The present invention deals with a water intake riser for an offshore platform.

It also deals with an assembly of such a platform and at least one such riser.

The platform is for example an FLNG (Floating Liquefied Natural Gas) unit, an FPSO (Floating Production Storage and Offloading) unit, or a semi-submersible unit, such as a SPAR (Single Point Anchor Reservoir). The platform floats on a body of water, generally a sea, an ocean or a lake.

The role of the riser is to bring cooling water from the body of water to production units on the platform. Currently there exists stiff metal risers hung under the platform with a length of 80-150 meters.

In order to improve the energy efficiency of the platform, one solution would consist in using cooler water. One way to obtain cooler water would be to rise water from deeper under the platform, typically from a 700 to 800 meter depth.

However, calculations show that, under the effect of waves and currents, a metal riser of a greater length than 80-150 meters would be subject to mechanical stress leading to reduced lifetime or rupture of the riser.

The use of flexible materials for the design of risers, such as rubber hoses, by nature flexible, allows designing risers that are compatible with the levels of stress encountered. However, such risers would be very expensive.

Some other flexible materials, such as high-density polyethylene (HDPE), are less expensive and could be used for deep risers. Calculations seem to show that such risers could cope with mechanical stress. However, uncertainties remain on the actual resistance of these materials and in particular potential creep effects.

An aim of the invention is thus to provide a reasonably priced solution allowing to improve the energy efficiency of the platform.

To this end, the invention proposes a water intake riser adapted to hang freely from an offshore platform in a body of water downwards along a main axis intended to be vertical, the riser being adapted to bring cooling water to the platform and comprising a plurality of rigid tubular segments successive along the main axis, the plurality of segments having an upper segment and a lower segment along the main axis, the upper segment being adapted to be connected to the platform with an upper ball-and-socket joint, and the lower segment defining a water inlet, the plurality of segments forming a water duct adapted for carrying the cooling water from the water inlet to the upper segment, the water duct having a length along the main axis greater than 200m, the plurality of segments respectively defining central axes, any two successive segments of the plurality of segments being connected to each other respectively with a ball-and- socket tubular joint, said tubular joint being adapted to allow a first segment of said any two successive segments to rotate with respect to a second segment of said any two successive segments around any rotation axis perpendicular to the central axis of the second segment between an aligned position, in which the central axis of the first segment is parallel to the central axis of the second segment, and an inclined position, in which the central axis of the first segment and the central axis of the second segment respectively define a pitch angle around said any rotation axis, the pitch angle being greater than or equal to 4°.

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

- the pitch angle is greater than or equal to 8;

- each segment of the plurality of segments has an internal diameter greater than

40cm;

- each segment of the plurality of segments has a length along the central axis of said each segment, the length being comprised between 20 and 120 times the internal diameter;

- each segment of said plurality of segments comprises more than 90wt% of steel;

- each segment of said plurality of segments comprises a peripheral wall having a thickness perpendicularly to the central axis of said each segment, said thickness being comprised between 1.27cm and 3.81cm;

- each segment of said plurality of segments has a stiffness in kN.m ² (kilonewton square meter), the tubular joint has a stiffness in kN.m/ ⁰ (kilonewton meter per degree), and the ratio of said stiffness of said each segment divided by said stiffness of said tubular joint is comprised between 5000 and 50000;

- said tubular joint is watertight with respect to the body of water;

- said tubular joint comprises an inner part at least partially formed by a taper end of the second segment, and an outer part comprising a flange formed by a fitting end of the first segment, and a body attached on the flange and abutting against said inner part along the central axis of the first segment in order to prevent the inner part from exiting the outer part, wherein the body comprises:

- a steel element fixed to the flange, and

- a flexible element sandwiched between a radially outer surface of the taper end and a radially inner surface of the steel element with respect the central axis of the first segment; - the flexible element comprises an elastomeric material, preferably rubber, in particular natural, nitrile or butadiene rubber;

- the flexible element comprises a hard portion fixed to radially outer surface of the taper end, and a soft portion comprising the elastomeric material, the soft portion being attached to the hard portion and to the radially inner surface of the steel element;

- in section along a plane including the central axis of the first segment, the radially inner surface of the steel element and a rotation center of said tubular joint define a construction axis forming an angle with the central axis of the first segment, the angle being comprised between 25° and 65°; and

- the first segment of said any two successive segments is located above said second segment of said any two successive segments along the main axis in the aligned position.

The invention also proposes an assembly of an offshore platform and such a riser, the riser freely hanging from the platform in a body of water downwards along the main axis, the main axis being vertical, and the upper segment of the riser being connected to the platform with the upper ball-and-socket joint.

The invention also proposes a method of bringing cooling water to an offshore platform comprising the following steps:

- providing the platform, and a water intake riser freely hanging from the platform in a body of water downwards along a vertical main axis, the riser comprising a plurality of rigid tubular segments successive along the main axis, the plurality of segments having an upper segment and a lower segment along the main axis, the lower segment defining a water inlet, the plurality of segments forming a water duct having a length along the main axis greater than 200m, the plurality of segments respectively defining central axes, and any two successive segments of the plurality of segments being connected to each other respectively with a ball-and-socket tubular joint,

- connecting the upper segment to the platform with a ball-and-socket upper joint,

- carrying the cooling water from the water inlet to the upper segment via the water duct, and

- using said tubular joint, allowing a first segment of said any two successive segments to rotate with respect to a second segment of said any two successive segments around any rotation axis perpendicular to the central axis of the second segment between an aligned position, in which the central axis of the first segment is parallel to the central axis of the second segment, and an inclined position, in which the central axis of the first segment and the central axis of the second segment respectively define a pitch angle around said any rotation axis, the pitch angle being greater than or equal to 4°.

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 of an offshore platform and a water intake riser according to the invention, the tubular joints of the riser being in the aligned position,

- Figure 2 is a schematic section view of one of the tubular joints shown in Figure 1 , the tubular joint being in the aligned position and

- Figure 3 is a schematic section view of the tubular joint shown in Figures 1 and 2, the tubular joint being in the inclined position.

In reference to Figure 1 , an assembly 10 according to the invention will be described.

The assembly 10 comprises an offshore platform 12, and at least one water intake riser 14.

As a variant (not shown), the assembly 10 comprises several water intake risers analogous the riser 14.

The platform 12 is for example an FLNG unit or an FPSO unit.

In other embodiments, the platform 12 is any kind of semi-submersible unit, such as a SPAR.

The platform 12 is adapted for floating on a body of water 16, for example a sea, an ocean or a lake. The platform 12 includes at least one production unit 18 requesting cooling water 20 brought from the body of water 16 via the riser 14.

For example, the production unit 18 is adapted to produce LNG (Liquefied Natural

Gas).

The body of water 16 lies on a seabed 22. The body of water 16 has a depth D for example comprised between 500m and 5km.

The body of water 16 may be subject to waves 24 and/or a water current 26 which can both cause movements the riser 14 with respect to the platform 12.

The riser 14 is adapted to freely hang downwards from the platform 12 in the body of water 16 along a vertical main axis V. The riser 14 is adapted to bring the cooling water 20 to the platform 12.

The riser 14 comprises a plurality of rigid tubular segments Si, S ₂ , ...SN successive along the main axis V, where N is the number of segments.

In the example shown, the riser 14 is only made of these segments. By “rigid” it is for example meant that the modulus of elasticity is greater than 100 GPa. The riser 14 is not connected or fixed to the seabed 22, so as to be usable in deep waters.

N is greater than or equal to two. In Figure 1 , five segments are represented, with a break on a segment S, meaning that there can be more segments in particular embodiments.

In other particular embodiments (not shown), the value of N may be 2, 3, 4, 6 or more.

The plurality of segments Si, S ₂ , ...S _N has an upper segment Si and a lower segment S _N along the main axis V, the upper segment Si being connected to the platform 12 with an upper ball-and-socket joint Ji, and the lower segment S _N defining a water inlet 28.

The plurality of segments Si, S ₂ , ...S _N forms a water duct 30 adapted for carrying the cooling water 20 from the water inlet 28 to the upper segment Si. The plurality of segments respectively defines central axes \L, V ₂ , ...V _N , all of which are vertical in Figure 1.

The water inlet 28 is advantageously located at the lower end of the lower segment SN-

The water duct 30 has a length E along the main axis V greater than 200m, preferably 450m, and more preferably 700m.

Any two successive segments Si, S, _+i taken from the plurality of segments Si, S ₂ , ...SN are connected to each other respectively with a ball-and-socket tubular joint J, _+i . The upper segment Si is connected to the second segment S ₂ with the tubular joint J ₂ . The second segment S ₂ is connected to the third segment S ₃ with the tubular joint J ₃ , and so on.

Each segment of the plurality of segments Si, S ₂ , ...S _N is advantageously has an axisymmetric general shape, and defines an internal space 32, for example cylindrical, for the cooling water 20. Each segment has an internal diameter D,, for example greater than 40cm.

Each segment of the plurality of segments Si, S ₂ , ...S _N respectively has a length L along its central axis V,, the length L being comprised between 20 and 120 times the internal diameter D,.

Each segment of said plurality comprises more than 90 wt% (90% per weight) of steel. Each segment of said plurality comprises a peripheral wall 34 having a thickness EE, perpendicularly to the central axis V, of said each segment, said thickness EE, being for example comprised between 1 27cm and 3.81cm.

As shown in Figures 2 and 3, the tubular joint J, _+i is adapted to allow a first segment S, of said any two successive segments Si, S, _+i to rotate with respect to a second segment S, _+i around any rotation axis R, _+i perpendicular to the central axis V _i+i of the second segment between an aligned position (Figures 1 and 2) and an inclined position (Figure 3).

In the aligned position (Figure 2), the central axis V, of the first segment S, is parallel to the central axis V _i+i of the second segment Si _+i .

In the inclined position (Figure 3), the central axis V, of the first segment S, and the central axis V _i+i of the second segment S, _+i define a pitch angle a, ₊ i around said any rotation axis R, _+i

The pitch angle a, ₊ i is greater than or equal to 4°, advantageously 8° This means that any segment S, _+i can deviate with respect to any segment S, in any radial direction and form the pitch angle a, _+i . The maximum deviation angle is at least 4°, and peferably at least 8°. The maximum deviation angle is for example 20°.

Advantageously the tubular joints J ₂ , ...JN, preferably Ji, J ₂ , ...JN, are structurally similar to each other.

Advantageously the segments of said plurality of segments Si, S ₂ , ...S _N , except the upper tubular joint Ji and the water inlet 28, are similar to each other. However, their lengths U may vary from one another.

The segments of said plurality of segments Si, S ₂ , ...S _N have a bending stiffness Ks in kN.m ² (kilonewton square meter).

The tubular joint J, _+i has a rotational stiffness K in kN.m/ ⁰ (kilonewton meter per degree), and the ratio of the stiffness K _s of the segments divided by the stiffness KJ of the tubular joint J, _+i is advantageously comprised between 5000 and 50000.

The tubular joint J, _+i is advantageously watertight with respect to the body of water

16.

As shown in Figure 2 and 3, the tubular joint J, _+i comprises an inner part 36 at least partly formed by a taper end 38 of the second segment Si _+i . The tubular joint J, _+i includes an outer part 40 comprising a flange 42 for example formed by a fitting end 44 of the first segment Si, and a body 46 attached on the flange and abutting against said inner part 36 along the central axis V, of the first segment S, in order to prevent the inner part from exiting the outer part 40. The tubular joint J, _+i advantageously comprises a thrust 48 fixed on the segment S, and defining a spherical surface 50. The spherical surface 50 has a center O located on the rotation axis R, _+i shown in Figures 2 and 3.

The body 46 comprises a steel element 52 fixed to the flange 42, and a flexible element 54 sandwiched between a radially outer surface 56 of the taper end 38 and a radially inner surface 58 of the steel element 52 with respect the central axis V, of the first segment S,.

The flexible element 54 comprises an elastomeric material 55.

In the example, the flexible element 54 comprises a hard portion 60 fixed to radially outer surface 56 of the taper end 38 with screws (not shown), and a soft portion 62 comprising the elastomeric material 55, the soft portion being attached to the hard portion and to the radially inner surface 58 of the body 46.

The flexible element 54 allows rotation of the segment S, _+i with respect to the segment S, around said any rotation axis R, _+i , i.e. around the center O which acts as a rotation center of the tubular joint J, _+i .

Under normal conditions, a clearance exists between the surface 50 and the taper end 38. However, if the axial tension in the tubular joint J, _+i becomes negative, the taper end 38 may abut against the surface 50.

The soft portion 62 for example comprises a plurality of steel reinforcements 63 alternating with layers 64 of the elastomeric material 55. The number of steel reinforcements 63 is equal to the number of layers 64 minus one.

The hard portion 60 and the soft portion 62 are for example bonded together by physico-chemical links along a contact surface 64.

The elastomeric material 55 is for example rubber, in particular natural, nitrile or butadiene rubber. For example, the elastomeric material 55 partly overmolds the hard portion 60.

The radially inner surface 58, the steel reinforcements 63 and the contact surface 64 are for example spherical and centered on the center O.

In section along a plane P including the central axis V,, the radially inner surface 58 and the center O define a construction axis D forming an angle b with the central axis V,, the angle b being advantageously comprised between 25° and 65?

The radially outer surface 56 is conical in the example.

The skilled person knows how to adapt the thickness (along the construction axis D) and the width (perpendicularly to the construction axis D in the plane P) of the steel reinforcements 63 and the layers 64 according to external mechanical load and angle capacity requirements. The first segment S, of said any two successive segments Si, S, _+i is for example located above said second segment S, _+i along the main axis V in the aligned position.

Advantageously the tubular joints J ₂ , ...JN are distributed along the riser 14 based on a modal analysis in order to keep movements and deformations of the riser within predefined limits.

A method of bringing the cooling water 20 to the offshore platform 12 using the riser 14 will now be briefly described.

The riser 14 as described above is provided. The upper segment Si is connected to the platform 12 with the upper joint Ji .

The cooling water 20 is carried from the water inlet 28 to the upper segment Si and then to the platform 12 via the water duct 30. Pumps (not represented) are for example used to displace the cooling water 20.

The cooling water 20 is used to cool the production unit 18, and is for example returned to the body of water 16 after use.

The tubular joints J ₂ , ...JN allow the first segment S, of any two successive segments Si, S, _+i in the plurality of segments Si, S ₂ , ...SN to rotate with respect to the second segment S, _+i around any rotation axis R, _+i perpendicular to the central axis V _i+i of the second segment between the aligned position and the inclined position. The tubular joints Ji advantageously allows the first segment Si to rotate with respect to the platform 12.

Thanks to the above features, the riser 14 and allows obtaining the cooling water 20 from deeper than previously possible. The cooling water 20 is colder than what it would be with an existing shorter riser, thus providing better energy efficiency to the platform. The temperature decrease in cooling water achieved thanks to the riser 14 is comprised between a few degrees Celsius and more than 10°C, ip to 12 or 14°C.

Despite the length of the riser 14, the mechanical stress along the riser due to waves and currents remains acceptable. As a result, the riser 14 is resistant, while less expensive than risers made of flexible material.

The riser 14 provides a reasonably priced solution allowing to improve the energy efficiency of the platform 12.