FLEXIBLE PIPE

TECHNICAL FIELD

The present invention relates to a flexible pipe comprising an internal pressure sheath encircling a bore, and at least one armor layer on the outer side of the internal pressure sheath and optionally an outer sheath on the outer side of said at least one armor layer.

BACKGROUND

Flexible pipes are frequently used as flexible risers or flexible flowlines for the transport of fluids including hydrocarbons such as oil and gas.

Moreover, unbonded flexible pipes are often used e.g. as riser pipes or flowlines in connection with the production of oil and gas via offshore floating systems or other subsea facilities.

The unbonded flexible pipes are constructed of a number of independent layers, such as helically laid steel and polymeric layers formed around a central bore for transporting fluids. A typical unbonded flexible pipe

comprises, from the inside and outwards, an inner armor layer known as the carcass, an internal pressure sheath surrounded by one or more armor layers, such as pressure armor and tensile armor, and an outer sheath. Thus, the internal pressure sheath forms a bore in which the fluid to be transported is conveyed. In some unbonded flexible pipes, the carcass may be omitted and when the carcass is omitted, the bore is denoted a smooth bore. When the carcass is present, the bore is denoted a rough bore. The annular space between the internal pressure sheath and the outer sheath is known as the annulus and houses the pressure armor and the tensile armor. The armor layers usually comprise or consist of one or more helically wound elongated armoring elements, where the individual armor layers are not bonded to each other directly or indirectly via other layers along the pipe.

When the armor layers are wound at an angle larger than 55° relative to the pipe center axis, they are classified as pressure armor layers, whereas armor layers wound with an angle of less than 55° are classified as tensile armor layers. By using unbonded wound elements the pipe becomes bendable and sufficiently flexible to be rolled up for transportation. The armor elements are very often manufactured from metallic material, such as carbon steel or stainless steel, and may, thus, be sensitive to corrosion.

Flexible unbonded pipes of the present type are for example described in the standard "Recommended Practice for Flexible Pipe", ANSI/API 17 B, fourth Edition, July 2008, and the standard "Specification for Unbonded Flexible Pipe", ANSI/API 17J, Third edition, July 2008. As mentioned, such pipes usually comprise an innermost sealing sheath - often referred to as an internal pressure sheath or inner liner, which forms a barrier against the outflow of the fluid which is conveyed in the bore of the pipe, and one or usually a plurality of armor layers. Normally the pipe further comprises an outer protection layer, often referred to as the outer sheath, which provides mechanical protection of the armor layers. The outer protection layer may also be a sealing layer sealing against ingress of sea water. In certain unbonded flexible pipes one or more intermediate sealing or non-sealing layers are arranged between armor layers. These intermediate layers can optionally be thermally insulating layers, anti-wear layers or anti-creep layers.

In general flexible pipes are designed to have a lifetime of 20 years or more.

The term "unbonded" means in this text that at least two of the layers including the armoring layers and polymer layers are not bonded to each other. In practice, the known pipe normally comprises at least two armoring layers located outside the internal pressure sheath and optionally an armor structure located inside the internal pressure sheath, which inner armor structure normally is referred to as the carcass.

The unbonded flexible pipes may carry the fluids between subsea wells that are in connection with a hydrocarbon reservoir located under the sea bed, and a floating structure. The fluid may be a hydrocarbon fluid, such as natural gas or oil, depending upon the nature of the hydrocarbon reservoir, or an injection fluid such as water. The fluids, which are transported to the floating structure, can be processed, for example by separation, compression and/or further treatment. When the floating structure is moored close to a hydrocarbon reservoir, it can be kept in fluid communication with the producing well heads via one or more flexible risers. The one or more flexible risers can convey fluids between the well heads of a hydrocarbon reservoir and the floating structure. Flexible risers may be configured as free-hanging catenaries or be provided in alternative configurations, such as lazy wave and lazy S types, using buoyancy modules.

Thus, a flexible riser can for example be connected at one end to the floating structure, and at another end to a riser base manifold, which can secure the flexible riser at the sea bed.

Examples of floating structures are vessels like FPSO's (floating production and storage offloading).

Unbonded flexible pipes are often used e.g. as riser pipes in the exploitation of oil or gas fields or in connection with other subsea applications. One of the difficulties during the production of oil and gas is that operational changes may lead to a sudden pressure drop or pressure increase. Such sudden changes of the pressure may be detrimental to the internal pressure sheath and cause the flexible pipe to become inoperational. Thus, to protect the internal pressure sheath it is common practice to apply a carcass in the bore of the pipe, i.e. on the inner surface of the internal pressure sheath and optionally one or more pressure armors on the outer surface of the internal pressure sheath. However, the use of such armor layers increases the weight of the flexible pipe. Moreover, the armor layer, which is typically made from metallic material, is vulnerable in respect of corrosion due to the harsh environment in offshore applications.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a collapse resistant flexible pipe with reduced weight and reduced risk of damage due to corrosion.

The present invention relates to a flexible pipe having a length and a longitudinal axis and comprising an internal pressure sheath encircling a bore, said flexible pipe further comprises at least one armor layer on the outer side of the internal pressure sheath and optionally an outer sheath on the outer side of the at least one armor layer and where a flexible hose is placed in the bore, the hose being adapted for transport of a first fluid and an interface between the outer side of the hose and the inner side of the internal pressure sheath is adapted for receiving and exchanging a second fluid.

The internal pressure sheath forms the bore with the inner side. The hose is placed in the bore and under certain conditions, e.g. when the hose is pressurized, the outer side of the hose will be in contact with inner side of the internal pressure sheath. However, as a general rule the outer side of the hose is not adhered or bonded to the inner side of the internal pressure sheath. The hose is able to move away from the inner side of the internal pressure sheath and form one or more cavities in the interface between the inner side of the internal pressure sheath and the outer side of the hose.

As the pipe does not have an internal armor layer or carcass, it is possible to produce a pipe with a reduced weight.

The longitudinal axis of the pipe is also denoted centre axis or simply the axis of the pipe. It should be emphasized that the term "comprises/comprising" when used herein is to be interpreted as an open term, i.e. it should be taken to specify the presence of specifically stated feature(s), such as element(s), unit(s), integer(s), step(s) component(s) and combination(s) thereof, but does not preclude the presence or addition of one or more other stated features.

The terms "inside" and "outside" a layer of the pipe are used to designate the relative distance to the axis of the pipe, such that inside a layer means the area encircled by the layer i.e. with a shorter radial distance to the axis of the pipe than the layer, and "outside a layer" means the area not encircled by the layer and not contained by the layer, i.e. with a longer radial distance to the axis of the pipe than the layer.

The term "radial distance" is used herein to mean the distance to the axis of the pipe determined perpendicularly to the axis when the pipe is substantially straight.

The term "inner side" of a layer is the side of the layer facing the axis of the pipe. The term "outer side" of a layer is the side of the layer facing away from the axis of the pipe.

Conveying hydrocarbons and other fluids through a hose located within the bore of a flexible pipe is a new design principle to avoid collapse of a submerged flexible pipe during service. Normally, the carcass provides the main collapse resistance of flexible pipes where the thickness of the carcass layer increases with water depth. The carcass is designed to withstand the differential pressure between the ambient hydrostatic sea water and the pressure in the pipe bore to resist pipe collapse when the bore pressure becomes less than the ambient pressure. With the present invention the pipe bore pressure equalizes with the ambient pressure thus making the carcass redundant.

A further object with the present invention is to provide a flexible pipe with an improved service life. Flexible pipes that convey fluids containing corrosive constituents like CO2 and H2S are susceptible to corrosion attack of their armor layers owing to permeation of the corrosive constituents through the pressure sheath, which in combination with the presence of water can result in a corrosive annulus environment. With the present invention it is possible to reduce or prevent the permeation of these corrosive gases into the pipe annulus using a permeation resistant hose material thus mitigating the corrosion attack. Furthermore, the interface space between the hose and the pressure sheath can be flushed with a fluid, which will further reduce the amount of CO2 and H2S that can permeate through the pressure sheath.

Furthermore, the new design principle makes it possible to operate the flexible pipe in a new and more effective manner. For example it is possible to empty the hose by simply applying a differential pressure over the hose, thus avoiding time consuming pigging operations. Also, emptying the hose can be used as a preventive measure to avoid hydrate formation during shutdown operations, e.g. for pipes transporting wet gas.

The flexible pipe is adapted for transport of oil and gas and mixtures of oil and gas and in an embodiment the first fluid is a hydrocarbon fluid. Thus, the hydrocarbon fluid comprising oil and/or gas is transported in the hose which is located in the bore of the pipe.

The flexible pipe is furthermore adapted for transport of non-hydrocarbon fluids and in an embodiment the first fluid is water or chemicals. The first fluid may also be a mixture of hydrocarbon and non-hydrocarbon fluids.

During operation of the flexible pipe one or more cavities may occur in the interface between the inner side of the internal pressure sheath and the outer side of the hose. These cavities are filled with a second fluid, and preferably the second fluid is selected from a gas or a liquid.

In an embodiment the second fluid is water, preferably sea water. Water and in particular sea water is readily available in offshore installations and can be utilized at low cost. Consequently, the flexible pipe provides an embodiment for keeping the second fluid in equilibrium with the ambient sea water pressure.

The one or more cavities may change in size and number and extent throughout the entire length of the pipe. The presence of cavities in the interface between the inner side of the internal pressure sheath and the outer side of the hose may be more or less permanent during the operation of the pipe, in particular, their number and size may vary depending on the pressure differential between the pipe bore and the hose. The term "cavity" or

"cavities" in this context should not necessarily be understood as empty space or voids in the pipe but rather as one or more cavities filled with the second fluid. The alternative to the cavity is that the outer side of the hose and the inner side of the internal pressure sheath are in contact without any cavities between them. Such a situation may occur when the pressure of the first fluid inside the hose is larger than the pressure of the second fluid outside the hose.

The cavities in the interface between the inner side of the internal pressure sheath and the outer side of the hose may occur due to a pressure drop in the fluid transported in the hose. When this happens, the cavity is filled with a second fluid such as sea water which will serve to equalize the pressure in the bore of the pipe.

When a cavity occurs it is advantageous to fill the cavity with the second fluid as fast as possible to avoid unfavorable pressure differential across the pressure sheath which otherwise could result in a collapse of the pressure sheath or ultimately the entire pipe. Thus, in an embodiment the flexible pipe comprises means for injecting the second fluid into occurring cavities between the outer side of the hose and the inner side of the internal pressure sheath. Thereby, it is possible to perform a rapid pressure equalization. The means for injecting the second fluid may be pumps and nozzles. The pipe also comprises means, such as nozzles, to remove the second fluid to facilitate expansion of the hose during its pressurization thereby decreasing the number and/or size of the cavities. Thus, the supply of the second fluid can be adjusted to maintain a hydrostatic pressure in the pipe bore which is substantially in equilibrium with the ambient hydrostatic pressure, such as the hydrostatic pressure of the ambient sea water.

The term "substantially" should herein be taken to mean that ordinary operational and/or product variances and tolerances are comprised.

Although the means for injecting and removing the second fluid may be placed anywhere along the length of the flexible pipe, the flexible pipe in an embodiment is terminated in an end-fitting and the means for injecting the second fluid is located in the end-fitting or in a spool piece connected to the end-fitting.

In an embodiment the means for injecting the second fluid is placed on the pipe along the length of the pipe.

The means for injecting the second fluid and also the means for removing the second fluid may be placed along the length of the pipe and in one or two end-fittings terminating the pipe. The means may in an embodiment be controlled by an electronic device, such as a computer, and the hydrostatic pressure affecting the flexible pipe may be measured by one or more sensors which may be connected with the electronic device.

The internal pressure sheath is typically made from a polymeric material, such as high density polyethylene (HDPE), cross linked polyethylene (PEX), polypropylene (PP), polyvinyl difluorid (PVDF) or polyamide (PA).

The flexible hose is made from a polymeric material which may be the same material as the internal pressure sheath, however, the material may be manufactured and treated differently and with additives which make the material more flexible. Thus, the hose may be made from the polymers high density polyethylene (HDPE), cross linked polyethylene (PEX), polypropylene (PP), polyvinyl difluorid (PVDF) or polyamide (PA). However, it is desirable that the hose is at least slightly more flexible and resilient than the internal pressure sheath. Such a difference may be obtained in several different ways.

In case the same type of polymer is used for the internal pressure sheath and the hose, different grades of the polymer, e.g. with different physical and mechanical properties, may be chosen for the internal pressure sheath and the hose, to obtain a hose with more flexibility than the internal pressure sheath. Alternatively, different types of polymers with different physical and mechanical properties may be used or the required flexibility of the hose may be obtained by using a thinner layer thickness than the pressure sheath.

Moreover, polymers with less crystallinity may be used for the hose to achieve a more flexible hose. Thus, the polymer material of the internal pressure sheath may have a crystallinity in the range of 60-80%, whereas the polymer material of the hose may have a crystallinity in the range of 25-55 %.

If the internal pressure sheath and the hose are made from a cross-linked polymer, the degree of cross-linking may be lower in the polymer material of the hose to achieve a more flexible hose. For example, the polymer material of the internal pressure sheath may have a cross-linking degree of in the range of 60-90 %, and the polymer of the hose may have a cross-linking degree in the range of 15-55 %.

Although the polymer materials of the inner liner and the hose are selected from the polymers which are frequently used in flexible pipes, such as polyethylene, polyamide or polyvinyl difluoride, the polymer of the internal pressure sheath and the hose may in principle be any of the polymer materials used in the various layers today. Consequently the polymers may be selected from the group consisting of polyolefins, such as polyethylene and poly propylene; polyamide, such as poly amide-imide, polyamide-11 (PA- 11) and polyamide-12 (PA-12); polyimide (PI); polyurethanes; polyureas; polyesters; polyacetals; polyethers, such as polyether sulphone (PES); polyoxides; polysulfides, such as polyphenylene sulphide (PPS);

polysulphones, such as polyarylsulphone (PAS); polyacrylates; polyethylene terephthalate (PET); polyether-ether-ketones (PEEK); polyvinyls;

polyacrylonitrils; polyetherketoneketone (PEKK); copolymers of the

preceding; fluorous polymers such as polyvinylidene diflouride (PVDF), homopolymers and copolymers of vinylidene fluoride ("VF2 "), homopolymers and copolymers of trifluoroethylene ("VF3 "), copolymers and terpolymers comprising two or more different members selected from the group consisting of VF2, VF3, chlorotrifluoroethylene, tetrafluoroethylene, hexafluoropropene, and hexafluoroethylene.

The hose may be layered and optionally comprise dedicated layers, which may serve to enhance thermal insulation and/or permeation resistance properties. A hose comprising two or more layers may also have an improved strength in respect of resisting loads such as radial pressure loads and axial tensile loads. In case the hose is layered, the different layers may be of the same or different polymer material and preferably the layers are bonded to each other, e.g. by chemical or physical bonding.

In an embodiment the hose is made from layered material. Thus, the wall of the hose is a layered, which may increase the strength of the hose. The layer may be e.g. a layer of impermeable polymer material, such as PE, PA or PVDF. Another layer may be a woven or non-woven layer of fiber material.

In an embodiment the hose is made from woven polyester coated on each side with a chlorinated cross-bound ethylene based polymer with

reinforcements and an internal impermeable layer of LDPE, PA, or PVDF. The coating is rubber like but vulcanizing is not required, and softeners are not leaching out as for normal PVC-coated materials.

The reinforcement may be fibres, such as fibres selected from basalt fibres, polypropylene fibres, carbon fibres, glass fibres, aramid fibres, steel fibres, polyethylene fibres, mineral fibres and/or mixtures comprising at least one of the foregoing fibres.

Both the material of the internal pressure sheath and the hose should be able to resist the effects of being exposed to the temperature and pressure loads from the first fluid, e.g. the hydrocarbon fluids transported in the hose. The polymer material should also be substantially impervious to gases in the hydrocarbon fluids such as CO2, H2S and CH ₄ . However, minor amounts of gases may be able to migrate through the material of the hose. These gases may be dissolved in the second fluid and removed from the flexible pipe with the second fluid. Thus, the risks of corrosion of the steel armor in the pipe annulus caused by gas constituents like CO2 and H2S are significantly reduced.

The internal pressure sheath has a thickness which ensures that the strength is sufficient to resist the forces affecting the internal pressure sheath. In an embodiment the internal pressure sheath has a wall thickness in the range 4 mm to 26 mm, such as in the range 4 mm to 18 mm, or such as in the range 5 mm to 16 mm.

The flexible hose should also have a thickness which provides a flexible and resilient hose which has a sufficient strength to withstand variable differential pressure loads and axial loads e.g. due to weight and inertia forces when installed in a riser. In an embodiment the flexible hose has a wall thickness in the range 0.4 mm to 4 mm, such as in the range 0.8 mm to 2 mm.

In an embodiment of the flexible pipe, the flexible hose in unloaded condition has an inner diameter in the range 5 cm to 50 cm. Thus, the hose may be capable of transporting rather large amounts of the first fluid. Moreover, the inner diameter of the internal pressure sheath is adapted to match the outer diameter of the flexible hose. In an embodiment the hose has a varying wall-thickness along its length in the longitudinal direction of the pipe. Thus, it is possible to design a flexible pipe with a low weight, but having sufficient strength to resist the forces which vary with the depth of the water in which the pipe is applied.

The different layers of the flexible pipe are terminated in end-fittings and in an embodiment the pipe comprises one or more end-fittings where the flexible hose is anchored and sealed in the one or more end-fittings. The hose may be anchored and sealed in the end-fitting by mechanical devices, such as screws, sleeve inserts, bushings or other squeezing means, or by use of an adhesive, such as a resin, e.g. epoxy.

To protect the armor layer around the internal pressure sheath, which armor layer may be made from metallic material, the flexible pipe in an embodiment comprises an outer sheath made from a polymeric material, such as polyethylene PE, polyamide PA, polypropylene PP or polyvinyldiene fluorid PVDF. The outer sheath may serve to prevent ingress of corrosive fluid, such as seawater to the armor layer. Thus, in an embodiment the outer sheath is impermeable to fluid.

The flexible pipe comprises at least one armor layer and in an embodiment the at least one armor layer is a pressure armor layer. The pressure armor layer serves to resist forces affecting the flexible pipe in radial direction. The pressure armor layer is preferably made from an elongate metallic strip or interlocking profiles wound around the internal pressure sheath with a winding angle of about 56° to about 89°. The pipe may comprise two pressure armor layers which may be wound in the same or opposite direction.

In an embodiment the at least one armor layer is a tensile armor layer, preferably the pipe comprises two tensile armor layers. The tensile armor layer serves to take up forces in the axial direction of the flexible pipe. The tensile armor layer is preferably made from an elongate metallic strip, wire, or profile wound around either the internal pressure sheath or one or more tensile armors with a winding angle of about 20° to about 55°.

In an embodiment the flexible pipe comprises at least one pressure armor layer and at least one tensile armor layers. Thus, the flexible pipe is armored against both radial and axial forces.

Preferably the flexible pipe is an unbonded flexible pipe. As mentioned, the hose is as such not bonded to the internal pressure sheath and may be retrofitted in the pipe. However, in some embodiments parts of the hose may be bonded to the internal pressure sheath.

The flexible pipe may comprise one or more sensors, such as pressure sensors, temperature sensors or sensors sensible to strain. In an embodiment the pipe comprises one or more sensors such as pressure sensors. The sensor may e.g. be optical sensors coupled to a control device comprising a computing unit.

The invention also relates to a method for equalizing the hydrostatic pressure in a flexible pipe bore with the external ambient environment having a length and a longitudinal axis and comprising an internal pressure sheath encircling a bore. The unbonded flexible pipe further comprises at least one armor layer on the outer side of the internal pressure sheath and optionally an outer sheath on the outer side of said at least one armor layer and a flexible hose is placed in the bore of the pipe. The method comprises the steps of:

- placing substantially the whole length of the flexible pipe in a liquid and allowing the liquid to surround the pipe,

- transporting a first fluid in the hose, and

- allowing a second fluid to enter the interface between the outer side of the hose and the inner side of the internal pressure sheath wherein the pressure of the second fluid corresponds substantially to the pressure of the liquid surrounding the flexible pipe. According to the method a second fluid is allowed to enter the interface between the outer side of the hose and the inner side of the internal pressure sheath and fill out cavities occurring in the interface. When a cavity between the outer side of the hose and the inner side of the internal pressure sheath occurs it may remain more or less permanent, however, the cavity may change volume depending on the differential pressure between the hose and the pipe. The cavity houses the second fluid and by letting the hydrostatic pressure there equalize to be in equilibrium with the external ambient pressure, the pipe is prevented from collapse when depressurizing the hose.

In an embodiment of the method a constant pressure is applied to the second fluid in addition to the equalized hydrostatic pressure. Thus, a method is provided to control the flow of the first fluid transported in the hose, e.g. to facilitate the emptying of the hose without using pigs and without

depressurizing the pipe system.

The method provides the possibility to "flush" the pipe and remove undesired gases, such as CO2 and H2S which may have permeated through the material of the hose from the first fluid transported in the hose. Thus, in an

embodiment a constant or intermittent flow of the second fluid flows through the pipe in occurring cavities in the interface between the outer side of the hose and the inner side of the internal pressure sheath. The second flow may flow from one end of the pipe to another end, thereby "flushing" the entire length of the pipe.

In an embodiment the liquid surrounding the pipe and the second fluid is water, such as seawater. Thus, the liquid surrounding the pipe and the second fluid can be the same and in a further embodiment the second fluid enters the occurring cavity between the outer side of the hose and the inner side of the internal pressure sheath via holes and/or perforations in the flexible pipe and/or holes in an end-fitting. In this embodiment there is a connection between the liquid surrounding the pipe, which may be seawater, and the fluid in the cavities in the interface between the outer side of the hose and the inner side of the internal pressure sheath whereby the hydrostatic pressure will be the same.

In an embodiment the pressurized second fluid is injected e.g. by means of pumps between the outer side of the hose and the inner side of the internal pressure sheath. In this embodiment it is possible to apply a higher pressure on top of the hydrostatic pressure.

It is also possible to form a flexible line which is assembled from two or more flexible pipes according to the invention. The flexible line may then comprise several sections, where each section is constituted by a flexible pipe according to the invention, and mutually connected by connection pieces, which may e.g. be end-fittings. The flexible pipes in each section may be identical or different. If the flexible pipes are different, the difference may e.g. be the length of the pipe, the wall thickness of the internal pressure sheath or the hose or the structure of the one or more armor layers. Thus, it is possible to design a flexible pipe system for offshore use, which has optimum properties at different water depths.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be explained more fully below with reference to the drawings in which:

Figure 1 shows a conventional unbonded flexible pipe;

Figure 2 shows an embodiment of a flexible pipe according to the invention;

Figure 3 shows a cross-section of a flexible pipe according to the invention; and

Figure 4 shows a cross-section of a flexible pipe according to the invention; Figure 5 shows an embodiment of a flexible pipe and an end-fitting according to the invention; and

Figure 6 shows a flexible line assembled from flexible pipes according to the invention.

The figures are schematic and simplified for clarity, and they show only details which are essential to the understanding of the invention, while other details are left out. The same reference numerals may be used for identical or corresponding parts.

Figure 1 shows a conventional flexible pipe 1 having a longitudinal axis x-x. From the inside to the outer side the flexible pipe comprises a carcass 10 supporting an internal pressure sheath 11. The carcass 10 is made from materials like duplex steel grades, which are resistant to the harsh

environment in the bore of the pipe in which the oil and gas flow. The internal pressure sheath 11 is made from extruded polymers, e.g.

polyethylene, and the carcass mainly serves to support the internal pressure sheath 11 in case of a sudden pressure drop in the bore. The internal pressure sheath 11 is reinforced with a pressure armor layer 12 and two tensile armor layers 13, 14. The outer surface of the flexible pipe 1 is constituted by a protective sheath 15.

Figure 2 shows an embodiment of a flexible pipe 100 according to the present invention which differs significantly from the conventional flexible pipe described above. From the inside and out the flexible pipe 100 comprises a flexible hose 101. The flexible hose 101 is located in the bore 110 encircled by the internal pressure sheath 102. The hose 101 is loosely mounted in the bore and not bonded to the inner surface of the internal pressure sheath 102.

The hose 101 is a layered hose including polyethylene material and the internal pressure sheath 102 is made from the same grade of polyethylene. The hose is approximately 1.2 mm thick and the wall thickness of the internal pressure sheath is approximately 12 mm.

On the outer surface of the internal pressure sheath 102 a pressure armor 103 is located. The pressure armor 103 is made from metal strips of stainless steel which are wound around the internal pressure sheath with a winding angle of about 80° in respect of the longitudinal axis X-X.

The pressure armor 103 is surrounded by a tensile armor 104 which is made from metal strips or wires of stainless steel. The tensile armor 104 is wound with a winding angle of about 30° in respect of the longitudinal axis X-X.

The tensile armor 104 is surrounded by an outer sheath 105 made from extruded polyamide. The hose 101, the internal pressure sheath 102 and the outer sheath 105 are substantially fluid-tight.

Between the internal pressure sheath 102 and the outer sheath 105 is formed an annulus in which the pressure armor 103 and the tensile armor 104 are located.

Figures 3 and 4 are cross-sections of the pipe 100 under different conditions.

In figure 3 the hose 101 is in a condition which may be referred to as fully load expanded in which condition the entire outer surface of the hose 101 is in contact with the inner surface of the internal pressure sheath 102 and all available space in the bore 110 is occupied by the hose 101.

However, in figure 4 the hose 101 only occupies a part of the bore 110 leaving vacancies 112 in the bore 110. The vacancies 112 are filled with a second fluid which will equalize the pressure of the first fluid within the hose 101. There may also appear a situation in which there is no contact between the outer surface of the hose 101 and the inner surface of the internal pressure sheath 102 and the hose 101 is surrounded by the second fluid in a vacancy surrounding the hose. The degree of surface contact between the outer surface of the hose 101 and the inner surface of the internal pressure sheath 102 depends on the pressure of the first fluid in the hose 101 and the pressure of the second fluid in the bore of the flexible pipe.

As it can be seen from the figures only the hose 101 has any substantial change of shape. The internal pressure sheath 102, the pressure armor 103, the tensile armor 104 and the outer sheath maintain their substantially circular shape.

Figure 5 illustrates how the flexible pipe 100 is terminated in an end-fitting part 200 comprising an end-fitting 201 and a spool piece 202.

The internal pressure sheath 102, the pressure armor 103, the tensile armor 104 and the outer sheath 105 are all terminated in the end-fitting 201.

However, the hose 101 is loosely placed in the bore 110 of the pipe. In the interface between the outer surface of the hose 101 and the inner surface of the internal pressure sheath 102 is a cavity 112 which can contain a second fluid. The cavity 112 communicates with the external environment via a hole 203 in the spool piece 202.

The hose 101 is anchored to the end-fitting 200 in the spool piece 202 at the back end 204. The back end 204 of the spool piece 202 is adapted for attachment of the hose 101. The hose 101 is anchored to the spool piece 202 by a combination of a sleeve insert or bushing, screws and a polyester resin, which provide a substantially liquid tight sealing between the hose 101 and the spool piece 202.

Via the hole 203 the cavity 112 may be connected with pumps, vents and other control means (not shown in the figure) which may serve to maintain the pressure around the hose 101 at a desired level. By use of such means it is possible to control and equalize the pressure in the hose and the bore, thereby avoiding damage on the pipe structure caused by difference in pressure.

Figure 6 shows a floating facility 301 at sea level 302. The floating facility 301 is connected to a production facility 303 at the sea bed 304 by the flexible line 300. The flexible line comprises three different sections with flexible pipes 300 A, 300 B and 300 C according to the present invention. The flexible pipes are connected by connection pieces 305A and 305B. At each end the flexible line is connected with end-fittings 305 which are connected to connectors on the floating facility 301 and the production facility. The connection pieces 305 A and 305 B and the end-fittings 305 allow communication between the environment and the interface between the internal pressure sheath and the hose within the pipes, thus, allowing equalization of the pressure.

In the flexible pipe 300 A the wall thickness of the layered hose is 0.84 mm and in the flexible pipe 300 B the wall thickness of the layered hose is 1.00 mm and in the flexible pipe 300 C the wall thickness of the layered hose is 1.2 mm. The thickness of the internal pressure sheath in the flexible pipes 300 A, 300 B and 300 C is the same, the thickness is 12 mm. Both the internal pressure sheath and the hose are made from polyethylene material grades. The flexible pipes also comprises an outer sheath made of polyamide and pressure armor and tensile armor made from stainless steel.

Thus, it is possible to apply a hose with different wall thickness in different sections of the pipe. Furthermore, in another embodiment the hose has varying wall thickness within the same pipe section, for example where the hose is thicker and stronger at the top end of a riser section compared to its lower end. Thereby it is possible to weight and cost optimize the design of the different sections of the flexible pipe/riser with hoses which will have sufficient wall thickness and strength to withstand the imposed axial and radial loads of the hose at varying water depth. Consequently the present invention provides a flexible pipe in which the carcass can be omitted and the pressure in the bore of the pipe can be equalized by e.g. surrounding sea water. By omitting the carcass a lighter flexible pipe can be achieved.

Moreover, the interface between the outer side of the hose and the inner side of the internal pressure sheath can be flushed by sea water which may remove harmful gases, such as CO2 and H2S which may permeate through the material of the hose. Thus, the risk of the gases reaching the annulus and the armor layers in the annulus is significantly reduced, and, thereby the risk of corrosion of the armor layers is reduced.