While deepwater subsea pipelines are designed to withstand a broad range of load patterns, including large hydrostatic pressure, high internal pressure, hydrodynamic loading and temperature induced axial stresses, the interaction between fishing gear and the pipeline is one of the most severe design cases for an offshore pipeline system.
When bottom trawl gear is towed across the surface-laid pipeline, the interaction can be divided in different stages: impact (very short duration, high forces, local response) and subsequent pull-over (longer time duration, global pipeline response). In special cases, hooking may occur, where the trawl equipment is stuck under the pipeline.
The damage, afflicted to the pipeline, depends on the type of fishing gear (weight and velocity, beam trawl or otter trawl), and the pipeline conditions (primarily wall thickness and coating system). The most important issue with respect to the design of fishing gear resistant pipelines is the ability to provide a realistic description of the applied loads and their time history, and the response of the as-laid offshore pipeline (including potential pipeline spans).
In this paper, numerical models are presented to simulate the displacements and corresponding stresses induced during trawl gear pull-over. The finite element analyses are compared with simplified analytical approaches to investigate whether the recommended practices used in the offshore industry provide conservative predictions of impact energy dissipation and pipeline integrity.