Sudden Subsidence and Its Assessment
On 7 May, 2001, a minor earthquake occurred in the vicinity of the Ekofisk oil-field (Fig. 1.23) in the North Sea, which comprises several offshore jackets and tripod structures connected by bridges. During this earthquake, a permanent vertical movement of platforms occurred. The people on board the platforms and the platform connecting bridges felt a strong sudden vertical drop, which is often referred to as sudden subsidence or sudden drop. The vertical movement is estimated to have been approximately 80 mm. Assuming that the vertical displacement took place in a single motion with the acceleration of gravity (free drop), the duration of the sudden subsidence is estimated to have been 0.13 s. This is the only reported sudden subsidence event to have occurred offshore thus far. Such events can cause failures of structural members or even a collapse of topside and supporting structures, and may also influence the capacity of the foundation and grout connection of pile clusters, etc.