This paper summarizes a three-year research on the overtrawlability of pipe-in-pipe, and develops the method to access it. Overtrawlablity is the ability of the pipe to resist the trawl gear impact and the pull-over force. The trawl gear will first impact the pipe and then pull over the pipe. If the overtrawlable capacity of the pipeline is insufficient, the trawl gear may damage the pipeline by denting or bending the pipe heavily. Overtrawlability of the pipeline has a direct bearing on whether the pipeline needs to be protected by burial with significant cost savings if burial is not required.
DNV-RP-F111 and the other industry guidelines give a method to estimate the overtrawlability, but currently there is no method specifically for pipe-in-pipe. The outer pipe gives the inner pipe extra protection, and it does not have to resist internal pressure and can therefore accommodate a greater level of indentation than a single-wall pressure-containing pipe. If we apply the approach for the single wall pipe to pipe-in-pipe, the results are likely to be conservative. Therefore, an assessment of the overtrawlability of pipe-in-pipe is necessary to be conducted before making any trenching decisions, and to avoid providing unnecessary protection.
With this motivation, a three-year JIP has been conducted at the National University of Singapore to better understand the overtrawlability of pipe-in-pipe, and to find the method to assess it. The research includes three different experiment programs to investigate the impact and pull-over responses. FE models of indentation and impact are developed and validated against first-hand experimental data. A new relationship of load-deflection relationship is developed to assess the impact response. External pressure is considered by the FE model using hydrostatic fluid element. Small scale pull-over experiments are conducted to study the pull-over force for different pipes and different conditions.
The research work altogether develops the way to analysis the response of pipe-in-pipe when it interacts with a trawl gear. Finally, A comparison between a pipe-in-pipe and a single wall pipe shows the difference between them and throws light on the issue of trenching for pipe-in-pipes to some extent.
