support these

WTGs. Positive buoyancy of the floating foundation may range, in practice, up to 20 kilotons or more. The foundations may have sides having a width (for instance the length of a section or piece of pipe forming a side of the foundation) exceeding 20 m. In practice, the sides of the foundation may have a width in the order or 30 to 40 m.

The floating foundation of the disclosure may have a weight in the order of 1 to 10 kilotons. Positive buoyancy of each floating foundation when submerged at least exceeds its weight. The floating foundations may be installed at a depth of, for instance, 5 to 50 meters, for instance at a depth in the order of 10 to 35 meter.

The ballasting frame 60 of the disclosure may have any size, shape or form. In a practical embodiment, the frame 60 has a lower base in the shape of a triangle. Said triangle may have sides having a length in the order of 20 to 60 meter. Alternative shapes are possible as well, such as a V-shape, truncated pyramid shape, etc., as described above.

The weight of the ballasting frame combined with the weights 94 may have a combined weight to provide sufficient negative buoyancy to allow to submerge the floating foundation in a controlled and stable manner.

Within details and structural variations of different embodiments of the frame and the method as described above, the frame may have a weight to at least provide some negative buoyancy compared to the positive buoyancy of the foundation, while providing structural strength. Weights 94 may be added to bring the total weight of the frame to a level exceeding the positive buoyancy of the floating foundation at any given time during the installation. Total weight, or weight of the frame, herein relates to the sum of the frame weight and the added weights 94. Frame weight is the weight of only the structural section of the frame 60. Adding weights allows to limit capital expenditure on structural steel and production costs. Also, the weights are generic and can relatively easily be removed from the frame 60 and re-used for another project or purpose.

The weights 94 may provide, for instance, at least 50%, for instance at least 60%, for instance at least 70%, for instance at least 80%, for instance at least 90%, for instance at least 95% of the total weight of the frame. The total weight at least exceeds the positive buoyancy of the foundation.

In a practical embodiment, weight of the structure of the frame is relatively minimal. For instance, the structure of the frame may weigh in the order of 600 mT (mT is metric ton, or 103 kg) for a combined weight of the ballasting frame of 4100 mT. Herein, 3500 mT is provided by clump weights 94, i.e. mass added to the structure of the frame. Depending on the buoyancy of the foundation, the frame weight and/or clump weights 94 can be adjusted.

Adding weights has the additional advantage of allowing the frame to be relatively slender. Making the frame 80 out of structural components providing the full weight required, especially for relatively large floating foundations, be relatively expensive and impractical.

This as the frame would become very bulky as structural steel typically has a large volume compared to weight (structural steel is typically made to be strong compared to its weight rather than vice-versa).

In a practical embodiment, the frame may have a weight in the range of 5 to 15 kilotons or more. In top view, sides of the frame may have a width (for instance meaning length of pipe sections constituting a respective side) in the range of 10 to 40 m or more, for instance about 20 to 40 m. The frame may have a height in the order of 5 to 40 meters.

In a practical embodiment, the heavy lift vessel 2 of the disclosure may be a semi- submersible crane vessel (SSCV). The vessel may be capable of everything from deepwater installation to monopiles to 14,000 metric ton topside installations. Alternatively, the vessel 2 may be equipped with at least two cranes of 10,000 mT lifting capacity each (wherein 1 mT is a metric ton or 1000 kg). The vessel may have a reinforced deck area in the order of 220 meters in length and 100 meters in width. Lift Capacity may exceed 20,000 metric tons. Draft of the vessel 2 may be in the order of 10 to 35 m.

Although the embodiment described above relate to floating foundations having a substantially triangular shape in top view, the system and method of the present disclosure are suitable for floating foundations of virtually any size or shape. The ballasting frame may be adapted to the shape of the foundation. Alternatively, the floating foundation may be provided with suitable connection elements to connect to the ballasting frame as available.

Alternative shapes in top view of the floating platform include, but are not limited to, round, oval, square, hexagonal, triangular but with a wind turbine connection on one of the extremities and not in the middle, etc.

Last but not least, the Merchant Marine Act of 1820 is a United States federal statute that provides for the promotion and maintenance of the American merchant marine. Among other purposes, the law regulates maritime commerce in U.S. waters and between U.S. ports.

Section 27 of the Merchant Marine Act is known as the Jones Act and deals with cabotage (coastwise trade). It requires that all goods transported by water between U.S. ports be carried on ships that have been constructed in the United States and that fly the U.S. flag, are owned by U.S. citizens, and are crewed by U.S. citizens and U.S. permanent residents.

Lately, the Jones Act has been interpreted broadly, resulting in some offshore construction locations also being covered by the phrase " U.S. port", meaning that transport of goods for the construction of offshore wind turbines may be covered. The system and method of the present disclosure are suitable to operate economically viable within the constraints set by the Jones Act, for instance by using US flagged transport vessels in combination with any suitable construction vessel.

Although the embodiments of submerged floating foundations above have been described with reference to wind turbines, alternative structures can be arranged on the submerged platform as well. For instance, substations may be arranged as well. Substations herein are electricity collection hubs typically included in wind farms. Almost in every wind farm a step-up substation is included to collect all the energy generated by the turbines and received through the cables. The exceptions are new wind farms or existing wind farms extensions built near a substation that can be upgraded to absorb the additional energy produced. In these cases, only a control centre with a supervisory control and data acquisition system (SCADA) and a medium voltage system may be realized. Although there are different possible technical solutions, normally a substation may comprise at least one or more of the following elements: Medium voltage system; High voltage system; Capacitors banks; Auxiliary services; Control, protection and metering system; Communication system; and Fire protection and intruders protection systems.

The scope of the present disclosure is not limited to the embodiments described above.

Many modifications therein are conceivable without deviating from the scope of the present invention as defined by the appended claims. In particular, combinations of features of respective embodiments or aspects of the disclosure can be made. An aspect of the invention may be further advantageously enhanced by adding a feature that was described in relation to another aspect of the invention. While the present invention has been illustrated and described in detail with reference to the figures, such illustration and description are illustrative or exemplary only.

In the claims, the word “comprising” does not exclude other steps or elements, and “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference numerals in the claims should not be construed as limiting the scope of the present invention.

Claims

P35511NLOO/LGO CONCLUSIONS

A method for installing a floating foundation in a body of water at a predetermined depth, the method comprising the steps of: providing a construction vessel (2) with a hoisting system (4); providing a floating foundation (62) with a positive buoyancy; providing a ballast frame (60) with a weight, lifting the ballast frame (60) using the hoisting system (4) and moving it to cover the floating foundation; connecting the ballast frame to the floating foundation; using the hoisting system to lift the ballast frame (80) and floating foundation (62) assembly and move the combination to an installation location; While the floating foundation (62) is not connected to a seabed (40), lowering the assembly into the water using the weight of the ballast frame (60) to submerge the floating foundation (62) until the floating foundation an installation depth is reached; connecting the submerged floating foundation (62) to subsea anchors (26) using one or more mooring lines (44), raising the frame (60) and foundation assembly (62) to a working depth at which the foundation is submerged and the one or more mooring lines are under tension; disconnecting the ballast frame (60) from the floating foundation.

2. Method according to claim 1, wherein the one or more mooring lines comprise one or more cables, steel wires, synthetic wires, hybrid steel and synthetic wires, chains or combinations thereof.

A method according to claim 1 or 2, wherein the floating foundation (62) is a tension leg platform (TLP).

A method according to any one of the preceding claims, wherein the floating foundation (62) and the ballast frame (60) are connected via compression, friction, a pin connection, a clamp connection or combinations thereof.

A method according to any one of the preceding claims, wherein the hoisting system (4) comprises at least two hoisting cranes (6, 8), the step of using the hoisting system to lift the combination of the ballast frame (60) and the floating foundation ( 62) involves lifting the combination in which the at least two cranes (6, 8) work together.

Method according to claim 5, wherein the frame (60) comprises at least two separately connected parts.

Method according to any of the preceding claims, comprising the step of providing the ballast frame (60) with one or more weights (94).

The method of claim 7, wherein the weights (94) offset at least 50% of the positive buoyancy of the floating foundation (62), while the combined weight of the frame (60) and the weights (94) offset the positive buoyancy of the exceeds the foundation.

A method according to any one of claims 7 or 8, wherein the frame weight is on the order of 500 to 1000 tons, and the lump weights (94) have a weight on the order of 3000 tons or more.

A method according to any one of the preceding claims, comprising the step of pumping a heavy fluid, for example drilling fluid, into the ballast frame (60) to increase the weight of the frame.

A method according to any one of the preceding claims, wherein the floating foundation (62) has a basic shape in plan view, and the ballast frame (60) has a frame shape in plan view that substantially corresponds to the shape of the basic shape in plan view.

Method according to claim 12, wherein the basic shape in top view and/or the frame shape in top view are mainly triangular.

A method according to any one of the preceding claims, wherein the floating foundation (62) comprises at least one ballast element (150), the method comprising the step of adjusting the positive buoyancy of the foundation (62) within a predetermined range to assist in submerging the floating foundation (62).

A method according to claim 13, wherein the ballast element (150) is provided with an opening (154) through which water can freely enter or leave the ballast element.

A method according to any one of claims 1 to 14, including the step of using the hoisting system (4) to install a wind turbine generator (120) on the submerged floating foundation (62).

A method according to any one of claims 1 to 14, wherein a wind turbine generator (120) is pre-installed on the submerged floating foundation (62).

A method according to any one of claims 1 to 16, wherein the hoisting system (4) controls the position and/or orientation of the floating foundation (62).

A method according to claim 17, wherein the hoisting system (4) improves the stability of the floating foundation (62) during the installation phase.

Method according to any of the preceding claims, comprising the step of attaching the ballast frame and/or the floating foundation to the construction vessel (2) during transport using the construction vessel (2).

A method according to claim 19, wherein the mounting step comprises the use of a rotatable subframe (130) connected to the construction vessel (2).

21. A system for mooring a floating foundation to multiple mooring lines, the system comprising: a construction vessel (2) comprising a hoisting system (4); a floating foundation (62) with a positive buoyancy; and a ballast frame (60) for lifting and ballasting the floating foundation (62), the frame having a weight sufficient to submerge the floating foundation to a predetermined depth, the frame having a first connection (82) releasably connectable to the floating foundation (62) and a second connection (86) releasably connectable to the hoisting system (4) allowing the hoisting system to lift the frame and floating foundation assembly.

A system according to claim 21, wherein the hoisting system (4) comprises at least two hoisting cranes (6, 8), the two cranes being adapted to cooperatively lift the combination of the ballast frame (60) and the floating foundation (62) .

System according to claim 22, wherein the ballast frame (60) comprises at least two interconnected parts.

System according to claims 21 to 23, wherein the first connection (82) for connecting the ballast frame (60) and the floating foundation comprises a connection based on compression, friction, a pin connection, a clamp connection or combinations thereof.

A system according to claims 21 to 24, wherein the floating foundation (82) is a tension leg platform (TLP).

A system according to any one of claims 21 to 25, wherein the ballast frame (60) has a lattice construction, a box construction, a V-shaped construction or a combination thereof.

A system according to any one of claims 21 to 26, wherein the floating foundation (62) has a basic shape in plan view, and the ballast frame {60) has a frame shape in plan view that substantially corresponds to the basic shape in plan view.

28. System according to claim 27, wherein the basic shape in top view and/or the frame shape in top view is substantially triangular.

A system according to any one of claims 21 to 28, comprising one or more mooring lines (44), the mooring lines having an end adapted to be moored to the seabed (40) and an opposite end connectable to the floating foundation ( 62), where the mooring lines are adapted to keep the floating foundation submerged at a predetermined depth.

A system according to any one of claims 21 to 29, wherein the floating foundation (82) comprises at least one ballast element (150), allowing the positive buoyancy of the base to be adjusted within a predetermined range.

The system of claim 30, wherein the ballast element includes an opening without valves through which water can enter and exit the ballast element.

System according to any one of claims 21 to 31, wherein the ballast frame (60) has one or more ballast compartments that can accommodate a fluid, preferably with a density greater than water, for example drilling mud, to support the weight of the ballast frame (60). increase.

A system according to any one of claims 21 to 32, wherein the ballast frame (60) is provided with one or more weights (94) to provide a combined weight of ballast frame and weights (94) that at least increases the positive buoyancy of the floating foundation (62), wherein the weight of the weights (94) exceeds at least 50% of the positive buoyancy of the floating foundation (62).

The system of claim 33, wherein the one or more weights (94) are adapted to lower the center of gravity of the frame (60).

A system according to any one of claims 33 or 34, wherein the one or more weights (94) are provided by either a solid material or a liquid or a combination thereof.

36. Assembly comprising: a positively buoyant floating foundation (62); and a ballast frame (60) having a weight sufficient to submerge the ballast frame and floating foundation assembly to a predetermined depth.

Assembly according to claim 36, wherein the ballast frame comprises a first connection (82) that can be releasably connected to the floating foundation (62) and a second connection (86) that can be releasably connected to a hoisting system (4).

38. Assembly according to claim 36 or 37, wherein the floating foundation is a tension leg platform (TLP).

Assembly according to any one of claims 36 to 38, wherein the weight of the ballast frame (80), including any added weights (94), exceeds 1500 tons.

An assembly according to any one of claims 36 to 39, wherein the ballast frame (60) comprises an opening (80) through which a foundation pile (68) of the floating foundation (62) can extend.

41. Assembly according to any one of claims 36 to 40, adapted for the system of claims 21 to 36 or the method according to claims 1 to 20.

42. Ballast frame for lifting and lowering a floating foundation to a predetermined depth.

43. Ballast frame according to claim 42, adapted for the system of claims 21 to 36 or the method of claims 1 to 20.

44. Ballast frame according to claim 42 or 43, wherein the ballast frame has a lattice construction.

The ballast frame according to any one of claims 42 to 44, having a substantially triangular shape or a V-shape in plan view.

A ballast frame according to any one of claims 42 to 45, comprising an opening for a foundation pile (68) of the floating foundation (62).

Ballast frame according to any one of claims 42 to 46, provided with first connectors (82) for connecting the frame (60) to corresponding connectors (84) of the floating foundation (62).

Ballast frame according to claim 47, wherein the first connectors (82) are provided at the bottom of three ends of the ballast frame (60).

Ballast frame according to any one of claims 42 to 48, provided with second connectors (86) to connect the frame (60) to a hoisting system (4).