Pipelines are often buried in the seabed using ploughs to protect against man-made and natural hazards. The depth of burial is influenced by the soil conditions and plough progress rates required to meet contract deadlines. Plough progress rates may be a function of the towing vessel capabilities. Being able to reduce plough tow forces, increase the rate of progress or increase ploughing depths without significant increases in tow forces may offer considerable commercial advantages to the ploughing industry. One possible means of improving pipeline plough performance is through inducing vibration in the plough and surrounding ground. This paper reports the findings of a pilot study looking at the effects of plough vibration on tow forces and plough depth. The results show that vibration at 77–102Hz resulted in increased plough depth by up to 38% in dense sand, with limited average increases in tow forces of 4–6%.

1. Introduction

Offshore pipelines may be installed below the seabed using ploughing techniques to ensure protection against human activity, such as fishing and anchoring. Burial also reduces thermal losses when pumping hot product and the associated problems of upheaval buckling (UHB) as a result of thermal expansion during pipeline start-up and operation. Ploughing in difficult and unforeseen ground conditions may require higher tow forces than anticipated, which may lead to slower ploughing rates than is commercially desirable. It may also result in single plough passes that are not at the required burial depth, and thus the need for multi-passing. It may also become difficult to control the ploughs depth which may result in trench and pipeline out-ofstraightness (OOS) that can increase the risk of UHB. This could require further additional mitigation measures, such as rock dumping, which may not have been anticipated when costing the project.

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