The present invention relates to liners for pipes. Background

Pipelines are used to transport fluids from one location to another. In the oil and gas industry, pipelines are used to carry fluids from a well, typically oil, gas, water and/or gas condensate, to a receiving facility. Often, these fluids are conveyed as a mixture, in a multiphase flow.

The fluids carried in such pipelines can be corrosive of typical pipe materials such as steel. In order to avoid corrosion various things can be done. For example, a corrosion inhibitor may be introduced into the fluid in the pipeline. An alternative is to use a pipe that is constructed from a non-corrosive metal alloy, which does not corrode in the presence of the relevant fluid.

For long distance pipelines in oil and gas fluid transport, forming the pipeline from a non-corrosive alloy, or adding an inhibitor, at least to cover its full length, is often prohibitively expensive. There are also added environmental risks associated with using chemical inhibitors.

Another known solution to avoid corrosion is to line the pipe with a polymer liner. This allows relatively low-cost steel pipes to be used for a pipeline. Lined oil and gas pipelines may employ solid polymer liners which are complete barriers to corrosion from fluids in the pipeline.

A lined pipeline of this type may be built up from a number of pipe sections which are joined together. A pipe section may be pre-prepared for joining to another section by inserting a pre-fabricated liner into the pipe section, and drawing the liner into position within the pipe. The liner has a tight fit within the pipe section, with an outer surface of the liner wall resting against an inner surface of the pipe section. The ends which are to be joined to adjacent pipe sections may be formed from a non-corrosive alloy, but the significant majority of the pipeline when constructed may be protected from corrosion by the presence of the liner in each section. However, the use of polymeric liners for corrosion protection particularly when carrying gaseous fluids has been problematic. In particular, a problem with such liners is that they can be susceptible to collapse due if a pressure increase occurs in a micro-space between the outer surface of the liner and the inner surface of the pipe section.

For example, the polymer liner may dissolve significant amounts of gas during service of the pipeli ne. In particular, C02 and methane may be dissolved. Upon decompression of fluid inside the pipeline the dissolved gases may escape both towards the main bore of the pipe where the pressure is low, but also towards the interface or micro-space between the liner and the pipe. As a result of the pressure differential across the liner, an inward force is exerted on the outside of the liner causing it to separate from the pipe and deform or collapse inwards. A rapid drop in pressure in the pipeline and consequent release of gas from the liner may also damage the internal structure of the pipe section.

It will be appreciated that solid polymer liners of this kind may contain and release dissolved gas by a process of diffusion. A previous solution for reducing the risk of liner collapse in flexible pipes has been to use a pipe carcass inside the liner. However, this solution adds complexity and cost.

Summary of the invention According to a first aspect of the invention, there is provided a pipe liner as set out in the claims appended hereto.

According to a second aspect of the invention, there is provided a pipe assembly as set out in the claims appended hereto, the pipe assembly comprising a pipe section and the pipe liner according to the first aspect of the invention.

According to a third aspect of the invention, there is provided a method of reducing corrosion in a pipe section as set out in the claims appended hereto. According to a fourth aspect of the invention, there is provided a method of forming a lined pipeline as set out in the claims appended hereto.

It is found that a pipe liner providing fluid connection through the liner wall between the first and second regions (in certain embodiments the first region being a micro-annulus) can bring surprisingly effective corrosion resistance to a pipe section whilst equalising pressure across the liner wall to prevent collapse of the liner.

Description of the invention

There will now be described, by way of example only, embodiments of the invention with reference to the accompanying drawings, of which:

Figure 1 is a cross-sectional representation of a pipe section fitted with a pipe liner according to an embodiment of the invention;

Figure 2 is an exaggerated close-up representation of area A of the pipe liner of Figure 1 ; and Figures 3A and 3B are representations of porous structures of a wall for the pipe liner of Figure 1 .

Referring to Figure 1 a pipe assembly 1 comprises a pipe section 2 and a pipe liner 3 fitted within the pipe section 2. The pipe liner 3 lines an inner surface 4 of the pipe section. The pipe liner 3 is formed as a tubular member to match the pipe section 2, and has an inner surface 6 defining a bore 7 for fluid to flow through the pipe section. The bore 7 defines an axis 8 extending therethrough, in this case the axis being a central axis of the pipe section and liner. The pipe section and liner may have a circular, square or other cross section across the direction of flow through the bore.

The pipe section in this example is formed of a metal such as steel or carbon steel. The liner 3 is formed of a polymer. The fluid transported through the pipe assembly for purposes of this example includes oil and gas. In addition, water and methane and/or carbon dioxide gas are present in the fluid; these may typically be recovered from a well together with the oil and gas. The fluid may in general be corrosive of the metal from which the pipe section.

As can be seen in Figure 1 , the pipe liner has an outer surface 5 facing toward the inner surface 4 of the pipe section. The inner surface 6 faces inward, toward the bore 7. The bore is defined by the inner surface 6 of the liner. The liner is fitted to the pipe such that the outer surface 4 of the liner lies adjacent to the inner surface of the pipe.

In general the liner lies in contact with the pipe section. However, fluid can access a small region (a "micro-space") 9 on the outside of the liner, defined between an outer surface 4 of the liner and the inner surface 5 of the pipe section, as seen in exaggerated scale in Figure 2. Fluid can be accommodated in the micro-space. By way of the micro-space, different portions of space 9a-9d along the pipe section outside of the liner (but inside the pipe section) may be fluidly connected with each other. Thus, the micro-space may take an annular or part-annular form.

The liner 3 comprises a liner wall 10 with a structure that allows fluid to pass through the wall from the bore 7 to the micro-space 9 and vice versa. Thus, passageways through the liner wall 10 are provided, fluidly connecting the space of the bore 7 with the micro annulus. A plurality of passageways are typically provided through the liner wall along the length of the pipe section. In use, fluid passes axially through the pipe section from an upstream end 1 1 to a downstream end 12 through the bore 7 whilst fluid communication is permitted, laterally of the axis 8, through the passageways in the liner wall. As a result of the communication through the passageways, the fluid pressure on either side of the liner and resultant forces exerted by the fluid on the liner are balanced. In practice therefore, it will be appreciated that the liner is located in contact with the pipe section, but provides for fluid connectivity to an outside of the liner via the micro annulus. If a drop in pipeline pressure occurs, resulting in a drop of pressure in the bore 7, the pressure in the micro-space drops correspondingly, by way of the fluid communication through the liner wall 10, to equalise the pressure across the liner. Any build up of pressure in the micro-space would be equalised out. In other words, fluid released to the micro-space toward the outside of the liner, would flow from the annulus through a passageway in the liner wall and vent to the interior of the bore 7. In this way, liner collapse or the risk of liner collapse can be avoided or at least reduced.

Preferably, the liner wall 10 is porous, comprising a porous structure to provide fluid communication through the wall. More specifically, the liner wall 10 is formed of a porous plastics material, or porous polymer, designed to provide the desired fluid connectivity. The porous structure limits the exposure of the pipe section to corrosive substances that are present inside the pipe section. Thus, it provides a high degree of corrosion protection, whilst allowing pressure to equalise by communication through the passageways through the structure.

The specific polymer used for forming the liner wall, and for the liner in general, may be polypropylene, polyethylene, a polyamide, p o l yvinylidinefluoride (PVDF) or polytetrafluoroethylene (PTFE). The porous structure may have pores suitable for letting through carbon dioxide gas, methane gas or other gases through the liner wall. Gas from the micro space can then vent to the inside of the liner. Also, the porous structure is configured for liquid to pass through the liner wall. If liquid is present in the pipeline, it may therefore access the liner wall through the pores. It will however be appreciated that whilst the porous liner does not prevent the liquid from penetrating through to the metal substrate (inner surface) of the pipe section, it (severely) restricts the access of liquid to the substrate.

The average pore diameter, for example average maximum pore diameter, is typically less than 100 micrometres, and preferably between 1 and 100 micrometers. In general, it is desired to have pores that are as small as possible in order to maximise corrosion protection, whilst permitting enough gas to escape through the wall to prevent liner collapse.

Porous polymeric materials are commercially available from Porex Corporation (http://www.porex.com).

The polymer may be selected based on temperature of the fluid passed through the pipe section . For example, a polymer is selected which will not break down at expected operational temperatures. The temperature properties of polymer materials are either known or can be tested for example by heating tests. Examples of textures for the porous structure for the liner wall are shown in Figures 3A and 3B. In Figure 3A, granules or particles 13 can be seen with pores 14 defined in between. The pores 14 are interconnected between the inner surface 6 and outer surface 5 of the liner to define typically tortuous passageways through the liner wall. The passageways through the liner wall 10 lead from the bore 7 and exit into the microspace 10. In Figure 3B, the material has a fibrous nature and elongated particles 15 which define passageways through the wall. The porous structure provides multiple microscopic paths for connecting an inner surface of the liner wall with an outer surface of the liner wall. The paths allow gas and liquid to flow therethrough. The porous structure comprises a continuum of partly fused polymer particles. The partly fused particles define pores therebetween. The pores in turn are interconnected to define the paths. In this way, upon flow through the paths or passageways, fluid passes over the surfaces of the (fused) particles or along surfaces of the material of the wall that define the paths.

In order to provide fluid connectivity and pressure equalisation, there is a trade off against corrosion resistance. Testing of the structure can be performed with different fluids to observe the amount of corrosion for liners with different pore sizes. The presence of the liner significantly reduces exposure and hence the net flow of water plus carbon dioxide to the inner surface of the pipe section, whilst the pores of the porous structure of the liner wall are limited in size typically being 1 -100 micrometres in diameter. Compared with conventional polymer liners which do not have a porous structure and provide complete corrosion protection, liners as described above using a porous wall may achieve around 90% of the corrosion resistance. The liner acts to reduce fluid flow-through speed to the metal pipe section. Accordingly, the exchange or access rate of liquid and gas to the metal pipe section is reduced. Corrosion rates are therefore reduced. However, good connectivity across the liner is sufficient to prevent liner collapse in the event of sudden pipeline pressure drops.

By use of the porous liner concept described, the liner will allow a corrosive medium inside the pipeline to contain gas without the risk of liner collapse upon rapid pressure release. As the liner is porous, no significant pressure build up between steel pipe and liner will occur as a result of release of any dissolved gases in the actual polymer.

The present solution of using porous liners provides a cost effective way to transport hydrocarbon fluids containing water and carbon dioxide gas, or other corrosive fluids, in conventional steel pipe sections.

It will be appreciated that upon laying a pipeline, the pipe section 2 may be joined to further pipe sections, for example at their upstream and downstream ends 1 1 , 12, to form a pipeline. Each further pipe section may be provided with a liner such as the liner 3 as described above in relation to pipe section 2. Typically, the pipe section 2 is pre-fitted with the liner 3 before it is joined to adjacent sections. The same may apply to other pipe sections. Pipe sections with lengths of up to around 500 m may be pre- fitted with the liner.

Various modifications and improvements may be made without departing from the scope of the invention herein described.