University of Cambridge > > Engineering Department Geotechnical Research Seminars > Overtrawlability and Mechanical Damage of Pipe-in-pipe System

Overtrawlability and Mechanical Damage of Pipe-in-pipe System

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Trenching is an effective, but often high cost, method to protect a pipeline from external damage such as trawl gear impact. In the early days of offshore field development, all pipelines in the North Sea were trenched. In order to avoid unnecessary trenching, trenching guidelines were developed later. Nowadays, an increasing number of pipe-in-pipe systems are used for transport of oil, because of their significantly better thermal insulation than the single pipe system. This development raises questions about the trenching decision for pipe-in-pipe, because the previous research and guidelines are aimed at single wall pipe. If we apply the approach of single wall pipes directly to pipe-in-pipes, it is likely to result in a conservative design, as the outer pipe is not required to resist internal pressure and can accommodate a greater level of indentation than a single, pressure containing pipe. Eliminating conservatism in this aspect of design has the potential to eliminate the need for trenching in areas of high fishing activity and can therefore have considerable economic benefits.

Therefore, a 3-year research project was set up to study the overtrawlability of pipe-in-pipes and to develop a basis for a rational trenching decision. When trawl gear interacts with a pipeline, it will impact the pipe first and then pull-over the pipe. Three different experimental programs were carried out to study the mechanical behavior of pipe-in-pipe under trawl gear crossing, including quasi-static indentation tests, impact tests and pull-over tests. Finite element models were built and verified against the test data. Semi-empirical models to estimate the indentation response were built. Pull-over response of pipe-in-pipes was studied and possible improvements of pull-over model test methodology were proposed.

This talk is part of the Engineering Department Geotechnical Research Seminars series.

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