The influence of relative density and trenching depth on the pitch a pipeline plough travels at have been investigated by means of 1/50th scale model plough testing. Tests were performed in dry sand at two relative densities with the model plough set up to form trenches of various depths. The relationships between plough pitch and depth are presented and compared between the two sand densities. The results show that relative density and depth can have a significant effect on plough pitch with possible repercussions for plough stability and trenching depth.
Small diameter offshore pipelines are frequently buried in the seabed to a depth of several pipeline diameters. Burial is used as protection to prevent external loading from fishing activity/snags or hydrodynamic loading and to prevent movement of the pipeline during thermal expansion on commissioning (Finch et al, 2000). One method of pipeline burial is ploughing. In this method, a pipeline plough is towed by a vessel to form a trench on the seabed into which the pipeline is placed. A second pass from a backfill plough is used to replace the soil from the spoil heaps above the pipeline to achieve an appropriate cover depth. The two main areas of commercial operational interest with offshore ploughing are the achievement of an appropriate cover depth (with a flat trench profile) in a single pass of the pipeline plough, and the rate at which the pipeline ploughing can be carried out. Clearly, either the requirement for multi-pass ploughing or slow plough speeds will increase necessary vessel time and therefore cost. Tow forces are believed to increase with both plough depth and velocity. In particular, plough tow forces increase with rate at typical ploughing velocities allowing for only partial drainage of material being sheared.