WO2011026801A1

WO2011026801A1 - Method of protecting a flexible pipe from corrosion and a flexible pipe

Info

Publication number: WO2011026801A1
Application number: PCT/EP2010/062607
Authority: WO
Inventors: Johannes Martinus Maria Out
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2009-09-01
Filing date: 2010-08-30
Publication date: 2011-03-10

Abstract

Method of protecting a flexible pipe (10) from corrosion comprising at least the steps of : (a) providing a flexible pipe (10) comprising: - an outer sheath (11); - an inner sheath (12) - an annular volume (16) bounded by the outer sheath (11) and the inner sheath (12) and comprising at least one metallic reinforcing layer (13, 15) and a dry annular void; - a pipe fluid passage (18), located within the inner sheath (12); - a first end (20) having a first end-fitting (40) comprising a first port (51) in fluid communication with the pipe fluid passage (18), and at least one protective liquid injection port (48) in fluid communication with the annular void; - a second end (30), having a second end-fitting comprising a second port in fluid communication with the pipe fluid passage (18), and at least one fluid release valve in fluid communication with the dry annular void; and (b) passing a protective liquid to the protective liquid injection port (48) of the first end-fitting (40), thereby filling the dry annular void within the annular volume (16) with protective liquid to provide a protected flexible pipe.

Description

METHOD OF PROTECTING A FLEXIBLE PIPE FROM CORROSION AND A

FLEXIBLE PIPE

The present invention provides a method of protecting a flexible pipe from corrosion, a flexible pipe for use in such a method and a corrosion protected flexible pipe. The flexible pipe can be a flexible riser or flexible flowline.

Such flexible pipes can carry pipe fluids between a hydrocarbon reservoir located under the sea bed and a floating structure. The pipe 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 pipe fluids, which are for instance passed to the floating structure, can be

processed, for example by compression and/or further treatment .

Natural gas is a useful fuel source, as well as being a source of various hydrocarbon compounds. It is often desirable to liquefy natural gas in a liquefied natural gas (LNG) plant at or near the source of a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a smaller volume and does not need to be stored at high pressure.

The Floating Liquefaction, Storage and Off-loading (FLSO) concept combines the natural gas liquefaction process, storage tanks, loading systems and other

infrastructure into a single floating unit. The Floating Liquefied Natural Gas (FLNG) concept is similar to that of the FLSO concept, but additionally provides natural gas treatment as well as the liquefaction process, storage tanks, loading systems and other infrastructure into a single floating structure.

Such concepts are advantageous because they provide off-shore alternatives to on-shore liquefaction plants. These vessels can be moored off the coast, or close to or at a gas field, in waters deep enough to allow off^¬ loading of the LNG product onto a carrier vessel. They also represent movable assets, which can be relocated to a new site when the gas field is nearing the end of its productive life, or when required by economic,

environmental or political conditions.

When the floating structure is moored close to a gas field or other hydrocarbon reservoir, it can be kept in fluid communication with the producing well heads via one or more flexible risers. The one or more flexible risers can convey fluids between the well heads of a hydrocarbon reservoir and the floating structure. Flexible risers can 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 can be connected at one end to the floating structure, and at another end to a riser base manifold, which can secure the flexible riser to the sea bed.

A sub sea pipeline can connect the riser base

manifold to the well heads either directly, or via a well manifold. The sub sea pipeline may be a metal or

composite tubular flowline, or a flexible flowline comprising flexible pipe. In such configurations, a production hydrocarbon, e.g. natural gas, from a

hydrocarbon reservoir, e.g. a gas field, can be passed along the sub sea pipeline from one or more well-heads, which can be in the same or different hydrocarbon

reservoirs, to the riser base manifold.

The riser base manifold is the point at which the production and any injection pipelines are connected to one or more flexible risers which convey the production hydrocarbon to the floating structure. The riser base manifold can provide the touchdown point at which the flexible riser reaches the sea bed. Alternatively, the flexible riser can reach the sea bed at a touchdown point distant from the riser base manifold to which it is connected. The flexible risers can be connected to the floating structure at a hangoff point. The hang-off point may be at a side of the floating structure, or situated within a moonpool in the floating structure, for example at the bottom of a turret. The floating

structure can be moored to the sea bed by a plurality of mooring lines which are anchored to the sea bed.

The flexible pipes, such as flexible risers and flexible flowlines, are constructed of a number of independent layers, such as helical laid steel and polymeric layers formed around a central bore for

carrying production or injection fluids.

The paper titled "Prevention and monitoring of fatigue-corrosion of flexible risers' steel

reinforcements" by Antoine Felix-Henry, OMAE2007-29186, published as part of the Proceedings of the 26^th

International Conference on Offshore Mechanics and Arctic Engineering, June 10-15 2007, San Diego, California, USA, discloses a flexible riser pipe comprising steel

reinforcing layers contained in an annular volume located between inner and outer polymer sheathes. The structure of the flexible riser is designed to seal and avoid any direct contact between the fluid carried in the bore and the steel reinforcing layers in the annular volume.

Although the inner and outer sheathes defining the annular volume are designed to be leak proof, under high temperature and pressure conditions, small amounts of gases can permeate through the inner sheath from the riser fluid. Corrosive gases such as carbon dioxide, dihydrogen sulphide and water vapour may be present in a hydrocarbon production fluid in the bore. Such corrosive gases may diffuse into the annular volume and attack the steel reinforcing layers. OMAE2007-29186 discloses that a gas venting system can be integrated inside the

flexible pipe end terminations to flush corrosive gases from the annular volume.

The flooding of the annular volume with water, such as from seawater ingress from a damaged outer sheath or water vapour diffusing from the bore fluids through the inner sheath and condensing in the annular volume, can result in the corrosion of the steel reinforcing layers. Such corrosion can result in a reduction in the fatigue life of the riser.

In addition, a flooded annular volume may also prevent gas from being vented from the annulus, leading to a pressure build up which may result in a rupture in the external sheath and further corrosion issues.

In response to such annular flooding as a result of damage during use, OMAE2007-29186 discloses that subsea clamps may be installed in the flexible riser to seal the outer sheath at any damaged area and provide a gas release valve from the annular volume. The gas release valve can be applied to a "hog bend" region of the flexible riser. The "hog bend" represents a local maximum in the height of a sinusoidal- or wave-shaped flexible riser. The annular volume can then be injected with an inert fluid to mitigate against corrosion and extend the fatigue life of the flexible riser.

Such a method may result in the leakage of the inert fluid from the annular volume into the surrounding environment. If the inert fluid is not environmentally benign, this method of corrosion mitigation may be associated with a mild degree of environmental damage.

WO-9840657-A1 discloses a riser wherein the annulus is continuously flushed with a lower pressure medium, such as warmed gas or oil.

WO-00/17479 discloses a riser wherein the annulus is connected to the inner channel of the riser via a flow path to prevent over-pressure of the annulus. The flow path includes a one-way valve or a pump so that fluid can flow from the annulus to the inner fluid channel, but not the other way around. The riser may include a further flow path to introduce fluids or gases in the annular space for cleaning and maintenance thereof.

FR-2 858 841 discloses a method for operating a riser. The method comprises injecting an entrainment gas under pressure in the annulus to force permeate gases in the annulus to flow along towards a vent, to be vented to the outside of the riser.

The present invention seeks to address the problem of corrosion of the metallic reinforcing layers in the annular volume of flexible pipes and to provide a less elaborate and more cost effective system.

In a first aspect, the present invention provides a method of protecting a flexible pipe from corrosion, comprising at least the steps of:

(a) providing a flexible pipe comprising:

an outer sheath; an inner sheath located within the outer polymer sheath;

an annular volume bounded by the outer sheath and the inner sheath, said annular volume comprising at least one metallic reinforcing layer and a dry annular void;

a pipe fluid passage, located within the inner sheath;

a first end, said first end having a first end- fitting, said first end-fitting comprising a first port in fluid communication with the pipe fluid passage, and at least one protective liquid

injection port in fluid communication with the annular void;

a second end, said second end having a second end- fitting, said second end-fitting comprising a second port in fluid communication with the pipe fluid passage, and at least one fluid release valve in fluid communication with the dry annular void; and

(b) passing a protective liquid to the at least one

protective liquid injection port of the first end- fitting, thereby filling the dry annular void within the annular volume with protective liquid to provide a protected flexible pipe.

The present invention protects a flexible pipe from corrosion by filling the annular void with a protective liquid, such as monoethylene glycol (MEG) , optionally supplemented by other fluids such as methanol. The protective liquid can prevent corrosive substances such as water, particularly electrolytic sea water, from reaching the at least one metallic reinforcing layer in the annular volume. Corrosion of metallic reinforcing layers such as steel may occur, for example by the oxidation of the iron in the alloy in the presence of oxygen, or by reaction with one or both of CO₂ and H₂S, and water. The

protective liquid in the annular void fills the air gaps in the annular volume, preventing corrosive substances from reaching the reinforcing layers.

The protective method of the present invention can be carried out on an undamaged flexible pipe, such as an uninstalled flexible pipe. For instance, the protective method may be carried out as part of the manufacture of the flexible pipe or prior to installation of the pipe at the latest. In addition, the annular void of the

flexible pipe in the present invention can be filled with protective liquid from the at least one injection port in the first end-fitting, with any gas occupying the annular void being vented from the at least one fluid release valve at the second end-fitting, providing an efficient filling operation.

The method of the present invention can be contrasted with the responsive methods disclosed in OMAE2007-29186. OMAE2007-29186 discloses a reactive method of corrosion mitigation, which is carried out only after damage has occurred during operation and the annular volume of a flexible riser is compromised with the presence of corrosive materials, such as after a rupture in the outer sheathe .

In particular, it is only after annular flooding that OMAE2007-29186 teaches that the annulus can be injected with inert fluid via a topside venting system. This operation is carried out in situ. The flexible riser must first be prepared by the fitting of two subsea clamps using a remote operated vehicle (ROV) . One of the clamps seals the rupture in the outer sheath. The second clamp provides a drilling site allowing the outer sheath to be drilled to allow a gas release valve to be inserted into the annular volume. The drilling site is normally located in the "hog bend" of a wave-shaped flexible riser. This is a complicated and potentially hazardous operation, which is avoided in the present invention because the annular void is provided already filled with protective fluid during manufacture or prior to

installation of the pipe at the latest. There is thus no requirement to utilise the topside venting system of the floating structure to inject the inert fluid.

Other prior art systems are dynamic. Herein, fluids or gases are actively pumped into or out of the annulus after installation of the riser, either for maintenance or protection.

The flexible pipe according to the invention

comprises an annulus which is filled with a protective fluid before installation of the pipe at a production site. After installation, the protective fluid remains in the annular space. The flexible pipe of the invention provides a static system, i.e. the protective fluid is static within the annulus. Consequently, the flexible pipe of the invention can be installed relatively easy and fast at the production site, as the annular space is already filled with protective fluid. Also, the pipe of the invention has lower operating costs than dynamic systems .

In a further aspect, the present invention provides a flexible pipe for use in a sub sea environment comprising at least :

- an outer sheath; - an inner sheath located within the outer polymer sheath;

- an annular volume bounded by the outer sheath and the inner sheath, said annular volume comprising at least one metallic reinforcing layer and a dry annular void;

- a pipe fluid passage, located within the inner sheath;

- a first end, said first end having a first end- fitting, said first end-fitting comprising a first port in fluid communication with the pipe fluid passage, and at least one protective liquid

injection port in fluid communication with the dry annular void;

- a second end, said second end having a second end-fitting, said second end-fitting comprising a second port in fluid communication with the pipe fluid passage and at least one fluid release valve in fluid communication with the dry annular void. The at least one protective liquid injection port is configured to allow the filling of the annular void of the flexible pipe with protection fluid. The annular void, which can comprise 5-10% of the annular volume, could optimally be filled at an acceptable rate as part of the manufacture or installation process.

If the annular void is filled as part of the

manufacturing process, the filling rate is not so important. However, if the annular void is filled with the protective fluid during installation, this should be carried out prior to the flexible pipe being submerged in water. In the latter case, the at least one injection port preferably allows the annular void to be filled at least at the rate at which the flexible pipe is deployed, for instance withdrawn from any storage reel.

In a further aspect, the present invention provides method of adapting a flexible pipe to provide a protected flexible pipe comprising at least the steps of:

- providing a flexible pipe comprising at least:

- an outer sheath;

- an inner sheath located within the outer sheath;

- a pipe fluid passage, located within the inner sheath;

- a first end, said first end having a first end- fitting, said first end-fitting comprising a first port in fluid communication with the pipe fluid passage, and at least one gas release valve in fluid communication with the annular void;

- a second end, said second end having a second end-fitting, said second end-fitting comprising a second port in fluid communication with the pipe fluid passage;

- replacing the at least one gas release valve of the first end-fitting with at least one protective liquid injection port;

- providing the second end-fitting with at least one fluid release valve in fluid communication with the dry annular void; and

- filling the dry annular void of the flexible pipe with a protective liquid from the at least one protective fluid injection port to provide a protected flexible pipe . The protected flexible pipe is preferably provided prior to installation. For the benefits of the

protective liquid to be obtained, the annular void should be dry and thus filled prior to the flexible pipe being submerged in water or brought into operation by passing pipe fluids through the pipe fluid passage. As used herein, the term "dry" is intended to mean substantially free of water.

Embodiments of the present invention will now be described by way of example only, and with reference to the accompanying non-limiting drawings in which:

Figure 1 is a first embodiment showing a typical flexible pipe according to the invention.

Figure 2 is a second embodiment showing a typical end-fitting of the method and flexible pipe according to the invention.

Figure 3 is a third embodiment showing the protected flexible pipe in use as a flexible riser.

For the purpose of this description, a single

reference number will be assigned to a line as well as a stream carried in that line. The same reference numbers refer to similar components, streams or lines.

Figure 1 shows a schematic diagram of a flexible pipe 10 as disclosed herein. The individual layers forming the flexible pipe have been sequentially removed to provide a 3-dimensional cross-section of the pipe. For simplicity, the end-fittings are not shown. These are discussed in detail in relation to Figure 2.

The flexible pipe 10 can be a flexible riser or a flexible flowline.

Flexible risers are configured to withstand dynamic loads. Such dynamic loads can arise as a result of the wave and wind movement of a body of water, for instance when the riser is suspended between a floating structure and the sea bed. The dynamic loads resulting from the operation of a flexible riser in the splash zone are of particular importance when designing such risers.

A detailed discussion of flexible risers can be found in the American Petroleum Institute's publication titled "Specification for unbonded pipe", API Specification 17J, Third edition, effective date January 2009.

Flexible flowlines are configured to withstand predominantly static loads, such as those experienced by a sub sea pipeline, for instance connecting a riser base manifold to a wellhead, or conveying pipe fluids between an undersea reservoir and an on-shore location.

The flexible pipe 10 shown in Figure 1 has a circular cross-section, which is preferred. However, such pipes may also have non-circular cross-sections, such as elliptical or ovoid. The individual layers forming the flexible pipe are preferably arranged concentrically, with each layer being equidistant from the longest axis of the pipe, which is at the centre of pipe fluid passage

18. One or more layers of the flexible pipe 10 can be present as sheathes, that is a tubular structure which can envelop other layers of smaller diameter.

Flexible pipe 10 comprises an outer sheath 11, which is preferably made of polymer, such as a thermoplastic polymer which is resistant to the protective liquid discussed below. The outer sheath 11 may comprise high density polyethylene (HDPE) , medium density polyethylene (MDPE) , or a polyamide such as PA-11 or PA-12. The outer sheath 11 should be leak-proof, to prevent ingress of water or other fluids.

The flexible pipe 10 further comprises an inner sheath 12. The inner sheath 12 is located within the outer sheath 11. The inner sheath is preferably

constructed of a polymer, such as a thermoplastic polymer which is resistant to the protective liquid discussed below. The inner sheath 12 may comprise high density polyethylene (HDPE) , cross-lined polyethylene (XLPE) , a polyamide such as PA-11 or PA-12, or a plasticized or unplasticized polyvinylidene fluoride alloy (PVDF-alloy) . Like the outer sheath 11, inner sheath 12 should be leak- proof against the transmission of fluid from the inside or outside.

A carcass layer 17 can be located within the inner sheath 12. The carcass layer 17 can provide the wall of the pipe fluid passage 18 in which the pipe fluid is conveyed by the flexible pipe 10.

The pipe fluid may be a hydrocarbon production fluid such as natural gas or oil from a hydrocarbon reservoir, or it may be an injection fluid to be passed to a

hydrocarbon or other reservoir. The pipe fluid may comprise one or more corrosive materials, such as carbon dioxide, hydrogen sulphide and water, which could damage metallic elements of the flexible pipe 10, were they to come into contact.

The outer sheath 11 and inner sheath 12 provide the boundaries of an annular volume 16, which extends the length of the flexible pipe 10. The annular volume 16 may comprise at least one layer comprising at least one metallic reinforcing layer, and other, optional layers selected from the group comprising: at least one

insulating layer and at least one anti-wear layer.

The at least one metallic reinforcing layer is present to provide the necessary strength, such as one or more selected from the group comprising hoop strength, axial strength and torsional strength. The at least one metallic reinforcing layer may also prevent loss of interlock to the flexible pipe. The metallic reinforcing layers can be helically laid metallic layer or a layer composed of interlocking metallic elements. There may be from one to eighteen metallic reinforcing layers present in the flexible pipe. The at least one metallic

reinforcing layer are preferably made of a metal or alloy, for instance steel, such as carbon steel.

The at least one metallic reinforcing layer may be selected from one or both of at least one tensile armour layer 13 and at least one pressure armour layer 15. A pressure armour layer 15 can provide hoop strength to the flexible pipe. A preferred pressure armour layer is an interlocking profiled layer, such as a zeta-shaped, T- shaped or C-shaped pressure vault layer. A tensile armour layer 13 can provide axial and torsional strength to the flexible pipe, optionally together with hoop strength if a separate pressure armour is not provided.

The at least one insulating layer can provide thermal insulation to the pipe fluid carried in the internal pipe fluid passage. The requirement for an insulating layer will depend upon the composition of the pipe fluid and the temperature of the subsea environment. For instance, an insulating layer may prevent condensate formation if the pipe fluid is a hydrocarbon stream from a hydrocarbon reservoir .

The at least one anti-wear layer may be present to assist the relative movement and provide wear resistance between other layers in the annular volume 16, especially the metallic reinforcing layers, which when constructed of helically laid or interlocking metallic elements can abrade adjacent layers. The annular volume 16 shown in the embodiment of Figure 1 comprises a tensile armour layer 13, an anti- wear layer 14 and a pressure armour layer 15.

In a further preferred embodiment (not shown) , the annular volume may comprise two tensile armour layers and a pressure armour layer, with an anti-wear layer between each of the former layers.

It will be apparent that annular volume 16 comprises spaces between the individual layers forming the annular volume 16 and within such layers. For instance, there may be gaps between the helical laid or interlocking metallic elements forming the at least one metallic reinforcing layer. This free space is termed the

"annular void" herein, and may account for between 5-10%, more particularly 7-8% of the annular volume 16. The annular void is a contiguous volume within the annular volume 16, which is unoccupied by the materials forming the layers therein. The annular void extends along the length of the flexible pipe 10 i.e. between the first and second ends of the flexible pipe.

In one embodiment, the volume of the annular void may account for 4.6 litres per meter of flexible pipe with internal diameter of 36.3 cm. Thus, for a flexible pipe of length 1500 m, the annular void may have a volume of 6.9 m³.

Conventionally the annular void can filled with a gas, such as air or maintained under vacuum. In the method of protecting the flexible pipe disclosed herein, the annular void is provided in a dry state i.e. free or substantially free of water and is filled with a

protective liquid. Thus, in one embodiment, a method of manufacturing a protected flexible pipe is disclosed, said method

comprising at least the steps of:

- providing a flexible pipe as described herein and

- filling the annular void of the flexible pipe with a protective liquid to provide a protected flexible pipe.

The protective liquid is an inert liquid which retards corrosion of the at least one metallic

reinforcing layer 13, 15 in the annular volume 16.

preferably a glycol, such as monoethylene glycol (MEG) .

Glycols such as MEG are preferred components because they function as dehydrating agents, capturing any absorbed water and preventing it from reaching the at least one metallic reinforcing layer 13, 15. The protective liquid may further comprise an alcohol, such as methanol.

In a preferred embodiment, the protective liquid comprises MEG and methanol having a ratio in the range of 3 to 5:1 MEG:methanol (by weight) . Such a proportion of components provide a protective liquid having a density in the range of 1.02 to 1.04 g/cm³ approximating that of sea water. By providing a protective liquid of similar density to that of sea water, the protective liquid is less likely to be displaced upon damage to the integrity of the outer sheath 11 allowing leakage of seawater into the annular void. In addition, the pressure difference between the annular volume and the sea water surrounding the outer sheath as a function of depth will remain equal to the difference at the sea surface. Thus, even if during installation the first end of the flexible pipe is located 30 m vertically above the surface of the sea, the pressure difference will be equal to 3 bar. This is well below the pressure range of 5.3 to 8 bar at which an outer sheath would be expected to rupture. It is possible to provide a protective liquid having a density greater than that of sea water. This would be advantageous because there would be an outward flow of protective liquid from the annular void, rather than an inward flow of seawater upon any puncture of the outer sheath 11.

The protective liquid may comprise at least one additive, such as one or more compounds from the group comprising :

- a corrosion inhibitor, such as an oxygen scavenger;

- a pH neutraliser, such as sodium hydrogen carbonate;

- a biocide, to restrict any organic growth in the

annular void upon damage of the flexible pipe; and

- a dye, such as a fluorescent dye, which would be

visible in the surrounding environment upon inspection with a ROV, providing an indication of leakage of the protective liquid and thus damage to the flexible pipe .

Taking the example of a flexible pipe having an annular void of 4.6 litre per meter, filling with a protective liquid having a density in the range of 1.02 to 1.04 g/cm³ would add approximately 4.8 kgf/ m to the weight of the pipe. The additional weight of the

protective liquid would account for 1-2 % of the mass of the flexible pipe with the pipe bore 18 filled with seawater. Such an increase in weight will have no more than a minor effect on the properties or operation of the flexible pipe 10.

The flexible pipe 10 is provided with end-fittings to allow termination of all the flexible pipe layers and transfer the functions, such as strength and sealing, of its constituent layers. The end fittings should be sufficient to prevent leakage, structural deformation and the pull-out of polymeric layers. The end-fittings can also allow the ends of the flexible pipe to be secured to further equipment to allow the transmission of pipe fluid through the pipe fluid passageway. The operation of a flexible pipe is discussed in greater detail in relation to the embodiment of Figure 3.

Figure 2 shows a generalised schematic diagram of an end-fitting 40 of a flexible pipe 10, such as a first end-fitting of a first end 20.

The end fitting 40 secures the layers of the flexible pipe 10. The carcass layer 17 is secured by carcass installation ring 41. The inner sheath 16 is secured by inner sheath seal ring 42. The tensile armour layer 13 can be secured by a wire holder 43. The outer sheath 11 can be secured by a sheath holder 44. Expansion of the pressure armour layer 15 is prevented by clamping means 50.

A ring seal 45 can provide a pressure tight seal between the outer sheath 11 and an end-fitting jacket 47. The end-fitting jacket 47 can be secured to an end- fitting head 46 which can hold the carcass installation ring 41 and inner sheath seal ring 42.

The end-fitting 40 can comprise at least one

protective fluid injection port 48, which is in fluid communication with the annular void of the annular volume

16. In the embodiment shown in Figure 2, a protective liquid injection port 48 is provided in end-fitting head 46. The protective liquid injection port 48 is in fluid communication with the annular void of the annular volume 16 via a protective liquid conduit 49. In an alternative embodiment not shown in Figure 2, the at least one protective fluid injection port and any protective liquid conduit may be provided in the end-fitting jacket 47. Protective liquid can be provided to liquid injection port 48 and passed to the annular volume 16 (through any protective liquid conduit 49, if present) where it can fill the annular void. As the protective fluid passes into the flexible pipe 10, it will displace any gas present in the annular void, causing a gas/liquid

interface to move along the length of the flexible pipe 10 until the interface reaches the second end of the flexible pipe 10 and the annular void is filled. Any gas present in the annular void can exit the annular volume via at least one fluid release valve, which is in fluid communication with the annular void. The at least one fluid release valve is present in the second end-fitting and can vent any gas beyond the flexible pipe 10.

The second end-fitting may be of similar construction to the first end-fitting and contains means to secure at least one of the layers of the flexible pipe. The second end-fitting may comprise a second port in fluid

communication with the pipe fluid passage 18, providing a second end of the flexible pipe 10.

The second end-fitting further comprises at least one fluid release valve. The at least one fluid release valve is in fluid communication with the dry annular void, in a similar manner to the at least one protective fluid injection port in the first end-fitting. The at least one fluid release valve allows the venting of any gas present in the annular void when this is filled with the protective liquid.

Further fluid release valves may additionally also be provided along the length of the flexible pipe 10.

The annular void of the flexible pipe 10 may be filled with the protective fluid either during

manufacture, or after manufacture but prior to being submerged in water. For instance, the annular void may be filled at the manufacturing plant or at the quayside prior to transport to the installation site.

The flexible pipe 10 should be provided with a dry annular void prior to filling with protective fluid.

This can be achieved by filling the annular void with dry gas, such as dry air, or by maintaining the annular void under lower than ambient pressure, for instance under vacuum .

The annular void can be filled with protective liquid by pumping the protective liquid to the at least one protective liquid injection port 48 in the first end- fitting 40. The protective liquid will fill the annular void, ejecting any gas present at the at least one fluid release valve in the second end-fitting. If the annular void is being maintained at lower than ambient pressure, this can be stopped prior.

In the case that the flexible pipe is a flexible riser, it may be present on a storage reel or carousel during the filling step.

It is preferred that the first end-fitting 40 and therefore at least one protective liquid injection port 48 is provided at the gravitationally lowest point of the flexible pipe 10, with the second end-fitting and

therefore at least one fluid release valve is provided at the gravitationally highest point of the flexible pipe 10 during the filling step. In this way, the interface between the protective liquid and any gas present may move vertically along the annular volume, maximising the displacement of any gas in the dry annular void to provide maximum filling of the annular void with

protective liquid. In a further embodiment, a method of adapting a flexible pipe is provided. It is known to include a gas venting system for the annular volume in the end-fitting of a flexible pipe. Such an end-fitting may be adapted by replacing the gas release valve in the end-fitting with a suitable protective liquid injection port, to provide a first end-fitting. If necessary widening of the conduit connecting the protective liquid injection port to the annular volume can also be carried out. Any widening of the conduit provides a protective liquid conduit of sufficient diameter to allow the filling of the annular void with protective liquid. The diameter of the conduit can thus be increased to provide an

acceptable rate of filling.

Conventional end-fittings of a flexible pipe can be machined from steel or other alloys or alternatively cast from a block of concrete. If necessary, in the method disclosed herein, the diameter of the conduit can be machined to increase the bore, providing a higher flow rate for the protective liquid.

The second end-fitting may also require adaptation to allow the filling of the annular void with the protective fluid. The second end-fitting should be provided with at least one fluid release valve which is in fluid

communication with the annular void.

The dry annular void of the flexible pipe can then be filled with a protective liquid from the at least one protective fluid injection port to provide a protected flexible pipe.

A discussion of flexible pipe operation can be found in the American Petroleum Institute's publication titled "Recommended Practice for Flexible Pipe", API Recommended Practice 17B, Fourth Edition, July 2008. Figure 3 shows a three dimensional cut-through section of a floating structure 100 having flexible pipes 10 in use. In this embodiment, the flexible pipes 10 are flexible risers. The flexible risers lOa-h convey a riser fluid between the at least one riser fluid

reservoir 250 underneath the sea bed and a floating structure 100 on the sea surface. Thus, the risers can be sub sea risers. As used herein, the term "sub sea" is intended to encompass both salt water and fresh water environments, and represents the region between the water surface and the bottom of the body of water.

The floating structure 100 can be a floating vessel, or an off-shore floating platform. A floating vessel may be any movable or moored vessel, generally at least having a hull, and usually being in the form of a ship such as a 'tanker' or a semi-submersible vessel, for instance having pontoons.

Such floating vessels can be of any dimensions, but may be elongated or substantially square. Whilst the dimensions of a floating vessel are not limited at sea, building and maintenance facilities for floating vessels may limit such dimensions. Thus, in one embodiment of the present invention, the floating vessel or off-shore floating platform is less than 600m long such as 500m, and a beam of less than 100m, such as 80m, so as to be able to be accommodated in existing ship-building and maintenance facilities.

An off-shore floating platform may also be movable, but is generally more-permanently locatable than a floating vessel.

The flexible pipe disclosed herein is for instance advantageous for applications in any water depth, such as water depths greater than 200 m, for instance 250 to 500 m, or greater than 1000 m. However, shallower water depths may also provide a dynamic and more oxygenated environment in which corrosion protection may also be required .

In one embodiment, the riser fluid is a hydrocarbon fluid such as natural gas, and the at least one riser fluid reservoir 250 are hydrocarbon fluid reservoirs such as natural gas reservoirs. In this embodiment, the hydrocarbon fluid could be conveyed from the hydrocarbon fluid reservoirs 250 under the sea bed 500 to the

floating structure 100, where the hydrocarbon fluid can be stored and preferably treated. When the hydrocarbon fluid is natural gas it is preferred that the floating structure 100 comprises natural gas treatment and/or liquefaction units such that the natural gas can be treated to remove unwanted impurities and cooled to provide liquefied natural gas.

In an alternative embodiment, the method and flexible riser disclosed herein can be used in the sequestration carbon dioxide.

Many hydrocarbon reservoirs, such as natural gas reservoirs, may contain carbon dioxide, for instance in contents of 6-10%. This carbon dioxide could be separated in the floating structure from the hydrocarbon fluid, such as natural gas, which is removed from the reservoir, and then re-injected into a riser fluid reservoir 250. The riser fluid reservoir can be any sealed subsurface geological formation such as a hydrocarbon reservoir, a depleted hydrocarbon reservoir, an aquifer or other sealed water containing layer. In this case, the riser fluid can comprise carbon dioxide, preferably as a dense phase, such as supercritical carbon dioxide i.e. carbon dioxide having a pressure and temperature above the critical point. In contrast to the previous embodiment, the riser fluid comprising carbon dioxide would for instance be conveyed from a separation unit on the floating structure 100 to the at least one depleted hydrocarbon reservoir 250 under the sea bed 500, where the riser fluid comprising carbon dioxide can be stored.

In a further alternative embodiment, also

illustrating a carbon dioxide sequestration method, the riser fluid comprising carbon dioxide passed to the riser fluid reservoir 250 can come from any source. For

example, the carbon dioxide may be generated at a

location different from the floating structure 100, such as an on-shore location, and transferred to the floating structure 100 for sequestration underneath the sea bed. The riser fluid and riser fluid reservoir 250 may be as defined in the previous embodiment.

In the exemplary embodiment of Figure 3, the floating structure 100 is a floating vessel. The floating vessel is held in position by a plurality of mooring lines 610 which are connected to the floating vessel at a mooring point, such as a turret, and maintain the mooring point of the floating vessel in a fixed position. Figure 1 shows a trigonal arrangement of three bundles 620a, 620b, 620c of mooring lines, each bundle comprising four mooring lines 610a, b, c, d. The mooring lines 610 are fastened securely to sea bed 500, for instance using anchor piles.

The at least one flexible riser 10 may be provided as a free-hanging catenary or in alternative configurations using buoyancy modules such as lazy wave and lazy S types. Each flexible riser 10 has a first end 20

connected to the floating vessel 100. Figure 1 shows eight flexible risers 10 a-h, arranged in first and second riser bundles of four, connected to the floating vessel 100 at first ends 20 a-h respectively.

The flexible risers 10 a-h each have a second end 30 a-h on the sea bed 500. The second ends 30 a-h of the flexible risers 10 a-h need not be in direct contact with the sea bed 500. It is preferred that the second ends 30 a-h of the flexible risers are adapted to be secured to the sea bed 500. In the embodiment shown in Figure 1, the second ends 30 of the flexible risers are connected to two riser base manifolds 300. Second ends 30 a-d of the first riser bundle are connected to first riser base manifold 300a, while second ends 30 e-h of the second riser bundle are connected to second riser base manifold 300b. The riser base manifolds 300 are fastened securely to sea bed 500, for instance using fixed piles. In this way, the first and second ends 20, 30 of the flexible risers 10 are secured to the floating vessel 100 and sea bed 500 respectively.

The riser base manifolds 300 provide a fluid

connection between the flexible risers 10 and at least one riser fluid transfer stream 210. The at least one riser fluid transfer stream conveys the riser fluid between the riser base manifold 300 and the riser well head 200. Figure 1 shows first riser base manifold 300a connected to four well heads 200 a-d via an optional well head manifold 220a. Four riser fluid transfer streams 210 a-d connect well heads 200 a-d to the well head manifold 220a. Two further riser fluid transfer streams 210 I, j connect the well head manifold 220a to the riser base manifold 300a. Similarly, second riser base manifold 300 is connected to four well heads 200 e-h via an optional well head manifold 220b. Four riser fluid transfer streams 210 e-h connect well heads 200 e-h to the well head manifold 220b. Two further riser fluid transfer streams 210 1, m connect the well head manifold 220 b to the riser base manifold 300b. The well heads 200 are in fluid communication with the at least one riser fluid reservoir 250 which lie beneath the sea bed 500.

In this way, riser fluid such as a hydrocarbon fluid can be conveyed from the at least one hydrocarbon reservoir 250 to the floating vessel 100. Similarly, a riser fluid comprising carbon dioxide can be conveyed from the floating vessel 100 to the at least one riser fluid reservoir for carbon sequestration.

The mooring lines 610 are intended to maintain the mooring point of the floating vessel 100 in a fixed position. However, the mooring lines 610, which may be steel chains, allow a degree of movement such that the mooring point of the floating vessel 100 can move in response to wave motion, such as the heave and/or pitch of the floating vessel 100.

The person skilled in the art will understand that the present invention can be carried out in many various ways without departing from the scope of the appended claims. For instance, it will be apparent that the flexible pipe and method disclosed herein will be applicable to the manufacture and protection of flexible flowlines as well as flexible risers.

Claims

C L A I M S

1. A method of protecting a flexible pipe (10) from

corrosion, comprising at least the steps of:

(a) providing a flexible pipe (10) comprising:

- an outer sheath (11);

- an inner sheath (12) located within the outer sheath (11) ;

- an annular volume (16) bounded by the outer sheath (11) and the inner sheath (12), said annular volume (16) comprising at least one metallic reinforcing layer (13, 15) and a dry annular void;

- a pipe fluid passage (18), located within the inner sheath (12);

- a first end (20), said first end having a first end-fitting (40), said first end-fitting (40) comprising a first port (51) in fluid communication with the pipe fluid passage (18), and at least one protective liquid injection port (48) in fluid communication with the annular void;

- a second end (30), said second end (30) having a second end-fitting, said second end-fitting

comprising a second port in fluid communication with the pipe fluid passage (18) and at least one fluid release valve in fluid communication with the dry annular void; and

(b) passing a protective liquid to the at least one

protective liquid injection port (48) of the first end-fitting (40), thereby filling the dry annular void within the annular volume (16) with protective liquid to provide a protected flexible pipe.

2. The method according to claim 1 wherein steps (a) and (b) are carried out prior to submerging the protected flexible pipe in water.

3. The method according to claim 1 or claim 2 wherein steps (a) and (b) are carried out on-shore.

4. The method of any of claims 1 to 3 wherein the

flexible pipe of step (a) and the protected flexible pipe of step (b) are provided with one or both of, preferably both of, the first end-fitting (40) and second end-fitting unconnected to other structures.

5. The method according to any of the preceding claims wherein the dry annular void in step (a) is provided by the step of :

- filling the annular void with dry gas; or

- maintaining the annular void at a pressure less than ambient pressure.

6. The method according to any of the preceding claims wherein the flexible pipe (10) is a flexible riser (10), further comprising the steps of:

- connecting the first end-fitting (40) of the flexible riser (10) to a floating vessel (100), after carrying out step (b) ;

- connecting the second end-fitting of the flexible riser (10) to a riser base manifold (300) on the sea bed (500) after carrying out step (b) .

7. A flexible pipe (10) for use in a sub sea environment comprising at least:

- an outer sheath (11);

- an inner sheath (12) located within the outer sheath (11) ;

- a pipe fluid passage (18), located within the inner sheath (12);

- a first end (30), said first end having a first end-fitting (40), said first end-fitting comprising a first port (51) in fluid communication with the pipe fluid passage (18), and at least one

protective liquid injection port (48) in fluid communication with the dry annular void;

- a second end, said second end having a second end-fitting, said second end-fitting comprising a second port in fluid communication with the pipe fluid passage (18) and at least one fluid release valve in fluid communication with the dry annular void .

8. The flexible pipe (10) according to claim 7 further comprising :

- a carcass layer (17) located between the inner sheath (16) and the pipe fluid passage (18) .

9. The flexible pipe (10) according to claim 7 or claim 8 wherein the at least one metallic reinforcing layer (13, 15) is selected from the group comprising:

- at least one metallic tensile armour layer (13); and - at least one metallic pressure armour layer (15) .

10. The flexible pipe (10) according to any one of claims 7 to 9 wherein the annular volume (16) further comprises at least one layer selected from the group comprising :

- at least one insulating layer; and

- at least one anti-wear layer (14) .

11. The flexible pipe (10) according to any one of claims 7 to 10, further comprising a protective liquid, said protective liquid filling the dry annular void.

12. The flexible pipe (10) according to claim 11 wherein the protective liquid comprises a glycol, preferably monoethylene glycol.

13. The flexible pipe according to claim 11 or claim 12 wherein the protective liquid comprises a corrosion inhibitor .

14. A method of adapting a flexible pipe to provide a

protected flexible pipe comprising at least the steps of:

- providing a flexible pipe comprising at least:

- an outer sheath (11);

- an inner sheath (12) located within the outer sheath (11) ;

- a pipe fluid passage (18), located within the inner sheath (12);

- a first end (20), said first end having a first end-fitting (40), said first end-fitting (40) comprising a first port (51) in fluid communication with the pipe fluid passage (18), and at least one gas release valve in fluid communication with the annular void;

comprising a second port in fluid communication with the pipe fluid passage (18); - replacing the at least one gas release valve of the first end-fitting with at least one protective liquid injection port (48);

- filling the dry annular void of the flexible pipe with a protective liquid from the at least one protective fluid injection port (48) to provide a protected flexible pipe.