Overtrawlability and Mechanical Damage of Pipe-in-Pipe

A pipeline on the seabed may be struck by moving trawl gear, and that may damage the pipeline. Trenching can be a useful but expensive way to protect the pipeline. Pipe-in-pipe and bundled pipeline systems are widely used in the offshore industry recently because of their high level of thermal insulation and because they lend themselves to rapid and economical installation. However, there is no clearly specified standard method to analyze the overtrawlability of pipe-in-pipe systems. If we apply the same method as for the single wall pipe, it is likely to result in a conservative design for the pipe-in-pipe. The objective of this paper is to investigate the overtrawlability of pipe-in-pipe, especially in the impact phase, and to fill this gap. In this study, the authors demonstrate that a quasi-static analysis can replace a dynamic analysis to some extent because the overall response does not show a big difference. The demonstration is based on both quasi-static indentation tests and impact tests for single wall pipe and pipe-in-pipe, as well as the corresponding finite element (FE) models. The FE models not only help to compare the responses but also offer a way to analyze the overtrawlability of the pipe-in-pipe. The quasi-static FE models are used for a further comparison between a pipe-in-pipe and a 406.4 mm (16 in.) single wall pipe to illustrate the overtrawlability of the pipe-in-pipe.