WO2020099228A1

WO2020099228A1 - A method for flushing a flexible pipe and an assembly of a flexible pipe and an end-fitting

Info

Publication number: WO2020099228A1
Application number: PCT/EP2019/080484
Authority: WO
Inventors: Kristian GLEJBØL
Original assignee: National Oilwell Varco Denmark I/S
Priority date: 2018-11-13
Filing date: 2019-11-07
Publication date: 2020-05-22

Abstract

Method for flushing a flexible pipe (1) comprising a first end and a second end and connected to an end-fitting at least at the first end, where the flexible pipe comprises an internal pressure sheath (11) and an outer sheath (15) surrounding said internal pressure sheath, arranged such that at least one annulus is formed between the internal pressure sheath and the outer sheath, and the at least one annulus comprises one or more armor layers (12). The annulus is in fluid connection with at least one venting channel (35) close to the first end of the pipe, and flushed with a rinse liquid from the second end to the first end of the pipe. The venting channel comprises means (36) to open and close thereby allowing removal of at least a portion of the rinse liquid from the first end of the pipe in such a way that the pressure in the annulus at the first end of the pipe is kept at least 3 bar above ambient pressure.

Description

A METHOD FOR FLUSHING A FLEXIBLE PIPE AND AN ASSEMBLY OF A

FLEXIBLE PIPE AND AN END-FITTING

TECHNICAL FIELD

The present invention relates to a method for flushing a flexible pipe and an assembly comprising an end-fitting and a flexible pipe where the flushing fluid from the annulus of the pipe exits via a venting channel which may be located in the end-fitting.

BACKGROUND

Flexible marine pipes generally are classified as either bonded or unbonded pipes. Flexible pipes comprise polymeric and metallic layers. A bonded pipe is a pipe in which the layers are bonded together by the polymer layers, which may be made from a vulcanized elastomeric material. An unbonded pipe generally comprise separate unbonded polymeric and metallic layers, allowing relative movement between layers. The present invention concerns unbonded flexible pipes. When the unbonded pipes are partly or fully reinforced by steel they are in the literature referred to as "steel armored flexible pipes".

Such steel armored flexible pipes are suitable for marine applications such as for transport of fluids in the petrochemical industry e.g. oil, gas, water or mixtures hereof 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 unbonded flexible pipes comprises multiple independent layers, such as helical wound steel and polymeric layers, as well as extruded polymeric layers formed around a central bore. A typical steel armored 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 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 armor is made from steel whereas the tensile armor is made from fibre reinforced polymer composites. In other unbonded flexible pipes the pressure armor and the internal pressure sheath may be integrated while the tensile armor is made from steel elements.

The annular space or spaces outside the internal pressure sheath, which houses the steel armor layers are usually referred to as the annulus or annuli.

The flexible pipes may for example be applied for carrying 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 armoring layers surrounding the internal pressure sheath may for example comprise one or more pressure armor layers comprising one or more armoring 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 armor layers primarily absorb radial forces in the pipe. The armoring layers surrounding the internal pressure sheath also comprise one or more tensile armoring 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 armor layers primarily absorb axial forces in the pipe. The armoring 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 armoring 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 armoring layer(s). This armoring layer is usually of very flat profiles e.g. in the form of fibre reinforced polymeric tapes 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 fluids may cause the pressure in the annulus to rise, which unmitigated will lead to bust of the outer sheath. To prevent bursting, vent valves are normally mounted in the termination of pipes, such that fluid pressure in the pipe is

automatically relieved when the pressure in the annulus significantly exceeds ambient pressure, as. e.g. described in API17B, 5.2.4.

Ingress of fluids, and in particular ingress of gas and water into the annulus result in simultaneous appearance of gas and water on the exposed steel surfaces of the armor. This may lead to localized corrosion of the steel armor elements located in the annulus, eventually leading to premature failure of the pipe.

To mitigate corrosion of the armor 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, where the pressure sheath is filled with a reactive compound which reacts with diffusing gasses before they penetrate the liner and reach the annulus and thus the armor 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. It is also known to flush the annulus with a gas or liquid e.g. from WO 2011/026801 and US 9523446. When liquid is used to flush gases from the annulus, it is important that the gas is fully absorbed, and bubble nucleation and bubble growth is supressed. Gas bubbles should be avoided for several reasons. Smaller gas bubbles may block the tiny channels in the annulus resulting in high pressure losses along the annulus and hence zones with high and shifting pressure losses, which may lead to erratic flow and burst of the outer sheath. Larger gas bubbles may create "dead" zones in the annulus hereby eliminate the local removal of water and corroding gasses and hence create local hotspots for corrosion.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a method by which the annulus of a flexible pipe can be flushed by a liquid in an efficient and cost- effective manner.

Moreover, it is an object of the present invention to provide a method by which the annulus can be flushed with a liquid without undesired bubble formation in the annulus.

Moreover, it is an object of the present invention to provide a simple method for restoring the gas dissolving capacity of the rinse liquid prior to reinjection in the annulus.

In a first aspect of the present invention relates to a method for flushing the annulus of a flexible pipe comprising a first end and a second end and connected to an end-fitting at least at the first 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, said at least one annulus comprises one or more armor layers, and said at least one annulus is in fluid connection with at least one venting channel close to the first end of the pipe, the method comprises the step of flushing said at least one annulus with a rinse liquid from the second end to the first end of the pipe, and arranging the at least one venting channel to open and close thereby allowing removal of at least a portion of the rinse liquid from the first end of the pipe in such a way that the pressure in the annulus at the first end of the pipe is kept at least 3 bar above ambient pressure.

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.

Throughout the description or claims, the singular encompasses the plural unless otherwise specified or required by the context.

The term "about" is generally used to include what is within measurement uncertainties. When used in ranges the term "about" should herein be taken to mean that what is within measurement uncertainties is included in the range.

The venting channel may be arranged in the end-fitting. However, the venting channel may also be connected to the annulus by other means. By maintaining the pressure in the annulus at the first end of the pipe at least 3 bar above the ambient pressure it is possible to suppress bubble nucleation and hence to ensure that gases absorbed in the rinse liquid is not released and, thus, form bubbles in the rinse liquid in the first end of the pipe.

Furthermore, upon exposing the pressurized rinse liquid to near atmospheric pressure, a large proportion of the dissolved gasses will boil off the rinse liquid and, thus, prepare the rinse liquid for reinjection in the flushing system.

Upon forced reinjection using a pump, the rinse liquid flows from the second end of the pipe to the first end of the pipe by flow partially forced by the gravimetric difference between the rinse liquid and flushing fluid.

As a general rule the higher the pressure of the rinse liquid, the more gas can be absorbed (Henry's law). Thus, there is a reverse proportionality between the pressure of the rinse liquid in the first end of the pipe and the amount of liquid that must be flushed through the pipe to keep the flow free of bubbles. The gases absorbed in the rinse liquid during normal operation are e.g. CO2, CH4 and H₂S

Ambient pressure is the pressure in the environment surrounding the end^¬ fitting and the venting channel at the first end of the pipe. If the first end of the pipe is connected to e.g. a platform or a ship via the end-fitting above the sea surface the ambient pressure will be the atmospheric pressure (1 bar), whereas ambient pressure at 10 meters of water depth will be 2 bars. In the text below bara is used to describe absolute pressure inside the pipe whereas barg is used to describe the local difference between inside pressure and ambient pressure.

The flexible pipe is normally mounted in such a way that the first end of the pipe is in a position which is higher than the position of the second end of the pipe with respect to a horizontal level. In rare cases the first end of the pipe and the second end of the pipe may be at substantially the same level, e.g. when the pipe is mounted between installations on a flat seabed. For deep-sea installations pipes are often supported by a mid-water arch, where the pipe supported directly by the arch is placed higher than adjacent pipe sections. In such a case it is essential that no bubbles form in the rinse liquid, since these bubbles may coalescence into gas pockets of substantial volume. This may reduce and even block the flow of rinse liquid through the pipe.

The venting channel can be fitted with a venting valve, which can be adjusted to open when the pressure in the first end of the pipe reaches a

predetermined value above 3 barg. The venting valve can close again when the pressure falls below 3 barg.

The mixture of rinse liquid and dissolved gas leaving the venting valve is optionally transported to a flash chamber where rinse liquid and gas are separated. The gas released from the rinse liquid can be transported to a flare and the rinse liquid can be reinjected without further processing.

According to the method the pressure in the first end of the annulus is kept above about 3 barg and up the about 30 barg. If the relative pressure in the annulus becomes very high, it may cause damage on the outer sheath, thus it is preferred to keep the pressure below 30 barg.

In an embodiment the pressure in the annulus at the first end of the pipe is controlled by the at least one venting valve. The control of the pressure in the annulus can be obtained by adjusting the venting valve to open and close in response to pre-selected pressures. Preferably, the pre-selected pressures are selected from the range 3-30 barg, such as 4-20 barg.

In an embodiment of the method, the pressure in the annulus is monitored at the first end of the pipe and the result of the monitoring is used to control the flow-rate of rinse liquid into the annulus.

In an embodiment the pressure in the annulus is monitored at the first end of the pipe and the result of the monitoring is used to control the venting means of the venting channel, such as the venting valve. In this manner it is possible to ensure that the pressure in the first end of the annulus is kept substantially between 3 and 30 barg.

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

The pressure at which the rinse liquid starts to form bubbles is an indication of how much gas has been absorbed. Thus, if bubbles form in the annulus in a high pressure flow path, such as in the annulus at the first end of the pipe, this indicates that the flow rate of the rinse liquid should be increased.

Formation of bubbles can be monitored using well-known optical devices.

If gas bubbles form in the annulus, the liquid column in the annulus may break down and the pressure at the first end of the pipe may become substantial and/or erratic. The increased pressure at the first end of the pipe increases the load on the outer sheath of the pipe and may ultimately cause damage to the pipe. Thus, in an embodiment the flexible pipe at the first end comprises a portion where the outer sheath is reinforced to resist internal pressure in the annulus by an exterior armor layer helically wound around the outer sheath. The exterior armor serves to protect the outer sheath at the first end of the pipe from damage caused the elevated pressure in the annulus.

The portion where the first end of the pipe is reinforced with an exterior armor layer, preferably begins at the end-fitting and continues 5 to 2000 m down the length of the pipe, such as 50 to 250 m down the length of the pipe.

In an embodiment the exterior armor layer is made from steel. Steel provides a strong armor layer and is easily processed. The steel can be processed to metal strips which can be wound around the outer sheath with winding angles in the range 50 to 85 degree in respect of the center axis of the pipe and form an exterior armor layer with good properties in respect of resisting internal pressure. The first end of the flexible pipe can also be protected against the internal pressure by winding a tape around the outer sheath and in an embodiment the flexible pipe at the first end comprises a portion with tape helically wound around the outer sheath, preferably the tape is made from metal or fiber reinforced polymer material.

The portion where the first end of the pipe is protected with a tape wound on the outer sheath, may start at the end-fitting and continue 5 to 2000 m down the length of the pipe, such as 50 to 250 m down the length of the pipe, until the pipe reaches a water-depth where the external water- pressure

compensate the anticipated maximum pressure in the pipe annulus.

For the purpose of providing extra protection to the flexible pipe, the flexible pipe in an embodiment comprises an additional outer extruded protective layer around the exterior armor layer or tape

In an embodiment of the method according to the invention the density of the rinse liquid is less than sea-water, i.e. less than 1025 kg/m³. 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 1025 kg/m³ (when measured at 20 °C). The relatively low density of the rinse liquid serves to ensure that the rinse liquid can have a negative pressure gradient along the pipe which will allow for a substantial flow while keeping the relative pressure sufficiently low, i.e. not more than 30 bars above ambient pressure. This prevents outer sheath rupture which will cause corrosion damage to the pipe armor structure. In an embodiment the density of the rinse liquid is in the range 950 to 650 kg/m³ (when measured at 20 °C), such as 890 kg to 720 kg/m³ (when measured at 20 °C).

The rinse liquid flows in the free volume in the annulus of the pipe. The free volume of the 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, which is not filled with armor layers and optionally other layers. The main function of the rinse liquid is to remove species, which may cause corrosion in the annulus of the pipe. The function may be improved further by adding inhibitors to the liquid.

The rinse liquid can also be 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.

In an embodiment the rinse liquid is selected from water, 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 water and ethanol or an alcohol mixed with diesel.

To keep the armor layers in the annulus protected against corrosion, the annulus should substantially be filled with rinse liquid from the second end of the pipe to the first end of the pipe. The free volume in the annulus should at least be filled 80 vol-% with rinse liquid, such as 90 vol-%, such as 95 vol-%, such as 100 vol-% with rinse liquid.

In an embodiment the annulus of the pipe is pumped substantially free of air during initial filling with rinse liquid to prevent formation of air pockets. Air pockets in the annulus are undesired because air pockets may give rise to corrosion and the previously mentioned erratic pressures.

The rinse liquid is transported to the second end of the flexible by means of one or more tubes fed by pumps. The tubes may be external in respect of the pipe or the tubes may be integrated in the pipe. In an embodiment the rinse liquid is transported from the first end of the pipe to the second end of the pipe in tubes embedded in at least one of the armor layers in the annulus.

Flexible pipes for offshore transport of oil and gas can have substantial lengths, such as more the 2000 m or even more than 3000 m. Sometimes it is advantageous that long pipes are assembled from two or more pipes sections. Pipe sections intended for use in deep water can be made with another structure than pipe sections intended for use near the sea surface. Thus, in an embodiment the flexible pipe from the first end to the second end may comprise two or more sections. As described above the two or more sections may be identical or different in structure, preferably the sections are connected by end-fittings.

The present invention also relates to an assembly comprising an end-fitting and a flexible pipe, said flexible pipe comprising a first end and a second end, and being connected to the end-fitting at the first 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, said at least one annulus comprises one or more armor layers, and said at least one annulus is in fluid connection with at least one venting channel close to the first end of the pipe, said flexible pipe is adapted for flushing of said at least one annulus with a rinse liquid from the second end to the first end of the pipe, and the at least one venting channel is adapted with means to open and close and allow removal of at least a portion of the rinse liquid from the first end of the pipe in such a way that the pressure in the annulus at the first end of the pipe is kept 3 bars or more above ambient pressure.

The assembly is connected to an end-fitting at the first end and in an embodiment of the assembly, the flexible pipe is connected to a second end^¬ fitting at the second end. The rinse fluid may e.g. be provided to the annulus at the second end of the pipe via the second end-fitting.

Although the venting channel can be arranged in connection with the annulus in several ways, e.g. by means of a ring mounted around the flexible pipe, the invention also includes an embodiment wherein the at least on venting channel is arranged in the end-fitting at the first end. The loads on the flexible pipe at the first end may be higher than the loads on the flexible pipe on parts on the pipe away from the first end and in an embodiment of the assembly a portion of the flexible pipe at the first end is reinforced with an exterior armor layer. The exterior armor layer can be designed to resist both radial loads and axial loads, preferably the exterior armor layer is designed to resist at least radial loads.

In an embodiment of the assembly a portion of the flexible pipe at the first end comprises an extruded polymer layer around an exterior armor layer. The extruded layer serves to protect the external armor layer, against mechanical wear and may also protect against corrosion.

The extruded polymer layer around an exterior armor layer is preferably made from a substantially fluid tight and temperature tolerant (i.e. capable of operating in a temperature range from about -30 °C to about 90 °C) polymer material. The polymer material is e.g. selected from polyolefines, e.g.

polyethylene or polypropylene; polyamides, 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. In and embodiment of the assembly, the annulus comprises at least one pressure armor layer.

The pressure armor layer can be made from metallic elongate members and in an embodiment the pressure armor layer 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.

In an embodiment of the assembly, the annulus comprises at least one tensile armor layer.

Preferably the tensile armor layer is made from metallic elongate members where 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.

The assembly comprises means, such as pumps and tubes for transporting rinse liquid to the second end of the pipe and in an embodiment the annulus comprises at least one tensile armor layer with integrated tubes extending from the first end of the pipe the second end of the pipe, where the tubes are adapted for transport of rinse liquid.

The tube for transport of rinse liquid 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 tube or tubes for transport of rinse liquid are wound with the same pitch or winding angle as one of the tensile armor layers of the flexible pipe. Thus, the tube or tubes are 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 tube or tubes in the armor layers of the pipe, which may serve to protect the feed channel against damage. All features of the invention and embodiments of the invention as described herein, including ranges and preferred ranges, may be combined in various ways within the scope of the invention, unless there are specific reasons not to combine such features.

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 an assembly of a flexible pipe and an end-fitting;

Figure 3 shows a flexible pipe in an offshore installation;

Figure 4 shows an end-fitting according to the invention;

Figure 5 shows a flexible pipe with exterior armor layer; and

Figure 6 shows a flexible pipe with an integrated tube for rinse liquid.

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 centre 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 is 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 of the annulus.

In the illustrated flexible pipe 1 the internal pressure sheath 11 is made from polyethylene and the outer sheath 15 is made from polyamide. The carcass 10 is made from stainless steel; the pressure armor 12 and the tensile armor 13, 14 is made from carbon steel.

Figure 2 illustrates an assembly comprising an unbonded flexible pipe 1 and an end-fitting 2.

The end-fitting 2 comprises a body part constituted by the outer casing 4 and the inner casing 5. The end-fitting also comprises a flange 6 for connection to a connector or another end-fitting. The flange 6 comprises holes 7 for bolts which may be used for connecting the end-fitting to other equipment.

The unbonded flexible pipe 1 is shown with only the outer sheath 15 visible. All the layers in the pipe 1 is terminated in the end-fitting in a well-known manner.

Figure 3 shows a flexible pipe 1 mounted in an offshore installation. The flexible pipe 1 is extending between the sea surface 23 and the sea bed 24.

At the sea surface 23 the flexible pipe is connected with a floating vessel 22 via an upper end-fitting 5a. The first end of the pipe 1 is indicated with "A" and the second end of the pipe is indicated with "B".

At the sea bed 24 the flexible pipe is connected to a production facility 27 via a lower end-fitting 5b. The flexible pipe 1 may have a length of several hundred meters and may have a length of one kilometre of more. The end- fitting 5b and the second end B of the pipeis subjected to ambient water pressure at installation depth.

Rinse liquid is transported in tubes integrated in the armor layers in the annulus of the pipe 1 - this will be explained in more details below. The rinse liquid enters the annulus of the pipe 1 at the second end, optionally via ducts formed in the end-fitting 5b. The rinse liquid will flow in the annulus of the pipe from the second end B to the first end A where used rinse liquid is taken out via the end-fitting 5a. The rinse liquid flows in the free volume of the annulus which constitutes from about 0.1 m³ to about 5 m³ per 1000 m pipe. The flow of the rinse liquid is preferably a forced flow established by means of one or more pumps installed on the floating vessel 22.

The ambient pressure affecting the flexible pipe is substantially higher at the second end B than at the first end A. Thus, the rinse liquid which has absorbed gas during the passage from the second end to the first end of the pipe is exposed to a varying ambient pressure which will decrease from the second end of the pipe to the first end of the pipe. When the rinse liquid has absorbed gases at high pressure, these gases will be released from the rinse liquid when the pressure decreases. The release of gases will in particular appear as bubble formation at first end A of the pipe 1. However, the bubble formation can be avoided if measures are taken to provide an increased pressure in the first end A of the pipe 1.

Figure 4 illustrates the interior of the end-fitting 2 where the layers of the pipe 1 are terminated. The carcass 10 is terminated at 30 by means of a carcass ring. The internal pressure sheath 11 is terminated at 31 by a seal. The pressure armor 12 is terminated at 32 by means of lock nuts. The tensile armors 13, 14 are terminated in the chamber 33 formed between the outer casing 4 and the inner casing 5 of the end-fitting 2. In the chamber 33 the tensile armors 13, 14 are fixed by means of cast epoxy. The outer sheath 15 is terminated at 34 by squeezing.

The annulus of the pipe is formed between the internal pressure sheath 11 and the outer sheath 15 and includes the pressure armor 12, the tensile armors 13, 14 and the free volume of the pipe. A channel 35 in the end^¬ fitting 2 connects the annulus with to an exhaust volume with pressure close to ambient. A valve device 36 is controls the connection between the annulus and the exhaust volume. The valve device 36 also controls the pressure in the annulus of the pipe 1, at least at the first end of the pipe. Thus, the valve 36 can be adjusted to only allow that rinse liquid is taken out of the annulus if the pressure in the annulus is elevated sufficiently with respect to ambient, such as more than 3 bars above ambient pressure. This will significantly reduce or even prevent bubble formation in the rinse liquid in the first end of the pipe.

In figure 4 the channel and valve device are shown as located in or

connected to the end-fitting. However, it is also possible to mount the channel and valve external to the end-fitting.

Figure 5 shows a flexible pipe 1 which has been reinforced with an exterior armor layer 40. The exterior armor layer 40 is formed from metallic strips which are wound helically around the outer sheath 15 of the pipe 1. The metallic strips forming the exterior armor layer 40 are made from carbon steel and are wound around the outer sheath with a winding angle about 55 degrees in respect of the centre axis of the pipe 1.

The exterior armor layer 40 is covered with a protective extruded sheath 41 made from polyamide. The exterior armor layer 40 and the protective sheath 41 need only to be applied at the first end of the pipe to protect the outer sheath 15 against the elevated pressure in the annulus at the first end of the pipe. The layers 40 and 41 can be terminated in the end-fitting or outside the end-fitting.

Figure 6 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 6 a tube 18 for rinse liquid is integrated in the inner tensile armor 13. The tube 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 tube 18.

Only one tube 18 is seen in figure 6, however two, three or more tubes may be provided on the flexible pipe 1. The one or more tubes are typically terminated in the end-fittings of the flexible pipe. In the first end-fitting the one or more tubes are connected to a source for rinse liquid and pumping means. In the second end-fitting the one or more tubes are connected to the annulus such that a flow of rinse liquid can flow through the annulus of the pipe from the second end-fitting to the first end-fitting.

As mentioned the figures are not accurate in every detail but only sketches intended to the show the principles of the invention. Thus, the unbonded flexible pipes according to the present invention may comprise several other layers and features, which are not shown in the figures. Such features may be insulating layers and intermediate layers, etc. The end-fittings may also be constructed different than shown in the figures and comprise further details and parts.

Claims

1. A method for flushing a flexible pipe comprising a first end and a second end and connected to an end-fitting at least at the first 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, said at least one annulus comprises one or more armor layers, and said at least one annulus is in fluid connection with at least one venting channel close to the first end of the pipe, the method comprises the step of flushing said at least one annulus with a rinse liquid from the second end to the first end of the pipe, and arranging the at least one venting channel with means to open and close thereby allowing removal of at least a portion of the rinse liquid from the first end of the pipe in such a way that the pressure in the annulus at the first end of the pipe is kept at least 3 bar above ambient pressure.

2. A method according to claim 1, wherein the pressure in the annulus is kept above about 3 bar and up the about 30 bar.

3. A method according to claim 1 or 2, wherein the venting channel comprises at least one venting valve and the pressure in the annulus at the first end of the pipe is controlled by the at least one venting valve.

4. A method according to any one of the preceding claims, wherein the pressure in the annulus is monitored at the first end of the pipe and the result of the monitoring is used to control the flow-rate of rinse liquid in the annulus.

5. A method according to any one of the preceding claims, wherein the pressure in the annulus is monitored at the first end of the pipe and the result of the monitoring is used to control the venting means of the venting channel.

6. A method according to any one of the preceding claims, wherein the flexible pipe at least at the first end comprises a portion where the outer sheath is reinforced to resist internal pressure in the annulus by an exterior armor layer helically wound around the outer sheath.

7. A method according to claim 8, wherein the exterior armor layer is made from steel

8. A method according to any one of the preceding claims, wherein the flexible pipe at least at the first end comprises a portion with tape helically wound around the outer sheath, preferably the tape comprises metal or fibre reinforced polymer material.

9. A method according to the claims 6-8, wherein the flexible pipe comprises an additional outer extruded protective layer around the exterior armor or tape

10. A method according to any one of the preceding claims, wherein the density of the rinse liquid is less than 1025 kg/m³.

11. A method according to any one of the preceding claims, wherein the rinse liquid is selected from water, methanol, ethanol, benzol, diesel, petroleum or a combination of two or more of the mentioned liquids.

12. A method according to any one of the preceding claims, wherein the annulus is substantially prefilled with rinse liquid from the second end of the pipe to the first end of the pipe prior to installation.

13. A method according to claim 12 wherein the annulus of the pipe is pumped substantially free of air prior to initial filling of the annulus with rinse liquid to prevent formation of air pockets.

14. A method according to any of the preceding claims wherein the rinse liquid is transported from the first end of the pipe to the second end of the pipe in tubes embedded in at least one of the armor layers in the annulus.

15. A method according to any one of the preceding claims, wherein the flexible pipe from the first end to the second end comprises two or more sections, said more section may be identical or different in structure, preferably the sections are connected by end-fittings.

16. An assembly comprising an end-fitting and a flexible pipe, said flexible pipe comprising a first end and a second end, and being connected to the end-fitting at the first 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, said at least one annulus comprises one or more armor layers, and said at least one annulus is in fluid connection with at least one venting channel close to the first end of the pipe, said flexible pipe is adapted for flushing of said at least one annulus with a rinse liquid from the second end to the first end of the pipe, and the at least one venting channel which is adapted with means to open and close and allow removal of at least a portion of the rinse liquid from the first end of the pipe in such a way that the pressure in the annulus at the first end of the pipe is kept 3 bar above ambient pressure.

17. An assembly according to claim 16, wherein the flexible pipe is connected to a second end-fitting at the second end.

18. An assembly according to claim 16 or 17, wherein the at least on venting channel is arranged in the end-fitting at the first end.

19. An assembly according to any one of the claims 16 to 18, wherein a portion of the flexible pipe at at least at the first end is reinforced with an exterior armor layer

20. An assembly according to any one of the claims 16 to 19, wherein a portion of the flexible pipe at the first end comprises an extruded polymer layer around an exterior armor layer.

21. An assembly according to any one of the claims 16 to 20, wherein the annulus comprises at least one pressure armor layer.

22. An assembly according to any one of the claims 16 to 21, wherein the annulus comprises at least one tensile armor layer.

23. An assembly according to any one of the claims 16 to 22, wherein the annulus comprises at least one tensile armor layer with integrated tubes extending from the first end of the pipe the second end of the pipe, said tubes being adapted for transport of rinse liquid.