Pipeline-Trawl Interaction: Effect of Trawl Clump Weights

This paper deals with clump weights, a new type of component in bottom trawl gear that needs to be considered in design and reassessment of pipelines. In recent years it has become popular to use two trawl bags, a so-called twin trawl, to increase the swept area and, thus, the efficiency of the trawler. In addition to the trawl boards at each side of the trawl bags, a heavy weight, a clump weight, is used between the bags to keep them close to the seabed. Clump weights can have a mass up to 9 tonnes, and the largest ones are shaped as spheres or cylinders inside a steel frame. Tests performed have revealed that the loads on pipelines from clump weights may exceed the corresponding loads from trawl boards. Thus, interaction with clump weights may govern the design and qualification testing of coating, and the design related to global pipeline response. This paper discusses loads from clump weights. Updated design approaches including pull-over load estimates are presented. The background in terms of small scale test results is also shown for some cases. The conclusion is that clump weights may govern the trawl design of pipelines, especially for trawl gear impact and pull-over.