An alternative to highly expensive corrosion resistant alloys used to transport aggressive hydrocarbons 1s to use cheaper steels lined with a thermoplastic This paper describes some key parameters for the material selection of the liners and the test protocols used to characterize them to ensure fitness for purpose.
The continuous need to reduce production costs IS a major economic objective both within British Gas and the Exploration and Production Industry In general One way of achieving this is to keep off-shore treatment of the produced oil or gas to a minimum. This results in harsher service conditions for flowlines Coupled with this is the ageing of existing fields and new more marginal f fields being exploited resulting In the subsea flowlines encountering increasing levels of CO2, and H2S This introduces additional costs to deal with the problems of internal corrosion in the sub-sea pipeline associated with the carrying of a wet aggressive hydrocarbon With conventional carbon steel pipelines, Internal corrosion 1s usually dealt with by using a combination of Internal corrosion allowance, chemical inhibition and fluid processing This has the attendant problems of requiring extra off-shore facilities and being difficult to optimize throughout the length of the pipeline Technical and economic evaluations of the various methods decide the optimum solution for a particular flowline With service conditions becoming harsher, new solutions for internal corrosion problems are required One approach IS the use of specified corrosion resistant alloys, such as Duplex stainless steel and Duplex clad carbon steel, but the high cost can prove prohibitive and may render some fields uneconomic It may be possible in some cases to use carbon steel with a large corrosion allowance and continuous chemical inhibition But, although this reduces the capital expenditure advantage over more exotic pipe material, it increases the operating costs to such an extent that a carbon steel flowline system is not cost-effective, and may even be impractical. An alternative, and potentially lower cost solution, is to Insert a thermoplastic liner into a conventional C-Mn steel pipe to act as a barner between the product and the steel The reduction in pipeline costs is particularly pertinent to British Gas since the proportion of costs associated with pipeline for gas field development are appreciably higher than for oil fields (Cooper, 1988)
The use of non-bonded hers for corrosion control has been exploited for a number of years m the chemical industries It is also found, on a more limited scale, In the oil and gas industries for both water and hydrocarbon service, particularly in the USA Thermoplastic lined pipe has been used for water injection in the North Sea but there is a need for performance and design data before it can be exploited for hydrocarbon service The lining of ‘low pressure’ (<10 bar) gas and water distribution pipes with polyethylene for the renovation of existing pipes using techniques such as British Gas's swagelining process is already well established