The influence of sea state on as-laid pipeline embedment: A case study

The as-laid embedment of an offshore pipeline is an important parameter for design as it affects lateral and axial stability, exposure to environmental loads, and thermal insulation. For soft clay seabeds, the as-laid embedment can be significantly higher than predicted using methods based on the static penetration resistance due to dynamic lay effects. There are very few published field surveys showing the as-laid embedment of a pipeline following installation. Back-analysis of field survey data has the potential to improve existing design methods, as well as gain insights into the effects of the installation conditions, including the vessel response, the lay angle and tension, weather and sea state, and downtime events. This study describes the as-laid field survey carried out for a 12-inch flowline installed on a soft clay seabed in the North Sea. The dynamic lay effects are examined by studying the influence of sea state, manifested through the vessel response which leads to pipeline motions at the seabed. The findings show that a clear correlation exists between sea state and pipeline embedment. The ratio between the mean observed embedment and the embedment that would be predicted using a conventional static penetration analysis (termed the dynamic embedment factor, Fdyn) ranged from 2.5 up to 4 for normal pipelay conditions, increasing with wave height. Downtime events, during which pipelay is temporarily suspended, significantly increase the embedment. In this study they led to maximum embedment values greater than one diameter, corresponding to Fdyn of up to 10, due to the larger numbers of cyclic pipeline motions at the seabed.