Model tests investigating trawl forces for free-spans of up to 6 m height are described. Results are presented for maximum warp force, maximum force applied to the pipeline and shape of the force-time trace. Effects of tow velocity, span and warp flexibility and trawl door type are quantified. The effects of trawl forces in limiting allowable span lengths and heights are illustrated.
The development of hydrocarbon fields off mid-Norway will present special design problems with respect to free-spanning pipelines. Water depths are typically 250–350 m and the bottom is very uneven being crisscrossed by numerous iceberg scour marks of depth up to 10 m and width up to 200 m or so. A pipeline laid in this area will not be able to entirely avoid such scour marks, even through careful routing, and the rectification of the resulting free-spans will involve considerable cost. Other areas offshore Norway are also characterised by similar extreme scour marking, e.g. Tromsøflaket. In order to develop appropriate rectification procedures for pipelines in areas with severe free-spanning it is necessary to assess the effects of trawling on free-spans for the various construction phases (just after laying, water-filled and in 9peration). This may be done, for example, through numerical simulation of spans, using as input appropriate time histories of forces from trawling, and determining allowable free-span lengths and heights for each phase. This information, together with similar information for dynamic behaviour (vortex-induced vibrations) and static stresses of free-spans, will determine the necessary rectification for each phase. Considerable investigation into beam trawling equipment has been conducted (Moshagen and Kjeldsen, 1980, Horenberg and Guijt, 1987, Guijt and Horenberg, 1987), however the beam trawl is not applicable in the water depths of interest to the present investigation and is not considered further here.
