GB2318399A

GB2318399A - Multilayer pipe for use as a pipe liner

Info

Publication number: GB2318399A
Application number: GB9621567A
Authority: GB
Inventors: Alan Lloyd Headford; Keith Andrew Wilson
Original assignee: Glynwed Pipe Systems Ltd
Current assignee: Glynwed Pipe Systems Ltd
Priority date: 1996-10-16
Filing date: 1996-10-16
Publication date: 1998-04-22
Anticipated expiration: 2016-10-16
Also published as: GB2318399B; GB9621567D0

Abstract

The multilayer pipe has a barrier layer formed from a first polyvinylidene fluoride material; and a structural layer formed from a second polyvinylidene fluoride material; the barrier layer being thinner than the structural layer, and having a greater resistance to hydrocarbons and other components of pipeline fluids than the structural layer. The pipe is deformed to a reduced diameter and then introduced into a pipeline for subsequent expansion.

Description

IMPROVEMENTS IN OR RELATING TO PIPELINES This invention relates to improvements in or relating to pipelines, and in particular to off-shore pipelines.

Off-shore pipelines, such as those used to pump oil and gas ashore from off-shore drilling rigs and terminals, typically are constructed so as to be capable of withstanding very high internal pressures and temperatures.

The pipelines used in such off-shore situations need to be strong and durable and therefore typically are formed from steel. However, one problem encountered with steel pipelines in general, and off-shore pipelines in particular, is the problem of corrosion. It is undesirable from an economic standpoint to construct pipelines of stainless steel, and therefore the somewhat cheaper carbon steels are often used for the construction of pipes. It is therefore necessary to include an allowance in the wall thickness for corrosion, or to protect the pipes against corrosion in some way, and, for example, the outer surface of the pipe may be protected by means of an appropriate surface treatment or covering. In order to protect the inner bore of the pipe from the corrosive effects of materials passing through the pipe, it has been proposed to provide a liner formed of a suitable polymeric material.

However, in order to be able to install a liner in an existing steel pipeline, the liner either needs to be considerably under-sized with respect to the pipeline, in which case the long term stability and integrity of the liner would be compromised, or the liner needs to be capable of being installed in a radially contracted or collapsed form and then expanded to full or nearly full size when in situ in the pipeline.

Techniques for temporarily reducing the cross-sectional area occupied by a liner pipe to enable it be installed in a pipeline are well known. The "Rolldown" process involves the progressive reduction in diameter of the pipe by means of radial compression using successive sets of compression rollers. The "Rolldown" process and similar processes are characterised by the fact that the liner pipe undergoes a significant degree of plastic flow during the size reduction and this produces a pipe which is dimensionally stable at or near the reduced diameter after the reduction process. The compressive strains imposed on the pipe wall by this process are typically in the region of up to about 20%.

For this type of process to operate successfuily, the polymer material must have a yield strain or elastic limit significantly lower than the strain imposed by the process such that the material is taken beyond its yield point thus imparting a degree of permanent plastic deformation. Materials which have a yield strain approaching or above 20% would behave predominantly in an elastic manner and would not retain any significant diameter reduction after the process. Such materials would therefore not generally be suitable for use in such a process.

In another type of process, the liner pipe is pulled through a diameter reducing die by means of axial tension on the pipe. In this case, the deformation of the pipe is more elastic in nature due to the lower strains involved, which tend to be up to about 10% to 1 5%. The diameter reduction is only achieved so long as the axial tension on the pipe is maintained.

Examples of this type of process are the techniques known as Swagelining, Die-drawing and Titeliner. For this type of process, it is beneficial if the material has a yield strain which is in the region of the strain imposed by the process, as the onset of yielding helps to stabilise the process and achieve a constant diameter reduction over the length of the pipe. If the yield strain of the pipe is greater than the strains imposed by the process, then the material is operating entirely within its region of predominantly elastic behaviour and consequently there is more scope for stress redistribution downstream from the forming die with consequent difficulties maintaining the reduced diameter of the pipe as installation progresses.

Yet another technique involves the deforming of the pipe to reduce its effective diameter by folding or collapsing it to a U-shaped or C-shaped cross section. In the collapsed form the pipe can be introduced into the bore of a pipeline and then expanded to regenerate its original circular cross section.

In this case peak surface strains in the folded form can be 30% or greater.

In each of the above diameter reducing or collapsing processes, high density polyethylene, which has a yield strain of around 10%, has the appropriate mechanical properties and has been used for liners in land-based pipelines carrying relatively benign materials such as mains water. However, the interior of an off-shore pipeline may be a very harsh and demanding environment and in such circumstances, polyethylene may not have the physical and chemical attributes necessary for use in such an environment.

Fluids obtained from offshore oil and gas wells typically contain mixtures of hydrocarbons, water and significant concentrations of contaminants such as carbon dioxide or hydrogen sulphide. Such fluids are often conveyed in sub-sea pipelines at temperatures of up to 1 300 Centigrade or more. It follows therefore that in order to be suitable for use in such conditions, the liner pipe needs to be heat resistant, i.e. it must adequately retain its properties and integrity at the operating temperatures.

In addition, it must be resistant to hydrolysis by the water in the fluids; it should be substantially impermeable to hydrocarbons and it should also be substantially impermeable to gases such as carbon dioxide. If permeation of hydrocarbons into the liner wall occurs, this can lead to softening and swelling of the polymer with consequent distortion of the liner wall, and increased solubility to gas, and even to loss of the integrity of the liner.

Carbon dioxide presents a further problem. At high temperatures and pressures, particularly at temperatures and pressures near its critical temperature and pressure, carbon dioxide can also act as a very good solvent for some polymers such as polyethylene and hence, if allowed to permeate into the liner wall, can also contribute to loss of integrity of the liner. A further and potentially very serious problem insofar as gases such as carbon dioxide is concerned is that if they do permeate into the liner wall in significant concentrations, and the pumping pressures are reduced suddenly, for example in the event of a sudden shutdown of the pipeline, this can lead to explosive decompression damage to the polymer liner which can have severe consequences with respect to structural integrity.

Polyvinylidene fluoride (PVDF) has been proposed as a liner material for hydrocarbon pipelines on account of its resistance to hydrocarbons and other associated pipeline fluids at temperatures of up to 1300 Centigrade or more. However, PVDF homopolymer is a relatively inflexible material and previous attempts by the present applicants to reduce or deform, and then expand or reform, PVDF liners have proven unsuccessful on the basis that the relatively stiff homopolymer is not compatible with the liner pipe installation techniques referred to above. Indeed initial attempts to use the homopolymer in the Rolldown method led to fracturing of the liner wall during reduction followed by total failure during subsequent expansion.

Plasticised PVDF, which has a yield strain in the region of 25%, has also been tried but has been found to be problematic in the die-drawing type of process and totally unsuitable for use in the "Rolldown" type of process, in both cases due to its excessively elastic characteristics.

It will be appreciated from the foregoing that the requirements for liners for use in offshore pipelines are very stringent indeed. Thus not only does the liner need to be able to resist the very harsh conditions found within pipelines, but it needs also to have the necessary flexibility and plastic/elastic properties to allow it to be temporarily deformed and then reformed during the commonly used pipe lining methods.

The present invention sets out to overcome the aforementioned problems by providing a liner pipe having a thin barrier layer of a PVDF material which is substantially resistant to hydrocarbons and the other components of offshore pipeline fluids, and a thicker structural layer of another PVDF material which may be less resistant to the pipeline fluids than the barrier layer but which has physical characteristics more compatible with the various installation techniques referred to above, the relative thicknesses and compositions of the layers being selected such that the deformability and reformability characteristics of the pipe are dictated by the properties of the structural layer.

Accordingly, in one embodiment of the invention, there is provided a multilayer pipe comprising a barrier layer formed from a first polyvinylidene fluoride material; and a structural layer formed from a second polyvinylidene fluoride material; the barrier layer being thinner than the structural layer, and having a greater resistance to hydrocarbons and other components of pipeline fluids than the structural layer.

The barrier layer is preferably formed from polyvinylidene fluoride homopolymer but can also be formed from a polyvinylidene fluoride copolymer or polymer blend in which vinylidene fluoride derived monomer units (VDF units) make up the greater proportion of the total number of monomer units in the copolymer or blend. Typically the VDF units make up more than 50% of the monomer units, preferably more than 60%, more preferably more than 70% and particularly preferably more than 80% of the monomer units. As an alternative to copolymers and blends, the barrier layer can also consist of a plasticised PVDF homopolymer, copolymer or blend.

The structural layer, which can be formed from a single layer or plurality of layers, is primarily responsible for the mechanical and deformability and reformability characteristics of the pipe. The structural layer can be arranged radially within the barrier layer, for example in circumstances where it is desired to minimise hydrocarbon permeation from the pipe exterior into the pipe. However, more usually the barrier layer is the radially innermost layer.

The structural layer can consist of only a single layer or it can comprise a plurality of layers. Where the structural layer comprises a plurality of layers, the properties of the layers can selected so that, for example, there is a gradation of resistance properties from the structural layer adjacent the barrier layer to the structural layer furthest from the barrier layer. For example, the resistance properties can decrease with distance from the barrier layer.

The barrier layer will in general be less permeabie to hydrocarbons than the structural layer. It may also be less permeable to gases such as carbon dioxide.

The structural layer can be formed from a polyvinylidene fluoride material which has a yield strain or elastic limit of less than 20%, for example less than 15%, but which is typically more than 7% and more usually greater than 9%, and has a high failure strain value, for example more than 100%. By contrast, the polyvinylidene fluoride material from -which the barrier material is formed can have a yield strain or elastic limit as low as 7% but with a lower failure strain value of, for example, 20-50%.

Relatively thin layers of such relatively brittle materials have been found to be less problematic due to the predominantly plane stress fields which may obtain when such layers are under conditions of tensile or compressive loading, for example.

The structural layer can be formed from a PVDF material which has a greater permeability to hydrocarbons than the barrier layer but which has greater fiexibility or ductility. The structural layer is typically formed from a plasticised PVDF or a PVDF copolymer or blend. For example, the PVDF material can be formed through the copolymerization of vinylidene fluoride and one or more haloalkene monomers. Examples of haloalkene monomers include fluoroalkenes such as tetrafluorethylene, hexafluoropropylene, and vinyl fluoride, chloroalkenes such as vinyl chloride and vinylidene chloride, and fluorochloroaikenes such as chlorotrifluoroethylene. A blend may comprise an intimate mixture of two or more polymers selected from plasticised and unplasticised PVDF homopolymer, and PVDF copolymers.

Examples of particular copolymers from which the structural layer can be formed include copolymers of vinylidene fluoride and hexafluoropropylene, and copolymers of vinylidene fluoride and chlorotrifluoroethylene.

The copolymers can be, for example, block copolymers or random copolymers but more usually they are random copolymers. By interrupting the vinylidene difluoride with other monomer units, the crystallinity of the polymer is reduced and hence it is more flexible.

Where the structural layer comprises polyvinylidene fluoride containing a plasticiser, the plasticiser should preferably be one which exhibits a low tendency to migrate between the layers. Examples of such plasticisers include rubbers such as nitrile rubber.

In general, the structural layer is disposed radially outwardly of the barrier layer. The structural layer is thicker than the inner layer; i.e. it comprises greater than 50% of the total wall thickness of the liner. The barrier layer typically constitutes from about 2.5% up to approximately 49.9% of the wall thickness, more usually 5 to 25% of the wall thickness, for example approximately 10% of the wall thickness.

The total wall thickness of the pipe will vary depending upon the intended use of the pipe. For example, if the pipe is intended as a liner for an existing pipeline, such as an offshore pipeline, the total wall thickness of the liner will depend to a large extent on the diameter of the pipeline. For such liner pipes, the wall thickness can vary, for example, within the range from about 3 to about 1 Omm. The barrier layer typically will vary between about 0.5 and 1 .5mm, e.g. approximately 1 mm. The thickness of the barrier layer should be sufficient to enable it to resist excessive physical damage from, for example, pipe line pigging operations.

The pipes of the present invention are particularly suited for use as liner pipes for existing pipelines, and in particular pipelines for carrying hydrocarbons and hydrocarbon mixtures at high temperatures of up to 1 300 Centigrade or more, such as offshore oil or natural gas pipelines. Therefore, according to another aspect of the invention, there is provided a liner pipe, e.g. a radially deformable or collapsible liner pipe, comprising a barrier layer and a structural layer as hereinbefore defined. By radially deformable is meant that the liner pipe is. capable of undergoing radial compression or being collapsed (e.g.to give a U-shaped or C-shaped profile) to give a reduced effective diameter suitable for installation in an existing pipeline, followed by expansion towards its original diameter, the contraction and expansion being accomplished without cracking or fracturing of the pipe.

By providing a liner pipe having a relatively thin barrier layer formed from a substantially hydrocarbon resistant PVDF material and a relatively thick main layer formed from a more flexible form of PVDF, it is contemplated that such liner pipes will possess the necessary ability to be radially contracted or collapsed and then expanded or reformed to allow for installation, whilst not losing the barrier properties and chemical and physical resistance properties needed to prevent damage to the liner pipe wall by the pipeline fluids.

By making both of the layers from a PVDF material, this considerably simplifies welding operations. If, for example, two liner pipes are welded in an end to end fashion, the welding process effectively takes place between the respective structural layers of the two liners, the respective barrier layers giving rise to a bead around the circumference of the pipe liner. If the liner is de-beaded, there is a small annular strip of exposed structural layer but, since the structural layer is formed from a PVDF-containing material, it still has relatively good impermeability and resistance characteristics and hence hydrocarbon permeation through the exposed region is minimal. Similarly, if the barrier layer is excessively damaged by pipeline pigging operations, such that a deep gouge in the barrier layer is formed, the relative impermeability of the structural layer will minimise any damage to the whole liner by the fluids conveyed therein.

In a further aspect, the invention provides a pipeline comprising an outer pipe (for example an iron or steel pipeline) and, disposed therein, a liner pipe as hereinbefore defined.

In a still further aspect, the invention provides a method of lining a steel pipe, and in particular an offshore pipeline, the method comprising deforming a liner pipe of the type hereinbefore defined such that the effective diameter of the deformed liner pipe is substantially less than the inner diameter of the pipeline, pushing or drawing a length of the deformed liner pipe through the metal pipeline, and causing or allowing the liner pipe to expand to or towards a diameter corresponding generally to its diameter prior to deformation.

in yet another aspect, the invention provides a method of lining an existing pipeline, the method comprising subjecting a liner pipe as hereinbefore defined to radially compressive forces so as to impose on the liner pipe a compressive strain which exceeds the yield strain of the liner pipe, thereby to cause predominantly plastic deformation of the liner pipe from an original diameter to a reduced diameter; inserting the reduced diameter liner pipe into the existing pipeline, and expanding the liner pipe following its insertion into the pipeline to or towards a diameter corresponding to the original diameter.

In another aspect, the invention provides a method of lining an existing pipeline, the method comprising pulling a liner pipe as hereinbefore defined through a diameter reducing die by the application of axial tension to reduce the diameter from its original diameter to a diameter small enough to be inserted into the pipeline; simultaneously drawing the reduced diameter liner pipe through the existing pipeline whilst maintaining the liner pipe under axial tension, and subsequently releasing the axial tension to allow the liner pipe to expand to or towards its original diameter.

Preferably, the die throat diameter is chosen such that the radial strain imposed on the liner pipe corresponds to or is greater (most preferably only just greater - e.g by no more than up to about 4%) than the yield strain of the liner pipe thereby to bring about the onset of plastic deformation, but not to cause excessive plastic deformation. By bringing about the onset of plastic deformation, the diameter reduction process is stabilised and this assists in maintaining a constant diameter reduction over the length of the pipe. Conversely, if the liner is plastically deformed by a significant amount, e.g. by more than 4%, release of the axial tension will not by itself allow or cause the liner to expand to be a close fit in the host pipeline.

The invention is illustrated, but not limited, by the following examples.

EXAMPLE 1 A liner pipe according to the invention is prepared by coextrusion through a 90mm single screw extruder for the structural layer and a 35mm single screw extruder for the barrier layer using a two layer co-extrusion die, the tooling and extruder screws etc being resistant to any corrosive effects resulting from contact with fluoropolymers. By using the aforementioned extrusion apparatus, there is produced a two layer pipe having an external diameter of 200mm, and consisting of an outer flexible layer of 6mm thick vinylidene fluoride - hexafluoropropylene copolymer (e.g. Kynarflex 2800 available from Atochem) and an inner barrier layer of 1 mm thick polyvinylidene homopolymer (e.g. Kynar 1000HD, also available from Atochem). A pipe of this construction is compatible with diameter reducing techniques such as the "Rolldown " method but has excellent chemical resistance properties and a very low or negligible permeability to hydrocarbons.

EXAMPLE 2 A two layer pipe having the same diameter and layer thicknesses as the pipe described in Example 1 is prepared in a similar manner except that the outer structural layer is formed from an alternating copolymer of vinylidene fluoride and chlorotrifluoroethylene (for example Solef 31508 available from Solvay).

It will readily be apparent that numerous modifications and alterations may be made to the embodiments described in the Examples above without departing from the principles underlying this invention. All such modifications and alterations are intended to be embraced by this Application.

Claims

1. A multilayer pipe comprising a barrier layer formed from a first polyvinylidene fluoride material; and a structural layer formed from a second polyvinylidene fluoride material; the barrier layer being thinner than the structural layer, and having a greater resistance to hydrocarbons and other components of pipeline fluids than the structural layer.

2. A multi layer pipe according to claim 1 wherein the barrier layer is formed from polyvinylidene fluoride homopolymer, or from a polyvinylidene fluoride copolymer or polymer blend in which vinylidene fluoride derived monomer units (VDF units) make up the greater proportion of the total number of monomer units in the copolymer or blend.

3. A multilayer pipe according to claim 2 wherein the barrier layer is formed from a polyvinylidene fluoride copolymer or polymer blend in which the VDF units make up more than 50% of the monomer units.

4. A multilayer pipe according to claim 2 wherein the barrier layer is formed from a plasticised PVDF homopolymer, copolymer or blend.

5. A multilayer pipe according to any one of the preceding claims wherein the structural layer is arranged radially within the barrier layer.

6. A multilayer pipe according to any one of claims 1 to 4 wherein the barrier layer is the radially innermost layer.

7. -A multilayer pipe according to any one of the preceding claims wherein the barrier layer is less permeable to hydrocarbons and gases such as carbon dioxide than the structural layer.

8. A multilayer pipe according to any one of the preceding claims wherein the structural layer is formed from a polyvinylidene fluoride material which has a yield strain or elastic limit of less than 20% and has a failure strain value of more than 100%.

9. A multilayer pipe according to any one of the preceding claims wherein the polyvinylidene fluoride material from which the barrier material is formed has a minimum yield strain or elastic limit of and has a failure strain value of 20-50%.

10. A multilayer pipe according to any one of the preceding claims wherein the structural layer is formed from a plasticised PVDF or a PVDF copolymer or blend.

11. A multilayer pipe according to claim 10 wherein the PVDF material is formed through the copolymerization of vinylidene fluoride and one or more haloalkene monomers.

12. A multilayer pipe according to claim 10 wherein the structural layer is formed from a blend comprising an intimate mixture of two or more polymers selected from plasticised and unplasticised PVDF homopolymer, and PVDF copolymers.

13. A multilayer pipe according to any one of the preceding claims wherein the barrier layer constitutes from about 2.5% up to approximately 49.9% of the wall thickness.

i4. A pipeline comprising an outer pipe (for example an iron or steel pipeline) and, disposed therein, a liner pipe which is a multilayer pipe according to any one of the preceding claims.

1 5. A method of lining a steel pipe, and in particular an off-shore pipeline, the method comprising deforming a liner pipe which is a multilayer pipe as defined in any one of claims 1 to 1 3 such that the effective diameter of the deformed liner pipe is substantially less than the inner diameter of the pipeline, pushing or drawing a length of the deformed liner pipe through the metal pipeline, and causing or allowing the liner pipe to expand to or towards a diameter corresponding generally to its diameter prior to deformation.

1 6. A method of lining an existing pipeline, the method comprising subjecting a liner pipe, which is a multilayer pipe as defined in any one of claims 1 to 13, to radially compressive forces so as to impose on the liner pipe a compressive strain which exceeds the yield strain of the liner pipe, thereby to cause predominantly plastic deformation of the liner pipe from an original diameter to a reduced diameter; inserting the reduced diameter liner pipe into the existing pipeline, and expanding the liner pipe following its insertion into the pipeline to or towards a diameter corresponding to the original diameter.

17. A method of lining an existing pipeline, the method comprising pulling a liner pipe, which is a multilayer pipe as defined in any one of claims 1 to 13, through a diameter reducing die by the application of axial tension to reduce the diameter from its original diameter to a diameter small enough to be inserted into the pipeline; simultaneously drawing the reduced diameter liner pipe through the existing pipeline whilst maintaining the liner pipe under axial tension, and subsequently releasing the axial tension to allow the liner pipe to expand to or towards its original diameter.

1 8. A multilayer pipe according to claim 1 7 wherein the die has a throat diameter such that the radial strain imposed on the liner pipe corresponds to or is greater than the yield strain of the liner pipe thereby to bring about the onset of plastic deformation, but not to cause excessive plastic deformation.