Operations of Subsea Systems (SOSS) - sea bed data from the physical test site in the Trondheimsfjord

Abstract

A shallow water area east of Trondheim harbour in the Trondheimsfjord has been used as test site to study the impact of towed bottom gear on seafloor sediments in the frame of the SOSS project (Safe Operations of Subsea Systems). This report presents detailed bathymetry, acoustic reflectivity and grain size analyses of seafloor sediments in the test area.\rSINTEF Fisheries and Aquaculture, in cooperation with Marine Scotland, performed towing tests with a sledge loaded with instruments in the period 31 th August 20 I 0 -6th September 20 10. The tests were performed from the NINU vessel RN Gunnerus. In September 2010 NGU mapped the test area with aGeoSwath 250 kHz interferometric sonar, collected high resolution sub-bottom profiler data with a Kongsberg S40 Topas parametric source, and aquired grab samples and Niemist6e cores at four locations. The survey was done with the NGU vessel FF Seisma.\rThe seabed in the test area comprises an upper sandy gravelly mud layer 5-10 cm thick resting on softer and more fine-grained muds. Comparisons of bathymetric datasets from 2004 and 2010 show that the test tows did not make significant scours into the sea bed.\rThe high content of coarse silt and sand makes the soil dilatant. The seabed will have a firm behavior if it is hit by a falling object like a gravity corer, while vibrations will cause objects to sink into the seabed. This infers further that an object that lies on the seabed vibrating or rocking, e.g. due to drag in a line may sink into the seabed. The sledge that was towed at constant speed over the seabed caused only minor influence on the seabed.