An Assessment on a Subsea Pipeline Subject to a Diagonal Trawl Impact

Abstract

In oil and gas offshore areas, flowlines in fishing waters are at risk of trawl gear impacts. In this regard, DNV-RP-F111 recommends a beam and spring-mass (BSM) model to assess the overtrawlability of pipelines. The BSM model is commonly employed to assess perpendicular trawl impacts on pipelines. However, in the case of a subsea pipeline under a non-perpendicular trawl impact, the BSM model does not incorporate the full diagonal load; the tangential component of the impact load is disregarded. Furthermore, the BSM model does not account for the progression of damage in a pipe (e.g., the plastic damage in a pipe that is imposed by a non-perpendicular trawl impact and is translated along the pipe during the impact). In this regard, a recent study showed that damage progression could decrease the structural resistance of a pipeline. Therefore, as the BSM model does not consider the damage progression effect, it could underestimate the resulting dent depth in a pipe. The present study uses an experimental investigation to examine the damage progression effect in a cylindrical specimen. Furthermore, using a hybrid shell-beam finite element model, an enhanced version of the BSM model, a scenario of a pipe subject to a diagonal impact is investigated. The experimental results show that the dent depth in a 5-inch pipe under a perpendicular 150 kN load could increase by 52%, where the imposed damage is translated for 300 mm along the pipe. In addition, the results obtained from the finite element analyses show that the dent depth in 5-inch and 14-inch pipelines is 20% higher under a full diagonal trawl impact with 30° angle from the pipe normal, compared to applying only the normal component of the diagonal impact. In conclusion, the resulting dent size in a pipe under a diagonal trawl impact is dependent on both normal and tangential components of the impact. Furthermore, the hybrid shell-beam model could be a suitable alternative to the BSM model for scenarios where a pipe is subject to a diagonal trawl impact.