WO2019141326A1

WO2019141326A1 - A method and a system for circulating a rinse liquid in a flexible pipe

Info

Publication number: WO2019141326A1
Application number: PCT/DK2019/050014
Authority: WO
Inventors: Kristian GLEJBØL
Original assignee: National Oilwell Varco Denmark I/S
Priority date: 2018-01-18
Filing date: 2019-01-16
Publication date: 2019-07-25

Abstract

The present invention relates to a method for circulating a rinse liquid in a flexible pipe (1) having an upper first (5) end and a lower second end (6). The flexible pipe (1) comprises an internal pressure sheath and an outer sheath surrounding said internal pressure sheath, arranged such that at least one annulus is formed between the internal pressure sheath and the outer sheath, each annulus comprises one or more armor layers. The flexible pipe (1) also comprises at least one feed channel having a first inlet end and a second outlet end and adapted for transportation of rinse liquid and extending from the upper first end (5) of the pipe (1) towards the lower second end (6) of the pipe (1), the second outlet end is in fluid communication with at least one annulus, where the pressure at the outlet of the feed channel at the lower second end (6) of the flexible pipe (1) is PLSE when the feed channel is filled with rinse liquid in static condition, and the ambient pressure, at the position of the outlet of the feed channel, outside the pipe (1) is PSE, wherein the inlet end of the feed channel at the upper first end (5) of the flexible pipe (1) is connected to a pressure pump, adapted for pumping the rinse liquid into the feed channel at a pressure P_pumb, where P_pumb <= (P_SE - P_LSE) * 1.1, and the pumped rinse liquid is transported back to the upper first end (5) of the pipe (1) through at least one pipe annulus

Description

A METHOD AND A SYSTEM FOR CIRCULATING A RINSE LIQUID IN A

FLEXIBLE PIPE

TECHNICAL FIELD

The present invention relates to a method and a system for circulating a rinse liquid in an armored flexible pipe hereby removing harmful and corrosive substances which may damage the armor.

BACKGROUND Marine pipes generally are referred to as bonded pipes or unbonded pipes. A bonded pipe generally is a pipe in which the reinforcement layers are bonded to the polymer layers, which may be made from a vulcanized elastomeric material. An unbonded pipe generally comprise separate unbonded polymeric and metallic layers, which allows relative movement between layers. The present invention generally concerns unbonded flexible pipes. When the unbonded pipes is made from polymers and steel they are in the literature referred to as "steel armored flexible pipes".

Such steel armoured flexible pipes are suitable for marine applications such as for transport of petrochemical fluids e.g. oil or gas or in a sub-sea environment.

Steel armored flexible pipes for offshore applications are generally known from the standard "Recommended Practice for Flexible Pipe", ANSI/API 17B, fifth Edition, May 2014 (hereafter API17B), and the standard "Specification for Unbonded Flexible Pipe", ANSI/API 17J, Fourth edition, May 2014

(hereafter API17J).

Such an unbonded flexible pipe may comprise a number of independent layers, such as helical wound steel and polymeric layers, as well as extruded polymeric layers formed around a central bore. A typical steel armoured flexible pipe comprises from the inside and outwards an inner armouring layer known as the carcass, an internal pressure sheath surrounded by one or more armouring layers, such as pressure armouring and tensile armouring, and an outer sheath. Thus, the internal pressure sheath defines the bore in which the fluid to be transported is conveyed and thereby ensures internal fluid integrity and stability. In some unbonded flexible pipes the carcass may be omitted. In some unbonded flexible pipes only the pressure armour is made from steel whereas the tensile armour is made from fibre reinforced polymer composites. In other unbonded flexible pipes the pressure armour and the internal pressure sheath may be integrated while the tensile armour is made from steel elements.

The annular space or spaces outside the internal pressure sheath, which houses the steel armour layers are usually referred to as the annulus or annuli. The flexible pipes may for example be applied for carrying the fluids between a hydrocarbon reservoir located under the seabed either to a junction point between subsea structures or from the seabed to a floating structure. The fluid may be a hydrocarbon fluid, such as natural gas or oil, water, CO2 or a mixture hereof depending upon the nature of the hydrocarbon reservoir. The fluid may also be an injection fluid such as water, CO2 or methanol.

In general, flexible pipes are expected to have a service time of about 20 years in operation.

Unbonded flexible pipes are e.g. used for the transport of oil and gas at large or intermediate sea depths. The mentioned construction is particularly well suited for the transport of oil and gas from subsea sources to installations at sea level where the oil and gas are being treated or forwarded for further processing such as for example by compression, filtering, separation, distillation and/or further treatment. The armouring layers surrounding the internal pressure sheath may for example comprise one or more pressure armour layers comprising one or more armouring profiles or strips, which are wound around the internal pressure sheath at a large angle, e.g. larger than 80°, relative to the centre axis of the pipe. This or these pressure armour layers primarily compensate for radial forces in the pipe. The armouring layers surrounding the internal pressure sheath may also usually comprise one or more tensile armouring layers which are wound at a relative small angle, such as between 10° and 50°, relative to the centre axis of the pipe. This or these tensile armour layers primarily compensate for axial forces in the pipe. The armouring layers are typically made of steel. A typical unbonded flexible pipe is for example described in WO 00/36324.

To avoid prohibitively large radial deformations of the tensile armouring layers due to torsion, axial compression and/or bending of the pipe, a holding layer may be wound at a large angle around the tensile armouring layer(s). This armouring layer is usually of very flat profiles in the form of fibre reinforced polymeric tapes e.g. as described in US2010/101675.

During operation water and gasses tend to diffuse from the bore of the pipe into the annulus or annuli of the pipe. Over time the diffused gasses may cause the pressure in the annulus to rise, which may lead to bust of the outer sheath. To prevent bursting, gas vent valves are normally mounted in the termination of pipes, such that the gas pressure in the pipe is relieved when the pressure in the annulus significantly exceeds ambient pressure, as. e.g. described in API17B, 5.2.4. Ingress of gas, and in particular ingress of gas and water into the annulus may result in simultaneous appearance of gas and water on the exposed steel surfaces of the armour. This may lead to localized corrosion of the steel armour elements located in the annulus, eventually leading to premature failure of the pipe. In order to mitigate corrosion of the armour layers attempts have been made to prevent aggressive gasses such as hydrogen sulphide and carbon dioxide from reaching the annulus. One strategy is described in US2011/120583A. Herein the pressure sheath is filled with a reactive compound which reacts with diffusing gasses before they reach the annulus and thus the armour wires. Another strategy for preventing gasses from reaching the annulus is described in WO 05/028198 according to which an impermeable film is applied between the bore and the annulus.

WO 2012/092931 discloses an unbonded flexible pipe in which the annulus can be ventilated by flushing the annulus with a maintaining fluid. The flexible pipe comprises maintaining passages to provide maintaining fluid to the annulus.

WO16/074681 discloses a method of installing an unbonded flexible pipe, wherein at least a part of the annulus is filled with a corrosion promoting liquid hereby forcing the corrosion to be even, which is less damaging to the pipe compared to localized corrosion.

US2011/195208 discloses a method of protecting the armour layers by applying a boron holding grease or oil for the purpose of increasing acid resistance and/or corrosion resistance of the armor elements in the annulus.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an alternative method and system for reducing corrosion of metallic armor layers located in the annulus of a flexible pipe. The invention provides a method by which harmful substances, such as CO2, H2O and H2S, which may have migrated into the annulus from the bore of the pipe, are efficiently removed from the annulus. The substances are removed without causing any undesired loads on the flexible pipe. The present invention also provides a method by which harmful substances can be removed from the annulus in a very safe manner by adjusting the pressure by which the rinse liquid is introduced into the annulus as well as the flow rate of the rinse liquid in the annulus. The method also makes it possible to avoid excessive pressure in the annulus, hereby preventing damage to the pipe structure due to internal pressure.

Normally, the pressure in the annulus of a flexible pipe is controlled by venting valves, however, it has now surprisingly been realized that it is possible to substantially control the pressure in the annulus by carefully controlling the pressure delivered by the feeding pump.

In a first aspect the invention relates to a method for circulating a rinse liquid in a flexible pipe having an upper first end and a lower second end, said flexible pipe comprising an internal pressure sheath and an outer sheath surrounding said internal pressure sheath, arranged such that at least one annulus is formed between the internal pressure sheath and the outer sheath, each annulus comprises one or more armor layers, and at least one feed channel having a first inlet end and a second outlet end and adapted for transportation of rinse liquid and extending from the upper first end of the pipe towards the lower second end of the pipe, the second outlet end is in fluid communication with at least one annulus, where the pressure at the outlet of the feed channel at the lower second end of the flexible pipe is PLSE when the feed channel is filled with rinse liquid in static condition, and the ambient pressure, at the position of the outlet of the feed channel, outside the pipe is PSE, wherein the inlet end of the feed channel at the upper first end of the flexible pipe is connected to a pressure pump, adapted for pumping the rinse liquid into the feed channel at a pressure Pp_Umb, where Pp_Umb < = (PSE - PLSE) * 1.1, and the rinse liquid is pumped into the inlet of the feed channel and to the outlet of the feed channel and from the outlet of the feed channel into the at least one annulus where the pumped rinse liquid is transported back to the upper end of the pipe through the at least one pipe annulus.

In an embodiment the pumped rinse liquid is transported back to the upper end of the pipe through at least one pipe annulus, where the rinse liquid flows in the annulus from the lower second end of the pipe to the upper first end of the pipe at a flow rate in the range 1 to 100 liter/hour.

The flexible pipe has an upper first end and lower second end. This means that the first end of the flexible pipe seen from a vertical direction is above the second end of the flexible pipe. In practice the first end of the pipe may be mounted to a sea surface installation such as a vessel or floating production platform and the second end of the pipe may be installed to a subsea facility which may be several hundred meters or even several thousand meters below the sea surface. Thus, the flexible pipe has a length and an axis extending along the length of the pipe. If the rinse liquid is incompressible, the pressure PLSE corresponding to the pressure at the bottom of a column of static rinse liquid have a magnitude proportional to the vertical distance between the first and the second end of the pipe.

The pressure PSE is the ambient pressure i.e. the pressure in the sea surrounding the flexible pipe at the level of the second end.

The pump may be any suitable pump for pumping a liquid, such as a centrifugal pump, piston pump, gear pump, screw pump or scroll pump. The rinse liquid is a liquid with a relatively low viscosity to ensure that the liquid flows unhindered in the annulus.

According to the method the pump should deliver a pressure Pp_Umb < = (PSE - PLSE) * 1.1, to ensure that rinse liquid can be pumped through through the feed channels and back through the annulus of the pipe without the annulus pressure becoming so high that it may cause damage in the pipe.

In an embodiment the pressure is Pp_Umb < (PSE - PLSE), this pressure is slightly lower, however, the pressure should still be sufficient to ensure that the rinse liquid has sufficient flow rate through the annulus. To obtain a good control of the pressure in the annulus, the pump pressure in an embodiment fulfils the requirement (PSE - PLSE) * 0.9 < = Pp_Umb < = (PSE - PLSE) * 1.1.

In an embodiment the pump pressure fulfils the requirement (PSE - PLSE) * 0.95 < = Ppumb < = (PSE - PLSE) * 1.05. In this embodiment it is also possible to obtain a good control of the pressure in the annulus.

The optimal flow rate of the rinse liquid depends on the exact application but should in the general case be in the range in the range 1 to 100 liter/hour to ensure a sufficient flow through the annulus to ensure that the rinse liquid can reduce the amount of harmful substances, such as CO2, H2O and H2S, to non-critical levels in the annulus. Harmful substances are here defined as substances which alone or in combination may cause corrosion or other degradation of metallic armor layers in the annulus. In the general case it has been found that the flow of rinse liquid is satisfactory when the flow rate of the rinse liquid is in the range 1 to 80 liter/hour, such as 2 to 65 liter/hour, such as 5 to 50 liter/hour.

The flexible pipe comprises at least one steel armor layer and in an

embodiment the flexible pipe comprises at least one tensile armor, which serves to take up tensile forces in the pipe. Preferably the tensile armor is made from metallic elongate members where the tensile armor the elongate members are wound around the pipe with a winding angle of 25 to 55 degrees in respect of the axis of the pipe.

In an embodiment the flexible pipe comprises at least one pressure armor to absorb pressure forces in the pipe.

The pressure armor can be made from metallic elongate members and in an embodiment the pressure armor comprises elongate members wound around the pipe with a winding angle of 55 to 89 degrees, such as up to 89.8 degrees in respect of the axis of the pipe. The tensile armor and the pressure armor can e.g. be made from carbon steel or stainless steel. In an embodiment the pressure armor comprises fibres embedded into a polymeric matrix. The fibre reinforced polymer can be provided as elongate members and be wound around the pipe with a winding angle of 65 to 89 degrees, such as up to 89.8 degrees in respect of the axis of the pipe. In another embodiment the wounded fibre reinforced polymer is fused to the liner.

The pressure armor and the tensile armor are located in the annulus between the internal pressure sheath and the outer sheath.

Preferably both the internal pressure sheath and the outer sheath is made from a substantially fluid tight and temperature tolerant (i.e. capable of operating in a temperature range from about -5 °C to about 150 °C) material and preferably the sheaths are made from polymer material. The polymer material is e.g. selected from polyolefins, e.g. polyethylene or polypropylene; polyamide, e.g. poly amide-imide, polyamide-11 (PA-11), polyamide-12 (PA- 12) or polyamide-6 (PA-6)); polyimide (PI); polyurethanes; polyureas;

polyesters; polyacetals; polyethers, e.g. polyether sulphone (PES);

polyoxides; polysulfides, e.g. polyphenylene sulphide (PPS); polysulphones, e.g. polyarylsulphone (PAS); polyacrylates; polyethylene terephthalate (PET); polyether-ether-ketones (PEEK); polyvinyls; polyacrylonitrils; polyetherketoneketone (PEKK); copolymers of the preceding; fluorous polymers e.g. polyvinylidene diflouride (PVDF), homopolymers or copolymers of vinylidene fluoride ("VF2 "), homopolymers or copolymers of

trifluoroethylene ("VF3 "), copolymers or terpolymers comprising two or more different members selected from VF2, VF3, chlorotrifluoroethylene,

tetrafluoroethylene, hexafluoropropene, or hexafluoroethylene; compounds comprising one or more of the above mentioned polymers, and composite materials, such as a polymer (e.g. one of the above mentioned) compounded with reinforcement fibers, such as glass-fibers, carbon-fibers and/or aramide fibers.

The internal pressure sheath and the outer sheath in a flexible pipe according to the invention can be made from the same or different polymer material.

In an embodiment the flexible pipe comprises a carcass. The carcass is the innermost layer of the pipe and supports the internal pressure sheath. The carcass is manufactured from one or more metallic elongate members and the elongate members are wound with a winding angle of about 70 to about 89 degrees with respect of the axis of the pipe to form a tubular member in the bore of the pipe. The carcass can e.g. be made from stainless steel.

The feed channel can be provided as a tube. The tube can be made from metal or polymer such as polyethylene, polypropylene, polyamide or polyvinyl difluoride. If made from polymer, the tube may be reinforced with fibres to enhance resistance to creep. A metallic tube can be made from carbon steel, stainless steel, titanium, aluminum, copper or a copper containing alloy.

In an embodiment, the feed channel is wound with the same pitch or winding angle as one of the tensile armor layers of the flexible pipe. Thus, the feed channel is wound around the pipe with a winding angle of 25 to 55 degrees in respect of the axis of the pipe. Thus, it is possible to incorporate the feed channel in the armor layers of the pipe, which may serve to protect the feed channel against damage. In an embodiment the feed channel is co-wound with one of the tensile armor layers. In this embodiment the feed channel is integrated with and protected by the tensile armor.

In an embodiment the feed channel is a pipe located outside the tensile armor of the pipe. The feed channel may be integrated in the pressure armor or the feed channel may be located outside the flexible pipe in the ambient environment. Thus, this embodiment provides great freedom for the application to the method and makes it possible to apply the method to existing offshore installations. In an embodiment the feed channel is one annulus in a multi annuli system of the pipe. The feed channel may e.g. be the inner annulus in a pipe

comprising two annuli. However, a flexible pipe may also comprise three or more annuli and in the case of three or more annuli anyone of the annuli may constitute the feed channel. When using an annulus in a multi annuli pipe for the feed channel the production of the flexible pipe can be simpler.

The flexible pipes used in the method may comprise several layers other than the internal pressure sheath and an outer sheath and at least one armor layer. As already explained the flexible pipe may comprise one or more tensile armors, one or more pressure armor and a carcass. The flexible pipe may also comprise one or more intermediate layers such as antifriction and antiskid layers. The intermediate layers may be fluid-tight and serve as barrier layers or sheaths between different annuli in the pipe. The

intermediate layers may be made from polymer material such as

polyethylene, polyamide or polyvinyl difluoride. The intermediate layers may be applied as tapes and wound around the pipe or applied by extrusion. All structural layers in the flexible pipe are terminated in an end-fitting by which the pipe can be connected to other installations or pipes. Consequently, in an embodiment of the method the flexible pipe comprises one or more

intermediate layers, preferably the one or more intermediate layers are fluid- tight, preferably the one or more intermediate layers are made from polymer material.

In an embodiment of the method the flexible pipe comprises several pipe sections, preferably these pipe sections are attached to each other by means of end-fittings. Each pipe section may be constituted by a flexible pipe terminated in an end-fitting at each end. Each pipe section may have a length in the range 100 meter to 2000 meter. The end-fittings which terminates the pipe may be equipped with connector units which may connect the annuli and feed channels of two adjacent flexible pipes. In some cases the means for connecting the annuli of adjacent pipes may be integrated directly in the attached end-fittings. In that case the end-fittings may also comprise valves and other equipment to control the flow in the flexible pipes and the annuli.

To obtain the best possible result using the method it has been found that the density of the rinse liquid should be less than about 1 g/cm³ (when measured at 20 °C), as the relatively low density of the rinse liquid serves to ensure that the rinse liquid can be transported through the annulus of the pipe while keeping the pressure sufficiently low to avoid damage on the pipe structure. In an embodiment the density of the rinse liquid is in the range 0,950 to 0,650 g/cm³ (when measured at 20 °C), such as 0,890 kg to 0,720 g /cm³ (when measured at 20 °C).

The rinse liquid should preferably be able to absorb or mix with water and hence promote active removal of ingressed water from the annulus as water in combination with e.g. CC>2 and hteS may form a corrosive environment in the annulus. In an embodiment the rinse liquid is mixable with water, preferably the rinse liquid is mixable with up to 5 vol-% water, such as up to 30 vol% water, such as up to 50 vol-% water or even more.

In an embodiment the rinse liquid absorbs CO2 and/or H2S. When the rinse liquid absorbs CO2 and/or H2S the rinse liquid is capable of removing the substances from the annulus and thereby serve to reduce the risk of corrosion.

Although the rinse liquid may be any liquid having a density about 1 g/cm³ or less (when measured at 20 °C), in an embodiment the rinse liquid is methanol, ethanol, benzol, diesel, petroleum or a mixture containing at least one or more of the mentioned liquids. When the rinse liquid is a mixture, it may e.g. be a mixture of water and methanol or ethanol.

In an embodiment the rinse liquid is functionalized by additives, either increasing the chemical or mechanical resilience of the pipe. The additives may e.g. be buffers to change the pH in the annulus, H2S or CO2 scavengers or corrosion inhibitors or additives modifying the frictional behavior of the pipe layers.

Even with armor layers in the annulus enough free volume for flushing should remain. In an embodiment the free volume of the one or more a annulus is in the range of 1 to 6 m³ for each 1000 meter of the flexible pipe. The free volume of the annulus is the volume left in the annulus when parts of the volume in the annulus is occupied by armor layers and optionally other layers. In an embodiment the flexible pipe is an unbonded flexible pipe.

The invention also relates to a pipe system for circulating a rinse liquid in a flexible pipe having an upper first end and a lower second end, said flexible pipe comprising an internal pressure sheath and an outer sheath surrounding said internal pressure sheath, arranged such that at least one annulus is formed between the internal pressure sheath and the outer sheath, each annulus comprises one or more armor layers, and a feed channel having a first inlet end and a second outlet end and adapted for transportation of rinse liquid and extending from the upper first end of the pipe to the lower second end of the pipe, the second outlet end is in fluid communication with at least one annulus, where the pressure at the outlet of the feed channel at the lower second end of the flexible pipe is PLSE when the feed channel is filled with rinse liquid in static condition, and the ambient pressure, at the position of the outlet of the feed channel, outside the pipe is PSE, wherein the inlet end of the feed channel at upper first end of the flexible pipe is connected to a pressure pump, adapted for pumping the rinse liquid into the feed channel at a pressure Pp_Umb, where Pp_Umb < = (PSE - PLSE) * 1.1, and the system is adapted to pump rinse liquid into the inlet of the feed channel to the outlet of the feed channel and from the outlet of the feed channel into the at least one annulus where the pumped rinse liquid can be returned to the upper end of the pipe through the at least one pipe annulus. To achieve an optimal flow in the annulus of the pipe the pumped rinse liquid is transported back to the upper end of the pipe through at least one pipe annulus, where the rinse liquid flows in the annulus from the lower second end of the pipe to the upper first end of the pipe at a flow rate in the range 1 to 100 liter/hour. As explained above, the upper first end of the flexible pipe in vertical direction is above the lower second end of the pipe. The vertical distance between pipe ends may be more than 1000 meter.

In an embodiment the rinse liquid is a liquid based on organic compounds and may have improved properties in respect of absorbing the substances CO2 and H2S e.g. relative to water . In an embodiment the rinse liquid is selected from methanol, ethanol, benzol, diesel, petroleum or a mixture of two or more of the mentioned liquids. To facilitate the flow of rinse liquid through the annulus the rinse liquid should have a relatively low density compared to sea water. In an

embodiment of the pipe system the density of the rinse liquid is less than 1,0 g/cm³, such as less than 0,9 g/cm³ (when measured at 20 °C) For the purpose of ensuring a smooth flow and facilitate the flow of the rinse liquid through the annulus it is desirable that the rinse liquid has a relatively low viscosity and in an embodiment the rinse liquid has a viscosity in the range 1,800 mPa*s to 0,400 mPa*s (measured at 20 °C), such as in the range 1,500 mPa*s to 0,500 mPa*s (measured at 20 °C). The viscosity can be measured according to the standards related to ISO 17.060.

To improve the properties of the rinse liquid, in an embodiment the rinse liquid comprises one or more additives selected from detergents, stabilizers, corrosion inhibitors, friction modifiers, buffers, FteS scavengers or CO scavengers. The flexible pipe in the system comprises at least one end-fitting, and in an embodiment the pump and the feed channel are connected via an end-fitting. The flexible pipe may comprise several sections which can be connected by means of end-fittings.

In an embodiment the feed channel is integrated in the flexible pipe. The feed channel may be integrated in a tensile armor layer, a pressure armor layer or an intermediate layer in the flexible pipe.

In an embodiment the feed channel is constituted by an annulus in the pipe. This embodiment is suitable for a multi annuli flexible pipe and may be less complicated to produce than a pipe with dedicated feed channels. The flexible pipe may comprise one or more sections. In an embodiment of the pipe system the flexible pipe comprises at least one section. Each section has a length in the range 100 m to 3000 m, such as in the range 250 m to 2500 m, such as in the range 500 m to 2000 m. When the flexible pipe comprises two or more sections, the sections may be coupled together by means of end-fittings. The end-fittings may comprise valves and connections for the feed channel and annuli.

To minimize cost and topside volume, the rinse liquid can preferably be re- generated and re-circulated several times. Thus, in an embodiment the system comprises means for re-generating the rinse liquid after having flown through the annulus.

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

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in further details with reference to drawings, in which: Figure 1 shows a flexible pipe;

Figure 2 shows a cross-section of a flexible pipe;

Figure 3 shows a flexible pipe with a feed channel; and

Figure 4 shows a flexible pipe used according to the invention. The drawings are only intended to illustrate the principles of the invention and are not dimensionally stable or accurate. The same reference numbers are used for the same parts in the drawings. Figure 1 shows a 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 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 an outer sheath 15. The internal pressure sheath 11 and the outer sheath 15 are made from material which is substantially fluid tight, and the annular space between the internal pressure sheath 11 and the outer sheath 15 are normally referred to as the annulus. The annulus houses the armor layers 12, 13 and 14. The volume in the annulus which is not occupied by the armor layers is the free volume.

Figure 2 is a cross section of the flexible pipe shown in figure 1. From the inside and out, i.e. from the bore 9, the pipe comprises a carcass 10, an internal pressure sheath 11, a pressure armor layer 12, a first inner tensile armor layer 13, a second outer tensile armor layer 14 and an outer sheath 15. The annulus is provided between the internal pressure sheath 11 and the outer sheath 15. A part of the volume in the annulus is occupied by the armor layers, the pressure armor layer 12, the first inner tensile armor layer 13, and the second outer tensile armor layer 14. The remaining volume in the annulus which is not occupied by the armor layers is the free volume.

Figure 3 shows a flexible pipe corresponding to the flexible pipe shown in figure 1 and 2. The flexible pipe comprises the carcass 10 supporting the internal pressure sheath 11. The internal pressure sheath 11 is reinforced with the pressure armor layer 12, the inner tensile armor layer 13, and outer tensile armor layer 14. The outer surface of the flexible pipe 1 is constituted by the outer sheath 15. In the flexible pipe of figure 3 a feed channel 18 for rinse liquid is integrated in the inner tensile armor 13. The feed channel 18 is wound around the flexible pipe 1 with the same winding angle as the inner tensile armor 13, i.e. approximately 30 degrees. Thus the inner tensile armor 13, together with the pressure armor 12 and the outer tensile armor forms a protection for the feed channel 18.

Only one feed channel 18 is seen in figure 3, however two, three or more feed channels may be provided on the flexible pipe 1. The one or more feed channels are typically terminated in the end-fittings of the flexible pipe. In the upper end-fitting the one or more feed channels are connected with a source for rinse liquid and pumping means. In the lower end-fitting the one or more feed channels are connected to the annulus such that a flow of rinse liquid can flow through the pipe from the lower end-fitting to the upper end-fitting.

Figure 4 shows a flexible pipe 1 mounted according to the method of the present invention. The flexible pipe 1 is extending between the sea surface 3 and the sea bed 4. At the sea surface 3 the flexible pipe is connected with a floating vessel 2 via an upper end-fitting 5.

At the sea bed 4 the flexible pipe is connected to a production facility 7 via a lower end-fitting 6. The flexible pipe 1 may have a length of several hundred meters and may have a length of one kilometre of more. Thus, a substantial pressure may exist in the environment around lower end-fitting.

EXAMPLE An unbonded flexible pipes is tested with ethanol (density ~ 788 kg/m³) The flexible pipe has the dimensions:

Inner diameter: 203mm

Outer diameter: 380mm

The internal pressure sheath is made from polyethylene. Thickness: 6mm

The outer sheath is made from polyamide. Thickness: 7mm In the annulus the flexible pipe comprises a pressure armor and two tensile armors. All the armor layers in the annulus are made from carbon steel.

The free volume in the annulus is 10 ³m³/m Pressures at 1000m water depth:

PSE: ~10.07 MPa (Ambient water pressure)

PLSE: ~7.93 MPa (Static pressure of liquid column in feed channel)

Ppump <= l.l*(10.07-7.93)[MPa] =; Pp_Ump < = 2.35 MPa

Consequently, the feed pressure of the ethanol from the pump into the feed channel is set to be approximately 2 MPa.

Claims

1. A method for circulating a rinse liquid in a flexible pipe having an upper first end and a lower second end, said flexible pipe comprising an internal pressure sheath and an outer sheath surrounding said internal pressure sheath, arranged such that at least one annulus is formed between the internal pressure sheath and the outer sheath, each annulus comprises one or more armor layers, and at least one feed channel having a first inlet end and a second outlet end and adapted for transportation of rinse liquid and extending from the upper first end of the pipe towards the lower second end of the pipe, the second outlet end is in fluid communication with at least one annulus, where the pressure at the outlet of the feed channel at the lower second end of the flexible pipe is PLSE when the feed channel is filled with rinse liquid in static condition, and the ambient pressure, at the position of the outlet of the feed channel, outside the pipe is PSE, wherein the inlet end of the feed channel at the upper first end of the flexible pipe is connected to a pressure pump, adapted for pumping the rinse liquid into the feed channel at a pressure Pp_Umb, where Pp_Umb < = (PSE - PLSE) * 1.1 , and the rinse liquid is pumped into the inlet of the feed channel and to the outlet of the feed channel and from the outlet of the feed channel into the at least one annulus where the pumped rinse liquid is transported back to the upper end of the pipe through the at least one pipe annulus.

2. A method according to claim 1, wherein the rinse liquid flows in the annulus from the lower second end of the pipe to the upper first end of the pipe at a flow rate in the range 1 to 100 liter/hour.

3. A method according to claim 1 or 2, wherein Pp_Umb < = (PSE - PLSE) .

4. A method according to anyone of the preceding claims, wherein (PSE - PLSE) * 0.9 <= Ppumb <= (PSE - PLSE) * 1.1

5. A method according to anyone of the preceding claims, wherein (PSE - PLSE) * 0.95 < = Ppumb < = (PSE - PLSE) * 1.05

6. A method according to claim 2, wherein the flow rate of the rinse liquid is in the range 1 to 80 liter/hour, such as 1 to 65 liter/hour, such as 1 to 50 liter/hour.

7. A method according to anyone of the preceding claims, wherein the flexible pipe comprises at least one tensile armor.

8. A method according to anyone of the preceding claims, wherein the tensile armor is elongate members wound around the pipe with a winding angle of 25 to 55 degrees in respect of the axis of the pipe

9. A method according to anyone of the preceding claims, wherein the flexible pipe comprises at least one pressure armor

10. A method according to claim 9, wherein the pressure armor comprises elongate members wound around the pipe with a winding angle of 55 to 89.8 degrees in respect of the axis of the pipe

11. A method according to claim 9, wherein the pressure armor comprises fibres embedded into a polymeric matrix

12. A method according to anyone of the preceding claims, wherein the flexible pipe comprises a carcass.

13. A method according to anyone of the preceding claims, wherein the feed channel is wound with the same pitch as one of the tensile armor layers of the flexible pipe

14. A method according to anyone of the preceding claims, wherein the feed channel is co-wound with one of the tensile armor layers.

15. A method according to anyone of the preceding claims, wherein the feed channel is a pipe located outside the tensile armor of the pipe

16. A method according to anyone of the preceding claims, wherein feed channel is one annuli in a multi annulus system of the pipe

17. A method according to anyone of the preceding claims, wherein the flexible pipe comprises several pipe sections, preferably the pipe section are attached to each other by means of end-fittings

18. A method according to anyone of the preceding claims, wherein the density of rinse liquid is in the range 0,950 to 0,650 g/cm³ (when measured at 20 °C), such as 0,890 kg to 0,720 g /cm³ (when measured at 20 °C)

19. A method according to anyone of the preceding claims, wherein the rinse liquid is mixable with water, preferably the rinse liquid is mixable with up to 5 vol-% water, such as up to 30 vol% water, such as up to 50 vol-% water or even more.

20. A method according to anyone of the preceding claims, wherein the rinse liquid absorbs CO2 and/or H2S

21. A method according to anyone of the preceding claims, wherein the rinse liquid is methanol, ethanol, benzol, diesel, petroleum or a mixture comprising one or more of the mentioned liquids

22. A method according to anyone of the preceding claims, wherein the rinse liquid is functionalized by additives, either increasing the chemical or mechanical properties and/or resilience of the pipe.

23. A method according to anyone of the preceding claims, wherein the free volume of the one or more annulus is in the range of 1 to 6 m³ for each 1000 meter of the flexible pipe

24. A method according to anyone of the preceding claims, wherein the flexible pipe comprises one or more intermediate layers, preferably the one or more intermediate layers are fluid-tight, preferably the one or more

intermediate layers are made from polymer material.

25. A method according to anyone of the preceding claims, wherein the flexible pipe is an unbonded flexible pipe.

26. A pipe system for circulating a rinse liquid in a flexible pipe having an upper first end and a lower second end, said flexible pipe comprising an internal pressure sheath and an outer sheath surrounding said internal pressure sheath, arranged such that at least one annulus is formed between the internal pressure sheath and the outer sheath, each annulus comprises one or more armor layers, and a feed channel having a first inlet end and a second outlet end and adapted for transportation of rinse liquid and extending from the upper first end of the pipe to the lower second end of the pipe, the second outlet end is in fluid communication with at least one annulus, where the pressure at the outlet of the feed channel at the lower second end of the flexible pipe is PLSE when the feed channel is filled with rinse liquid in static condition, and the ambient pressure, at the position of the outlet of the feed channel, outside the pipe is PSE, wherein the inlet end of the feed channel at upper first end of the flexible pipe is connected to a pressure pump, adapted for pumping the rinse liquid into the feed channel a pressure Pp_Umb, where Pp_Umb <= (PSE - PLSE) * 1.1 , and the system is adapted to pump rinse liquid into the inlet of the feed channel to the outlet of the feed channel and from the outlet of the feed channel into the at least one annulus where the pumped rinse liquid can be returned to the upper end of the pipe through the at least one pipe annulus.

27. A pipe system according to claim 26, wherein the rinse liquid flows in the annulus from the lower second end of the pipe to the upper first end of the pipe at a flow rate in the range 1 to 100 liter/hour.

28. A pipe system according to claim 26 or 27, wherein the rinse liquid is a carbon-containing liquid, preferably the rinse liquid is selected from methanol, ethanol, benzol, diesel, petroleum or a mixture comprising one or more of the mentioned liquids

29. A pipe system according to anyone of the claims 26 to 28, wherein the density of the rinse liquid is less than 1,0 g/cm³, such as less than 0,9 g/cm³ (when measured at 20 °C)

30. A pipe system according to anyone of the claims 26 to 29, wherein the rinse liquid has a viscosity in the range 1,800 mPa*s to 0,400 mPa*s

(measured at 20 °C), such as in the range 1,500 mPa*s to 0,500 mPa*s (measured at 20 °C)

31. A pipe system according to anyone of the claims 26 to 30, wherein the rinse liquid comprises one or more additives selected from detergents, stabilizers, buffers, H2S scavengers

32. A pipe system according to anyone of the claims 26 to 31, wherein the system comprises at least one end-fitting

33. A pipe system according to anyone of the claims 26 to 32, wherein the pump and the feed channel are connected via an end-fitting

34 A pipe system according to anyone of the claims 26 to 33, wherein the feed channel is integrated in the flexible pipe

35. A pipe system according to anyone of the claims 26 to 34, wherein the feed channel is constituted by an annulus in the pipe.

36. A pipe system according to anyone of the claims 26 to 35, wherein the flexible pipe comprise at least one section, each section has a lenght in the range 100 m to 3000 m, such as in the range 250 m to 2500 m, such as in the range 500 m to 2000 m 37. A pipe system according to anyone of the claims 26 to 36, wherein the system comprises means for re-generating the rinse liquid after having flown through the annulus.