Fellow’s Corner

Fellows Corner

Non-metallic materials are an essential element of facilities engineering in upstream E&P operations, being widely used in a range of functions from seals and corrosion barriers to piping and structural elements. This short article offers a brief insight into the capabilities of some of the available options.

In common with metallic materials, the selection and successful use of non-metallics is dependent on a detailed understanding of the way in which each material responds to the service environment over the life of the component, or system. Degradation of the capabilities of non-metallic materials can occur through a range of physical and chemical processes.

Elastomers (or rubbers), are widely used in oilfield sealing applications. These are highly elastic, polymeric materials, used in compression seals in a range of downhole, subsea, topsides and pipeline applications. Various nitrile and fluorocarbon-based materials are typically used to span the range of temperatures, pressures and fluid environments, met in oilfield operations. Processing of these materials involves “vulcanisation” or curing, using small amounts of sulphur, amines or peroxides to create highly flexible and extendable polymers.

A number of key failure modes can affect elastomer seals.  Some relate to the elastomer material being used outside its working temperature range, or in fluids with which it is incompatible. This can lead to chemical embrittlement, softening, compression set, large volume changes, and loss of elasticity at low temperature – any or all of which can lead to a seal failing. Pressure related failure modes can also be important. Extrusion damage occurs when a rubber seal is forced into the gap which it is sealing as a result of the applied pressure. Gas decompression damage occurs primarily in dry gas duty, being qualitatively similar to the “Bends” suffered by divers when returning to surface.

Qualification of seals and the material’s performance in any component or system, is typically carried out using a combination of materials and system testing, taking account of the time and temperature dependent properties of the materials involved. Finite element analysis (FEA) modelling of the complex, non-linear and time dependent materials properties of elastomers has proven vital in understanding some applications.

Further applications of a range of elastomer materials are to be found in hydraulic and transfer hoses, and in the flexible joints that are incorporated in metal drilling and catenary risers.

Thermoplastic materials, such as polyethene and nylon, find wide application in controlling the internal corrosion of steel pipelines. Such materials are fundamentally different in nature to elastomers, having a much smaller elastic range, and the way in which they are used is therefore somewhat different. As their name suggests, these materials are reversibly melt-processible, often being formed by extrusion for oilfield use. Materials are typically differentiated by their maximum service temperature capabilities, and their resistance to particular service fluids. So, for example, polyethylenes are typically used in water duties to a maximum of 60°C or so, while nylons can be used in hydrocarbon production service up to 90°C.

Thermoplastic liners have an extensive track record, both onshore and offshore, in providing a corrosion barrier within carbon steel pipelines, particularly for water injection service, where suitable metallic options are typically much more expensive or much less reliable. There are a number of “pull-through” liner technologies which can offer cost effective solutions to mitigate internal corrosion challenges in both new build projects and in rehabilitation. Often these involve “tight-fit” polyethylene liners, which have their outer diameter mechanically reduced, while they are pulled into a steel pipe. Release of the pulling force allows the polymer to relax back against the internal diameter of the pipe, which remains the structural element of the pipeline. Another option is the use so-called Reinforced Thermoplastic Pipes (RTPs) which are used, with good economic benefit, as loose fit, “slip liners” or even as stand-alone pipelines, in a range of production and injection services. These are composites in which glass, aramid or carbon fibre, or wire, reinforcement, is wound over a plastic pipe, in order to increase its pressure capability.

Thermoplastic materials additionally find wide application in unbonded flexible pipes, importantly being used as the internal and external sheathes in these complex pipe structures. The flexibility of these pipes often enables faster or more convenient offshore installation and hook-up, and provides excellent fatigue resistance in a range of harsh environments. Several thermoplastic materials are used as internal pressure sheathes, responsible for primary containment. Nylon materials are widely used in production service up to 60 – 90°C, with fluoropolymers used at high temperature, to around 130°C. In water injection, polyethylene is normally used. External, or outer, plastic sheathes contain the whole pipe structure, helping to keep the high strength steel wire reinforcement out of contact with seawater. Typically, this sheath is made of polyethylene for static pipes and nylon for pipes used in dynamic service.

Thermosetting materials, such as epoxy and phenolic resins, form the basis of a further set of related oilfield corrosion protection technologies. These materials employ a chemical hardener to permanently, and irreversibly, “set” the polymeric resin, often with temperature applied during curing to accelerate that reaction.

Fusion bonded epoxy (FBE) is used very widely as an external pipeline coating. It is applied as a powder to a carefully prepared surface, and melted and cured in situ to give a coating approximately 0.5 mm thick.

FBE also finds wide use as the base layer in multilayer coatings with polyethylene and polypropylene. Further, options for subsea insulation involving the incorporation of glass microspheres into thermoplastic layers are also widely accepted, for use on subsea pipelines. The thermoplastic nature of these materials allows the pipelines to be reeled for transport and installation, where required. Where more rigid insulation is acceptable, systems incorporating glass-microspheres into epoxy resins can be applied, for example to subsea manifolds.

Epoxy resins, and similar materials such as vinyl esters, find wide use in the painting and external protection of structures and equipment, as well as in the internal coating of vessels, typically in combination with glass flake fillers.

Glass reinforced epoxy pipes find a range of downhole, piping and pipeline applications, mostly in water service. Typically, this kind of pipe is rated to 16 bar design pressure, although some small diameter products can go much higher than this, for example in downhole tubing applications. A range of adhesively bonded, mechanically jointed and threaded connections are used across the industry. Qualification of composite pipes, and other non-metallic pipe options, is normally undertaken through a series of full-scale pipe tests, including: pressure rating using long term (10,000 hr) testing of pipe and end fittings, characterisation of minimum bend radius for storage, transportation and operation, characterisation of axial load capability, testing of capability of the product to handle gas service, and performance of the product in UV. A range of other engineering design issues also need to be worked through with each product, such as internal surface roughness, heat transfer co-efficient, and pipe expansion due to pressure and temperature.

Glass reinforced epoxy pipes can also be used as a liner, with composite lined downhole tubing having a long track record of successful onshore use in a range of corrosive services, and in offshore water injection. Some composite liners are capable of continuous service at up to 80°C in water-based applications. Insertion of the stiff liner into the steel host, on a joint-by-joint basis, leaves a small annulus between the liner and the host which is typically filled with cement, to transfer mechanical and pressure loads to the carbon steel host. Modified tubing connections allow the liner to be properly terminated, with thermoplastic corrosion barriers providing continuity of corrosion performance.

Finally, it is worth mentioning the use of external epoxy composite wraps to repair and reinforce topsides piping. This is a very convenient repair technology that does not involve hot work and which can be used to seal thinned, cracked or holed piping, at very least as a temporary solution until full repair can be affected.

The use of non-metallic components is an integral part of the materials selection challenge in oil & gas production. Given their frequent role in maintaining a primary or secondary containment, selection and use of these materials should be as carefully scrutinised as with the metallic components within any well, processing facility or pipeline.

For additional information see,  B Kermani and D Harrop, Corrosion and Materials in Hydrocarbon Production; A Compendium of Operational and Engineering Aspects, Wiley, 2019, Chapters 9 & 15.