TECHNICAL FIELD

The present invention relates to an unbonded flexible pipe having a length and a longitudinal direction along the length and comprising from the inside and out a bore surrounded by at least an internal pressure sheath, one or more armour layers and an outer sheath. Moreover, the present invention relates to a method of regulating the temperature of the surface of the unbonded flexible pipe.

BACKGROUND

Unbonded flexible pipes are frequently used as flexible risers or flexible flowlines for transport of fluid hydrocarbons such as oil and gas.

Moreover, unbonded flexible pipes are often used e.g. as riser pipes or flowlines in the production of oil or other subsea applications.

The unbonded flexible pipes are constructed of a number of independent layers, such as helical 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 armoring layer known as the carcass, an internal pressure sheath surrounded by one or more armoring layers, such as pressure armoring and tensile armoring, 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. 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 armoring and the tensile armoring. The unbonded flexible pipes may carry the fluids between 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, is processed, for example by compression and/or further treatment. When the floating structure is moored close to a gas field or 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 convey fluids between the well heads of a hydrocarbon reservoir and the floating structure. Flexible risers may be configured as free-hanging catenaries or provided in alternative

configurations, such as lazy wave and lazy S types, using buoyancy modules. Thus, a flexible riser may be connected at one end to the floating structure, and at another end to a riser base manifold, by which the flexible riser is secured to the sea bed.

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 an 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 armoring 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 be a sealing layer sealing against ingress of sea water. In certain unbonded flexible pipes one or more intermediate sealing layers are arranged between armor layers.

In general flexible pipes are expected to have a lifetime of 20 years in operation. 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 armoring layers comprise or consist of multiple elongated armoring elements that are not bonded to each other directly or indirectly via other layers along the pipe. Thereby the pipe becomes bendable and sufficiently flexible to roll up for transportation.

One problem which frequently arises when the unbonded flexible pipe is used offshore for transport of fluids of hydrocarbons is that the fluid is cooled by the surrounding sea water to such a degree that the fluid becomes highly viscous and difficult to transport. This problem is normally addressed by applying thermal insulation layers or active heating, such as electric heating.

However, another problem may also arise, that is if the fluid is too hot. In such cases the fluid may heat the material in the unbonded flexible pipe to a degree which may cause damage, e.g. too much heat may cause the polymer material in pipe to degrade by hydrolysis, which eventually will destroy the properties of the material. The pipe is particularly vulnerable for over-heating where it is not cooled by sea water, i.e. at the part of the pipe which is above the water line and connected to the floating structure for collecting the hydrocarbon.

The end-fittings, which connect the unbonded flexible pipes with the storage in the floating structure, have sometimes been equipped with cooling means. Moreover, it has also been tried to apply a jacket on the pipes for the purpose of cooling. Although the known methods for cooling function satisfactorily, they are, however, features which make the unbonded flexible pipes more costly and more complicated to produce.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide an unbonded flexible pipe where the risk of subjecting the outer sheath to too high or too low

temperatures which potentially result in damaging of the pipe is reduced and which pipe simultaneously can be produced at relatively low cost.

The present invention provides a solution for local cooling or heating of at least a part of an unbonded flexible pipe which is easy to apply and which may be used on both new pipes and existing pipes for offshore production of hydrocarbons.

In an embodiment the invention relates to an unbonded flexible pipe having a length and a longitudinal axis along the length and comprising at least an internal pressure sheath defining a bore, one or more armour layers and an outer sheath, wherein the unbonded flexible pipe in at least a length section comprises at least one tube wound around the outer sheath with an inclination of at least about 60° in respect of the longitudinal axis. The tube has a first end and a second end, where the first end of the tube is adapted for connection to a fluid source.

An unbonded flexible pipe always comprises an internal pressure sheath which is substantially fluid tight, and the internal pressure sheath may comprise two or more sub-layers. The armour layers are wound around the internal pressure sheath with different angles in respect of the longitudinal direction of the pipe. The longitudinal direction of the pipe is coincident with the axis of the pipe and "the longitudinal direction of the pipe" and "the axis of the pipe" may be used interchangeably. The unbonded flexible pipe also comprises an outer sheath serving to protect the armour layers. Moreover, an unbonded flexible pipe may also comprise one or more thermal insulating layers, which may conveniently be placed in the annulus formed between the internal pressure sheath and the outer sheath. Moreover, an unbonded flexible pipe may comprise an armour layer in the bore, referred to as a carcass, serving to reinforce the internal pressure sheath.

As mentioned the term "unbonded" means that at least two of the layers including the armour layers and polymer layers are not bonded to each other. In practice the known pipe normally comprises at least two armour layers located outside the internal pressure sheath. These armour layers are not bonded to each other directly or indirectly via other layers along the pipe. Thereby the pipe becomes bendable and sufficiently flexible to roll up for transportation. Moreover, the armour layers adjacent to the internal pressure sheath may not be bonded to internal pressure layer. In a similar manner an armour layer adjacent to the outer sheath may not be bonded to the outer sheath.

The outer sheath normally forms the outer surface of the unbonded flexible pipe, i.e. the surface which normally has the direct contact with the

environment, such as seawater. This outer surface of the unbonded flexible pipe is also the outer surface of the outer sheath, and the at least one tube is wound around this outer surface. When the tube is wound around the outer surface and optionally covered with a protective cover, the tube and/or the protective cover will be in direct contact with the environment. The protective cover is not necessarily water proof and may have an open structure, such as a net.

In this context the outer surface should be understood as the outer surface of the outer sheath, i.e. the surface of the outer sheath facing away from the bore of the unbonded flexible pipe. The outer sheath is the outermost fluid impermeable layer which extends in the entire length of the pipe and protects towards ingress of water into the annulus. Normally, the outer surface of the unbonded flexible pipe is simply referred to as the surface of the unbonded flexible pipe.

Both the internal pressure sheath, the outer sheath and the optional thermal insulation layers are normally made from polymer material, such as e.g. polyethylene, polypropylene or polyamide. Such polymer materials are vulnerable to extreme temperatures, i.e. in this respect temperatures below about 0°C and temperatures above about 100°C. Low temperatures may make the material brittle and cause formations of cracks which will eventually destroy the material. High temperatures may make the materials soft and cause them to lose their strength. Moreover, a long term of exposure to high temperatures may cause degradation of the polymer material by hydrolysis.

The incidents of exposure to extreme temperatures may only appear on local parts of the unbonded flexible pipe. In particular the unbonded flexible pipes for offshore production are locally exposed when they are used as risers between a seabed production facility and a floating unit on the sea surface. The length of the pipe which is above the sea surface and connected to the floating storage unit e.g. via an end-fitting may tend to become too warm. This is due to the fact that the hydrocarbons from the seabed production facility, which are transported to the floating unit, may have a considerable temperature, such as about 100°C or higher. When the unbonded flexible pipe is surrounded by sea water, the sea water will serve to cool the outer surface of the pipe. However, the length of the pipe which is above the sea surface will be surrounded by atmospheric air, which has a much lower thermal conductivity than sea water and thus will not have any significant cooling effect on the outer surface of the unbonded flexible pipe.

Consequently, in some situations the temperature in the length section of the unbonded flexible pipe which is above the sea surface may reach a critical temperature, which may lead to damage of the material in the pipe. However, critical situations may also appear in which the pipe or parts of the pipe is cooled too much and the temperature in the materials of the pipe becomes so low that it is critical and may cause damage to the materials. Such situations may e.g. appear during a rapid decompression of the system where gasses in the pipe are cooled due to adiabatic expansion.

Consequently, the present invention aims to remedy damage which may appear in such situations. It has been found that is possible to provide sufficient cooling or heating to the surface of an unbonded flexible pipe by winding at least one tube around the outer sheath of the unbonded flexible pipe with windings having an inclination of at least 60° in respect of the axis of the pipe. In one end the pipe is adapted to be connected with a source for fluid. The fluid may be a cold fluid when cooling is required and a hot fluid when heating is required.

The windings of the tube should have an inclination of at least 60° in respect of the axis of the pipe to ensure that a sufficient heat-transport for a sufficient period of time between the fluid in the tube and the surface of the unbonded flexible pipe is established. In some embodiments the inclination of the tube in respect of the axis of the pipe may be in the range of about 60° to about 89.99°, such as from about 65° to about 89.99°, conveniently from about 65° to about 89.8°, suitable from about 70° to about 89.8°.

In an embodiment the first end of the tube is adapted for connection with a source for fluid. The connection may be a valve and e.g. a piping system comprising a pump. The second end of the tube may also be connected with a valve. Alternatively the second end is open to allow a fluid to flow freely.

The tube is wound around the surface of the unbonded flexible pipe i.e. the outer sheath, and in an embodiment the unbonded flexible pipe further comprises a protective cover around the wound tube. The protective cover serves to protect the tube, but may also serve to ensure that the tube remains in place without displacement relative to the outer sheath, and also to press the tube towards the outer sheath to achieve a better contact and heat-transfer.

In an embodiment of the pipe according to the invention the tube is wound with contacting windings. The adjacent windings are in contact with each other and it is possible to have the maximum amount of windings per surface area unit. Thereby it is possible to achieve a good transport of heat between the outer sheath and the fluid in the tube.

In an embodiment of the unbonded flexible pipe the tube is wound with a distance between adjacent windings. In this embodiment it is possible to have other items between the windings, e.g. the windings of a second tube. In such an embodiment it is possible to send the fluid in opposite directions in the two tubes to obtain a very efficient transfer of heat.

The fluid source can in principle be any kind of fluid, such as water, steam or oil. However, in an embodiment the fluid source is water, such as seawater. Seawater is easy accessible in offshore applications and has excellent cooling capacities. In case heating is required it might by advantageous to use oil having a high boiling point, such as above 150°C at 1 atm. pressure.

In principle the tube may have any desired cross section and any desired diameter. However, in some embodiments the tube has a substantially circular cross section and an inner diameter in the range of 3 mm to 50 mm and an outer diameter in the range of 5 mm to 60 mm. The inner diameter will allow a sufficient amount of fluid to pass and the difference between the inner and the outer diameter will ensure that the wall thickness of the tube is sufficient to provide a satisfactory strength of the tube and at the same time ensure a good heat transport.

The term "substantially" should herein be taken to mean that ordinary product variances and tolerances are comprised. When a tube having a circular cross section is wound around the unbonded flexible pipe, the cross section may change from circular to become slightly oval, however, this has no impact on the cooling capacity.

In an embodiments the tube is made from a metallic material such as stainless steel, which has good properties in respect of heat conductivity. Alternatively the tube may be made from a polymer material, such as polyethylene or polypropylene which are materials having good properties in respect of machinability and resistance to corrosion.

The present invention also relates to a method for regulating the temperature of a surface part of an unbonded flexible pipe having a length and a longitudinal axis along the length and comprising from the inside and out at least an internal pressure sheath defining a bore, one or more armour layers and an outer sheath. The method comprises at least:

- winding at least one tube having a first end and a second end around the outer sheath in at least a length part of the unbonded flexible pipe with windings having an inclination of at least 60° in respect or the longitudinal direction;

- connecting the first end of the tube with a source for a fluid; and sending the fluid through the wound tube.

The tube is wound around the outer sheath. Thus, the tube is wound around the outer surface of the outer sheath, which surface may also be construed as the outer surface of the unbonded flexible pipe. Consequently, in this context the surface should be understood as the outer surface of the outer sheath, which is the outermost fluid impermeable layer which extends in the entire length of the pipe and protects towards ingress of water into the annulus

It has been found that when flowing through the tube the fluid affects not only the outer sheath of the pipe. In an embodiment also one or more layers inside the outer sheath are affected by being temperature regulated. In an embodiment the unbonded flexible pipe will be affected in an area from the outside and inwards towards the bore. Thus, the part in which the

temperature is regulated is actually an area in the outer periphery of the unbonded flexible pipe. This area may have a thickness in the range from about 0.5 cm to about 4 cm, depending on the actual configuration of the unbonded flexible pipe and the tube. The thickness of the area is measured in a plane perpendicular to the axis of the unbonded flexible pipe from the outer surface of the outer sheath and inwards towards the axis. The area is referred to as "the surface part".

The tube is wound around the outer sheath of the unbonded flexible pipe with windings having an inclination of at least about 60° in respect of the axis of the unbonded flexible pipe. The relative steep inclination provides for a good exchange of heat between the fluid in the tube and the surface of the unbonded flexible pipe. Moreover, the relative steep inclination also serves to provide sufficient flexibility of the unbonded flexible pipe having the tube wound around the outer sheath.

In an embodiment more than one tube may be wound around the outer sheath on different length sections of the unbonded flexible pipe.

To ensure a proper hold of the wound tube the method may comprise the further step of providing a protective cover around the wound tube.

When the temperature regulation of the surface is a cooling process the fluid sent through the wound tube is advantageously a liquid having a temperature in the range of from about minus 2 to about 30 degree centigrade.

Conveniently the temperature is in the range from about 0 to about 15 degree centigrade.

If the temperature regulation of the surface, in contrast to cooling, is a heating process the fluid sent through the wound tube is preferably a liquid having a temperature in the range from about 70 to about 100 degree centigrade.

According to an embodiment of the method the fluid sent through the wound tube is preferably water, such as sea water. In offshore production sites sea water is easily accessible and will have a temperature which is suitable for cooling. However, if the temperature regulating process is a heating process it may sometimes be preferred to use oil as the fluid, i.e. an oil having a higher boiling point than water. The oil is then heated to a suitable

temperature and sent through the wound tube.

In an embodiment the second end of the tube allows the fluid to freely flow out of the tube. This embodiment is particularly suitable when the fluid is water or seawater. The water may then be sent into the tube from the first end and flow through the tube and run out of the second end, e.g. into the sea. However, the second end of the tube may also be connected with a valve and/or a pump which will control the out flow of liquid from the second end of the tube. The first end of the tube may in an embodiment be connected with a valve and/or a pump which may control the inflow of liquid to the tube. The pump and the valve may be connected to temperature sensors located on the outer surface of or inside the unbonded flexible pipe. In response to signals from the temperature sensors the pump and the valve may send a fluid through the tube which will cool or heat the surface of the unbonded flexible pipe, respectively.

Although the entire length of the pipe may have one or more tubes wound around the outer sheath, it is, however, in many situations preferred that only a limited part of the length is wound with a tube around the outer sheath. The limited part of the length of the unbonded flexible pipe may e.g. be the part of the unbonded flexible pipe which is above the sea level and connects a subsea structure with a floating facility. The length section of the unbonded flexible pipe which is wound with a tube around the outer sheath is suitable from about 1 m to about 100 m, such as from about 2 m to about 75 m, such as from about 3 m to about 50 m.

When winding the tube around the pipe it may in an embodiment be advantageous to co-wind a thermal sensor. Any sensor that fits in the free space between the wound tube and the outer sheath may be used to give a suitable signal. The most preferred sensor is a glass fiber based optical sensor which will allow detailed thermal mapping along the pipe length.

According to an embodiment the unbonded flexible pipe comprises at least one end-fitting terminating one end of the unbonded flexible pipe, and the tube is wound on a part of the length of the unbonded flexible pipe adjacent to the end-fitting. If the end-fitting is the end-fitting which connects the unbonded flexible pipe with the floating facility, a part of the unbonded flexible pipe will be located between the end-fitting and the sea level and surrounded by air. As mentioned the air has a heat capacity which is lower than the heat capacity of water and extra cooling may be required on the length of the pipe between the end-fitting and the sea level.

However, the unbonded flexible pipe may also be terminated in an end-fitting connecting the unbonded flexible pipe with the subsea structure at the sea bed. Adjacent to this end-fitting a length of the unbonded flexible pipe may require cooling due to the fact that the hydrocarbons conveyed in the unbonded flexible pipe may have a very high temperature. Thus, a length of the pipe adjacent to this end-fitting may also be wound with a tube to cool the length of the pipe.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in further details with reference to embodiments shown in the drawing in which:

Figure 1 shows an unbonded flexible pipe according to the invention; Figure 2 shows an application of the unbonded flexible pipe; and

Figure 3 shows an alternative embodiment of the application.

The figures are not accurate in every detail but only sketches intended to the show the principles of the invention. Details which are not a part of the invention may have been omitted. In the figures the same reference signs are used for the same parts.

Figure 1 shows an embodiment of an unbonded flexible pipe 1 according to the invention. The unbonded flexible 1 comprises from the inside and out an internal pressure sheath 2, which is substantially pressure tight. The internal pressure sheath is made from a polyethylene composition and extruded to form to form the internal pressure sheath 1.

On the outer side of the internal pressure sheath 2 is wound a pressure armour 3. The pressure armour 3 serves to protect the pipes against damage which may arise if the pressure in the hydrocarbons conveyed in the bore of the pipe suddenly increases.

Outside the pressure armour 3 a first tensile armour 4 and a second tensile armour 5 are located. The tensile armours 4 and 5 serve to protect against tensile forces which otherwise may damage the unbonded flexible pipe.

The pressure armour 3 and the tensile armors 4 and 5 are made from elongate strips wound in a helical pattern to form a tubular structure. The strips may e.g comprise steel, stainless steel, aluminium or fibre reinforced plastics such as carbon or glass reinforced epoxy.

Outside the layers of tensile armors 4 and 5 is placed a layer of thermal insulation 6. The insulating layer 6 can be made from polypropylene, and in this particular embodiment the insulating layer 6 is an extruded layer.

On the outer side of the insulating layer 6 the outer sheath 7 is located. The outer sheath 7 serves to protect the unbonded flexible pipe 1. The outer sheath 7 is made from extruded polymer such as polyamide 11 and is fluid tight and water tight to prevent sea water from entering the annulus formed between the internal pressure sheath 2 and the outer sheath 7. In the annulus sea water may cause the armour layers to corrode.

A tube 8 made from a cross-linked polyethylene material is wound around the surface of the outer sheath 7 with consecutive windings and in close contact with the surface of the outer sheath. The tube 8 is capable of transporting fluids which may exchange heat with the surface of the outer sheath 7.

Normally the fluid will be water and in particular seawater. If the seawater has a temperature which is below the temperature of the outer sheath 7, the water in the tube 8 will absorb heat and cool the surface. If a heating of the outer sheath 7 is desired, hot water, e.g. with a temperature about 100°C, may be conveyed through the tube 8 and transfer heat to the outer sheath 7. The water flows into the tube 8 from one end by means of a pump and a valve, and the water flows out of the tube at the other end which also has a valve to control the flow.

The tube 8 is wound with an angle of about 80° in respect of the longitudinal direction of the pipe 10. The longitudinal direction of the pipe 10 corresponds to the axis of the pipe with extends in the center of the bore 11. The relative steep inclination of the tube 8 in respect of the axis of the unbonded flexible pipe 1 ensures that the fluid flowing in the tube will have sufficient time to exchange heat with the surface of the outer sheath.

In this embodiment of the unbonded flexible pipe 1 according to the invention the tube 8 is surrounded by a protective cover 9 made from either polyethylene or polyamide. As it is understood this protective cover 9 serves to protect the pipe 1 and the tube 8 wound around the pipe. However, the protective cover 9 may also serve to press the tube 8 towards the surface of the outer sheath 7 to establish a good contact between the outer sheath 7 and the tube 8. In other embodiments the protective cover may simply be an open net-like structure. The protective cover does not as such prevent e.g. seawater from coming into contact with the outer sheath. Figure 2 shows an unbonded flexible pipe 1 connected to a vessel 22 for collection and storage of hydrocarbons. The pipe 1 is connected with the vessel 22 via the end-fitting 20 and the bearing structure 21. The vessel 22 is floating in the sea 23 with sea surface 24.

The part of the pipe 1 which is above the sea surface is covered with a protective cover 9 and below this protective cover is a tube wound around the pipe. The tube is covered with the protective cover and is therefore not visible. The tube serves to cool the pipe in the length of the pipe which is above the sea surface and the tube receives sea water via the coupling device 12 which is connected with a valve and a pump located inside the bearing structure.

Thus, in operation the pump pumps sea water from the sea 23 and into the tube via the coupling device 12. The sea water flows in the windings of the tube and cool the surface of the pipe 1. Just above the sea surface the portion of the pipe 1 which is wound with a tube stops and an outlet 13 is established. In this embodiment the outlet 13 does not comprise a valve and the sea water in the tube flows freely back to the sea 23.

Even if the outlet 13 was below the sea surface the water would still be able to flow out into the sea 23 due to the action of the pump.

Figure 3 shows the situation where the unbonded flexible pipe 1 is connecting a subsea structure 25 with a vessel 22 floating in the sea 23. The pipe 1 is connected with the vessel 22 via the end-fitting 20 and bearing structure 21. In the opposite end the pipe 1 is connected with the subsea structure 25 via the end-fitting 26. In each end a length of the pipe 1 comprises a tube wound around the outer sheath and covered with a protective cover 9. The unbonded flexible pipe 1 forms an S-bend facilitated by buoyancy modules 27 between the vessel 22 and the subsea structure 25. In the length adjacent to the end-fitting 20 the purpose of applying the tube is to cool the pipe sufficiently due the fact that the pipe 1 is not immersed in seawater.

In the length adjacent to the end-fitting 26 the purpose is simply to provide extra cooling as the hydrocarbons coming from the well and entering the subsea structure 25 have a very high temperature.