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Subsea pipelines are used extensively throughout the world’s oceans to transport oil and gas from offshore facilities to land, often hundreds of miles. These pipelines range in diameter from three to sixty inches and are installed in deeper depths every year, currently as deep as 2,900 meters. Pipeline construction and installation costs are a large percentage of offshore projects and thus, methods toward reducing costs is an imperative objective. With pipeline installation projects taking place in harsher environments, vessel operability is vital. This work presents an improved method for determining limiting criteria for pipelay operations to more effectively plan and execute offshore projects. This improvement is based on the consideration of total effective pipeline stresses as the limiting criterion rather than the traditionally used limiting pitch angle. Limiting sea curves based on a sample dynamic pipeline analysis are shown and their incorporation in workability planning is discussed.

Pipeline ploughs are commonly used to bury offshore pipelines for their protection from loading by currents, damage from fishing trawler vessels and to provide thermal insulation to the line allowing the product to flow more efficiently. The rate of progress of pipeline ploughs in sand is complicated by a rate effect which causes the required tow force to increase drastically with velocity. In this research plough performance in sand is investigated by means of physical scale model tests. Scale model tests are the most practical method by which to conduct a parametric study on plough behaviour as full sized testing would be prohibitively expensive. Scale model tests also provide accurate control of sand conditions which allows investigation of the effect of soil parameters on plough behaviour. Model ploughs were manufactured at 50th, 25th and 10th scale so that scale effects could be explored. Each of the model ploughs had a detachable forecutter to allow its effect on plough performance to be observed. The forecutter was found to reduce the rate effect but increase the non-velocity dependant resistance of the plough. Ploughing tests were conducted at various relative densities in three sands of different permeability. The effects of ploughing rate on model plough behaviour under these various conditions was explored using an instrumented model plough, with particular attention paid to the resulting tow force. Results from the model ploughing tests were interpreted to determine the effect of permeability, relative density and plough depth on the tow forces generated during ploughing. The rate effect was found to increase strongly with reduction in permeability of the sand. Increasing the relative density of the sand was found to increase the rate effect but had little influence on the passive resistance of the plough. The test results were compared to an empirical model developed by Cathie and Wintgens (2001). New coefficients (Cw, Cs and Cd) have been proposed and therefore design procedures modified which may allow trenching contractors to make better predictions of plough performance in sands.

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