New Design Code for Interference Between Trawl Gear and Pipelines: DNV RP-F111

The offshore petroleum and the fishing industries are often operating in the same areas. Fishing, in particular bottom trawling, is of concern to pipeline integrity. Such trawling is mainly conducted with two types of trawl gear: otter or beam trawls. The otter trawl boards are steel, more or less rectangular boards which keep the trawl bag open by hydrodynamic drag forces. While the beam trawls use a 10–20 meters long beam to keep the trawl bag open. These trawl gears are dragged along the seabed and represent a potential hazard to pipelines. This paper gives a brief description of types and dimensions of trawl gear. Further it deals with methods for calculating the pipeline response when interacting with bottom trawl gear and finally adresses acceptance criteria for pipeline design and assessment during operation. The calculation methods and acceptance criteria given in this paper are based on test results and research done during the last decades including results from the Kristin, Sno̸hvit and Ormen Lange projects. The above mentioned trawl data, analysis methodology and acceptance criteria are taken from the new DNV Recommended Practice, DNV RP-F111 (2006). This Recommended Practice is an update of the former design code DNV GL 13 (1997). The update was performed mainly due to new types of trawl gear, updated trawl gear data, various experience from application of GL 13 in projects and a general harmonizing process with the pipeline code, DNV OS-F101, and the Hotpipe project, ref Collberg et al. (2005). Large sums are often spent in pipeline projects to protect against trawl gear interaction in terms of concrete coating, trenching or burial. On the other hand the costs of not providing a sufficient protection could be extremely high, with costs related to leakages, failures, stop in production and repair/replacement. This Recommended Practice provides a rational tool to optimize the costs related to trawl gear interference and still ensure that the integrity of the pipeline becomes acceptable.