Study of Trawling Effect on Subsea Pipeline in China Sea

摘要

There are busy trawling activities in China Sea. Recently more andmore subsea pipelines are laid in this offshore area and most of themare not buried. Trawling activities are high risk for subsea structuresespecially for unburied subsea pipelines and pipeline damage eventsexist in this area. It is important to study the trawling effect on subseapipelines to ensure the safety of them. There are mainly two types oftrawling gear in China Sea, otter trawl gear and beam trawl gear.Numerically analysis and model experiments are carried out to studytrawling effect on subsea pipelines. The study focuses on impact andpull-over forces of trawl gear onto the subsea pipeline. Impact and pulloverforces induced by otter trawl gear and beam trawl gear arecalculated theoretically according to DNV-RP-F111. Meanwhile,model experiments are carried out both in air and underwater tomeasure the forces. Influences of trawling velocity, fishing equipmenttype, pipeline span height on impact and pull-over force are studied inmodel experiment and the results were also compared with thenumerical results as verification. Results show that DNV results aregenerally higher than experimental results. For impact and pull-overforce of otter trawl board and beam trawl, there is no significantdifference between in air and underwater experiment cases. For all thecases, impact and pull-over forces increase with the increase oftrawling velocity. The impact and pull-over forces of otter trawl boardincrease with the increase of pipeline span height obviously. However,span height doesn’t show significant influence on beam trawl impactforces and pull-over forces. Compared with experimental results, DNVRP-F111 overestimates the force, which is conservative for engineeringdesign, and it will cost more money on the engineering facilities. So itis suggested to consider the DNV rules and the experimental resultscomprehensively in engineering design.