Pipeline plough performance in sand waves. Part 2: kinematic calculation method

Offshore pipelines can be buried in the seabed by ploughing a trench, placing the pipe at the base and then backfilling. The presence of sand waves or mega-ripples on the seabed surface can affect the progress of the plough and prevent the plough from generating a level trench with a uniform trench depth. A calculation method has been presented which makes assumptions about the motion of the plough in order to predict the kinematics of ploughs through regions of non-uniform seabeds. Results from the calculation methodology are compared to those from small scale model tests with good agreement, and the detailed kinematics of ploughs are then examined. The calculation method suggests that as a plough moves through a sand wave field, the oscillation of the plough about the skids results in the trench based being formed alternately by the share tip and heel. The new method allows prediction of likely offshore plough performance given known plough geometry, sand wavelength and wave amplitude and may be used as a tool for assessing the feasibility of pipeline ploughing in zones of sand waves or megaripples.