EFFECT OF PRE-STRAIN ON BENDING STRAIN CAPACITY OF MECHANICALLY LINED PIPE

The high demand for subsea transportation of corrosive wellhead produced fluids has created interest in economical mechanically lined pipes (MLP) made of external carbon steel and a thin internal layer of corrosion resistant alloy (CRA). The bending strain capacity of an MLP, where a CRA liner is adhered to a carbon steel host pipe by means of an interference fit, is often governed by the liner wrinkling limit state. Although the strain capacity of the MLP with a typical 3 mm thick liner is enough to withstand bending to strains encountered during installation with the S-lay or J-lay method, the liner is at risk of wrinkling when the MLP is subjected to higher bending strains during reel-lay. To allow reeled installation, the liner strain capacity is enhanced by either increasing the liner thickness or pressurizing the MLP during installation. In the former approach, the required liner thickness is proportional to the pipe diameter. For larger diameter MLPs, it is therefore often more economical to select a 3 mm thick liner and flood and pressurize an MLP to ensure liner stability during reeling. However, the MLP may need to be depressurized and partially drained during installation to allow welding a structure, performing reel-to-reel connection or pipeline recovery which impose bending strain on a plastically pre-strained and depressurized pipeline. Furthermore, reeled pipelines may be depressurized subsea while subjected to bending loads from operation. Although there is a history of research into the limit loads and failure modes of MLPs, there is still no comprehensive guidance on determining the risk of liner wrinkling in plastically pre-strained MLPs. In this paper, an approach is proposed for evaluating the strain capacity and assessing the risk of liner wrinkling after an MLP, subjected to plastic bending during reeled installation at elevated pressure, is depressurized and subjected to installation loads during offshore intervention or operational loading in service. The combined effect of strain history at elevated pressure, reeling-induced ovality, bending direction after depressurization, differential pressure, temperature and residual strain is discussed. The recommendations for further work are also given.