GB2504065A

GB2504065A - Subsea flexible riser

Info

Publication number: GB2504065A
Application number: GB1211625.7A
Authority: GB
Inventors: Kristin Nedrelid
Original assignee: Statoil Petroleum ASA
Current assignee: Equinor Energy AS
Priority date: 2012-06-29
Filing date: 2012-06-29
Publication date: 2014-01-22
Also published as: GB201211625D0

Abstract

The invention relates to a subsea flexible riser (12 figure 1) used for conveying fluids, such as hydrocarbons. The riser comprises an internal fluid tight liner 21, a load bearing structural layer 22 arranged to withstand internal and external pressure, at least one external load bearing structural layer 24 and an outer protective layer 25. The internal load bearing structural layer and the internal liner comprise fusible polymer matrix materials and the internal load bearing structural layer is bonded to the internal liner. The internal load bearing structural layer comprises a fibre reinforced composite material. The at least one external load bearing structural layer is a tensile armour comprising wound metal wires. A later embodiment relates to a method of manufacturing a subsea flexible riser by extrusion.

Description

COMPOSITE FLEXIBLE PIPE

TECHNICAL FIELD

The present invention relates to a composite flexible pipe, such as a flow line or a riser, for conveying hydrocarbons, chemicals or water.

BACKGROUND ART

Conduits or pipes for transferring materials from the seafloor to production and drilling facilities at the surface, as well as from the facility to the seafloor, are commonly termed risers. Subsea flexible risers are a type of flexible pipe developed for this type of vertical transportation. Risers can serve as production or import/export means and are the connection between the subsea field developments and production and drilling facilities. Similar to pipelines or flow lines, risers transport produced hydrocarbons, as well as production materials, such as injection fluids, control fluids and gas lift.

Usually insulated to withstand seafloor temperatures, risers can be either rigid or flexible A flexible riser is a hybrid that can accommodate a number of different situations, Flexible risers can withstand both vertical and horizontal movement and handle large curvatures, making them ideal for use with floating facilities. This flexible riser was originally used to connect production equipment aboard a floating facility to production and export risers, but now it is found as a primary riser solution as well. There are a number of configurations for flexible risers, including the steep S and lazy S that utilize anchored buoyancy modules, as well as the steep wave and lazy wave that incorporates buoyancy modules Other configurations include free hanging (catenary) risers.

In general, prior art flexible risers comprise a central stainless steel or metal-alloy carcass covered by a thermoplastic sheath that provides pressure containment. Numerous layers of flexible armour surround the sheath, or pressure vault, to provide tensile-and hoop-stress strength.

The armour layers are usually separated by cushioning layers of composite or thermoplastic material to prevent them from rubbing against one another. The number of armour layers is a function of the pressure and tensile strength specifications imposed by the particular application for which the riser is designed. A final thermoplastic outer sheath provides protection from external sources such as seawater ingress, attack from marine growth, or abrasion.

A problem with this type of riser is the large weight, which creates a relatively high load on both the riser itself and on the topside equipment used for supporting and handling the riser.

An alternative riser addressing this problem is described in WO 2000/70256, which describes a lightweight flexible riser consisting entirely of various polymer matrix layers. A problem with this type of composite riser is that when it is submersed in seawater the composite riser will have a weight per meter close to zero dependent on the internal fluid. When subjected to hydrodynamic loads, a flexible riser of this type can become unstable and display an unacceptable drifting behaviour.

A further problem is that fully composite pipes are stiffer in bending than traditional flexible pipes, and hence cannot be used for all riser configurations as flexible pipes. It is difficult to achieve a high tensile strength in a thermoplastic composite pipe and in the same time have reasonable flexibility in bending. Tensile fibre reinforcement will also give a higher bending stiffness.

In addition, a traditional flexible pipe has a carcass as the inner layer, wherein the rough bore inner layer results in increased flow resistance and potential acoustic flow induced vibrations.

Hence, there is a need for a flexible riser that avoids the above problems.

The object of the invention is to provide an improved flexible riser arrangement that solve the above weight related problems and structural problems.

DISCLOSURE OF INVENTION

The above problems are solved by an arrangement and a method as described in the attached claims.

In the subsequent text, the term "riser" or "sub sea riser" is defined as comprising a flexible riser.

The invention relates to a subsea flexible riser used tor conveying fluids, such as hydrocarbons, the riser comprising an internal liner, an internal load bearing structural layer, at least one external load bearing structural layer and an outer layer. The internal load bearing structural layer and the internal liner comprise fusible polymer matrix materials, which polymer materials will be described in further detail below. The internal load bearing structural layer is bonded to the internal liner, to form a single unit. The internal load bearing structural layer comprises a fibre reinforced composite material, wherein fibres have been added to the polymer matrix. The internal load bearing structural layer is arranged to prevent collapse of the inner liner, to withstand external hydrostatic pressure, and to act as a fluid tight pressure barrier. In some cases, when a pressure armour is not applied as an external load bearing structural layer, the internal load bearing structural layer shall also prevent bursting and withstand the internal pressure.

The bonded internal liner and internal load bearing structural layer can be encased by at least one external load bearing structural layer. The tirst external load bearing structural layer is a tensile armour comprising wound metal wires. The tensile armour comprises layers cross-wound in pairs and is used to resist tensile load on the flexible riser. Tensile armour layer typically comprise flat, rectangular wires wound at an angle between 15 and 60 degrees, often between 28 and 55 degrees to the longitudinal axis. For instance, a winding angle of 55 degrees relative to the longitudinal axis results in a torsionally balanced riser and is used when the tensile armour takes up the hoop stress, whereby no pressure armour is required. Tensile armour layers shall support the weight of all the riser layers and transfers the load through an end fitting to a vessel structure or similar. In addition the tensile armour is arranged to withstand the axial forces and bending moments from external loads. High tension in a deepwater riser may require two tensile armour layers (two pairs of cross-wound layers) instead of one.

The material for the tensile armour wires is typically high strength carbon steel, as for the pressure armour. If a pressure armour is applied it will be applied before the tensile armour.

According to a first example, the bonded internal liner and internal load bearing structural layer form a combined fluid tight pressure barrier and a pressure armour. In this example, the internal load bearing structural layer comprises a fibre reinforced composite material with sufficient hoop strength designed to withstand the hoop stress in the riser wall caused by the inner fluid pressure and the external hydrostatic pressure. Such a bonded internal liner and internal load bearing structural layer is encased by a tensile armour, as described above, and an outer layer.

According to a second example, the bonded internal liner and internal load bearing structural layer form a combined fluid tight pressure barrier provided with a separate pressure armour. In this example, the internal load bearing structural layer comprises a fibre reinforced composite material with sufficient hoop strength designed to withstand the hoop stress in the riser wall caused by the external pressure and some of the internal pressure. The second external load bearing structural layer shall withstand the additional internal pressure load. This second external load bearing structural layer is a pressure armour comprising wound wires. These wires can comprise metal or an alternative suitable material. The pressure armour is arranged between the internal load bearing structural layer and the tensile armour, formed by the first external load bearing structural layer. Such a bonded internal liner and internal load bearing structural layer is encased by separate pressure and tensile armours, as described above, and an outer layer.

The pressure armour in the second example is arranged to withstand the hoop stress in the riser wall caused by the inner fluid pressure. The pressure armour is wound around the pressure barrier and comprises interlocking wires. The wires have a profile that allows bending flexibility and controls the gap between the armour wires to prevent extrusion of the polymer pressure barrier through the armour layer. For additional hoop strength, a layer of back up flat wire spirals can be wounded around the interlocking wires. In order to resist hoop stress in the rest of the riser wall, the pressure armour is wound at a relatively small angle. The winding angle relative to the longitudinal axis of the riser can be between 80 and 90 degrees. The material for the pressure armour is typically high strength carbon steel, where the quality is chosen depending on the fluid in the riser (sweet or sour service).

Each of the first external load bearing structural layer, or the first and the second external load bearing structural layer, comprises one or more pairs of wire layers. The first external load bearing structural layer, or tensile layer, is wound at alternating positive and negative angles relative to the longitudinal axis of the riser. The second external load bearing structural layer, or pressure layer, comprises a pair of wires wound at a different angle relative to the longitudinal axis of the riser.

Alternatively, the internal load bearing structural layer comprises a fibre-reinforced thermoplastic low module material in which the reinforcement fibres are distributed in and bonded to the polymer matrix.

In this case the fibre-reinforced thermoplastic material comprises prefabricated tape or strips wound onto the internal liner at an angle of 55 degrees to 90 degrees to the longitudinal axis of the riser. The tape or strips are bonded to the internal liner, which liner comprises an extruded liner of the same or a similar thermoplastic matrix material as the material in the internal load bearing structural layer. The bonding method is preferably adhesive, but can also be a thermal bond.

The polymer matrix material can be selected based on fluid exposure concentration and fluid temperature. Common materials used include polyamide-1 1, high density polyethylene (HDPE), cross-linked polyethylene (XLPE), polyether ether ketone (PEEK) and polyvinylidene difluoride (PVDF).

The material selected may have to withstand fluid temperatures up to 200 °C.

For flexible risers, the maximum allowable strain (%) must also be considered.

Both the internal liner and the outer layer, forming a protective sheath, can comprise a suitable thermoplastic material. The outer layer or protective sheath is an outer polymer sheath that can be made from the same material as the pressure barrier. The outer layer is a barrier against seawater and provides a level of protection for the armour against chafing with other objects or during installation of the riser.

Additional, minor layers may include anti4riction tapes around the armour layers, to reduce wear on internal load bearing structural layer and external load bearing structural layer caused by friction between structural layers, and intermediate tape layers used to secure the position to the armour wires.

The relative weight ratio between the internal load bearing structural layer and the at least one external load bearing structural layer can be selected to give the riser a predetermined negative buoyancy. The dimensioning factor for at least one of the external load bearing structural layers may be to give the riser a specific weight in water.

By combining a composite internal structural layer with one or more metal external structural layers in the manner stated above, the invention allows the use of a lightweight composite material in both the pressure load bearing structural layer and the axial and bending load bearing structural layers of the riser. This results in a flexible riser having a lower weight than a traditional riser, but a higher weight than a full composite riser. Because of the low density of the internal structural layer, the riser will have a relatively low submerged weight while maintaining a predetermined negative buoyancy.

Due to the relatively low weight, the riser according to the invention will allow operations at greater water depths than what is permitted with traditional flexible pipes. In addition to the operational benefits, the relatively low weight of the riser will facilitate the installation, by reducing the size of equipment required to lift and bend the riser. This invention primarily relates to risers, but can also be applied to static pipelines.

For typical riser applications, the relatively low weight of the riser design allows easier optimization of the riser configuration. As opposed to lightweight composite risers, it is not necessary to add additional weight along the length of the riser. This is in contrast to a heavier riser comprising a metal carcass, which riser has a greater weight along the entire length of the riser. At the same time, the amount of buoyancy required to support the pipe, will be reduced. These factors result in a riser system which is easier to adapt to a variety of applications.

The invention further relates to a method for manufacturing a subsea flexible riser. The method comprises the steps of -extruding a polymer material into a cylindrical liner; -winding a fibre-reinforced polymer material tape around the liner and bonding it to the liner, to form a internal load bearing structural layer around the liner; -winding a metal wire into at least one tubular external load bearing structural layer around the internal load bearing structural layer, and -extruding an outer fluid tight, thermoplastic layer over the outermost external load bearing structural layer.

The method further involves winding the fibre-reinforced polymer material as prefabricated tape or strips onto the internal liner at an angle of 55 degrees to degrees to the longitudinal axis of the riser.

According to a first example, a first external load bearing structural layer forming a tensile armour is wound around the internal load bearing structural layer. The method involves winding each first external load bearing structural layer in pairs of metal wire layers wound at alternating positive and negative angles between 15 degrees and 60 degrees to the longitudinal axis of the riser.

According to a second example, an additional external load bearing structural layer, forming a pressure armour is wound around the internal load bearing structural layer, prior to the first external load bearing structural layer. The method involves winding a external load bearing structural layer with one or two wires wound at an angle between 80 degrees and 90 degrees to the longitudinal axis of the riser.

BRIEF DESCRIPTION OF DRAWINGS

In the following text, the invention will be described in detail with reference to the attached drawings. These schematic drawings are used for illustration only and do not in any way limit the scope of the invention. In the drawings: Figure 1 shows a riser extending from the sea floor to a production vessel; Figure 2 shows a schematic illustration of the layers making up a flexible riser according to a first example of the invention; Figure 3 shows a schematic illustration of the layers making up a flexible riser according to a second example of the invention.

Figure 4A shows a cross-section through a strip making up an internal load bearing structural layer; and Figure 4B shows a cross-section through a strip making up an alternative internal load bearing structural layer.

EMBODIMENTS OF THE INVENTION

Figure 1 shows a riser extending from the sea floor to a production vessel.

Oil is produced by subsea wells via a manifold, which passes through rigid flow lines and then flexible risers into a floating production, storage and offloading system. The vessel shown in this figure is a ship, but the arrangement is applicable on any type of floating (floater), semi-submersible of permanent production platform.

As shown in Figure 1, the riser assembly comprises a riser base 11 located on the seabed, connected to the lower end of the flexible riser 12. The flexible riser 12 comprises a carcass, a pressure barrier, pressure and tensile armours, and an outer protective sheath (not shown). Figure 1 indicates a lazy wave arrangement of the flexible riser.

Figure 2 shows a schematic illustration of the layers making up a pipe in the form of a flexible riser according to a first example of the invention. In this example, the riser comprises, from the inner to the outer layer, an internal liner 21 arranged to form a fluid tight pressure layer, an internal load bearing structural layer 22, an external load bearing structural layer 24 and an outer layer 25 or protective sheath.

The liner 21 forms the innermost layer of the flexible riser and is made from fusible polymer matrix material. The internal load bearing structural layer 22 is also made from a fusible polymer matrix material and is bonded to the internal liner 21. The polymer matrix material of the internal load bearing structural layer 22 contains fibres to make up a fibre reinforced composite material. The internal load bearing structural layer 22 preferably comprises a fibre-reinforced low modulus polymer matrix material in which the reinforcement fibres are evenly distributed in and bonded to the polymer matrix material. The fibre reinforced composite matrix material comprises high density polyethylene, cross-linked polyethylene, polyamide of polyvinylidene fluoride (PVDF), or other suitable thermoplastic matrix materials and the reinforcing fibres comprise aramid fibres, glass fibres or carbon fibres.

Alternatively, the internal load bearing structural layer comprises a fibre-reinforced low modulus polymer matrix material which is reinforced by bundles of fibres extending in the direction around the liner. Each bundle of fibres contains multiple mutually movable fibres and is encased in, but not bonded to the surrounding polymer matrix material. The materials making up the fibre reinforced composite material are the same as in the previous

example.

In both cases, the fibre-reinforced thermoplastic material comprises prefabricated tape or strips wound onto the internal liner at an angle of 55 degrees to 90 degrees to the longitudinal axis of the riser. The tape or strips are preferably adhesively bonded to the internal liner during winding, which liner comprises an extruded liner of the same or a similar thermoplastic matrix material as the material in the internal load bearing structural layer.

Alternatively, the tape can be thermally bonded to the liner.

In this way, the bonded internal liner 21 and internal load bearing structural layer 22 form a combined fluid tight layer, a pressure barrier and a pressure amour arranged to withstand the hoop stress in the riser wall caused by the inner fluid pressure.

The bonded internal liner 21 and internal load bearing structural layer 22 at least one external load bearing structural layer 24 in the form of a tensile armour. The example shown in Figure 2 comprises one external load bearing structural layer 24. The tensile armour 24 comprises pairs of metal wire layers wound at alternating positive and negative angles between 15 degrees and 60 degrees to the longitudinal axis of the riser 20.

The tensile armour 24 is encased by the outer layer 25 or protective sheath.

The internal liner 21 and the outer layer 25 comprise the same polymer matrix material, in this case a thermoplastic material.

Figure 3 shows a schematic illustration of the layers making up a flexible riser according to a second example of the invention. In this example, the riser comprises, from the inner to the outer layer, an internal liner 31 arranged to form a fluid tight pressure layer, an internal load bearing structural layer 32, an external load bearing structural layer 34 and an outer layer 35 or protective sheath.

The liner 31 forms the innermost layer of the flexible riser and is made from fusible polymer matrix material. The internal load bearing structural layer 32 is also made from a fusible polymer matrix material and is bonded to the internal liner 31. The polymer matrix material of the internal load bearing structural layer 32 contains fibres to make up a fibre reinforced composite material. The internal load bearing structural layer 32 preferably comprises a fibre-reinforced low modulus polymer matrix material which is reinforced by bundles of fibres extending in the direction around the liner. Each bundle of fibres contains multiple mutually movable fibres and is encased in, but not bonded to the surrounding polymer matrix material. The fibre reinforced composite material comprises high density polyethylene, cross-I inked polyethylene, polyamide of polyvinylidene fluoride (PVDF), or other suitable thermoplastic matrix materials and the reinforcing fibres comprise aramid fibres, glass fibres or carbon fibres.

Alternatively, the internal load bearing structural layer comprises a fibre-reinforced low modulus polymer matrix material in which the reinforcement fibres are evenly distributed in and bonded to the polymer matrix material.

The materials making up the fibre reinforced composite material are the same as in the previous example.

In both cases, the fibre-reinforced thermoplastic material comprises prefabricated tape or strips wound onto the internal liner at an angle between 80 degrees to 90 degrees to the longitudinal axis of the riser. The tape or strips are adhesively or thermally bonded to the internal liner during winding, which liner comprises an extruded liner of the same or a similar thermoplastic matrix material as the material in the internal load bearing structural layer.

In this way, the bonded internal liner 31 and internal load bearing structural layer 32 form a combined fluid tight layer and a pressure barrier arranged to withstand the hoop stress in the riser wall caused by the inner fluid pressure.

The bonded internal liner 31 and internal load bearing structural layer 32 is enclosed in at least one first external load bearing structural layer 34 in the form of a pressure armour at least one first external load bearing structural layer 33 in the form of a tensile armour 34. The example shown in Figure 3 comprises one first and second external load bearing structural layer 33, 34.

The pressure armour 33 is arranged to withstand the hoop stress in the riser wall caused by the inner fluid pressure. The pressure armour 33 is wound around the internal load bearing structural layer 32 and comprises interlocking wires. The pressure armour 33 comprises pairs of wire layers wound at an angle between 80 degrees and 90 degrees to the longitudinal axis of the riser 30. The tensile armour 34 comprises pairs of metal wire layers wound at alternating positive and negative angles between 15 degrees and 60 degrees to the longitudinal axis of the riser 30.

The tensile armour 34 is encased by the outer layer 35 or protective sheath.

The internal liner 31 and the outer layer 35 comprise the same polymer matrix material, in this case a thermoplastic material.

As described above, the internal load bearing structural layer 22, 32 as shown in Figures 2 and 3 comprises fibre-reinforced thermoplastic material.

According to a first example, the fibre-reinforced thermoplastic material comprises prefabricated tape or strips wound onto the internal liner.

As described above, the internal liner is a thermoplastic tube designed to make the riser fluid-tight. The liner tube may be prefabricated by extrusion, and wound onto storage spools before assembly of the riser components, or it may be extruded simultaneously with the assembly of the riser. The thermoplastic material of the liner may be chosen based on the operating conditions. Possible materials are high density polyethylene, cross-linked polyethylene, polyamide of polyvinylidene fluoride (PVDF), or other suitable thermoplastic materials.

The internal liner is preferably bonded to the structural layers. Adhesive bonding is preferable, as it does not require the use of a liner material which is the same as or a similar material as the matrix of the internal load bearing structural layer. Alternatively, co-extrusion can be used, wherein a layer of a temperature resistant and chemically resistant polymer is extruded within a tube of another material which is the same as or similar to the matrix of the internal load bearing structural layer. Alternatively, the liner may be bonded to the structural layers by chemical or mechanical means.

The internal load bearing structural layer according to the invention comprises a solid layer of a polymeric material which is reinforced by long continuous fibres extending around the liner. In this manner there is achieved a material having a high strength in the riser hoop direction, and which is able to withstand high axial strains, i.e. transversely to the fibres.

Figure 4A shows a cross-section through a strip making up the internal load bearing structural layer, where the reinforcement fibres are supplied in yarn bundles consisting of thousands of individual fibres. The fibres may consist of any high stiffness and strength fibres. The most suitable fibre types are aramid fibres, glass fibres or carbon fibres, having a diameter in the range 5-micrometers. The composite material is formed by combining the fibre bundles with the polymer matrix. The fibres can either be individually embedded in the matrix, or bundles of the fibres may be surrounded by fibre material, so that the fibres are free to move relative to each other (bundle or cord reinforced material). In the example shown in Figure 4A, the first structural layer comprises a wound strip 41 of a thermoplastic low module material 42 reinforced by bundles 43 of fibres extending around the internal layer 21, 31 (see Figures 1 and 2), each bundle 43 containing a large number of relatively movable fibres and being surrounded by, but not bonded to the thermoplastic matrix material 42.

The internal load bearing structural layer 22, 32 (see Figures 1 and 2) can also be manufactured from a thermoplastic low module material in which the reinforcement fibres are evenly distributed and bonded to the thermoplastic matrix, as indicated in Figure 4B. The example in Figure 4B shows a cross-section through such an alternative strip 51, comprising a thermoplastic low module material 52 with distributed reinforcement fibres 53. This results in a composite material with higher stiffness and higher strength than a cord-reinforced material, but with lesser capacity to withstand axial strain.

The tape or strips of fibre-reinforced thermoplastic material used in the internal load bearing structural layer 22, 32, is prefabricated in tape form, the material consisting of long lengths spooled onto reels. The matrix material may be any of the polymeric materials mentioned above for the internal liner.

If the matrix material of this layer is the same as or a similar material as the one used for the inner liner, the liner can be adhesively or thermally bonded to this layer. If the matrix material is different from the liner material, the liner can be adhesively, chemically or mechanically bonded to this layer.

The prefabricated tapes of cord-reinforced or fibre-reinforced thermoplastic material are assembled to form a solid reinforced layer over the internal liner.

The layer is assembled by applying discrete tapes onto a liner at an angle between 80 degrees to 90 degrees to the longitudinal axis of the riser.

Bonding of these tapes to the internal layer below can be performed in a number of ways. The tapes may be wound onto a liner and adhesively bonded to the liner. Alternatively, the tapes may be wound onto a liner and thermally bonded to the liner at the points of contact using any of a number of heating methods, including hot air, a flame or infrared radiation. Alternatively, the tapes may be wound onto the liner without heating. In this case all layers or plies are thermally bonded simultaneously by passing the component through a series of ovens providing external heat to the component wall.

This first structural layer comprises several plies wound at angles between degrees to 90 degrees to the longitudinal axis of the riser. The plies which have a winding angle at or close to 90 degrees provide resistance to internal pressure. The plies having a lower winding angle, provide some axial stiffness and strength. The design of this structural layer with regard to the number of layers, the thickness of the layers, and the angles of each layer is optimized for the design loads of each application.

The method for manufacturing a subsea flexible riser according to the invention comprises the steps of -extruding a fusible polymer matrix material into a cylindrical liner; -winding a fibre-reinforced fusible polymer matrix material strip around the liner and bonding it to the liner, to form a internal load bearing structural layer around the liner; -winding a profiled metal wire into at least one tubular external load bearing structural layer around the internal load bearing structural layer, and -extruding an outer fluid tight, thermoplastic layer over the outermost external load bearing structural layer.

The fibre-reinforced polymer matrix material is wound as prefabricated tape or strips onto the internal liner at an angle between 80 degrees to 90 degrees to the longitudinal axis of the riser.

At least one first external load bearing structural layer 24, forming a tensile armour, is wound around the internal load bearing structural layer 22. Each first external load bearing structural layer 24 is wound in pairs of metal wire layers wound at alternating positive and negative angles between 15 degrees and 60 degrees to the longitudinal axis of the riser.

Alternatively, an additional, second external load bearing structural layer 23, forming a pressure armour, is wound around the internal load bearing structural layer 22 prior to the first external load bearing structural layer 24.

Each second external load bearing structural layer 23 is wound in pairs of wire wound at an angle between 80 degrees and 90 degrees to the longitudinal axis of the riser.

Claims

CLAIMS1. Subsea flexible riser used for conveying fluids, such as hydrocarbons, the riser comprising an internal liner, an internal load bearing structural layer, at least one external load bearing structural layer and an outer layer, characterized in that the internal load bearing structural layer and the internal liner comprise fusible polymer matrix materials; that the internal load bearing structural layer is bonded to the internal liner; that the internal load bearing structural layer comprises a fibre reinforced composite material; and that at least one external load bearing structural layer is a tensile armour comprising wound metal wires.
2. Subsea flexible riser according to claim 1, c h a r a c t e r i z e d i n that the bonded internal liner and internal load bearing structural layer form a fluid tight pressure barrier and a pressure armour.
3. Subsea flexible riser according to claim 2, c h a r a c t e r i zed i n that each tensile armour layer comprises pairs of metal wire layers wound at alternating positive and negative angles between 15° and 600 to the longitudinal axis of the riser.
4. Subsea flexible riser according to claim 1, c h a r a c t e r i z e d i n that the bonded internal liner and internal load bearing structural layer form a fluid tight pressure barrier.
5. Subsea flexible riser according to claim 4, c h a r a c t e r i z e d i n that at least one additional external load bearing structural layer is a pressure armour comprising wound wires.
6. Subsea flexible riser according to claim 5, c h a r a c t e r i z e d i n that each pressure armour layer comprises one or two wires wound at an angle between 800 and 900 to the longitudinal axis of the riser.
7. Subsea flexible riser according to any one of claims 1-6, c h a r a c t e r i z e d i n that the internal load bearing structural layer comprises a fibre-reinforced thermoplastic material in which the reinforcement fibres are distributed in and bonded to the polymer matrix.
8. Subsea flexible riser according to claim 7, c h a r a c t e r i z e d i n that the fibre-reinforced thermoplastic material comprises prefabricated tape wound onto the internal liner at an angle of 55° to 900 to the longitudinal axis of the riser.
9. Subsea flexible riser according to claim 8, c h a r a c t e r i z e d i n that the tape is adhesively bonded to the internal liner, which liner comprises an extruded liner of the same or a similar thermoplastic matrix material as the material in the internal load bearing structural layer.
10. Subsea flexible riser according to any one of claims 1-9, c h a r a c t e r i z e d i n that, when submersed in seawater, the composite riser will have a weight per meter close to zero dependent on the internal fluid.
11. Method for manufacturing a subsea flexible riser, which method comprises the steps of -extruding a fusible polymer material into a cylindrical liner; -winding a fibre-reinforced fusible polymer material tape around the liner and bonding it to the liner, to form a internal load bearing structural layer around the liner; -winding a metal wire into at least one tubular external load bearing structural layer around the internal load bearing structural layer, and -extruding an outer fluid tight, thermoplastic layer over the outermost external load bearing structural layer.
12. Method according to claim 11, ch a ía cte rized by winding the fibre-reinforced polymer matrix material as prefabricated tape onto the internal liner at an angle of 55° to 900 to the longitudinal axis of the riser.
13. Method according to any one of claims 11-12, characterized by winding a first external load bearing structural layer forming a tensile armour around the internal load bearing structural layer.
14. Method according to claim 13, c ha ra cte r i z e d by winding each first external load bearing structural layer in pairs of metal wire layers wound at alternating positive and negative angles between 15° and 60° to the longitudinal axis of the riser.
15. Method according to claim 14, characterized by winding an additional external load bearing structural layer, forming a pressure armour is wound around the internal load bearing structural layer, prior to the first external load bearing structural layer.
16. Method according to claim 15, characterized by winding each additional external load bearing structural layer in pairs of wires wound at an angle between 80° and 90° to the longitudinal axis of the riser.