SCOUR UNDER A VERTICALLY OSCILLATING LEG

Abstract

Mishaps with jack-up oil rigs could be caused by scour beneath their legs as they oscillate vertically, either on site or when being shifted to or from site. This action was replicated in the laboratory by moving cylindrical feet to and from a sandy bed at frequencies appropriate to the scale of the model. By retaining equal Reynolds numbers for the sand grains, whose size was commensurate with prototype material, the period of oscillation had to vary with the leg diameter. By adopting a suitable size scale, a range of periods was determined from knowledge of resonant motions of rigs in relatively deep water. Erosion was recorded which appeared sufficient to cause mal-distribution of load in the structure. Even when oscillations occurred in a leg penetrating the bed, no support was available due to liquifaction of the soil. The two pilot studies herein described indicate the need for a comprehensive research program on the subject, due to the large investments in jeopardy.