C118

In 1969, a small but intense storm, Camille, struck the Mississippi delta area off the Louisiana coast, a prolific oil and gas producing area. In the aftermath of the storm, one platform was found toppled and two other platforms were damaged severely, all in the So Pass area of the Mississippi delta (Bea, 1971). Subsequent investigations showed that the damage occurred not as a result of wind and wave forces on the platforms, but rather from wave-induced mudslides in the soft, underconsolidated sediments prevalent in the area. Although platform damage and the pipeline breakages were well known in the area prior to Camille, this was the first major destruction of a large platform, and it sparked a multitude of studies on the topic of wave seabottom interaction. One of the first studies reported was by Henkel (1970) who used standard slope stability analysis with the forcing function of being wave-induced bottom pressures. The only sediment properties required in this analysis were unit weight and undrained shear strength. Subsequently, Wright and Dunham (1972). Developed a finite element approach which took into account the actual sediment properties at So Pass 70, the location were the platform was destroyed during Camille. Other analytical studies have been conducted, many of which used Biot’s formulation for poro-elastic bodies. In addition, a significant number of geological studies were conducted which developed the extent and magnitude of seafloor slides in the Mississippi delta area, as well as their morphology (e.g. Coleman et al., 1978 and 1980).

The study reported herein utilized results from research conducted for Chevron, Gulf and Mobil oil companies in response to needs for design of mudslide resistant platforms in the So. Pass 57-77 area. This research took three main paths: a) the development of constitutive relationships for the soft sediments involved, b) determination of sediment drag forces on platform members, mostly for piles, and c) development of the governing equations for sediment movement and numerical solution of these equations. The end product was a computer program which provided cyclic and downslope movement of the sediments in response to wave forces and calculated the drag forces of the sediment on platform members.