Trenching study for flexible pipeline subject to trawl gear interaction

The objective of this study was to determine whether a subsea flexible water-injection pipeline subject to overtrawling fishing activities should be trenched.

The study involved finite element analysis to determine peak stresses, displacements and minimum bend radii of the flexible when subjected to trawl gear interaction. 

This analysis was based on the methodology outlined in HSE publication OTH 561 "Guidelines for Trenching Design of Submarine Pipelines", and considered loads during pullover, impact and hooking of the fishing trawl gear.  Jee had previously developed these guidelines for a joint industry project (JIP), and has subsequently used them on many projects involving trawl gear interaction of subsea pipelines.

Other considerations during the project included the frequency of fishing activity in the North Sea, and a research study to determine the track record of untrenched flexible pipelines and whether there were any recorded incidents relating to overtrawling.

Benefits to our client included:

• Early recommendation, based on Jee’s engineering judgement, that gave our client the justification to proceed with the project.
• Technical justification for the decision on whether to trench the flexible pipeline.