WO2020008042A1

WO2020008042A1 - Fairings for aquatic structures

Info

Publication number: WO2020008042A1
Application number: PCT/EP2019/068118
Authority: WO
Inventors: Clément BOIREAU
Original assignee: Total SE
Current assignee: TotalEnergies SE
Priority date: 2018-07-06
Filing date: 2019-07-05
Publication date: 2020-01-09
Anticipated expiration: 2021-01-06
Also published as: AR115711A1; WO2020008224A1

Abstract

The invention relates to an aquatic structure (4) equipped with a fairing (1), said fairing (1) comprising a flexible waterproof tarpaulin (2) configured to define an interior space (8) and an aperture (3) configured to put said interior space (8) into fluid communication with the environment, said interior space being configured to inflate when filled with water, wherein the aquatic structure (4) has a main direction, and the ratio of the maximum dimension of the fairing (1) orthogonal to the main direction of the aquatic structure (4), when the interior space of the tarpaulin (2) is inflated, relative to the diameter of the aquatic structure (4) orthogonal to the main direction is lower than or equal to 6. The invention also relates to a method for installing such a fairing (1) on an aquatic structure (4).

Description

FAIRINGS FOR AQUATIC STRUCTURES

TECHNICAL FIELD

The present invention relates to new fairings for aquatic structures and to aquatic structures comprising such new fairings.

TECHNICAL BACKGROUND

Marine pipes and cables and structures which extend from a ship or platform on the surface of the ocean to the ocean floor, such as production risers, as well as many other structures in contact with water, may be subjected to strong water currents. These currents lead to the creation of vortex-induced-vibrations and high drag forces in the aquatic structures which destabilize them. In addition, current direction may vary according to water depth. Water temperature may also broadly vary, for example from 30°C to 0°C.

In order to reduce the vibrations and drag forces, aquatic structures may be fitted with fairings having specific shapes, which makes it possible to canalize the water flow around the pipes or cables. Fairings may be used preferably at a depth of from 0 to 6000 m but could even go deeper.

As of today, various fairing profiles exist. However, theses fairings are rigid structures.

For example, US 4,398,487 describes a fairing having a rigid body made of plastic, optionally reinforced with fiberglass, of metal or of a multilayered material consisting of a syntactic foam core sandwiched between fiberglass outer coverings.

US 6,048,136 describes fairings for drilling risers made from fiberglass.

The extension of rigid fairings from the aquatic structures is limited by space constraints. Indeed, marine pipes or cables, for example, are often used in offshore platforms with limited available space. The equipment present on these platforms needs to have a volume which is as low as possible.

Rigid fairings, with a short profile, exhibit poor hydrodynamic qualities, such as a relatively high drag coefficient, and have insufficient efficiency in reducing vortex-induced-vibrations and/or drag forces when water currents are strong. There is thus a need for improved fairings for aquatic structures having high hydrodynamic performance without being cumbersome before their installation in the operating environment, e.g. the ocean or a river.

SUMMARY OF THE INVENTION

It is a first object of the invention to provide a fairing for an aquatic structure, comprising a flexible waterproof tarpaulin configured to define an interior space and an aperture configured to put said interior space into fluid communication with the environment, said interior space being configured to inflate when filled with water.

The invention also relates to a fairing for an aquatic structure, said fairing comprising a flexible waterproof tarpaulin configured to define an interior space and an aperture configured to put said interior space into fluid communication with the environment, said interior space being configured to inflate when filled with water, wherein the aquatic structure has a main direction, and the ratio of the maximum dimension of the fairing orthogonal to the main direction of the aquatic structure, when the interior space of the tarpaulin is inflated, relative to the diameter of the aquatic structure orthogonal to the main direction is lower than or equal to 6.

The invention also relates to an aquatic structure equipped with a fairing, said fairing comprising a flexible waterproof tarpaulin configured to define an interior space and an aperture configured to put said interior space into fluid communication with the environment, said interior space being configured to inflate when filled with water,

wherein the aquatic structure has a main direction, and the ratio of the maximum dimension of the fairing orthogonal to the main direction of the aquatic structure, when the interior space of the tarpaulin is inflated, relative to the diameter of the aquatic structure orthogonal to the main direction is lower than or equal to 6.

In some embodiments, the fairing comprises a rigid structure configured to be rotatably disposed on the aquatic structure, the tarpaulin being attached to the rigid structure.

In some embodiments, the rigid structure is in the shape of a tube.

In some embodiments, the rigid structure is a rigid fairing.

In some embodiments, the tarpaulin is attached to the rigid structure via bolts, snap buttons, Velcro straps, zips, grooves, tumblers, by heat-sealing, heat- bonding or a combination thereof. In some embodiments, the tarpaulin is configured to be directly attached to the aquatic structure, the tarpaulin being preferably further configured to be rotatably disposed on the aquatic structure.

In some embodiments, the fairing is configured to rotate around the aquatic structure under the effect of flow when it is in water.

In some embodiments, the tarpaulin is made of polyamide, polyester, polytetrafluoroethylene, polypropylene, polyethylene, oilcloth, such as a cotton or linen cloth with a coating of boiled linseed oil, or a combination thereof.

In some embodiments, the aperture comprises a sieve.

In some embodiments, the transverse cross section of the fairing has one substantially pointed extremity when the interior space is filled with water.

In some embodiments, the interior space is divided into several cells by internal partitions.

In some embodiments, the fairing comprises at least two fairing elements placed one after the other according to an axis, each fairing element comprising a flexible waterproof tarpaulin and an aperture.

In some embodiments, each fairing element is independently configured to be rotatably disposed on the aquatic structure.

In some embodiments, the fairing surrounds the aquatic structure.

In some embodiments, the aquatic structure comprises at least one translation stop for preventing translation of the fairing along the aquatic structure.

In some embodiments, the translation stop is a flange or a clamp or a collar or a sleeve or a clip fixed to the aquatic structure.

In some embodiments, the fairing is attached to another, rigid, underlying fairing.

In some embodiments, the ratio of the maximum dimension of the fairing orthogonal to the main direction of the aquatic structure, when the interior space of the tarpaulin is inflated, relative to the diameter of the aquatic structure orthogonal to the main direction is lower than or equal to 5, preferably lower than or equal to 4.5.

In some embodiments, the aquatic structure is chosen from the group consisting of drilling risers, production risers, umbilicals, flexibles pipes, flow lines or control lines going from the water surface to the sea floor, anchoring structures and fixed structures, such as girders, horizontal structures, cross beams, tendons, floating structures such as semi-submersible structures and horizontal beams of floating platforms.

It is another object of the invention to provide a method for installing a fairing on an aquatic structure, comprising the steps of: providing a fairing as described above,

attaching said fairing to an aquatic structure as described above.

In some embodiments, the fairing is attached to the aquatic structure on a ship or offshore platform or onshore or on a subsea structure.

It is another object of the invention to provide a use of the fairing as described above for reducing vortex-induced-vibrations and/or drag forces in an aquatic structure.

In some embodiments, the aquatic structure, in the abovementioned use, is selected from the group consisting of drilling risers, production risers, umbilicals, flexibles pipes, flow lines or control lines going from the water surface to the sea floor, anchoring structures and fixed structures, such as girders horizontal structures, cross beams, tendons, floating structures such as semi-submersible structures and horizontal beams of floating platforms.

The present invention enables to meet the abovementioned need. In particular the invention provides a fairing for an aquatic structure exhibiting high hydrodynamic performances, which makes it possible to efficiently reduce vortex- induced-vibrations and/or drag forces, while having a reduced size before its installation.

This is achieved by the use of a flexible tarpaulin which defines an interior space capable of inflating under water pressure and by the presence of an aperture in the fairing so as to put the interior space into fluid communication with the environment surrounding the fairing, that is to say the water in which the aquatic structure is immersed, as well as by a specific ratio of the maximum dimension of the fairing orthogonal to the main direction of the aquatic structure (when the interior space of the tarpaulin is inflated) relative to the diameter of the aquatic structure orthogonal to the main direction.

When the fairing according to the invention is not in its operating environment, such as the ocean or a river, the interior space defined by the flexible tarpaulin may be deflated and the tarpaulin part of the fairing may be, for example, folded down to reduce the volume of the fairing. But, when the aquatic structure equipped with the fairing according to the invention is installed in water, water flows through the aperture of the fairing and fills its interior space. The interior space inflates under the pressure of said water, resulting in an increase in volume of the fairing and an increase in the extension of the fairing away from the aquatic structure (such as a cable or pipe) that it equips.

This makes it possible to use larger (thus more efficient) fairing profiles than those of the rigid fairings of the prior art so as to effectively reduce vortex- induced-vibrations and/or drag forces in the aquatic structures fitted with the fairings.

However, surprisingly, the fairing with a flexible tarpaulin must not be too large. The specific ratio of the maximum dimension of the fairing of the invention relative to the diameter of the aquatic structure on which the fairing is mounted makes it possible to further reduce drag forces in the aquatic structure. In addition, if the fairing is too large, the efforts on the tarpaulin when the fairing is used may be too high, cavitation may appear and in extreme cases the tarpaulin may even tear. Therefore, a larger dimension of the tarpaulin could only be achieved by stiffening the tarpaulin. The specific ratio of the maximum dimension of the fairing of the invention relative to the diameter of the aquatic structure also enables to overcome this inconvenience.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic view of a transverse cross section of an example of fairing according to the invention.

Figure 2 is a schematic longitudinal view of an example of fairing according to the invention.

Figure 3 is a schematic view of a transverse cross section of another example of fairing according to the invention.

Figure 4 is a schematic view of a transverse cross section of another example of fairing according to the invention.

DESCRIPTION OF EMBODIMENTS

The invention will now be described in more detail without limitation in the following description.

Making reference to Figures 1 and 2, a fairing 1 according to the invention is intended to be used for a marine pipe or cable, or for any other aquatic structure 4.

By“aquatic structures", is meant any mechanical structure having at least a part of it immersed in water. The aquatic structures generally have an overall elongated shape. This makes it possible to define a main (longitudinal) direction of an aquatic structure. Preferably, the aquatic structure has a cross section in a plane orthogonal to the main direction of the aquatic structure which is circular.

In the following description, for reasons of conciseness and clarity, the invention will mainly be described in relation with a pipe. However, this description applies to any aquatic structure in a similar way. The aquatic structure of the invention may be placed in e.g. a lake or a river. More preferably, it is placed in a sea or ocean. The main direction of the aquatic structure may extend from a shore to off-shore, for example substantially horizontally; or it may preferably extend substantially vertically from the surface of the water; or it may extend, for example from the surface of the water, at any angle between the horizontal and the vertical.

The fairing 1 according to the invention comprises a flexible, waterproof tarpaulin 2 which is configured to define an interior space 8.

By“flexible tarpaulin" is meant that the tarpaulin can be deformed under water pressure. In particular, the interior space 8 is able to inflate when filled with water.

The tarpaulin 2 may be made of any material with the proviso that it remains flexible and waterproof. Advantageously, the material of which the tarpaulin is made is a biocide material, in particular a bactericide material, and/or an abrasion- resistant material and/or a salt-resistant material. It may be a woven or a non- woven material, or a plastic material produced by molding, extrusion or the like. For example, the tarpaulin may be made of polyamide, polyester, polytetrafluoroethylene, polypropylene, polyethylene, or a combination thereof. It may also contain fibers, such as textile fibers, carbon fibers or metal fibers, or any fibers capable of increasing resistance of the tarpaulin, and may be treated, for example coated with any known coating material to make it water-impervious. By way of example, the tarpaulin may be made of or comprise an oilcloth (such as a cotton or linen cloth with an oil coating, such as a boiled linseed oil coating). The tarpaulin may be made of a biodegradable material. This is particularly advantageous when the fairing is needed only for a certain period, for example when the fairing is needed only for the laying of the aquatic structure: thus, there is no need to withdraw the fairing which is degraded and removed from the aquatic structure by itself over time. The tarpaulin may be made of a single material or of a plurality of different materials, such as the abovementioned materials, for example arranged in successive layers.

The tarpaulin may also comprise a reinforcing structure, such as a braided structure, for instance a metallic braided structure.

In order to define the interior space 8, the tarpaulin 2 may be, for example, sewn and/or heat-sealed.

The fairing according to the invention also comprises an aperture 3 configured to put the interior space 8 into fluid communication with the environment. By“environment’ is meant the medium in which the fairing 1 is placed, for example, ocean water.

By“fluid communication" is meant that any fluid from the environment is able to reach the interior space 8, through the aperture 3. In operation, water in which the pipe 4 is immersed flows through the aperture 3 and fills the interior space 8. Under the pressure of said water, the interior space 8 inflates to confer its operational shape to the fairing 1 .

The aperture 3 may comprise a sieve or grid. In some embodiments, the aperture consists of a sieve. Such a sieve makes possible to prevent objects exceeding a certain size from entering into the interior space 8. The sieve may be made of any material compatible with use in the ocean or more generally in water. The aperture 3 may also comprise a means to stay open and avoid collapsing despite the pressure from the current, for example the sieve could be made out of rigid material to prevent the aperture 3 from collapsing. Rigid rings could also be used.

For example, the sieve may be made of stainless steel.

In some embodiments, the aperture 3 may be configured such that water is able to flow through the aperture 3 in only one direction. For example, the aperture 3 may comprise a check valve.

The fairing 1 according to the invention may comprise only a single aperture 3 putting the interior space 8 into fluid communication with the environment.

Alternatively, the fairing 1 may comprises several apertures 3. Such an embodiment is shown in Figure 3. Each of these apertures 3 may be, for example, a sieve or a check valve. The fairing 1 may further comprise a pressure relief valve 10. This embodiment is particularly advantageous when the fairing 1 may be subjected to strong current, in order to prevent any damage to the fairing.

The fairing 1 may comprise a rigid structure 5. By“rigid structure" is meant a structure which cannot be deformed by the pressure of water naturally existing in oceans, seas, rivers or the like. The rigid structure 5 may comprise, or be made of, any material compatible with the materials with which the rigid structure 5 is intended to be in contact, such as the materials of the aquatic structure. Examples of suitable materials are high-density polyethylene (FIDPE), polyurethane, polytetrafluoroethylene, any metal or alloy such as steel, wood, or a combination thereof.

In some embodiments, the rigid structure 5 is in the shape of a tube. Preferably, this rigid tube is intended to surround the pipe 4. The rigid structure 5 is preferably free to rotate with respect to the pipe 4. This enables the fairing 1 to weathervane around the pipe 4 (about the main direction thereof), under the effect of flow. Therefore, a change in the direction of the current will make the fairing 1 weathervane around the pipe 4. The freedom in rotation may be conferred by means of ball bearings or bearings made of materials having a low friction coefficient with the pipe’s materials. In other embodiments, the rigid and free to rotate structure 5 is directly in contact with the pipe 4 and the rotation is performed by sliding of the rigid structure 5 with respect to the pipe 4. The rigid structure 5 may be for example in polyoxymethylene.

In some embodiments, the rigid structure 5 is able to allow water to flow to the interior space 8 of the tarpaulin 2 through the rigid structure 5. The rigid structure 5 may have grooves, holes or any system to allow water from the environment to flow to the interior space 8 of the tarpaulin 2.

The rigid structure 5 may extend to form a minimalist fairing comprising a rigid aperture 3, preferably it extends inside the interior space 8. Such an embodiment is shown in Figure 4. In these embodiments, the aperture 3 is positioned in the rigid structure 5. The tarpaulin 2 is attached to the rigid structure 5 so that the interior space 8 of the tarpaulin 2 is in fluid communication with the environment, through the rigid structure 5, the tarpaulin 2 being able to inflate when the fairing is used. The rigid structure 5 is able to allow water to flow from the aperture 3 to the interior space 8 through the rigid structure 5 by any means. An advantage of such embodiments is that, in case of destruction of the tarpaulin 2, a smaller rigid fairing remains, allowing drag force reduction. In such embodiments, the rigid structure 5 is preferably able to rotate with respect to the pipe 4.

In some embodiments, the rigid structure 5 may be discontinuous along the pipe 4. For example, it may be made of rigid rings or bearings distributed along the pipe 4, leaving space between each of them. These embodiments may be particularly advantageous to keep the aperture 3 open when the fairing 1 is immerged into water. Indeed, the aperture(s) 3 may be present in the tarpaulin 2 surrounding the pipe 4. The presence of distinct rigid rings distributed along the pipe and positioned between the pipe 4 and the tarpaulin 2 that comprises the aperture(s) 3 and that surrounds the pipe 4 makes it possible to keep a gap between the pipe 4 and the tarpaulin 2 (and therefore between the pipe 4 and the aperture 3 in the tarpaulin 2) at the portions of the pipe 4 between each rigid rings. This prevents the aperture 3 in the tarpaulin 2 from being pressed against the pipe 4, which would lead to the closing of the aperture 3. In other embodiments, the aperture 3 is part of the rigid structure 5, for example as shown in figure 4.

The rigid structure 5, for example the rigid rings, may comprise a rigid part extending away from the pipe 4 inside the interior space 8. This may help to rigidify a little bit the fairing structure and help the fairing 1 to rotate with respect to the pipe 4 under the effect of flow by providing a longer lever arm for the flow.

The structure 5, for example the rigid rings, may also help the fairing 1 to rotate with respect to the pipe 4 under the effect of flow by reducing the friction between the fairing 1 and the pipe 4.

The tarpaulin 2 may comprise attachment elements for attaching the tarpaulin 3 onto the rigid structure 5. Suitable attachment elements are bolts, snap buttons, Velcro straps, zip or a combination thereof. The tarpaulin 2 may also be attached onto the rigid structure 5 by heat-sealing and/or heat-bonding. The tarpaulin may also be attached through a tumbler or a groove. The attachment elements are preferably watertight.

In other embodiments, the tarpaulin 2 is directly attached onto the pipe 4, i.e. no intermediate structure is provided between the tarpaulin 2 and the pipe 4. The tarpaulin 2 may be attached onto the pipe 4 by means of bolts, snap buttons, Velcro straps, tumblers, grooves, heat-sealing, heat-bonding or a combination thereof. Preferably, in these embodiments, the tarpaulin 2 is free to rotate with respect to the pipe 4 (about the main direction thereof).

The fairing 1 is advantageously able to rotate (about the main direction of the pipe 4) under the effect of flow when it is in its operating environment, i.e. water.

In an advantageous variant, the (or one) aperture 3 is located in the fairing 1 such as to face, or substantially face, the current. Preferably, the fairing 1 is able to rotate (about the main direction of the pipe 4) under the effect of flow when it is in water such that the (or one) aperture 3 always faces, or substantially faces the current. In other terms, the fairing 1 is preferably configured so as to orient itself in water depending on the direction of the current so that maximum water pressure on the fairing 1 is exerted at the position of the (or one) aperture 3.

If the (or one) aperture is not located in the fairing 1 such as to face, or substantially face, the current, this aperture is preferably located in the fairing such as not to face any depression area 1 1 which may form next to the pipe 4 due to the position of the current streamlines 12 around the pipe 4. The depression areas are caused by water flow velocity, when the water flow velocity is higher than the water current velocity. Depression areas are complex to evaluate and depend on the water velocity and the fairing geometry. The fairing 1 may also comprise a water intake structure such as but not limited to a NACA inlet to overcome the depression area and to fill the interior space 8 of the fairing 1 .

In operation, when the interior space 8 is filled with water and inflated, the transverse cross section of the fairing 1 (i.e. the cross section transverse to the main direction of the pipe) may have an elongated shape, with one end conforming to the pipe 4 or rigid structure 5 (such as a partly circular part), and the opposite end extending away from the pipe 4. In some embodiments, the opposite end extends away from the pipe in a substantially pointed manner. In other embodiments, the opposite end may have a substantially flat shape, or curved shape (for instance a concave shape or a convex shape or a shape having both convex and concave portions) or any other suitable shape. Preferably, the aperture 3 is located in the part conforming to the pipe 4. In some embodiments, the shape of the partly circular part is induced by the tubular rigid structure 5. For example, the tarpaulin 2 surrounds at least part or all of the rigid structure 5.

The interior space 8 may comprise internal partitions which divide it into several cells. These internal partitions make it possible to maintain a predefined shape for the interior space 8 when the tarpaulin 2 is inflated. Preferably, the internal partitions are made of the same materials as the tarpaulin 2. Each cell contains at least one aperture to enable water to fill it, the apertures being preferably in the internal partitions, with the proviso that the fairing 1 contains at least one aperture 3 directly open onto the environment as described above. Therefore, all cells are in fluid communication with the environment, directly or indirectly. In advantageous embodiments, the interior space 8 may comprise internal partitions forming a honeycomb structure in the interior space 8. This makes it possible to increase the overall rigidity of the fairing 1 , in particular when the fairing 1 is subjected to high speed currents or when it has relatively high dimensions, while keeping a relative flexibility.

The pipe 4 equipped with the fairing 1 may comprise at least one translation stop for preventing the fairing 1 from moving along the pipe 4. The stops may be flanges 7. Alternatively, or in addition, the stops may be clamps, collars, sleeves, or clips. The flanges 7 (or the clamps or the collars, or the sleeves, or the clips) are fixed to the pipe 4. The flanges (or the clamps or the collars, or the sleeves, or the clips) may be made of any metal or alloy such as steel, polymer material, for example rubber, such as synthetic rubber or natural rubber, or a combination thereof.

If any aquatic structure 4 is equipped with a fairing 1 , the at least one translation stop could be made of the structure material. For example, on concrete piles, the at least one translation stop could be made incorporated on the aquatic structure 4 and in concrete.

The length L1 of the fairing 1 is preferably lower than or equal to 6 times the diameter L2 of the pipe 4. In other words, the ratio of the length L1 of the fairing 1 relative to the diameter L2 of the pipe 4 is preferably lower than or equal to 6 times (in the present description, these two wordings are equivalent). By “length L1 of the fairing", is meant the maximum extension of the fairing in operation, i.e. when the interior space 8 is filled with water. By“diameter L2 of the pipe (or of the aquatic structure) 4", is meant the maximum dimension of the pipe (or aquatic structure) in a plane orthogonal to the main direction of the pipe (or aquatic structure). The length L1 of the fairing is the longer dimension that is perpendicular to the axis of the pipe. Preferably, length L1 is higher than or equal to twice the diameter L2. Length L1 may be from twice to 2.5 times the diameter L2, or from 2.5 times to 3 times the diameter L2, or from 3 times to 3.5 times the diameter L2, or from 3.5 times to 4 times the diameter L2, or from 4 times to 4.5 times the diameter L2, or from 4.5 times to 5 times the diameter L2, or from 5 times to 5.5 times the diameter L2, or from 5.5 times to 6 times the diameter L2. More preferably, the length L1 of the fairing 1 is lower than or equal to 5 times, more preferably lower than or equal to 4.5, the diameter L2 of the pipe 4.

The fairing 1 may comprise at least two fairing elements 9. A fairing 1 comprising two fairing elements 9 is shown in Figure 2. The fairing elements 9 are preferably successively placed along the main direction of the pipe 4. Each of the fairing elements 9 comprises a flexible waterproof tarpaulin 2 and an aperture 3 as described above and may comprise independently all the abovementioned features. When the fairing 1 comprises several fairing elements 9, the sentence that the fairing comprises“only a single aperture" must be understood as meaning that each of the fairing elements contains only a single aperture and the sentence that the fairing comprises“several apertures" must be understood as meaning that at least one fairing element comprises several apertures, and that in particular all of the fairing elements may comprise several apertures, where all the fairing elements may not have the same number of apertures. These embodiments are particularly advantageous for ease of installation of the fairing along a long pipe 4. Preferably, the fairing elements 9 are each and independently free to rotate with respect to the pipe 4. Thus, the fairing elements 9 are each and independently able to rotate under the effect of flow when they are in their operating environment, i.e. water. This is advantageous as the direction of the current may vary along the main direction of the aquatic structure. For example, the direction of the current may vary according to water depth i.e. the direction of the current may vary from the surface of the water to the floor. In such cases, each fairing element 9 is able to orient itself depending on the current direction to which it is subjected, which may reduce the shear effect in the fairing.

The fairing elements 9 may be completely independent from each other or may be tied to each other. When the fairing elements 9 are tied to each other, they are preferably able to move (preferably, in rotation) with respect to each other. For example, they may rotate with respect to each other by an angle a.

In some embodiments, the fairing of the invention may be attached to another rigid fairing, such as a fairing of the prior art. This makes possible to extend the length of the rigid fairing and, consequently, to reduce the drag coefficient of the system comprising the pipe equipped with the fairings. In such embodiments, the rigid, underlying, fairing is an example of rigid structure 5 of the fairing of the invention.

Advantageously, the pipe 4 equipped with the fairing 1 has a drag coefficient measured in situ lower than or equal to 1 , or lower than or equal to 0.9, or lower than or equal to 0.8, or lower than or equal to 0.7, or lower than or equal to 0.6, or lower than or equal to 0.5, or lower than or equal to 0.4, or lower than or equal to 0.3, or lower than or equal to 0.2, or lower than or equal to 0.1 .

The aquatic structure 4 may be chosen from the group consisting of, but not limited to, drilling risers, production risers, umbilicals, flexibles pipes, any flow lines or control lines going from the water surface to the sea floor, anchoring structures and fixed structures, such as girders horizontal structures, cross beams, tendons, floating structures such as semi-submersible structures and horizontal beams of floating platforms.

The invention also relates to a method for installing a fairing 1 on an aquatic structure 4. The method comprises the steps of:

- providing a fairing 1 as disclosed above;

- attaching said fairing 1 to the aquatic structure 4 as disclosed above.

By“attaching said fairing to the aquatic structure", is meant that the fairing

1 may be attached directly or indirectly to the aquatic structure 4, for example through another rigid fairing.

The fairing 1 which is provided may be preassembled, or the constituting parts of the fairing may be provided, these parts being assembled when the fairing is attached to the aquatic structure.

As mentioned above, the fairing 1 may comprise several fairing elements 9. In this case, the fairing elements 9 may be each attached to the aquatic structure 4, one after the other.

Alternatively, the fairing may be continuous and made of one piece. The fairing, either continuous or comprising several fairing elements 9 tied to each other, may be provided wound on a drum or a reel. Then, the fairing may be reeled out and attached along the aquatic structure 4 (such as a riser or a pipeline).

The fairing 1 may be attached to the aquatic structure 4 just before the aquatic structure 4 is dropped into the water, or in an upstream stage, or when the aquatic structure 4 is already in its operating environment. For example, the fairing 1 may be attached to the aquatic structure 4 in an upstream stage and the aquatic structure 4, for example a cable (or any aquatic structure able to be wound), equipped with the fairing may be wound on a drum or reel, for example in order to be provided to the place where the aquatic structure is intended to be used. Thus, the aquatic structure 4 equipped with the fairing 1 is“ready to use” and may be reeled out to be used.

The fairing 1 may be attached to the aquatic structure 4 on a ship or offshore platform or onshore or on a subsea structure.

The invention also relates to the use of a fairing 1 as disclosed above for reducing vortex-induced-vibrations and/or drag forces in an aquatic structure 4.

The aquatic structure 4 may be at a set position or may move through water. In some embodiments, the fairing 1 may be used for reducing vortex- induced-vibrations and/or drag forces in an aquatic structure 4 when said aquatic structure 4 is moved in water from a place to another place. For example, the fairing 1 may be used for reducing vortex-induced-vibrations and/or drag forces in an aquatic structure 4 (for example any aquatic structure that is part of a floating production platform) that is towed toward the position where it is intended to be used. Fairings 1 may be used on towing equipment such as cables, dragging platforms... In some embodiments fairings 1 could be used to control buoyancy when the aquatic structure 4 is moved when immersed.

EXAMPLES

The following examples illustrate the invention without limiting it.

Two fairings were manufactured, installed on a riser and tested in a basin.

Fairing No. 1 and fairing No. 2 were identical except that fairing No. 1 had a higher length (i.e. the maximum dimension of the fairing orthogonal to the main direction of the riser).

The fairings were manufactured using a flexible tarpaulin made of a material having a weight per area of 400 g/m², a fold ratio (measuring the stiffness of a material) of 0.35 and a tear resistance of 10 dN.

The features of the fairings were the following:

The apertures were made in the tarpaulin, and positioned at the leading edge of the fairing, one below the other (along the main direction of the riser).

The riser used for the tests was an aluminum pipe of 3 m in length and 500 mm in diameter.

The riser fitted with fairing No.2 had a ratio length of the fairing/diameter of the riser of 4.4 and was an aquatic structure according to the invention, the riser fitted with fairing No.1 had a length of the fairing/diameter of the riser of 6.5 and was a comparative example.

For the resistance tests, the bare riser, the riser equipped with fairing No.1 or the riser equipped with fairing No.2 were tested at three different speeds: 0.55 m/s, 1.00 m/s or 2.75 m/s. The drag coefficient CD of the tested structures was calculated with the formula:

C_D = F_x / ((1/2)^*pV^*A)

wherein F_x is the measured force, p is the fresh water density, v is the fresh water viscosity. As the area A, the bare riser area was taken: A = L*D with L being the length in meter and D being the diameter in meter.

Both faring No.1 and faring No.2 make it possible to reduce the drag coefficient with respect to the riser alone. Flowever, fairing No.2, that has a lower ratio length/bare riser diameter than fairing No.1 , makes it possible to improve drag force reduction. Indeed, compared to fairing No.1 , fairing No.2 makes it possible to achieve a reduction of the drag coefficient of from 15 to 38 %, it is therefore much more efficient in reducing drag forces.

Claims

1. An aquatic structure (4) equipped with a fairing (1 ),

said fairing (1 ) comprising a flexible waterproof tarpaulin (2) configured to define an interior space (8) and an aperture (3) configured to put said interior space (8) into fluid communication with the environment, said interior space being configured to inflate when filled with water,

wherein the aquatic structure (4) has a main direction, and the ratio of the maximum dimension of the fairing (1 ) orthogonal to the main direction of the aquatic structure (4), when the interior space of the tarpaulin (2) is inflated, relative to the diameter of the aquatic structure (4) orthogonal to the main direction is lower than or equal to 6.

2. The aquatic structure (4) of claim 1 , wherein the fairing (1 ) comprises a rigid structure (5) configured to be rotatably disposed on the aquatic structure (4), the tarpaulin (2) being attached to the rigid structure (5).

3. The aquatic structure (4) of claim 2, wherein the rigid structure (5) is in the shape of a tube.

4. The aquatic structure (4) of claim 2 or 3, wherein the tarpaulin (2) is attached to the rigid structure (5) via bolts, snap buttons, Velcro straps, zips, grooves, tumblers, by heat-sealing, heat-bonding or a combination thereof.

5. The aquatic structure (4) of claim 1 , wherein the tarpaulin (2) is configured to be directly attached to the aquatic structure (4), the tarpaulin (2) being preferably further configured to be rotatably disposed on the aquatic structure (4).

6. The aquatic structure (4) of any one of claims 1 to 5, wherein the fairing (1 ) is configured to rotate around the aquatic structure (4) under the effect of flow when it is in water.

7. The aquatic structure (4) of any one of claims 1 to 6, wherein the tarpaulin (2) is made of polyamide, polyester, polytetrafluoroethylene, polypropylene, polyethylene, oilcloth, such as a cotton or linen cloth with a coating of boiled linseed oil, or a combination thereof and/or the tarpaulin (2) is made of a biodegradable material.

8. The aquatic structure (4) of any one of claims 1 to 7, wherein the aperture (3) comprises a sieve.

9. The aquatic structure (4) of any one of claims 1 to 8, wherein the transverse cross section of said fairing (1 ) has one substantially pointed extremity when the interior space (8) is filled with water.

10. The aquatic structure (4) of any one of claims 1 to 9, wherein the interior space (8) is divided into several cells by internal partitions.

11. The aquatic structure (4) of any one of claims 1 to 10, wherein the fairing (1 ) comprises at least two fairing elements (9) placed one after the other according to an axis, each fairing element (9) comprising a flexible waterproof tarpaulin (2) and an aperture (3).

12. The aquatic structure (4) of claim 11 , wherein each fairing element (9) is independently configured to be rotatably disposed on the aquatic structure (4).

13. The aquatic structure (4) of any one of claims 1 to 12, wherein the fairing (1 ) surrounds said aquatic structure (4).

14. The aquatic structure (4) of any one of claims 1 to 13, comprising at least one translation stop for preventing translation of the fairing (1 ) along the aquatic structure (4).

15. The aquatic structure (4) of claim 14, wherein the translation stop is a flange (7) or a clamp or a collar or a sleeve or a clip fixed to the aquatic structure (4).

16. The aquatic structure (4) of claim 2 to 15, wherein the rigid structure 5 is a rigid fairing.

17. The aquatic structure (4) of any one of claims 1 to 16, wherein the ratio of the maximum dimension of the fairing (1 ) orthogonal to the main direction of the aquatic structure (4), when the interior space of the tarpaulin (2) is inflated, relative to the diameter of the aquatic structure (4) orthogonal to the main direction is lower than or equal to 5, preferably lower than or equal to 4.5.

18. The aquatic structure (4) of any one of claims 1 to 17, chosen from the group consisting of drilling risers, production risers, umbilicals, flexibles pipes, flow lines or control lines going from the water surface to the sea floor, anchoring structures and fixed structures, such as girders, horizontal structures, cross beams, tendons, floating structures such as semi-submersible structures and horizontal beams of floating platforms.

19. A method for installing a fairing (1 ) on an aquatic structure (4), comprising the steps of:

providing a fairing (1 ) as defined in any one of claims 1 to 12, attaching said fairing (1 ) to an aquatic structure (4) as defined in any one of claims 1 to 18.

20. The method of claim 19, wherein the fairing (1 ) is attached to the aquatic structure (4) on a ship or offshore platform or onshore or on a subsea structure.

21. The use of the fairing (1 ) as described in any one of claims 1 to 12 for reducing vortex-induced-vibrations and/or drag forces in an aquatic structure (4) as described in any one of claims 1 to 18.