The effect of buried fibres on offshore pipeline plough performance

Ploughing is a technique often used to bury offshore pipelines in the seabed. During this process the operator must ensure that a sufficiently deep, level trench is produced while towing the plough with the available bollard pull of a suitable trenching support vessel. This paper reports experimental work investigating the effect that encountering fibres or reinforcing elements such as buried tree branches in the soil (e.g. relict debris from deltaic flood washout) may have on the ploughing operation. It is shown that fibres in soil can have a reinforcing effect and hinder plough progress by both increasing tow force and leading to potential 'ride-out' of the plough (significant loss of trenching depth). This behaviour is correlated with the percentage of fibre reinforcement volume in sand and a simple method is provided to estimate changes in tow force and plough inclination during ploughing operations.