Patent Description:
It also deals with a process of installing such an assembly in a predefined location.

Such a floating platform is known as a Tensioned-Leg Platform or TLP. When installed, it is permanently moored by tendons, usually connected to corners of the platform structure. TLPs are used for offshore production of oil and gas, and as bases for offshore wind turbines.

Typically, the floating platform may be designed so that the assembly (platform plus wind turbine) can freely float in a stable enough manner for being towed to the predefined location of operations. The design constraint of the hull to be stable during the wet tow operation with the wind turbine integrated onto the hull results in the hull steel weight being increased and, thus, in the overall cost of the floating structure being increased.

Another solution is to include temporary disjoint floating tanks connected by a rigid structure. With this type of floating platform, the assembly (platform plus the wind turbine) can freely float in a stable enough manner for being towed to the predefined location of operations. With this configuration, the cost of the floating platform can be reduced, but the additional cost of the temporary floating tanks together with their installation and removal offshore increases the overall cost and project risks.

Motions of the installed assembly is also a major issue for smooth operation of the wind turbine and for reducing mechanical stress in the assembly. Using inclined mooring tendons (not parallel to each other) has proven to be a good solution to reduce the horizontal motions of the assembly, in particular at the elevation of the rotor-nacelle assembly of the wind turbine. However, using inclined tendons reduces the capacity to withstand the vertical loads driven by waves, particularly in extreme conditions. As a consequence, the capacity of the mooring has to be increased. <CIT> refers to a floating structure provided with a hollow body prearranged to support a wind generator and suitable to be installed in deep water by a plurality of arms which can be stably connected to the ground by anchoring members for each arm and a connection member arranged between each anchoring member and a respective arm, distance adjustment means being provided in order to vary the inclination of the hollow body with respect to the ground.

Therefore, the above design constraints result in a high cost of the assembly.

An aim of the invention is thus to provide an offshore electricity production assembly that is less expensive to produce.

To this end, the invention proposes an offshore electricity production assembly according to claim <NUM>.

In other embodiments, the assembly comprises one or several of the features corresponding to claims <NUM> to <NUM>, taken in isolation or any claimed combination.

The invention also deals with a process according to claim <NUM>.

The invention and its advantages will be better understood upon reading the following description, given solely by way of example and with reference to the appended drawings, in which:.

An offshore electricity production assembly <NUM> according to the invention will be described with reference to <FIG> and <FIG>.

The assembly <NUM> comprises a floating platform <NUM> floating on a body of water <NUM>, a wind turbine <NUM> (partially represented in <FIG>) fixed to the floating platform, and inclined mooring tendons 18A, 18B, 18C connecting the floating platform to a seabed <NUM>.

The assembly <NUM> comprises a first plurality of connecting tendons 22A, 22B, 22C. Furthermore, the assembly <NUM> comprises a second plurality of connecting tendons 24A, 24B, 24C.

The floating platform <NUM> comprises a tubular central buoyant column <NUM> extending along a longitudinal axis Z intended to be vertical, and a plurality of tubular radial buoyant pontoons 28A, 28B, 28C protruding from the column along radial axes R1, R2, R3 spaced around the longitudinal axis Z.

The floating platform <NUM> is subject to a buoyancy force F, the column <NUM> and the pontoons 28A, 28B, 28C being configured to provide at least <NUM>% of said buoyancy force, preferably more than <NUM>% of the buoyancy force F. In other words, buoyancy is achieved by the column <NUM> and the pontoons 28A, 28B, 28C without any other significant element(s).

The column <NUM> has an immersed portion <NUM> defining a first average external diameter D1. Each of the pontoons 28A, 28B, 28C defines a second average external diameter D2, the pontoons being fully immersed in the body of water <NUM> when the mooring tendons 18A, 18B, 18C are tensioned.

By "average external diameter" of a tubular element, it is meant the external diameter of a cylindrical tube having the same volume.

The first average external diameter D1 is larger than the second average external diameter D2, wherein the ratio D1/D2 is comprised between <NUM> and <NUM>.

For example, the immersed portion <NUM> of the column <NUM> and the pontoons 28A, 28B, 28C have constant cross-section diameters along the longitudinal axis Z and the radial axes R1, R2, R3 respectively. As a consequence, their average external diameters D1, D2 are equal to their external diameters.

Advantageously, the column <NUM> defines only one or two internal compartment(s) (not shown).

The column <NUM> and the wind turbine <NUM> have a weight W1, and the column, considered alone, is subject to a buoyancy force W2, said buoyancy force W2 being for example comprised between <NUM>% and <NUM>% of said weight W1.

Advantageously, the plurality of pontoons 28A, 28B, 28C consists of three to five pontoons, preferably three pontoons.

For example, each of the pontoons 28A, 28B, 28C defines only one internal compartment (not shown).

Advantageously, the radial axes R1, R2, R3 are regularly spaced around the longitudinal axis Z. Seen from above (<FIG>), the radial axes R1, R2, R3 define angles αN of <NUM>°/N between themselves, N being their number. In the example shown, the angle αN is approximately <NUM>°.

The radial axes R1, R2, R3 also defines an angle β with a transverse plan P perpendicular to the longitudinal axis Z, the angle being comprised between -<NUM>° and +<NUM>°. For example, the angle β is about <NUM>°, which means that the radial axes R1, R2, R3 are perpendicular to the longitudinal axis Z.

The transverse plan P for example contains the volumetric center C of the plurality of pontoons 28A, 28B, 28C.

The floating platform <NUM>, considered without the wind turbine <NUM> and the mooring tendons 18A, 18B, 18C, is advantageously configured to float on the body of water <NUM> during wet tow to the predefined location of operations on the body of water <NUM> with a water line <NUM> defining a plan P1 parallel to the transverse plan P.

The plan P1 is advantageously located below the transverse plan P, and for example defines a distance D between them, said distance being smaller than <NUM>% of the second average external diameter D2. This ensures stability of the floating platform <NUM> without the wind turbine <NUM> on it during wet tow.

In the example, there are three mooring tendons 18A, 18B, 18C.

The mooring tendons 18A, 18B, 18C are tensioned when the assembly <NUM> is in operation (position shown in <FIG>).

The mooring tendons 18A, 18B, 18C are inclined with respect to the longitudinal axis Z (considered vertical for this matter), and converge upwards. For example, the mooring tendons 18A, 18B, 18C form angles with the longitudinal axis Z that are all comprised between <NUM>° and <NUM>° when the floating platform <NUM> is balanced without environmental loads (wind, waves, current). These angles may differ from each other and evolve with time, depending on the movements of the floating platform <NUM>. In <FIG>, only the angle γ formed by the mooring tendon 18B is represented.

The connecting tendons 22A, 22B, 22C of the first plurality extend between the column <NUM> and one of the pontoons 28A, 28B, 28C respectively for applying upward forces F1, F2, F3 on the pontoons. The connecting tendons 22A, 22B, 22C are advantageously pre-tensioned after the assembly of the pontoon elements 28A, 28B, 28C to the column <NUM>. Their function is mainly to reduce the maximum bending moment in the pontoons 28A, 28B, 28C once the floating platform <NUM> is connected to the mooring tendons 18A, 18B, 18C.

Any two successive pontoons 28A, 28B, 28C around the longitudinal axis Z are connected to each other by one of the connecting tendons 24A, 24B, 24C of the second plurality.

A process for installing the assembly <NUM> at a predefined location, according to the invention, will now be described.

First, the floating platform <NUM>, the mooring tendons 18A, 18B, 18C, and the wind turbine <NUM> are provided separately (not assembled).

The floating platform <NUM> is then moved to the predefined location, the floating platform either floating on the body of water <NUM> and being towed by at least one ship (not shown), or being transported on board a floating vessel (also not shown). If the floating platform <NUM> floats on the body of water <NUM>, it does it with a water line <NUM> corresponding to the plan P1, which provides stability during transportation of the floating platform.

Once the floating platform <NUM> is in the predefined location, the mooring tendons 18A, 18B, 18C are connected to the seabed <NUM> and to the floating platform, and tensioned.

The wind turbine <NUM> is installed onto the floating platform <NUM> using at least one lift or a crane vessel (not shown).

In operation, the mooring tendons 18A, 18B, 18C are tensioned and the floating platform <NUM> is subject to swell conditions.

Swell creates a vertical load on the pontoons (curve C1 in <FIG>) and a vertical load on the column (curve C2) that has an opposite sign. Curve C3 represents the sum VL of these vertical loads.

The curves C1, C2, C3 are calculated using the below example. Vertical loads, in kN/m (kilonewton per meter of wave amplitude), are expressed as a function of the wave period T in seconds. The floating platform <NUM> is designed in such way that the vertical load VL (curve C3) is close to zero for a period T1 (so called cancellation period), corresponding to the typical swell peak period encountered in extreme conditions at the location of operations, thus reducing the extreme tension in the mooring tendons 18A, 18B, 18C. The cancellation period T1 can be shifted by adjusting the D1/D2 ratio.

In the predefined location where the assembly <NUM> is, the extreme swell conditions (typically the swell having a <NUM>-years return period) corresponds, for example, to a wave significant height of <NUM> and a peak period of <NUM>.

As it can be seen in curve C3, the total vertical load VL is close to zero (at least below <NUM> kN) for that particular swell peak period. Thanks to the diameter D1 being larger than the diameter D2 and adjusted to the right ratio, the vertical load applied by swell on the column <NUM> compensates the vertical load applied by swell on the pontoons 28A, 28B, 28C. Thus, the cancellation period T1 matches the peak period of the extreme swell conditions, which minimizes the extreme tensions in the mooring tendons 18A, 18B, 18C.

Removing the constraint of the floating platform to be stable in pre-service conditions (i.e. prior to the connection to the mooring tendons) with the wind turbine integrated onto the floating platform allows for further optimization of the design with regards to its volume distribution between the column and the pontoons leading to a reduction of the wave induced excitation force in the vertical axis (i.e. heave excitation force).

This results in a virtuous circle when designing the floating platform and the mooring tendons reducing the required floating platform volume (and thus its weight) and the extreme tension in the mooring tendons. Indeed, the pre-tension of the mooring tendons shall be high enough to prevent negative tension under extreme environmental conditions. The heave excitation force is one of the main contributors of the mooring tendon dynamic load. Reducing this heave excitation force by optimizing the floating platform design (i.e. its volume distribution) allows for a reduction of the pre-tension, as well as a reduction of the extreme tension in the mooring tendons. The pre-tension being ensured by the buoyancy of the floating platform, the reduced pre-tension allows for further reduction of the floating platform size which in turn reduces the heave excitation force.

Claim 1:
An offshore electricity production assembly (<NUM>) comprising a floating platform (<NUM>), a wind turbine (<NUM>) fixed to the floating platform (<NUM>), and inclined mooring tendons (18A, 18B, 18C) connecting the floating platform (<NUM>) to a seabed (<NUM>), the floating platform (<NUM>) comprising:
- a tubular central buoyant column (<NUM>) extending along a longitudinal axis (Z) intended to be vertical, the column (<NUM>) having an immersed portion (<NUM>) defining a first average external diameter (D1) which is the external diameter of a cylindrical tube having the same volume as the immersed portion (<NUM>) of the column (<NUM>), and
- a plurality of tubular radial buoyant pontoons (28A, 28B, 28C) protruding from the column (<NUM>) along radial axes (R1, R2, R3) spaced around the longitudinal axis (Z), each of the pontoons (28A, 28B, 28C) defining a second average external diameter (D2) which is the external diameter of a cylindrical tube having the same volume as the considered pontoon (28A, 28B, 28C), the pontoons (28A, 28B, 28C) being immersed in a body of water (<NUM>), wherein the first average external diameter (D1) is larger than the second average external diameter (D2),
wherein the floating platform (<NUM>) is subject to a buoyancy force (E), the column (<NUM>) and the pontoons (28A; 28B, 28C) being configured to provide at least <NUM>% of said buoyancy force (F),
wherein each of the radial axes (R1, R2, R3) defines an angle (β) with a transverse plan (P) perpendicular to the longitudinal axis (Z), the angle (β) being comprised between - <NUM>° and +<NUM>°, and
wherein the first average external diameter (D1) divided by the second average external diameter (D2) is comprised between <NUM> and <NUM>.