The present invention deals with an offshore electricity production assembly comprising a floating platform, a wind turbine fixed to the floating platform, and mooring tendons connecting the floating platform to a seabed.

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.

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 comprising a floating platform, a wind turbine fixed to the floating platform, and inclined mooring tendons connecting the floating platform to a seabed, the floating platform comprising: - a tubular central buoyant column extending along a longitudinal axis intended to be vertical, the column having an immersed portion defining a first average external diameter, and

- a plurality of tubular radial buoyant pontoons protruding from the column along radial axes spaced around the longitudinal axis, each of the pontoons defining a second average external diameter, the pontoons being immersed in a body of water, wherein the first average external diameter is larger than the second average external diameter.

In other embodiments, the assembly comprises one or several of the following features, taken in isolation or any technically feasible combination:

- the first average external diameter divided by the second average external diameter is comprised between 1 .4 and 2.3;

- the immersed portion of the column and the pontoons have constant cross-section external diameters along the longitudinal axis and the radial axes respectively;

- the column defines only one or two internal compartment(s);

- each of the pontoons defines only one internal compartment;

- the floating platform is subject to a buoyancy force, the column and the pontoons being configured to provide at least 90% of said buoyancy force;

- the column and the wind turbine have a weight, and the column, considered alone, is subject to a buoyancy force, said buoyancy force being comprised between 85% and 115% of said weight;

- the plurality of pontoons consists of three to five pontoons;

- the plurality of pontoons consists of three pontoons;

- each of the radial axes defines an angle with a transverse plan perpendicular to the longitudinal axis, the angle being comprised between 80° and 100°;

- the floating platform, considered without the wind turbine and the mooring tendons, is configured to float on the body of water with a water line defining a plan parallel to the transverse plan, the plan being lower than the transverse plan, the transverse plan including a volumetric center of the plurality of pontoons;

- the electricity production assembly comprises a first plurality of connecting tendons or connecting braces extending between the column and one of the pontoons respectively for applying upward forces on the pontoons;

- the electricity production assembly comprises a second plurality of connecting tendons or of connecting braces, any two successive pontoons of the plurality of pontoons around the longitudinal axis being connected to each other by one of said connecting tendons or said connecting braces of the second plurality; and - the radial axes are regularly spaced around the longitudinal axis.

The invention also deals with a process for installing the electricity production assembly described above in a predefined location, comprising the following steps:

- providing the floating platform, the mooring tendons, and the wind turbine separately,

- moving the floating platform to the predefined location, the floating platform either floating on the body of water and being towed by at least one ship, or being transported on board a floating vessel,

- connecting the mooring tendons the seabed and to the floating platform, and tensioning the mooring tendons, and

- fixing the wind turbine to the floating platform using at least one lift from a crane vessel.

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:

- Figure 1 is a schematic side view of an assembly according to the invention in operation,

- Figure 2 is a schematic upper view of the floating platform of the assembly shown in Figure 1 ,

- Figure 3 is a graphic showing vertical loads applied by swell on the column and on the pontoons of the platform shown in Figures 1 and 2, as a function of the swell period,

- Figure 4 is a schematic side view of an assembly forming a variant of the assembly represented in Figures 1 and 2, and

- Figure 5 is a schematic upper view of the floating platform of the assembly shown in Figure 4.

An offshore electricity production assembly 10 according to the invention will be described with reference to Figures 1 and 2.

The assembly 10 comprises a floating platform 12 floating on a body of water 14, a wind turbine 16 (partially represented in Figure 1 ) fixed to the floating platform, and inclined mooring tendons 18A, 18B, 18C connecting the floating platform to a seabed 20.

Advantageously, the assembly 10 may comprises a first plurality of connecting tendons 22A, 22B, 22C. For example, the assembly 10 comprises a second plurality of connecting tendons 24A, 24B, 24C.

The floating platform 12 comprises a tubular central buoyant column 26 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 12 is subject to a buoyancy force F, the column 26 and the pontoons 28A, 28B, 28C being advantageously configured to provide at least 90% of said buoyancy force, preferably more than 95% of the buoyancy force F. In other words, buoyancy is achieved by the column 26 and the pontoons 28A, 28B, 28C without any other significant element(s).

Advantageously, the floating platform 12 has an operating draught H1 that is smaller than 35 meters, and more advantageously smaller than 30 meters.

The column 26 has an immersed portion 30 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 14 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. Preferably, the ratio D1/D2 is comprised between 1 .4 and 2.3.

For example, the immersed portion 30 of the column 26 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 26 defines only one or two internal compartment(s) (not shown).

The column 26 and the wind turbine 16 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 85% and 1 15% 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 (Figure 2), the radial axes R1 , R2, R3 define angles ON of 3607N between themselves, N being their number. In the example shown, the angle a _N is approximately 120°.

The radial axes R1 , R2, R3 also defines an angle with a transverse plan P perpendicular to the longitudinal axis Z, the angle being comprised between 80° and 100°. For example, the angle p is about 0°, 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 12, considered without the wind turbine 16 and the mooring tendons 18A, 18B, 18C, is advantageously configured to float on the body of water 14 during wet tow to the predefined location of operations on the body of water 14 with a water line 32 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 25% of the second average external diameter D2. This ensures stability of the floating platform 12 without the wind turbine 16 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 10 is in operation (position shown in Figure 1 ).

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 7° and 17° when the floating platform 12 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 12. In Figure 1 , only the angle y formed by the mooring tendon 18B is represented.

The connecting tendons 22A, 22B, 22C of the first plurality extend between the column 26 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 26. Their function is mainly to reduce the maximum bending moment in the pontons 28A, 28B, 28C once the floating platform 12 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 10 at a predefined location, according to the invention, will now be described.

First, the floating platform 12, the mooring tendons 18A, 18B, 18C, and the wind turbine 26 are provided separately (not assembled). The floating platform 12 is then moved to the predefined location, the floating platform either floating on the body of water 14 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 12 floats on the body of water 14, it does it with a water line 42 corresponding to the plan P1 , which provides stability during transportation of the floating platform.

Once the floating platform 12 is in the predefined location, the mooring tendons 18A, 18B, 18C are connected to the seabed 20 and to the floating platform, and tensioned.

The wind turbine 16 is installed onto the floating platform 12 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 12 is subject to swell conditions.

Swell creates a vertical load on the pontoons (curve C1 in Figure 3) 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 12 is designed in such way that the vertical load VL (curve C3) is close to zero for a period T 1 (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.

Example:

Floating platform 12

Operating draught H1 28m

Wind turbine blade air gap /MSL H2 22m

Central column 26

Column diameter D1 1 1.0m

Cross section 95.0m2

Buoyancy 2725ton

Weight 950ton

Pontoons 28A, 28B, 28C

Pontoon length 40m Pontoon diameter D2 6m

Cross section 28.3m2

Total buoyancy (x3) 3474ton

T otal weight (x3) 1018ton

Wind turbine 16

T otal mass considered 1800ton

In the predefined location where the assembly 10 is, the extreme swell conditions (typically the swell having a 50-years return period) corresponds, for example, to a wave significant height of 12m and a peak period of 16s.

As it can be seen in curve C3, the total vertical load VL is close to zero (at least below 50 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 26 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, for example the operating draught H1 , which in turn reduces the heave excitation force.

This virtuous circle leads to an optimization of the floating platform and the mooring tendons design. The assembly is thus less expensive to produce. With reference to Figures 4 and 5, an assembly 100 constituting a variant of the assembly 10 will now be described. The assembly 100 is analogous to the assembly 10 shown in Figures 1 and 2. The elements that are similar bear the same numeral references and will not be described again. Only the differences will be described hereafter.

Instead of the connecting tendons 22A, 22B, 22C and the connecting tendons 24A, 24B, 24C, the assembly 100 comprises a first plurality of connecting braces 122A, 122B, 122C, and advantageously a second plurality of connecting braces 124A, 124B, 124C.

According to other variants (not shown), the connecting tendons 24A, 24B, 24C are not replaced by the connecting braces 124A, 124B, 124C and remain, or are just not present without being replaced by braces.

The connecting braces 122A, 122B, 122C of the first plurality extend between the column 26 and one of the pontoons 28A, 28B, 28C respectively for applying the upward or downward forces F1 , F2, F3 on the pontoons. The connecting braces 122A, 122B, 122C provide structural reinforcement to the floating platform 12.

For example, the connecting braces 122A, 122B, 122C are tubes having an external diameter D3.

Advantageously each of connecting braces 122A, 122B, 122C has an uppermost part 126 which is located at a depth H3 when the assembly 100 is in operation.

Advantageously, the external diameter D3 is smaller than or equal to half of the second average external diameter D2, or smaller than 3 meters.

The depth H3 is for example greater than 5 meters, or greater than 5 to 30% of the draught H1 . This allows a ship (not shown) to come near the column 26 without bumping in the connecting braces 122A, 122B, 122C.

In the present document, heights and depths that are relative to the sea level are to be understood as relative to the mean sea level, also known as “MSL”.

The connecting braces 124A, 124B, 124C also provide structural reinforcement to the floating platform 12.

For example, the connecting braces 124A, 124B, 124C are tubes having an external diameter D4.

The external diameter D4 is advantageously smaller than or equal to half of the second average external diameter D2, and for example equal to the external diameter D3.

For example, each of the pontoons 28A, 28B, 28C has a length L1 , and radially extends further than the connecting braces 122A, 122B, 122C, by a distance L2 that advantageously represents 25% to 30% of the length L1 . The assembly 100 and its variants operate in the same way as the assembly 10 and provides the same advantages, plus additional rigidity brought by the braces versus tendons.