Pipe in pipe section

Abstract

A pipe-in-pipe section 2 of a pipeline (4) for subsea transport of hydrocarbons, the section comprising: an outer pipe section 10; an inner pipe section 12 disposed within the outer pipe section; and

Description

In oil and gas-producing areas throughout the world, there are pipelines required to carry what are termed ‘aggressive’ or ‘dirty’ hydrocarbons flows. These hydrocarbons contain compounds that are typically very corrosive to traditional steel pipelines, such as hydrogen sulphide, and carbon dioxide. This problem can be addressed in two main ways. The first option is to use a high-grade corrosion resistant material for pipeline construction. However, such materials are very expensive, up to ten times the cost of steel. The cost premium means that this option is increasingly not preferred for any significant length of pipeline is involved. A second option is to use a less expensive corrosion resistant liner, typically a plastic liner. Thus, there are known plastic-lined-pipes (PLPs). As the liner fitting is never absolute, there is a micro-annulus between the liner and the inner surface of the outer steel pipe. However, such plastic materials are permeable to the corrosive compounds. The plastic material allows gas molecules to permeate out of the fluid stream and into the microannulus. Such gas becomes pressurised over time, and in the event that fluid pressure in the pipeline is released, as inevitably happens from time to time for operational reasons, the gas pressure in the vent-annulus can cause the liner to collapse. In such circumstances, it is probable that the liner will not re-inflate without damage. Possible solutions include strengthening the liner so that it is less susceptible to collapse, or making the liner impermeable to gases by including a thin metallic layer within the plastics liner. Both these solutions however are likely to require the production of special liner materials and / or the use of special installation techniques. WO2023 / 002101 discloses controlling the collapse of the liner so that the liner is not damaged when it collapses, and the liner can recover its initial shape when the pipe is pressurized again. But the control is complex and not reliable enough, especially when the pipe is subject to many cycles of depressurization I re-pressurization. It is also known to prevent gas build up inside the micro-annulus by evacuating and venting such gases outside the micro-annulus. Conference paper OTC19937 (Plastic Liners for Hydrocarbon Transport: A Qualified and Cost-Efficient Alternative to CRAs - S.J. Hall, D.J. Hill, P. Dang - Offshore Technology Conference, Houston, May 2009) discloses a plastic lined pipe where the liner comprises longitudinal external grooves which are used to vent the micro-annulus and evacuate the gases toward the pipe end terminations. But this arrangement is complex and there is a risk that polymer creep will cause closure of the grooves over the service life of the pipe. WO00 / 08368 shows an apparatus for use in venting plastics lined pipeline comprising a vent assembly for through fitment in a pipeline wall and having means for resisting deformation of the plastics lining into the vent assembly. Gas build up is vented radially through the outer steel pipe wall.to the ambient environment. A gas relief valve allows gas circulation from the micro-annulus to the sea water and prevents sea water from entering inside the microannulus. But this is not suitable for deeper water applications due to venting occurring only when the pressure inside the micro-annulus becomes larger than the hydrostatic pressure. WO2004 / 011840 shows a pipe liner connector suitable for use with pipe sections having an internal liner, the pipe liner connector comprising a substantially cylindrical sleeve having opposed open ends for sealed attachment to the internal liner of a pipe section, and one or more vents for balancing a pressure differential between a micro-annulus, formed between the internal liner and the pipe sections, and a bore defined by the connected pipe sections. WO2023 / 041917 discloses a variant for balancing the pressure differential between the micro-annulus and the bore, wherein the venting means are integrated inside the liner itself instead of the connector. These designs are not suitable when hydrocarbons are very corrosive, as such hydrocarbons can circulate from the bore to the micro-annulus through the venting means and initiate local corrosion of the outer steel pipe. The object of the invention is to overcome these drawbacks and provide an improvement in this field of technology. Summary According to a first aspect of the present invention there is provided a pipe-in-pipe section for use in a pipe-in-pipe pipeline for subsea transport of hydrocarbons, the pipe-in-pipe section comprising: an outer pipe section; an inner pipe section disposed within the outer pipe section; a vent-annulus defined between the outer pipe section and the inner pipe section; a polymer liner disposed within the inner pipe section; a micro-annulus defined between the polymer liner and the inner pipe section; and a plurality of vents configured to allow a fluid to flow from the micro-annulus into the vent-annulus. According to a second aspect of the present invention there is provided a method of manufacturing a pipe-in-pipe section for use in a pipe-in-pipe pipeline for subsea transport of hydrocarbons, the method comprising at least the steps of in any order: providing an outer pipe section; providing an inner pipe section disposed within the outer pipe section to create a vent-annulus defined between the outer pipe section and the inner pipe section; providing a polymer liner disposed within the inner pipe section; providing a micro-annulus defined between the polymer liner and the inner pipe section; and providing a plurality of vents configured to allow a fluid to flow from the micro-annulus into the vent-annulus. According to a third aspect of the present invention there is provided a method of venting a pipe-in-pipe pipeline for subsea transport of hydrocarbons comprising a pipe-in-pipe section as defined herein, comprising the step of venting a fluid from the micro-annulus into the ventannulus defined between the outer pipe section and the inner pipe section. According to a fourth aspect of the present invention there is provided a pipe-in-pipe pipeline for subsea transport of hydrocarbons, the pipe-in-pipe pipeline comprising a pipeline comprising a plurality of the pipe-in-pipe sections as defined herein, wherein the plurality of pipe-in-pipe sections are connected to provide a continuous vent-annulus between the plurality of inner pipe sections and the plurality of outer pipe sections. Brief description of the drawings The invention will be explained more fully below in connection with a preferred embodiment and with reference to the drawings in which: Figure 1 is a longitudinal cross-sectional view along a pipe-in-pipe section according to one embodiment of the present invention; Figure 1a is a portion of Figure 1 showing a fluid flow path; Figure 2 is a longitudinal cross-sectional view of a pipe-in-pipe section according to another embodiment of the present invention; Figures 3 and 4 are a longitudinal cross-sectional view and a radial cross-sectional view respectively of a pipe-in-pipe section according to another embodiment of the present invention; Figures 5 and 6 are schematic perspective views of two inner pipe sections for use in a pipe-in-pipe section according to further embodiments of the present invention; Figures 7 and 8 are different longitudinal cross-sectional views of the ends of a two different pipe-in-pipe sections and a bulkhead, according to further embodiments of the present invention; Figure 9 is a longitudinal cross-sectional view of a pipe-in-pipe pipeline comprising two pipein pipe sections connected together according to another embodiment of the present invention; Figure 10 is a schematic view of a pipe-in-pipe pipeline end and PLET according to another embodiment of the present invention; and Figure 11 is a longitudinal cross-sectional view of the end of a pipe-in-pipe pipeline connected to a single pipe according to another embodiment of the present invention. Detailed description Plastic Lined Pipes (PLPs), wherein an outer pipe generally formed of carbon steel forms a ‘host’ pipe and is lined with a polymer for corrosion protection, are well known in the art. PLPs are conventionally used for the transportation of water and other ‘produced fluids’ for water injection applications. Recently, PLPs have been considered for the transportation of hydrocarbons, as PLPs present a highly cost attractive solution relative to the traditional use of corrosion resistant alloys as liner in steel host pipelines. Such PLPs must rely on the use of available polymers that can provide some corrosion resistance to the corrosive compounds in the hydrocarbon flow. However, even then, such PLPs are still permeable to certain compounds, especially at high pressure. As the outer host pipe in a PLP is generally impermeable, the plastic liner allows the smaller gas molecules to permeate out of the fluid stream and into the microannulus between the liner and the host pipe, which becomes pressurised over time. In the event that fluid pressure in the pipeline is released or sufficiently lowered, this could result in collapse of the liner, for example if the pipeline is to be depressurised or otherwise drained during shutdown. Re-pressurisation of the pipeline does not lead to a perfect re-positioning of a now deformed liner. One possible solution to release any pressure build-up in the micro-annulus is to perforate the plastic liner. However, such perforations result in a small but direct contact of the sour grade steel typically used for the outer or host pipe, and the hydrocarbon flow, which may contain carbon dioxide and hydrogen sulfide. This can lead to corrosion, and the risk of carbon dioxide induced stress corrosion cracking, or hydrogen induced cracking (HIC). Carbon steel surfaces, including weld materials, can also be damaged by extended exposure to hydrogen sulphide, which will lead to sulphide stress cracking (SSC), and local corrosion. Furthermore, hydrogen sulphide is highly soluble in water, so that any water vapour present may also pose an issue. Thus, conventionally, there are still considered limitations placed on the allowable levels of certain compounds such as carbon dioxide and hydrogen sulphide, in a hydrocarbon flow that can be transported by a PLP. According to one aspect of the present invention, there is provided a pipe-in-pipe section for use in a pipe-in-pipe pipeline for subsea transport of hydrocarbons. The pipe-in-pipe section comprises an outer pipe section, an inner pipe section disposed within the outer pipe section, and a vent-annulus defined between the outer pipe section and the inner pipe section. The pipe-in-pipe section further comprises a polymer liner disposed within the inner pipe section, a micro-annulus defined between the polymer liner and the inner pipe section, and a plurality of vents configured to allow a fluid to flow from the micro-annulus into the vent-annulus. The term “section” as used herein relates to any suitable length for subsequently joining together with other sections to form a pipe-in-pipe (PIP) pipeline. A section may be a single pipe joint typically being 12m or 24m long, or a longer length. The present invention is not limited to the length of the or each section, and the term “section” may include a number of pipe joints already joined together. The term pipe-in-pipe pipeline for subsea transport of hydrocarbons generally includes any arrangement comprising a flow line, sometimes termed an inner flow line, within another pipe, typically termed an outer pipe. Such pipeline are commonly abbreviated to “PIP” pipelines. The forming of PIP pipelines is well known in the art, and one example is shown in US 2012 / 0138183A for reference purposes only. The outer pipe section of the present invention may be formed of any suitable material, and may have any suitable thickness or dimension, typically intended to achieve a certain degree of flexibility in the formed pipeline. The flexibility can include pipeline-reelability, to assist with reel-laying. The outer pipe section is typically formed of steel, in particular carbon steel. The inner pipe section of the present invention may be formed of any suitable material, and may have any suitable thickness or dimension, typically intended to achieve a certain degree of flexibility in the formed pipeline. The flexibility can include pipeline-reelability, to assist with reel-laying. The inner pipe section is typically formed of steel, in particular carbon steel. The polymer liner useable with the present invention may be formed of any suitable material, generally being a plastics material. Suitable materials include PE (polyethylene) such as PE100, PERT (polyethylene raised temperature), high-density polyethylene (HDPE), PEDF polyamide, polypropylene, polyvinylidene fluoride (PVDF), and other polyolefin grades. Optionally, the polymer liner is high-density polyethylene. The polymer liner may comprise one or more layers. The one or more layers may be of different materials, including two or more types of plastics or polymer. Optionally, the polymer liner includes reinforced fibres. The reinforced fibres may be part of or integrated with a plastics layer, or form one of more layers in the polymer liner. The reinforced fibres could be aramid fibres, aramid copolymer fibres, glass fibres, carbon fibres, polyester fibres, PBO fibres, crystalline PE fibres, or a combination of different fibres. Optionally, the polymer liner comprises one or more plastics layers and one or more reinforced fibre layers. According to one embodiment of the present invention, the polymer liner may comprise a polymer inner layer, a high strength fiber reinforcement layer, for example woven aramid fibers, and a polymer outer sheath. The polymer liner may be of any thickness from 1 mm to 50mm or greater, including for example in the ranges 8mm-30mm or 12mm-25mmm or 17mm-20mm. The polymer liner is optionally extruded, or formed in layers in a manner known in the art. The polymer liner may be located or disposed within the inner pipe section so as to be wholly or substantially pressed against the inside face or surface or inner circumference of the inner pipe section. The polymer liner may be located or disposed within the inner pipe section to be generally fitted therewith, generally by compression or interference fit. Optionally, the external diameter of the liner is ‘oversized’ relative to the inner or internal diameter of the inner pipe section by at least 1%, optionally higher. The degree of tightness between the liner and the inner pipe section can assist avoiding any slippage during installation and use. Optionally, a number of inner pipe sections are joined together, and a continuous polymer liner is inserted into the connected inner pipe sections so as to form a continuous inner liner along such joined sections. As discussed herein, the fit between the polymer liner and the inner pipe section may be tight or close but there is typically no physical bond thereinbetween. As such, a small microannulus therefore exists between the polymer liner and the inner pipe section. In the present invention, a plurality of vents are configured to allow a fluid to flow from the micro-annulus into the vent-annulus. In this way, any pressure build-up in the micro-annulus is relieved by passage of fluid causing the pressure build-up into the vent-annulus. The fluid flowing from the micro-annulus into the vent-annulus is usually a gaseous fluid or a gas. For example, the fluid flowing from the micro-annulus into the vent-annulus can include one or both of hydrogen sulphide and carbon dioxide. In one embodiment of the present invention, at least some of the plurality of vents comprise through-holes extending through the inner pipe section. Optionally, some or all of the plurality of vents comprise through-holes extending through the inner pipe section. The through-holes may be simply formed through the inner pipe section after manufacture of each inner pipe section, or after the joining of a number of inner pipe sections to form a longer pipe section, and optionally prior to disposal of a polymer liner within the or each inner pipe section. Optionally, through-holes forming one or more of the plurality of vents may be lined. In another embodiment of the present invention, the through-holes have a diameter in the range 2mm and 15mm, inclusive, and including a diameter of 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm, 11mm, 12mm, 13mm and 14mm. In another embodiment of the present invention, the through-holes have a polygonal shape, (such as square, rectangle etc. 2-10mm in length), or have a slot shape (rectangle with rounded ends). Optionally, controlling any creeping of the liner inside the hole or the valves can be mitigated through counter boring the hole / slot with the smaller diameter hole (2-4mm) mouth opening on the bore side. Further, the use of polymer, sintered material or a metallic plug below a vent can be used to prevent creep of the liner which may otherwise block the vent from operation. According to another embodiment of the present invention, the plurality of vents are disposed at two or more axial positions along the longitudinal axis of the inner pipe section. The present invention is not limited by the regularity or non-regularity of the positions of the plurality of vents along the longitudinal axis of the inner pipe section. Optionally, each axial position is spaced from a next or nearest axial position of between 2m to 50m inclusive. One such possible spacing is 3m or 4m or 5m or 10m or 12m or 18m or 24m or 36m or 48m but the present invention is not limited by such spacing being regular or irregular along the longitudinal axis of the inner pipe section. Optionally, the plurality of vents are disposed at two or more different angular positions around the circumference of the inner pipe section. The present invention is not limited by the regularity or non-regularity of the angular positions of the plurality of vents around the circumference of the inner pipe section. Optionally, the pipe-in-pipe section of the present invention has a first vent provided at a first axial position (along the length of the section), and a first angular position (around the circumference of the section), and a next nearest vent provided at a second axial position and a second angular position, wherein the first axial position and the second axial position are different and the first angular position and the second angular positions are different. The skilled person can see that the present invention allows any suitable pattern for the plurality of vents to be arranged or configured to allow a fluid to flow from the micro-annulus into the annulus. Where the plurality of vents comprises through-holes extending through the inner pipe section, or a series of inner pipe sections conjoined, the skilled user can see that the plurality of vents can have any suitable pattern because of the expected wholly or substantially continuation of the micro-annulus between the polymer liner and the inner pipe section or the inner pipe sections conjoined. Optionally, the present invention comprises a plurality of vents provided at a plurality of first defined axial positions along the length of a section, and defined angular positions around the circumference of the section, with a second or further series of vents provided at second or further axial positions and second angular positions, wherein said first and second or further axial and angular positions are different. In one embodiment of the present invention, the plurality of vents are arranged along a pipein-pipe section or section in the form or pattern of a helix extending along the outer surface of the inner pipe section. Optionally, the helix comprises at least two turns. Optionally, each turn of the helix comprises a set of two or more vents distributed at two or more different angular positions and two or more different axial positions. Optionally, each turn of the helix has the same number of vents at the same angular positions. Optionally, at least some of the axial positions of the plurality of vents comprise a set of two or more vents, and each of the two or more vents are disposed at different angular positions around the inner pipe section. Optionally, each axial position comprises the same number of vents at the same angular positions. In one embodiment of the present invention, the pipe-in-pipe section further comprises a tube disposed within the vent-annulus, wherein the tube is in communication with the plurality of vents such that fluid from the micro-annulus can flow into the tube. Optionally, the tube extends along the outer surface of the inner pipe section and within the space between the outer pipe section and the inner pipe section defined as the vent-annulus. Optionally, such a tube comprises a main tube body extending along the outer surface of the inner pipe section and a plurality of vent branches, wherein each branch extends from the main tube body to a vent, to provide a flow path from the vent to the main tube body. Optionally, the vent is a through-hole extending through the inner pipe section, and each branch is received within one of the through-holes. In another embodiment of the present invention, at least some of the plurality of vents comprise a one-way valve. Various one-way valves are known in the art, and typically comprise a moveable biased poppet or plug within a housing. Typically, the biasing is by means of a spring, often a helical spring, arranged within the housing to allow movement of the poppet to open and close, typically to open upon a pre-determined or threshold pressure acting on a part of the one-way valve. One-way valves are available from manufacturers such as Tamo Limited and Hoke, and are configured to prevent the flow of fluid returning from the vent-annulus into the micro-annulus. Optionally, the pipe-in-pipe section of the present invention further comprises an insulator layer disposed within the vent-annulus, and configured to cover at least part of the outer surface of the inner pipe section. Optionally, the insulator layer comprise an insulator material is permeable to the fluid flowing through the plurality of vents. Optionally, the insulator layer covers at least part of the outer surface of the inner pipe section, but not all the outer surface. For example, there may be spaces or spacing in or along the insulator layer, allowing tight but imperfect fitment of the insulator layer around the outer surface of the inner pipe section to allow fluid still to travel from the plurality of vents to the vent-annulus. Optionally, the insulator material comprises an inert material, being inert to the corrosive substances or compounds forming part of the fluid to be vented along the vent-annulus. Optionally, the insulator layer comprise an insulator material constructed from one or more materials, including those comprising the group being: nano-porous such as silica aerogel, micro-porous, such as compacted powder or fibres, or mineral wool. Optionally, the insulator layer is formed by one or more pouches or bags or other outer layers comprising or housing an insulator material. Optionally, such pouches or bags are formed of inert material as defined herein. Optionally, the insulation layer is made in layers of varying thicknesses (typically 5mm per layer) and packaged or bagged. If there are any cables to be provided with the ventannulus, the cables can be provided and installed prior to adding any insulation, and the insulation design may be adjusted to ensure proper fitment accounting for cable thicknesses in the underlying layers. Where the insulator layer is formed by one or more pouches or bags comprising insulating material, such pouches or bags may be wrapped and secured around the outer surface of the inner pipe section but not bonded thereto, so as to be formed as a tight but imperfect fitment, to maintain permeability of the insulating layer to the fluid flowing through the plurality of vents. Optionally, the insulator layer of the present invention may be arranged or adapted to accommodate any cables, such as electrical, signal cables or trace-heating cables, to be disposed or otherwise installed on an outer surface of the inner pipe section, prior to disposal of the insulating layer therearound or thereover. Optionally, the pipe-in-pipe section of the present invention further comprises a plurality of spacers or centralisers disposed within the vent-annulus. Spacers can also be termed centralisers, and are typically constructed from a range of polyamide-based polymers or other materials or combination of materials, such as nylon, polymers, wood and metal. Spacers can be formed in any suitable shape, size or design, generally comprising a number of sections, such as for example two half-shells, which can be disposed or installed around an inner pipe section prior to location of an inner pipe section within an outer pipe section. The outer diameter of a spacer is typically smaller than the inner diameter of the outer pipe section. Centralisers and spacers are known in the art, and can be designed or adjusted to allow passage of cables and tubes along the surface of the inner pipe section, and to accommodate any insulation or insulator layer. Spacers or centralisers are typically installed periodically along the length of a pipe section, section or pipeline, and may also assist insertion of the inner pipe section into the outer pipe section, and help support the inner pipe section within the outer pipe section in use or service. Spacers or centralisers can also prevent damage, such as during pipe reeling, to any components installed along the outside of the inner pipe section. Optionally, the or each spacer is configured such that the annulus has a continuous cavity through which the fluid can flow unimpeded. In one embodiment of the present invention, spacers (once formed on the inner pipe section) can have a diameter less than the internal diameter of the outer pipe section. In this way, there is provided a continuous vent-annulus defined between the outer pipe section and the inner pipe section. Alternatively or additionally, one or more spacers are not solid, either by design or pattern, and / or by the addition of longitudinal apertures or holes therethrough, to again provide a continuum of the vent-annulus between the outer pipe section and the inner pipe section. Optionally, the pipe-in-pipe section of the present invention include one or more acid gas getters within the vent-annulus, such as hydrogen and / or hydrogen sulfide and / or carbon dioxide getters. A suitable hydrogen getter comprises zinc oxide (ZnO) and is embedded in polymer components located within the vent annulus, for example as part of the spacer design or centraliser materials, such as polyethylene doped with ZnO. Optionally, the minimum extent or thickness of the vent-annulus is at least 5mm, such as 6mm or more. According to a second embodiment of the present invention there is provided a method of manufacturing a pipe-in-pipe section for use in a pipe-in-pipe pipeline for subsea transport of hydrocarbons, the method comprising at least the steps of in any order: providing an outer pipe section; providing an inner pipe section disposed within the outer pipe section to create a vent-annulus defined between the outer pipe section and the inner pipe section; providing a polymer liner disposed within the inner pipe section; providing a micro-annulus defined between the polymer liner and the inner pipe section; and providing a plurality of vents configured to allow a fluid to flow from the micro-annulus into the vent-annulus. In one embodiment of the present invention, the method of manufacturing a pipe-in-pipe section is to manufacture a pipe-in-pipe section as defined herein. The nature of the outer pipe section, polymer liner are defined herein, and the steps of disposing an inner pipe section within an outer pipe section to create an annulus therein between, is well known in the art and not further described herein. Such annulus that is formed provides the vent-annulus of the pipe-in-pipe section as defined herein. The provision of a micro-annulus defined between the polymer liner and the inner pipe section is as discussed herein above, and is a result of the fitting of a polymer liner within a pipe section or section. Such fitting is typically imperfect, so as to leave a residual gap, which may be regular or irregular, between the polymer liner and the inner pipe section. Providing a plurality of vents configured to allow a fluid to flow from the micro-annulus into the vent-annulus can be carried out using one or more of the methods as defined herein, such as drilling a series of through-holes through the inner pipe section, optionally prior to providing a polymer liner disposed within the inner pipe section. Optionally, one or more through holes includes a one-way vent. The skilled reader can see that one or more of the above steps can be provided in a nonsequential manner as listed, and that the present invention is not limited to the sequential listing of the steps as defined above. According to another aspect of the present invention, there is provided a method of venting a pipe-in-pipe pipeline for subsea transport of hydrocarbons comprising a pipe-in-pipe section as defined herein, comprising the step of venting a fluid from the micro-annulus into the ventannulus defined between the outer pipe section and the inner pipe section. In some uses or arrangements of the pipe-in-pipe section in a pipe-in-pipe pipeline for subsea transport of hydrocarbons, the conditions of use of the PIP pipeline may be such as to create a natural flow or circulation or convection along the vent-annulus towards a suitable end or collection point as discussed herein after. Optionally, the method further comprises the step of applying an over-pressure or of applying and under-pressure along the vent-annulus. Such a change in pressure assist to increase the passage of fluid being vented along the vent-annulus. An over-pressure along the vent-annulus can be applied using a suitable pressure means to achieve an active venting action, such as a pump, either at one end, or along the length of the vent-annulus, or a combination of same, to create a relatively positive gaseous flow. Optionally, nitrogen, or any suitable inert gas, could be used as a carrier medium by pumping from one end of the pipeline, with discharge from an end, (optionally to topside) through the aid of umbilical tubing or a PIP riser as described herein. In this method, the ends of the pipeline can be terminated with bulkheads containing inlet / outlet ports (opening into the PIP annulus also as described herein) connected with associated hoses and / or tubing. This solution could also be coupled with a vacuum pump to ensure annulus pressure remains at or below 1 bar. In a similar manner, an under-pressure can be applied along the vent-annulus by the application of a suitable vacuum means, such as a vacuum pump, either at one end of the vent-annulus, or along the length of the vent-annulus or a combination of same, to create a relatively negative pressure along the vent-annulus. A passive venting solution could be provided using one-way valves, either along the ventannulus, or seated in the web of a bulkhead at a connected riser base, or both. Once above a pre-determined pressure (e.g. >1.0bar) exists, the gas pressure would overcome the valve closing force in a one-way valve, and such gas would migrate to the other side of a valve or bulkhead having such a valve. The present invention is not limited by disposal of the fluid passing along or through the vent-annulus. Vented fluid may be collected by suitable fluid equipment, or vented, either to atmosphere, or using further venting equipment, typically arranged either topside of the subsea environment, or through a suitable apparatus or unit such in a pipeline end termination (PLET). According to one embodiment of the present invention, there is provided a conduit connected to the vent-annulus, when the conduit is configured to transport fluid in the ventannulus to a venting pipeline directed towards or approximate to the surface of the sea. Such conduit may be an umbilical, such as a riser-annulus of a pipe-in-pipe riser system, wherein the pipe-in-pipe riser system comprises an outer casing, an inner casing and tubing arranged in the riser-annulus formed between the outer casing and the inner casing. Optionally, the conduit is connected to the vent-annulus via a bulkhead, wherein the bulkhead has a pipe-in-pipe configuration with an inside pipe, outside pipe, and a bulkhead annulus disposed therein, said bulkhead annulus optionally comprising a one-way valve to control the flow of fluid from the vent-annulus to the conduit. Optionally, any such conduit could include a vacuum to ensure flow of the vented gases to topside. According to another embodiment of the present invention, there is provided a pipe-in-pipe (PIP) pipeline a pipe-in-pipe pipeline for subsea transport of hydrocarbons, the pipe-in-pipe pipeline comprising a plurality of the pipe-in-pipe sections as defined herein, wherein the plurality of pipe-in-pipe sections are connected to provide a continuous vent-annulus between the plurality of inner pipe sections and the plurality of outer pipe sections. Optionally, such a pipe-in-pipe pipeline further comprises a conduit connected to the ventannulus, wherein the conduit is configured to transport the fluid in the continuous ventannulus to a venting pipeline proximate to the surface of the sea. Optionally, such a conduit is a riser vent-annulus of a pipe-in-pipe riser system, wherein the pipe-in-pipe riser system comprises an outer casing, an inner casing and tubing arranged in the riser vent-annulus formed between the outer casing and the inner casing. Optionally, in a pipe-in-pipe pipeline according to the present invention the continuous ventannulus is connected to a vacuum such that the fluid actively flows from the micro-annulus to the vent-annulus due to a pressure differential. Joining the ends of outer sections and inner sections of two PIP pipelines is well known in the art, and typically involves an assembly of one or more interconnection pieces, typically a bulkhead, or two end bulkheads, connected together, optionally also using one or more further intermediate shell connector plates or pieces. Joining together the ends of PLP is also well known in the art, and typically involves an assembly of one or more interconnection pieces, such as liner connections or rings, optionally with a sleeve thereinbetween to create the continuum of the hydrocarbon flow along the combined sections. Thus, according to another embodiment of the present invention, there is provided a pipe-in-pipe pipeline having at least two of the pipe-in-pipe sections as defined herein which are connected using liner connectors and one or more pipe-in-pipe bulkheads, wherein said one or more bulkheads are able to provide a continuous vent-annulus between the pipe-in-pipe sections. The or each bulkhead could include insulation as described herein. The said one or more bulkheads may be able to provide the continuous vent-annulus between the pipe-in-pipe sections by having one or more apertures therethrough. Such apertures may comprise a one-way valve to maintain the flow of the fluid from one pipe-in-pipe section to another pipe-in-pipe section, or to provide the space for a vent-tube as described herein to continue from one pipe-in-pipe section to another pipe-in-pipe section. Optionally, the connection from one pipe-in-pipe section to another pipe-in-pipe section described herein further includes a clad overlay between the liner connectors. A clad overlay assists the continuum of the purpose of the polymer liner, in providing protection for the bulkhead inner surface as the pipeline content flows from one pipe-in-pipe section to another pipe-in-pipe section, and to minimise any gas in the pipeline content to escape at the connection. Optionally, the clad overlay overlaps with the polymer liner of each pipe-in-pipe section. According to another embodiment of the present invention, the pipe-in-pipe pipeline as defined herein having at least two of the pipe-in-pipe sections further includes a sleeve such as a pigging sleeve, between the liner connectors. Sleeves and pigging sleeves are well known in the art, and typically comprise a bore to extend across a joint between the ends of pipe sections, typically to assist the passage of a fluid or a pig therethrough. A pigging sleeve could be lined, for example with a polymer liner being the same or different to the polymer liners described herein. Optionally, the sleeve provides a smooth continuum of the bore of the polymer liner between the polymer liner of each pipe-in-pipe section. According to another embodiment of the present invention, the pipe-in-pipe pipeline as defined herein has at one end a single pipe connection, and said single pipe connection is configured to allow a fluid to flow from the vent-annulus. Such configuration may comprise one or more suitable apertures able to allow fluid flow therethrough. Turning to the drawings, Figure 1 shows a pipe-in-pipe section 2 having a longitudinal axis 3, and comprising an outer pipe section 10, an inner pipe section 12 disposed within the outer pipe section 10, a vent-annulus 14 defined between the outer pipe section 10 and the inner pipe section 12, and a polymer liner 16 disposed within the inner pipe section 12. A number of the pipe-in-pipe sections 2 of Figure 1 can be conjoined to form a pipe-in-pipe (PIP) pipeline for subsea transport of hydrocarbons. The conjoining of the pipe-in-pipe sections 2 may be carried out in a known manner, typically butt-welding the inner pipe sections 12 and the outer pipe sections 10 together, either directly, or using one or more bulkheads as described hereinafter. The polymer liner 16 can be disposed within the inner pipe section 12 in a manner known in the art. One method is to join a member of inner pipe sections 12 together to form a longer length, (either per se, or as part of the sequence of forming a pipe-in-pipe pipeline), and to extend a polymer liner 16 therewithin. As exact contact between all of the polymer liner 16 and the inner circumferential surface of the inner pipe sections 12 is rarely absolute, a microannulus 18 becomes defined between the polymer liner 16 and the inner pipe section 12 after fitting, (and / or becomes defined over time, as gases from the hydrocarbon flow leak or penetrate the polymer). As discussed hereinbefore, hydrocarbon ‘flows’, or fluids that flow through a pipe-in-pipe pipeline, can contain one or more compounds, in particular one or more gases, which are corrosive. Particular examples of corrosive gases in hydrocarbons are carbon dioxide and hydrogen sulphide. Such gases can permeate through the polymer liner 16 ,and so collect and build-up within the micro-annulus 18 as the inner pipe sections 12 are impermeable, (generally being formed from a grade of steel). Such build-up of gases in the micro-annulus 18 can become pressured over time, such that in the event that fluid pressure in the pipeline is released or sufficiently lowered, this could result in collapse of the polymer liner 16 inwardly, for example if the pipe-in-pipe pipeline is drained during shut down. Repressurisation may not lead to the polymer liner 16 returning to exactly how it was before collapse. Collapse of the polymer liner 16 can therefore be a catastrophic failure of the pipein-pipe pipeline This situation is more critical if there are repeated de-pressurisation and repressurisation steps or cycles in the PIP pipeline. The present invention provides apparatus and methods to achieve venting of built-up gases in a micro-annulus of a PIP pipeline in a controlled manner, optionally in a continuous manner, and optionally using controlled apparatus and methods of disposal of such gases. In the present invention, a plurality of vents 20 are configured or provided to allow a fluid, in particular one or more gases leaking or escaping from the main hydrocarbon flow passing through the PIP pipeline, to flow from the micro-annulus 18 into the vent-annulus 14. In the example shown in Figure 1, the plurality of vents 20 are through-holes extending through the inner pipe sections 12. Thus, the plurality of vents 20 allow fluids such as the gases described above that may be collecting in the micro-annulus 18, to flow into the vent-annulus 14, for subsequent disposal in a manner described herein. The present invention minimises or avoids possible collapse of the polymer liner 16 should there be pressure reduction of the fluid flowing through the PIP pipeline 14, and therefore avoids catastrophic failure of the PIP pipeline. The through-holes can be created by drilling, laser, etc, at any time during the manufacturing process of either an inner pipe section 12 per se, or as part of the overall PIP pipeline assembly process, and optionally prior to installation of the polymer liner 16 within the inner pipe section or sections 12. Figure 1 shows the pipe-in-pipe section 2 further comprising an insulator layer 22 disposed within the annulus 14, and configured to cover at least a part of the outer surface of the inner pipe section 12. The insulator layer 22 may be formed of a suitable material that is directly permeable to the fluid flowing through the vent 20, and / or may be formed into one or more pouches (not shown) comprising insulator material, wherein such pouches are fitted around the inner pipe section 12 to allow the flow of a fluid from the micro-annulus 18 to go therearound. Figure 1 also shows the pipe-in-pipe section 2 further comprising a number of first centralisers or spacers 24 disposed within the vent-annulus 14, and extending circumferentially around the inner pipe section 12 substantially towards the outer pipe section 10. Each spacer 24 may be formed of one or more parts, sections or portions, which together, once fitted, form a suitable shaped collar around the inner pipe section 12 to maintain the distance between the inner pipe section 12 and the outer pipe section 10 in a manner known in the art. Figure 1 shows an example of a spacer 24 having an outer diameter that is less than the inner diameter of the outer pipe section 10, so as to leave a gap thereinbetween. Such gap allows flow of a fluid along the vent-annulus 14. Figure 1a shows a portion of the pipe-in-pipe section 2 of Figure 1, with a series of black ‘arrows’ showing the path of a fluid, usually a gaseous flow, along the micro-annulus 18 towards a vent 20, through the vent 20, and then along the vent annulus 14, in a manner of an embodiment of a method of the present invention. The fluid in the vent annulus 14 can then be vented to a remote and safe location or environment as further discussed hereinafter. Such locations may be above the sea surface, typically termed ‘topside’. Figure 2 shows a similar pipe-in-pipe section 2’ as Figure 1, (comprising an outer pipe section 10, an inner pipe section 12 disposed within the outer pipe section 10, a ventannulus 14, a polymer liner 16, a micro-annulus 18 and vents 20). Figure 2 shows a number of second centralisers or spacers 24’ disposed within the vent-annulus 14, and extending circumferentially around the inner pipe section 12. Such spacers 24’ now include one or more slots or apertures 25 therethrough, so as to additionally or alternatively allow flow of a fluid along the vent-annulus 14. The slots 25 may not extend around the whole circumference of the inner pipe section 12 as shown in more detail in Figure 4 hereinafter. Optionally, there are provided a regular number of spacers 24, 24’ disposed along the length of either the inner pipe section 12, or the combination of a number of inner pipe sections 12 to form a pipe-in-pipe pipeline. Figures 1 and 2 show a series of three spacers 24, 24’. Figure 2 also shows a further variant wherein some of the vents 20 include a one-way valve 28. The one-way valves 28 may comprise an arrangement having a moveable poppet, valve piece or plug, within a housing, and arranged to fit within the vent 20. The one-way valves 28 ensure the one-way flow of fluid from the micro-annulus 18 to the vent- annulus 14, without any return flow. All the vents 20 may comprise one-way valves 28. Figure 3 shows a similar pipe-in-pipe section 12 as Figure 2, (comprising an outer pipe section 10, an inner pipe section 12 disposed within the outer pipe section 10, a ventannulus 14, a polymer liner 16, a micro-annulus 18 and vents 20). Figure 3 also shows a number of the slotted second centralisers or spacers 24’ disposed within the vent-annulus 14, and extending circumferentially around the inner pipe section 12. Figure 3 further shows a vent tube 30 arranged along the outer circumference of the inner pipe section 12, (and therefore within the vent annulus 14). Optionally, the vent tube 30 is configured to be arranged in a pattern over each of the vents 20. Additionally or alternatively, the vent tube 30 comprises a series of branches or branch tubes 32 locatable over or within the vents 20. The vent tube 30 provides a direct pathway for fluid flowing through the vents 20 from within the micro-annulus 18. The vent tube 30 can be arranged to travel through the slots 25 of the second spacers 24’. Figure 4 is a radial cross-sectional view of the embodiments shown in Figure 3 along line AA. Figure 4 shows the second spacer 24’ being formed in two half shells, for ease of location around the inner pipe section 12 during manufacture, and with suitable connections 40 thereinbetween. Figure 4 also shows the slots 25 in the second spacers 24’, either for the vent tube 30, or as a separate channel, or both. The skilled reader can see that the inner pipe section of the present invention can have various arrangements or patterns for the vents along its longitudinal axis, which pattern may be regular or irregular or both. Figure 5 shows a schematic perspective view of a second inner pipe section 12’ useable in the present invention to demonstrate patterns of some embodiments. The second inner pipe section 12’ in Figure 5 has a longitudinal axis 3, an axial origin of a cylindrical coordinate system 21, and an angular origin 23 of a cylindrical coordinate system 23. The inner pipe section 12’ shows a plurality of vents 201,202, 203 arranged at different positions along the longitudinal axis of the inner pipe section 12’. The first vent 201 is arranged at a first distance d1 from the axial origin 21, and at a first angle a1 from the angular origin 23. The second vent 202 is arranged at a second distance d2 from the axial origin 21, and at a second angle a2 from the angular origin 23. The third vent 203 is arranged at a third distance d3 from the axial origin 21, and at a third angle a3 from the angular origin 23. The skilled reader can see that the inner pipe section 12’ can have any arrangement or patterns of vents along the longitudinal axis 3, which pattern may be regular or irregular or both, to allow a fluid to flow from the micro-annulus within the inner pipe section 12’ (not shown in Figure 5) to a vent-annulus outside the inner pipe section 12’ (also not shown in Figure 5). For example, each vent 201, 202, 203 could have an equal and opposite vent along the length of the inner pipe section 12’. Also, in Figure 5, the first distance d1 and the first angle a1 are different to the second distance d2 and the second angle a2, the second distance d2 and the second angle a2 are different to the third distance d3 and the third angle a3, etc. Figure 6 shows an example of a third inner pipe section 12” for use in a pipe-in-pipe section of the present invention, comprising a plurality of vents 20 in a regular circumferential pattern along the third inner pipe section 12”. Figure 6 also shows a vent tube 30 arranged around the outer circumference of the third inner pipe section 12”, and configured to be arranged in a helical pattern over each of the vents 20. Optionally, vent tubes 32 form a series of branches from the vent tube 30, locatable within the vents 20. The vent tube 30 provides a direct pathway or flow for fluid flowing through the vents 20 from within the third inner pipe section 12”. Figure 7 shows an end of the first pipe-in-pipe section 2 shown in Figure 1. Attached to the ends of the inner pipe section 12 and the outer pipe section 10 is a first bulkhead 42, having an inner pipe part 41 and an outer pipe part 43 on each side of a bulkhead web 45. The inner pipe part 41 can be directly welded 52 to the inner pipe section 12. In the example shown in Figure 7, the outer pipe part 43 is welded 53 to an outer pipe sleeve 50, which is then welded 51 to the outer pipe section 10, all in a manner known in the art. The bulkhead 43 includes a slot 46 in the bulkhead web 45 having a one-way valve 47 therein, to maintain and to direct the flow of the fluid in the vent annulus 14 from one pipe-in-pipe section 12’ to the vent annulus 14 in a next pipe-in-pipe section (not shown) connected to the other side of the bulkhead 43. Figure 7 also shows a liner connector 48, known in the art and typically metallic, having a tapered end to engage with the polymer liner 16 and provide a secure connection therewith. A second liner connector (not shown) would engage with the polymer liner of the next pipein-pipe section. Figure 7 also shows a clad overlay 44 positioned between and over the liner connector(s) 48. A clad overlay, or weld overlay, is a type of cladding that uses a welding process to melt a corrosion resistant metallic material onto the surface of another different metallic material. This cladding protects the other different material, generally steel, against corrosion. The clad overlay 44 covers and protects the inner surface of the inner pipe 41 of the bulkhead 42. It also covers and protects the inner surface of the end of the inner pipe section 12. A clad overlay assists the continuum of the purpose of the polymer liner 16, in providing protection for the bulkhead inner surface as the pipeline hydrocarbon content flows from one pipe-in-pipe section 2 to the next pipe-in-pipe section. This also minimises any gas in the pipeline hydrocarbon content escaping at the joint. Figure 7 shows the clad overlay 44 overlapping with the polymer liner 16 of the pipe-in-pipe section 2 to minimise any fluid leakage thereinbetween. Figure 7 also shows the weld 49 required between the clad overlay 44 and the liner connector 48. Figure 8 shows a similar arrangement to Figure 7, with variants. Firstly, Figure 8 shows the use of slotted second spacers 24’ as described hereinabove. Secondly, Figure 8 the use of the vent tube 30 and branches 32 described hereinabove. Thirdly, Figure 8 shows using the slot 46 in the bulkhead web 45 for a passage of the vent tube 30 therethrough, to maintain the flow of the fluid in the vent tube 30 from one pipe-in-pipe section 12 to the next pipe-in-pipe section (not shown) connected to the other side of the bulkhead 43. Figure 9 shows an embodiment of a pipe-in-pipe (PIP) pipeline 4 of the present invention, formed from a number of the pipe-in-pipe sections according to other embodiments of the present invention, including the examples shown hereinbefore. Figure 9 shows first and second pipe-in-pipe sections 2a, 2b, similar to the first pipe-in-pipe section 2 shown in Figure 1, having a longitudinal axis 3, and comprising outer pipe sections 10a, 10b, inner pipe sections 12a, 12b disposed within the outer pipe sections, ventannuluses 14a, 14b defined between the outer pipe sections 10a, 10b and the inner pipe sections 12a, 12b, and polymer liners 16a, 16b disposed within the inner pipe sections 12a, 12b, respectively. A micro-annulus 18a, 18b is defined between each polymer liner 16a, 16b and the inner pipe sections 12a, 12b after fitting. A plurality of vents 20a (only one shown) are configured or provided along the inner pipe sections 12a, 12b to allow a fluid flow from the micro-annulus 18a, 18b into the vent-annulus 14a, 14b. Insulator layers 22a, 22b and a number of centralisers or spacers 24a, 24b are disposed within the vent-annuluses 14a, 14b, extending circumferentially around the inner pipe sections 12a, 12b. The first and second pipe-in-pipe sections 2a, 2b are connected together using two bulkheads 42a, 42b having inner pipe parts 41 a,42a and outer pipe parts 43a, 43b, outer sleeve parts 50a, 50b, and a further intermediate bulkhead sleeve 54. These are welded together to form the pipe joint of the PIP pipeline 4 in a manner known in the art, and generally discussed hereinabove. Figure 9 also shows two liner connectors 48a, 48b having tapered ends to engage with the ends of the polymer liners 16a, 16b, and to provide a secure connection therewith. Figure 9 also shows two clad overlays 44a, 44b forming a singular progression between and beyond the liner connectors 48a, 48b as discussed above. Figure 9 also shows a pigging sleeve 56 located between the liner connectors 48a, 48b, to provide a smooth continuum of the polymer liner effect between the ends of the polymer liners 16a, 16b across the pipe joint. Figure 9 also shows a bulkhead insulation 22c to assist continuum of the insulation 22a,22b, of the pipe-in-pipe sections 2a, 2b. Figure 9 also shows a series of black ‘arrows’, showing the path of a fluid, usually gaseous flow, from the vent 20 in the first pipe-in-pipe section 2a, along the first vent annulus 14a, through the slot 46 and one-way valve 47 in the first bulkhead 43a, through the annulus between the first and second bulkheads 43a, 43b, through a slot 46b in the second bulkhead 43b, and then into the vent annulus 14b in the second pipe-in-pipe section 2b. In this way, the present invention provides a continuum of a directed pathway for the fluid being vented from the micro-annulus 18a along further pipe-in-pipe sections, such as second pipe-in-pipe section 2b, and so along the pathway of the pipe-in-pipe pipeline 4, optionally for venting in a remote and safe location and environment. Such locations may be above the sea surface, typically termed ‘topside’, and a dedicated venting pipeline may extend directly to a topside location or apparatus to vent fluid from the PIP pipeline 4. Figure 10 shows an arrangement for a method of venting another PIP pipeline 70 according to a further embodiment of the present invention. Figure 10 shows a PIP pipeline 70 passing into a pipeline end termination (PLET) 74. Within the PLET 74 can be an annulus connection 76, a pressured transducer 78, and a suitable interface 72 along a riser or to topside. The annulus connection 76 feeds fluid to be vented from the PIP pipeline 70 into a manifold 80, arranged to work with suitable vent ports with the interface 72, to allow for a dewatering system to split gaseous and liquid phases being vented through the annulus connection 76 prior to topside pumping. The pressure transducer 78 assists providing information and detail concerning the pressure within the annulus connection 76, and the manifold 80 may include a pump or pumps able to arrange a vacuum, serving to draw a flow through the annulus connection 76, and therefore through the vent-annulus 71 in the PIP pipeline 70. Figure 11 shows a further variant for second pipe-in-pipe section 2’ shown in Figure 2, comprising a number of second spacers 24’ having one or more slots or apertures 25 therethrough for passage of a vent tube 30 having branches 32 in the vents 20. Figure 11 shows the pipe-in-pipe section 2’ having at one end a single pipe 60 connected via a second bulkhead 62 in a manner known in the art. The single pipe connection 60 has an inner clad overlay and / or an inner metallic liner 61 made with a corrosion resistant alloy.. The bulkhead 62 has a transverse slot 64 configured to allow passage of the vent tube 30 through the second bulkhead 62, and so allow a directed pathway for the fluid being vented from the micro-annulus 18 along the single pipe 60, optionally for venting in a remote and safe environment. A fully active PIP annulus flushing method could also be performed by pumping. Nitrogen, or any other suitable inert gas, could be used as a carrier medium by pumping from the far end of a PIP pipeline 4 shown in Figure 9 via an attached umbilical (not shown), with discharge from the near end (to topside) through the aid of umbilical tubing or a PIP riser in a manner as described above. In this method, the ends of the PIP pipeline 4 can terminate with bulkheads 43 containing inlet / outlet ports (opening into the PIP vent annulus 14b, also as described above) connected with associated hoses and / or tubing. This arrangement could also be coupled with a vacuum pump (not shown) to ensure annulus pressure remains at or below 1 bar. The present invention provides a pipe-in-pipe section for use in a pipe-in-pipe pipeline for subsea transport of hydrocarbons, which flow of hydrocarbons may include one or more gases, in particular corrosive gases, and in particular gases which are permeable to the material used as the polymer liner within the pipe-in-pipe section. Gas build-up between the polymer liner and the inner pipe section can be vented through suitable vents along the inner pipe section into the vent annulus created between the inner pipe section and the outer pipe section, and such gases can be carried to a place of venting or otherwise disposal using a suitable over-pressure or under-pressure. Such gases can be continuously carried to a place of venting or otherwise disposal. Such gases can be removed rapidly to prevent the formation of corrosive fluids. Thus, the present invention provides an improved method of venting fluids, in particular gases, within the micro-annulus formed in a PLP, which PLP forms part of a PIP pipeline, which PIP pipeline is still reelable for laying in a subsea environment.

Claims

1. A pipe-in-pipe section (2) for use in a pipe-in-pipe pipeline (4) for subsea transport of hydrocarbons, the pipe-in-pipe section comprising:an outer pipe section (10);an inner pipe section (12) disposed within the outer pipe section;a vent-annulus (14) defined between the outer pipe section and the inner pipe section;a polymer liner (16) disposed within the inner pipe section;a micro-annulus (18) defined between the polymer liner and the inner pipe section; anda plurality of vents (20) configured to allow a fluid to flow from the micro-annulus into the vent-annulus.

2. The pipe-in-pipe section of claim 1, wherein at least some of the plurality of vents comprises through-holes extending through the inner pipe section.

3. The pipe-in-pipe section of claim 2, wherein the through-holes have a diameter in the range between 2 mm and 15 mm.

4. The pipe-in-pipe section of claim 1, 2 or 3, wherein the plurality of vents are disposed at two or more axial positions (aO, a1, a2) along the longitudinal axis (L) of the inner pipe section (12).

5. The pipe-in-pipe section of claim 4, wherein each axial position is spaced from a next nearest axial position by between 2 m to 50 m inclusive.

6. The pipe-in-pipe section of any preceding claim, wherein the plurality of vents are disposed at two or more angular positions (r1, r2) around the circumference of the inner pipe section.

7. The pipe-in-pipe section of any one of claims 4, 5 or 6, wherein a first vent is provided at a first axial position (ai) and a first angular position (h), and a next nearest vent is provided at a second axial position (a?) and a second angular position (r2), wherein the first axial position and the second axial position are different and the first angular position and the second angular positions are different.

8. The pipe-in-pipe section of any preceding claim, wherein the plurality of vents are arranged along a helix extending along one or more inner pipe sections (A,B,C), and wherein the helix comprises at least two turns.

9. The pipe-in-pipe section of claim 8, wherein each turn of the helix comprises a set of two or more vents distributed at two or more different angular positions and two or more different axial positions.

10. The pipe-in-pipe section of claim 9, wherein each turn has the same number of vent at the same angular positions.

11. The pipe-in-pipe section of any preceding claim, further comprising a vent tube (52) disposed within the vent-annulus, wherein the vent tube is in communication with the plurality of vents such that fluid from the micro-annulus is able to flow into the vent tube.

12. The pipe-in-pipe section of any preceding claim, wherein at least some of the plurality of vents comprise a one-way valve (26).

13. The pipe-in-pipe section of any preceding claim, wherein the polymer liner comprises HDPE.

14. The pipe-in-pipe section of any preceding claim, wherein the polymer liner includes reinforced fibres.

15. The pipe-in-pipe section of any preceding claim, further comprising an insulator layer (22) disposed within the vent-annulus and configured to cover at least part of an outer surface of the inner pipe section.

16. The pipe-in-pipe section of claim 15, wherein the insulator layer is permeable to the fluid flowing through the plurality of vents.

17. The pipe-in-pipe section of any one of claims 15 and 16, wherein the insulator layer is formed by one or more pouches comprising insulator material.

18. The pipe-in-pipe section of any preceding claim, further comprising a plurality of spacers (24) disposed within the vent-annulus extending circumferentially around the inner pipe section.

19. The pipe-in-pipe section of claim 18, wherein the spacers (24) include one or more apertures therethrough to provide a continuum of the vent-annulus.5 20. A method of manufacturing a pipe-in-pipe section for use in a pipe-in-pipe pipelinefor subsea transport of hydrocarbons, the method comprising at least the steps of in any order:providing an outer pipe section;providing an inner pipe section disposed within the outer pipe section to create a10 vent-annulus defined between the outer pipe section and the inner pipe section; providing a polymer liner disposed within the inner pipe section;providing a micro-annulus defined between the polymer liner and the inner pipe section; andproviding a plurality of vents configured to allow a fluid to flow from the micro-annulus 15 into the vent-annulus.

21. A method of manufacturing a pipe-in-pipe section as defined in any one of claims 1 to C\l -|8.00Qo 22. A method of venting a pipe-in-pipe pipeline for subsea transport of hydrocarbons comprising a pipe-in-pipe section as defined in any one of claims 1 to 18, comprising the “ step of venting a fluid from the micro-annulus into the vent-annulus defined between the outer pipe section and the inner pipe section.25 23. A method as claimed in claim 22 further comprising the step of applying anoverpressure or an underpressure along the vent-annulus.

24. A pipe-in-pipe pipeline for subsea transport of hydrocarbons, the pipe-in-pipe pipeline comprising a plurality of the pipe-in-pipe sections as claimed in any one of claims 1-18,30 wherein the plurality of pipe-in-pipe sections are connected to provide a continuous ventannulus between the plurality of inner pipe sections and the plurality of outer pipe sections.

25. The pipe-in-pipe pipeline of claim 24 further comprising a conduit connected to the vent-annulus, wherein the conduit is configured to transport the fluid in the continuous vent-35 annulus to a venting pipeline proximate to the surface of the sea.

26. The pipe-in-pipe pipeline of claim 25, wherein the conduit is a riser vent-annulus of a pipe-in-pipe riser system, wherein the pipe-in-pipe riser system comprises an outer casing, an inner casing and tubing arranged in the riser vent-annulus formed between the outer casing and the inner casing.

27. The pipe-in-pipe pipeline of any one of claims 24 to 26, wherein the continuous ventannulus is connected to a vacuum such that the fluid actively flows from the micro-annulus to the vent-annulus due to a pressure differential.10 28. The pipe-in-pipe pipeline of any one of claims 24 to 27, wherein at least two of thepipe-in-pipe sections are connected using liner connectors (48) and one or more pipe-in-pipe bulkheads (42), said one or more bulkheads able to provide the continuous vent-annulus between the pipe-in-pipe sections.

29. The pipe-in-pipe pipeline of claim 28, further including a clad overlay (44) between the liner connectors (48).

30. The pipe-in-pipe pipeline of claim 29, wherein the clad overlay (44) overlaps with the polymer liner of each pipe-in-pipe section.

31. The pipe-in-pipe pipeline of any one of claims 28 to 30, further including a pigging sleeve (56) between the liner connectors (48).

32. The pipe-in-pipe pipeline of claim 31, wherein the pigging sleeve (56) provides a25 smooth continuum of the polymer liner between the polymer liner of each pipe-in-pipe section.

33. The pipe-in-pipe pipeline of any one of claims 24 to 32 having at one end a single pipe connection, said single pipe connection configured to allow a fluid to flow from the vent-30 annulus.

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