Flexible liners for corrosion protection of pipelines

Flexible plastic liners are sometimes installed into new and existing oil and gas pipelines to prevent corrosion of the pipe wall. A practical difficulty of this method is that the plastic liners are permeable to gases, which can collect and form an annular space between the liner and the pipe. If the operating pressure in the pipe decreases then the collected gas can cause the liner to collapse and block the pipe. One method for overcoming this problem is to insert vents at intervals along the liner to allow the gas to escape into the pipe during depressurisation. However, there is concern that this arrangement might lead to excessive corrosion beneath the vent where the pipe wall is exposed. The rate of corrosion is expected to be controlled by the vent size but this principle needs to be confirmed by experiment. The work described in this thesis is aimed at investigating this corrosion by experiment for a range of conditions typical of oil and gas production. A novel crevice corrosion cell was designed, consisting of an X100 carbon steel plate and a sheet of transparent Perspex, separated by a thin gasket. A small hole in the Perspex simulated a liner vent and allowed carbon dioxide to reach the steel surface. Tests were carried out in 3.5% NaCl solutions saturated with carbon dioxide at 1 bar partial pressure. Corrosion rates along the length of the annular space were measured using the Linear Polarisation Resistance (LPR) technique on pairs of insulated X100 electrodes set into the plate. The corrosion rates within the annular space have been shown to be small compared to those in the bulk solution and to diminish rapidly with distance from the vent. Mathematical modelling, based on the transport of carbon dioxide, is described to explain these findings and support the experimental work. The effectiveness of the LinerVentTM, installed over the vent, in a turbulence pipeline was demonstrated. The benefit of applying cathodic protection within the annular space was also demonstrated. The results are discussed in terms of the fundamental corrosion principles and their practical implications