Prediction Of Storm-Induced Sea Bottom Movement And Platform Forces

Abstract

ABSTRACT A procedure is described for calculating storm wave induced movement of soft sea floor sediment and prediction of the associated drag forces exerted on offshore structures used in the production of gas and oil. The analytical model used to predict movement and the state of stress in the sediment is based on a rigorous viscoelastic analysis and a generalization of the method of equivalent linearization. In contrast to previously published work in this area, the procedure described accounts for the effects of soil inertia, nonlinear material damping, rate-dependence of the soil properties, and down-slope movement induced by wave action. Representative predictions of the sea bottom movement are shown. Finally, experimental work performed in support of this analysis and used to develop a model for predicting forces on cylinders is summarized. INTRODUCTION The Mississippi River daily carries more than one million tons of sediment through the mouths of its birdfoot delta distributaries. The great bulk of this material is deposited on the continental shelf in areas where there is major oil and gas production. As a result of river channelization which was initiated in the 1870's, some of these areas are now receiving sedimentation at a rate exceeding one foot per year (Coleman, 1976). Owing to the rapid rate of deposition, the sediment does not have an opportunity to fully consolidate or settle under the weight of the overlying material. This underconso1idated sediment most of it highly plastic clay, has a very low shear strength extending to significant depths below the mudline. A complete series of sediment movements including slumps, mud flows and faults has been described in this mass of weak, under consolidated clay (Garrison, 1974; Coleman, 1976). Pipeline breaks and displacements have been ascribed to these movements, but they were not considered a particular hazard to offshore drilling and production platforms. In August of 1969, Hurricane Camille struck the Louisiana and Mississippi coasts with winds of over 200 miles per hour. Offshore, three platforms almost directly in the path of the storm suffered major damage. One platform was found on its side on the bottom and the other two underwent enough displacement to cause their abandonment. Subsequent investigations, including determination of bottom contour and sediment shear strength changes, showed that the damage was due, not to wind and wave action alone (Bea, 1971), but to significant sediment movements along the bottom. Prior to Camille, the forces against the platform legs produced by these large movements had not been considered in platform design. PREVIOUS STUDIES ON SEA FLOOR MOVEMENT Geologists and engineers have long recognized the existence of sea floor movement or slides. Bea (1971) lists several mechanisms which might trigger such slides including earthquakes, erosion, unusual currents and rapid deposition of weak sediments on steep slopes. Henkel (1970) used conventional slope stability analysis to show that the differential bottom pressures produced by large storm waves would be high enough to cause shear failure and subsequent sliding of weak bottom sediments. Fig. 1 illustrates the loads acting on the sea floor sediment which were utilized in his predictions.