Low Cycle Fatigue of Subsea Mechanically Lined Pipeline With Liner Imperfections

Abstract

There is an ever-increasing demand for subsea transport of corrosive constituents which requires the use of corrosion resistant pipelines. This has generated interest in mechanically lined pipe (MLP) which consists of carbon steel pipe lined with a thin layer of corrosion resistant alloy (CRA), typically stainless steel. The CRA liner is adhered to the backing pipe by means of an interference fit. MLPs have been traditionally installed subsea using low strain methods such as towing, S-lay or J-lay. More recently, the efficient reel-lay method, typically used for pipelines up to 18” (457.2 mm) in diameter, has also been considered. To prevent damage to the MLP during high strain bending (i.e. wrinkling of CRA the liner) and thus allow reel-lay installation, TechnipFMC has qualified reeling of MLPs at ambient and elevated pressures. The ambient reeling approach, where the liner thickness is increased to prevent wrinkling during reeling, is appropriate for smaller diameter MLPs. For larger pipelines, it is generally more cost-effective to pressurise the MLP during reeling. Concerns have been expressed that liner imperfections such as small dents or wrinkles, introduced during manufacturing, installation or service, may compromise the integrity of the MLP subjected to high in-service cyclic loading. Therefore, this study was undertaken to examine the criticality of such flaws and determine the low cycle fatigue endurance of reeled MLPs with imperfections. First, a numerical study was undertaken to estimate in-service stress/strain ranges in the MLPs with liner flaws. Subsequently, small scale tests were carried out to quantify the fatigue performance of such MLPs. The obtained results confirmed that there is a negligible risk of failure of MLP flowlines due to crack initiation at liner imperfections and subsequent breach of the CRA layer, even for pipelines subjected to very severe inservice cyclic loading.