Corrosion Resisting Liners - The Results of an Investigation into Plastic Lining of Pipelines for Corrosive Hydrocarbon Service

Abstract

Abstract While onshore oilfield experience has established that internal plastic liners sucessfully prevent corrosion of carbon steel pipe transporting water and hydrocarbon based fluids. The same experience has demonstrated that in case of multiphase production service, with associated gas, plain plastic liners can have a limited life. One significant failure mode of plastic liners involves the permeation of (associated) gas through the plastic liner and accumulation of that gas in the annulus between the liner and steel host. This annular gas expands during depressurizations of the bore leading to liner collapse. The use of vents onshore has proven, at best, partially successful in managing this failure mode. The Corrosion Resistant Liners (COREL) Joint Industry Project (JIP) has addressed methods for preventing annular gas induced collapse. Phase I of the JIP was devoted to brainstorming potential technical solutions, the grooved liner and the perforated liner concepts being taken forward. Phase II carried out desk studies and small-scale tests to determine whether these solutions were feasible. Phase III placed both solutions, each applied to Polyethylene (PE) and Polyamide-11 nylon (PA- 11), in a dynamic test loop, simulating multiphase production service, at temperatures up to 80°C and pressures up to 100bar, with regular pressure fluctuations over a 26-week period. The combined results of all this work demonstrate that the grooved liner and the perforated liner concepts both prevent liner collapse and mitigate corrosion, opening the way for wider application of plastic liners in production flowline applications. Introduction Corrosive produced fluids can be handled using a number of established technologies, including chemically inhibited carbon steel and corrosion resistant alloys (CRAs). Another option, internal plastic liners in carbon steel flowlines, has been extensively and successfully used for water handling, onshore in North America and The Middle East, and offshore in the North Sea. The plastic liner acts as a corrosion barrier, while the carbon steel host pipe carries the pressure, and other loads. Plastic lining of carbon steel production flowlines potentially removes either the requirement to chemically treat fluids, leading to large savings in operating costs, or the need for CRAs, typically saving 20% to 50% in initial capital expenditure. Liner collapse is the major technical hurdle preventing the wider use of this technology for gas service or hydrocarbon multiphase service. In this context, two collapse mechanisms need to be considered: longitudinal buckling and circumferential collapse. Differential thermal expansion and swelling, due to uptake of hydrocarbons, can under certain conditions lead to longitudinal buckling, in which the liner forms "knots" within the steel host. This is the predominant failure mode of "loose" liners, and "tight" liners were developed to overcome this failure mode. "Tight" liners have a positive interference fit between the relaxed liner outside diameter and the steel inside diameter, typically 1% to 3%. However, in "tight" fit liners, circumferential collapse can be a major issue.