WO2015059480A1

WO2015059480A1 - A flexible pipe body

Info

Publication number: WO2015059480A1
Application number: PCT/GB2014/053164
Authority: WO
Inventors: Anthony Mark LAYCOCK
Original assignee: Ge Oil & Gas Uk Limited
Priority date: 2013-10-25
Filing date: 2014-10-23
Publication date: 2015-04-30
Also published as: GB201318914D0

Abstract

A method of producing a flexible pipe body, said method comprising extruding a polyethylene composition comprising a carbon-carbon thermal initiator to form a tubular layer, cross-linking the tubular layer, and applying an armour layer around the cross-linked tubular layer.

Description

A Flexible Pipe Body

The present invention relates to method of producing a flexible pipe body.

Traditionally flexible pipe is utilised to transport production fluids, such as oil and/or gas and/or water, from one location to another. Flexible pipe is particularly useful in connecting a sub-sea location (which may be deep underwater, say 1000 metres or more) to a sea level location. The pipe may have an internal diameter of typically up to around 0.6 metres. Flexible pipe is generally formed as an assembly of a flexible pipe body and one or more end fittings. The pipe body is typically formed as a combination of layered materials that form a pressure-containing conduit. The pipe structure allows large deflections without causing bending stresses that impair the pipe's functionality over its lifetime. The pipe body is generally built up as a combined structure including metallic and polymer layers.

Unbonded flexible pipe has been used for deep water (less than 3,300 feet (1 ,005.84 metres)) and ultra-deep water (greater than 3,300 feet) developments. It is the increasing demand for oil which is causing exploration to occur at greater and greater depths where environmental factors are more extreme. For example, in such deep and ultra-deep water environments, ocean floor temperature increases the risk of production fluids cooling to a temperature that may lead to pipe blockage. Increased depths also increase the pressure associated with the environment in which the flexible pipe must operate. As a result, the need for high levels of performance from the layers of the flexible pipe body is increased.

Flexible pipe may also be used for shallow water applications (for example less than around 500 metres depth).

Flexible pipes generally include polymer layers, such as polyethylene, polyamides and polyvinylidene fluoride (PVDF), which may be formed by extrusion. Most polymers will have a certain maximum allowable strain above which the risk of damage to the material is much greater. In flexible pipes where a polymer layer lies adjacent an armour layer (such as an internal pressure sheath adjacent a metallic pressure armour layer), the polymer layer may be subjected to non-uniform, highly localised strain. Accordingly, it is desirable to for such polymer layers to have a relatively high elastic modulus to provide the necessary resistance to deformation under such strain. A relatively high elastic modulus may also be desirable to improve the high temperature performance of the polymer layer, as polymer materials having a relatively high elastic modulus may be less likely to creep when exposed to high temperatures. On the other hand, if the elastic modulus of the initial polymer is too high, extrusion of the polymer layer may be difficult as the polymer may be resistant to flow. In such situations, high extrusion temperatures may be required, increasing the risk of high temperature degradation of the polymer during the extrusion step.

WO2007/042049 describes a process for producing a flexible pipe comprising a metal carcass and an internal sealing sheath that is extruded onto the carcass. The sheath is formed by extruding a non-crosslinked polyethylene material onto the heated metal carcass. The polyethylene material comprises a peroxide initiator. Once extruded, the material is crosslinked by exposing polymer to electromagnetic radiation. WO2007/042049, however, relies exclusively on the use of peroxide initiators.

According to the present invention, there is provided a method of producing a flexible pipe body, said method comprising:

extruding a polyethylene composition comprising a carbon-carbon thermal initiator to form a tubular layer,

cross-linking the tubular layer, and

applying an armour layer around the crosslinked tubular layer.

Advantageously, the carbon-carbon thermal initiators employed in the present invention are compatible with polyethylene compositions that are suitable for use in the manufacture of flexible pipe bodies. Accordingly, polyethylene compositions comprising effective amounts of carbon-carbon thermal initiators are suitable for extrusion. The carbon-carbon thermal initiators may also have sufficient thermal stability and/or sufficiently low volatility to remain homogeneously dispersed in the polyethylene composition, for example, during the extrusion step. Accordingly, the bulk of the initiator incorporated into the polyethylene composition remains substantially in its unreacted form during extrusion. The carbon-carbon thermal initiators of the present invention may also be less prone to degradation than, for example, initiators, such as azo compounds that degrade to form gaseous by-products (e.g. nitrogen) that expand and cause foaming in the tubular layer.

Preferably, the carbon-carbon initiator has the formula:

wherein n is an integer of 1 to 10,

X_! to X₈ are independently selected from at least one of hydrogen, halogen, alkyi, alkoxy, cyano, nitro, nitrile, hydroxyl and amino groups;

Zi to Z₆ are independently selected from at least one of hydrogen, halogen, alkyi, alkoxy, aryl, aryloxy, cycloalkyi, vinyl, phenyl, cyano, nitro, nitrile, hydroxyl, amino, carboxyl, ester, amide, thio, epoxide, silyl and silyloxy groups.

Preferably, n is 1. In one embodiment, X_! to X₈ and and Z₆ are each hydrogen and Z₂ to Z₅ are each an alkyi group. For example, Z₂ to Z₅ may each be an alkyi group having 1 to 10 carbon atoms.

In one embodiment, the carbon-carbon initiator has the formula:

(m),(m +1)-dialkyl-(m),(m+1)-diphenylalkane, wherein m is an integer of 2 to 10.

Each alkyi group may be independently selected from an alkyi group having 1 to 4 carbon atoms. In one embodiment, the alkyi group may be a methyl, ethyl, propyl, n-butyl, i-butyl or t-butyl group. The diphenylalkane may be selected from a diphenyl(C₄ -C₂o)alkane, for example, a diphenylbutane or a diphenylhexane. In a preferred embodiment, the carbon-carbon initiator is selected from 2, 3-dimethyl-2,3- diphenyl butane and 3,4-dimethyl-3,4-diphenyl hexane. Most preferably, the carbon-carbon initiator is 3,4-dimethyl-3,4-diphenyl hexane.

The carbon-carbon initiator may be a hydrocarbon. Hydrocarbons are preferred, as they are compatible with the polyethylene that requires crosslinking.

The carbon-carbon initiator may be liquid. This may be advantageous as a liquid initiator may be conveniently impregnated into the polyethylene composition, for example, by soaking. In one embodiment, the polyethylene composition is provided in particulate form, for example, as pellets or powder. The particulate composition may be soaked with the liquid initiator, for example, as a precursor step to provide the polyethylene composition for extrusion.

The carbon-carbon initiator may also be solid. Solid initiators may be compounded with, for example, particles (e.g. pellets or powder) of the polyethylene composition to provide the polyethylene composition for extrusion.

The carbon-carbon initiator may have a 10 hour half-life degradation temperature of at least 150 degrees. Preferably, the minimum 10 hour half-life degradation temperature is at least 160 degrees C, more preferably at least 180 degrees C. The maximum 10 hour half-life degradation temperature may be at most 250 degrees C, more preferably at most 230 degrees C, most preferably at most 210 degrees C. In one embodiment, the 10 hour half-life temperature is, for example, 160 to 250 degrees C. Advantageously, the 10 hour half-life degradation temperature of the carbon-carbon thermal initiator is sufficiently high to allow extrusion to be performed without a significant risk of extensive crosslinking taking place during the extrusion step. An advantage of the present invention is that relatively high extrusion temperatures may be employed. At the same time, the 10 hour half-life temperature of the carbon-carbon thermal initiator is sufficiently low to allow crosslinking to be performed at temperatures that do not cause significant thermal degradation of the polyethylene composition.

In one embodiment, the carbon-carbon initiator has an activation energy of at least 140 kJ/mole, preferably at least 148 kJ/mole, more preferably at least 150 kJ/mole. In one embodiment, the activation energy is at most 230 kJ/mole, preferably, at most 210 kJ/mole. In a preferred embodiment, the activation energy is in the range of 145 to 230 kJ/mole, for instance, 150 kJ/mole to 205 kJ/mole.

In one embodiment, the method of the present invention comprises the step of preparing the polyethylene composition comprising the carbon-carbon thermal initiator. This step may comprise mixing the carbon-carbon thermal initiator with a polyethylene composition. As discussed above, where the carbon-carbon thermal initiator is liquid, this liquid may be impregnated into particles (e.g. pellets or powder) of the polyethylene composition. Where the carbon-carbon thermal initiator is a solid (e.g. a powder or granules), the solid may be compounded with the polyethylene composition.

In general, the amount of carbon-carbon thermal initiator in the polyethylene composition may be at least 0.1 % by weight of the polyethylene composition, for example, between 0.2 and 5 % by weight of the polymer composition (including the initiator). In one embodiment, the polymer composition contains initiator in an amount of from 0.1 to 2.5 % by weight, and preferably from 0.3 to 1.5 % by weight of the total polymer.

The polyethylene composition comprising the carbon-carbon thermal initiator is extruded (e.g. in an extrusion zone) to form a tubular layer. This extrusion step may be carried out using any suitable extrusion method. For example, the composition may be extruded onto a tubular carcass. The carcass may optionally form the innermost layer of the flexible pipe body. The extruded tubular layer may form the internal pressure sheath of the flexible pipe body. Alternatively or additionally, the extruded tubular layer may form any intermediate polymer layer of the flexible pipe body.

Prior to extruding the polyethylene composition onto a region ("receiving region") of the carcass, the receiving region of the carcass may be heated. For example, the receiving region of the carcass may be heated to a temperature of at least 150 degrees C. The temperature, however, should preferably not exceed the activation temperature of the carbon-carbon thermal initiator as this may initiate crosslinking prematurely. Crosslinking initiated by heat transferred from the carcass may result in undesired tensioning in the crosslinked polyethyelene composition, which may compromise the mechanical properties of the extruded tubular layer. For example, in one embodiment, the temperature is less than 180 degrees C, preferably less than 170 degrees C.

Extrusion of the polyethylene composition may be performed at a temperature of 100 to 240 degrees C, preferably 110 to 200 degrees C. The extrusion temperature should desirably be high enough to allow the polyethylene composition to soften or melt sufficiently for extrusion to take place. However, the extrusion temperature should be low enough to avoid crosslinking taking place in the extrusion step to any significant degree. Where the polyethylene composition is extruded onto a heated carcass, the carcass should preferably be heated to a temperature that is equal to or less than the temperature of the extrusion step, for example, equal to or at most 10 degrees C less than the temperature of the extrusion step.

Any suitable polyethylene may be used to form the polyethylene composition that is extruded to form the tubular layer. In a preferred embodiment, high density polyethylene (HDPE) is used.

The polyethylene composition may comprise at least 50 % by weight, preferably at least 70 % by weight, more preferably at least 85 % by weight of polyethylene. The polyethylene composition may also optionally include up to about 40 % by weight, preferably up to about 20 %, more preferably up to about 10 % by weight of additional polymer(s) other than polyethylene. The additional polymer(s) may be selected from thermoplastics, for example, thermoplastic elastomers. Examples include block copolymers, such as SEBS, SBS, SIS, TPE-polyether-amide, TPE-polyether-ester, TPE-urethanes, TPE PP/NBR, TPE-PP/EPDM, TPE-vulcanisates and TPE-PP/IIR; rubbers, such as butadiene rubber, isoprene rubber, nitrile rubber, styrene-butadiene rubber and urethane rubber; polyolefins, such as

polypropylene and polybutylene including its isomers; liquid crystal polymers; polyesters; polyacrylates; polyethers; polyurethane; thermplastic vulcanisates; and liquid silicone rubber.

In addition to the initiator, the polyethylene composition may also contain other additives, including, for example, pigments, heat stabilisers, process stabilisers, metal deactivators, flame-retardants and/or reinforcement fillers. It is preferred to keep the amount of such additives low to reduce the risk of blistering and stress induced cracking. The reinforcement fillers may e.g. include glass particles, glass fibres, mineral fibres, talcum, carbonates, mica, silicates, and metal particles.

Prior to crosslinking (and, optionally, prior to the incorporation of the initiator), the

polyethylene composition may have a density of at least 920 g/cm³, preferably at least 940 g/cm³. In one embodiment, the polyethylene has a density between 945 and 955 g/cm³. Prior to crosslinking (and, optionally, prior to the incorporation of the initiator), the

polyethylene composition may have an elastic modulus of 400 to 800 MPa. The tubular layer may be extruded from a single polyethylene composition. Alternatively, the tubular layer may be co-extruded from two or more polymer compositions, at least one of which comprises a carbon-carbon thermal initiator.

The tubular layer may have a thickness of at least 4 mm, for example, at least 6 mm or at least 8 mm. The tubular layer may have a thickness of 25 mm or less, for example, 20 mm or less. In one example, the tubular layer may have a thickness of 15 mm or less.

Once extruded, the tubular layer is crosslinked, for example, in a crosslinking zone. The crosslinking zone may be located downstream, for example, adjacent the extrusion zone. Crosslinking may be achieved using any suitable method. For example, crosslinking may be achieved by heating the tubular layer to at least the activation temperature of the initiator. In one embodiment, crosslinking is achieved by heating the tubular layer to a temperature that is 0 to 60 degrees C greater, preferably, 20 to 30 degrees C greater than the 10 hr half-life activation temperature. In one embodiment, crosslinking is achieved by heating the tubular layer to a temperature greater than 200 degrees C, for example, from 200 to 300 degrees C, preferably at least 240 degrees C. Heating may be achieved using any suitable method. For example, a salt bath may be employed. Alternatively or additionally, radiation may be used. For example, the tubular layer may be exposed to electromagnetic radiation (e.g. infrared or microwave radiation) for sufficient time to raise the temperature of the tubular layer to initiate crosslinking.

In one example, crosslinking may be achieved by exposing the extruded polymer material to electromagnetic waves. Suitable forms of electromagnetic radiation may have wavelengths in the range of 1 km to 10^"2 nm. Suitable forms of electromagnetic radiation include radio, near infrared, infrared, microwave and UV radiation. In one embodiment, the cross-linking is activated by application of infrared radiation comprising wavelengths in the range 0.5-10, preferably in the range 0.8-6.0 μηι, for example in the range 1.0-5.0 μηι.

In one embodiment, the pressure in the crosslinking zone is raised to reduce the risk of formation of bubbles and irregularities. The pressure applied in this embodiment is applied on the outer side of the tubular layer.

The crosslinked tubular layer may be subsequently cooled. In one embodiment, the cooling step is performed using a coolant, such as air or water. In one example, the tubular layer is cooled in a water bath. The crosslinked tubular layer may have a density of 925 g/cm³, preferably at least 940 g/cm³. In one embodiment, the crosslinked tubular layer has a density between 945 and 955 g/cm³.

The crosslinked tubular layer may have a notch sensitivity/impact strength that is greater than that of the uncrosslinked tubular layer. Without wishing to be bound by any theory, this is believed to be because of the lower crystallinity of the crosslinked polymer. This reduction in crystallinity may improve the ductility of the crosslinked polymer at low temperature. As a result, the notch sensitivity/impact strength of the polymer may be improved.

The crosslinked tubular layer may have improved fluid compatibility than the uncrosslinked tubular layer. Without wishing to be bound by any theory, crosslinking may improve the swelling resistance of the polymer. This, in turn, may benefit the aging characteristics of the tubular layer, as the layer may be less prone to swelling in hydrocarbon media.

The crosslinked tubular layer may have an elastic modulus of 500 to 2000 MPa. The elastic modulus of the crosslinked tubular layer is advantageously greater than that of the tubular layer prior to crosslinking. For example, 10 to 50 % increase in elastic modulus may be observed. As a result of this increase in elastic modulus, the mechanical properties of the tubular layer may be improved.

In the method of the present invention, the tubular layer is surrounded by one or more armour layers. The armour layer may be a pressure armour layer and/or a tensile armour layer. The armour layer may be formed of metal or of ceramic material.

In one embodiment, the tubular layer is surrounded by a pressure armour layer. The pressure armour layer may be formed from an elongate strip of carbon steel having a generally Z-shaped cross-sectional profile. The strip is formed from a wire rolling process to have corresponding male and female connector portions such that as the strip is wound over the polymeric layer adjacent windings interlock. When pressure is applied within the bore of the tubular layer, the tubular layer may press against the pressure armour layer. The polymer in the crosslinked tubular layer, however, has a relatively higher elastic modulus. Accordingly, it is more resistant to flow and the risk of creep into the gaps in the armour layer is reduced. By increasing the elastic modulus of the tubular layer (e.g. by crosslinking), it may also be possible to improve the high temperature resistance of the flexible pipe. As the elastic modulus of the tubular layer is relatively high, it is resistant to softening upon exposure to high temperatures. Advantageously, this may reduce the risk of creep and, ultimately, layer thinning. As a result the high temperature performance of the pipe may be improved. . In one embodiment, the crosslinked tubular layer may have a range of operation of -60 to 95 degrees C.

Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

Fig. 1 illustrates a flexible pipe body; and

Fig. 2 illustrates a riser assembly.

In the drawings like reference numerals refer to like parts.

For example, it will be understood that a flexible pipe is an assembly of a portion of a pipe body and one or more end fittings in each of which a respective end of the pipe body is terminated. Fig. 1 illustrates how pipe body 100 may be formed from a combination of layered materials that form a pressure-containing conduit. It is to be noted that the layer thicknesses are shown for illustrative purposes only.

As illustrated in Fig. 1 , a pipe body may include an optional innermost carcass layer 101. The carcass provides an interlocked construction that can be used as the innermost layer to prevent, totally or partially, collapse of an internal pressure sheath 102 due to pipe decompression, external pressure, and tensile armour pressure and mechanical crushing loads. As is known in the technical field, there are 'smooth bore' operations (i.e. without a carcass) as well as 'rough bore' applications (with a carcass). The carcass layer may be formed from helically wrapped metallic tape having a shaped cross section to allow interlocking of adjacent wrapped tape portions. The internal pressure sheath 102 may be formed using a method according to the present invention.

The internal pressure sheath 102 acts as a fluid retaining layer and comprises a polymer layer that ensures internal fluid integrity. It is to be understood that this layer may itself comprise a number of sub-layers. It will be appreciated that when the optional carcass layer is utilised the internal pressure sheath is often referred to by those skilled in the art as a barrier layer. In operation without such a carcass (so-called smooth bore operation) the internal pressure sheath may be referred to as a liner. In addition, and not shown in Fig. 1 , there may also be included a wear layer between the carcass layer and internal pressure sheath. The wear (or sacrificial) layer may be a polymer layer (often extruded but sometimes in tape form) intended to provide a smoother surface or bed for the barrier layer to be extruded onto than would be the case over the carcass layer, which may have undulations and gaps between wraps; this smoother wear layer surface may allow the barrier layer to experience higher levels of general strain (extension) as a result of bending and pressure because what local stress concentrations remain are relatively small and insignificant. Without such a wear layer the extruded polymer barrier may exhibit an undulating inner surface with protruding cusps of material that have naturally flowed into gaps in the carcass layer during the extrusion process; these cusps act as stress concentrators when the polymer is strained.

A pressure armour layer 103 is a structural layer that increases the resistance of the flexible pipe to internal and external pressure and mechanical crushing loads. The layer also structurally supports the internal pressure sheath, and typically consists of an interlocked construction of wires with a lay angle close to 90°.

The flexible pipe body also includes an optional first tensile armour layer 105 and optional second tensile armour layer 106. Each tensile armour layer is used to sustain tensile loads and internal pressure. The tensile armour layer is often formed from a plurality of metallic wires (to impart strength to the layer) that are located over an inner layer and are helically wound along the length of the pipe at a lay angle typically between about 10° to 55°. The tensile armour layers are often counter-wound in pairs.

The flexible pipe body shown also includes optional layers of tape 104 which help contain underlying layers and to some extent prevent abrasion between adjacent layers.

The flexible pipe body also typically includes optional layers of insulation 107 and an outer sheath 108, which comprises a polymer layer used to protect the pipe against penetration of seawater and other external environments, corrosion, abrasion and mechanical damage. Each flexible pipe comprises at least one portion, sometimes referred to as a segment or section of pipe body 100 together with an end fitting located at least one end of the flexible pipe. An end fitting provides a mechanical device which forms the transition between the flexible pipe body and a connector. The different pipe layers as shown, for example, in Fig. 1 are terminated in the end fitting in such a way as to transfer the load between the flexible pipe and the connector.

The internal pressure sheath 102 of the pipe body 100 of Figure 1 may be formed of a tubular layer comprising a polymer, wherein the polymer is crosslinked such that the elastic modulus at an outer region of the tubular layer is greater than the elastic modulus at an inner region of the tubular layer.

Fig. 2 illustrates a riser assembly 200 suitable for transporting production fluid such as oil and/or gas and/or water from a sub-sea location 201 to a floating facility 202. For example, in Fig. 2 the sub-sea location 201 includes a sub-sea flow line. The flexible flow line 205 comprises a flexible pipe, wholly or in part, resting on the sea floor 204 or buried below the sea floor and used in a static application. The floating facility may be provided by a platform and/or buoy or, as illustrated in Fig. 2, a ship. The riser assembly 200 is provided as a flexible riser, that is to say a flexible pipe 203 connecting the ship to the sea floor installation. The flexible pipe may be in segments of flexible pipe body with connecting end fittings.

It will be appreciated that there are different types of riser, as is well-known by those skilled in the art. Embodiments of the present invention may be used with any type of riser, such as a freely suspended (free, catenary riser), a riser restrained to some extent (buoys, chains), totally restrained riser or enclosed in a tube (I or J tubes).

Fig. 2 also illustrates how portions of flexible pipe can be utilised as a flow line 205 or jumper 206.

It will be clear to a person skilled in the art that features described in relation to any of the embodiments described above can be applicable interchangeably between the different embodiments. The embodiments described above are examples to illustrate various features of the invention. Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

1 . A method of producing a flexible pipe body, said method comprising:

cross-linking the tubular layer, and

applying an armour layer around the crosslinked tubular layer.

2. A method as claimed in claim 1 , wherein the carbon-carbon initiator has the formula:

wherein n is an integer of 1 to 10,

X_! to X₈ are independently selected from at least one of hydrogen, halogen, alkyl, alkoxy, cyano, nitro, nitrile, hydroxyl and amino groups;

Ζ·_\ to Z₆ are independently selected from at least one of hydrogen, halogen, alkyl, alkoxy, aryl, aryloxy, cycloalkyi, vinyl, phenyl, cyano, nitro, nitrile, hydroxyl, amino, carboxyl, ester, amide, thio, epoxide, silyl and silyloxy groups.

3. A method as claimed in claim 1 or 2, wherein n is 1.

4. A method as claimed in claim 2 or 3, wherein X_! to X₈ and Zi and Z₆ are hydrogen and Z₂ to Z₅ are each an alkyl group.

5. A method as claimed in any one of claims 2 to 4, wherein Z₂ to Z₅ are each an alkyl group having 1 to 10 carbon atoms.

6. A method as claimed in any one of the preceding claims, wherein the carbon-carbon initiator is selected from 2, 3-dimethyl-2,3-diphenyl butane and 3,4-dimethyl-3,4-diphenyl hexane.

7. A method as claimed in any one of the preceding claims, wherein the carbon-carbon initiator is liquid.

8. A method as claimed in claim 7, wherein the polyethylene composition is prepared by impregnating a polyethylene composition with the liquid carbon-carbon initiator.

8. A method as claimed in claim 1 , wherein the carbon-carbon initiator is an (m),(m +1)- dialkyl-(m),(m+1)-diphenylalkane, wherein m is an integer of 2 to 10.

9. A method as claimed in claim 8, wherein each alkyl group is independently selected from an alkyl group having 1 to 4 carbon atoms and the diphenylalkane is selected from a diphenyl(C₄ -C₂o)alkane.

10. A method as claimed in any one of the preceding claims, wherein the carbon-carbon initiator has a 10 hour half-life degradation temperature above 150 degrees C.

11. A method as claimed in any one of the preceding claims, wherein the carbon-carbon thermal initiator has an activation energy in the range of 145 to 230 kJ/mole.

12. A method as claimed in any one of the preceding claims, wherein the tubular layer is crosslinked by exposing the tubular layer to electromagnetic radiation.

13. A method as claimed in any one of the preceding claims, wherein the tubular layer is crosslinked by heating the tubular layer to a temperature of at least 240 degrees C.

14. A method as claimed in any one of the preceding claims, wherein the crosslinked tubular layer has an elastic modulus of 500 to 2000 MPa.

15. A method as claimed in any one of the preceding claims, wherein the carbon-carbon initiator is present in a concentration of 0.1 to 5.0 weight % of the polyethylene composition prior to the crosslinking step.