The Failure of the South Pass 70 Platform B in Hurricane Camille

Abstract

In Aug. 1969 Shell Oil Co.'s Platform B, located in the OCS-designated South Pass Block 70, went down in Hurricane Camille. An intensive study was undertaken to determine the cause of platform failure. The data conclusively show that the structure failed primarily because of sea-floor soil movement. Introduction South Pass Block 70 is located about 15 miles southeast of the Main Pass on the Mississippi River (see Fig. 1). In preparation for the installation of a platform at this location, soil borings were taken by the well-known wireline technique. While experience in the Delta region led us to expect low soil strengths, the extent and depth of the very weak clays that were found in the boring program were unexpected. An immediate concern revolved around appropriate design approaches to withstand lateral and axial pile loadings. A second, and ultimately much more serious problem, was soon hypothesized; the soils were so weak that soil movement caused by slumping or sliding actions might take place. Thus, while the design proceeded, a program of research and field work to evaluate the probability of sea-floor movements was undertaken. The first public discussion of the potential problem was presented in 1970. Since that time, other investigators have studied the problem. The major conclusion of the initial engineering study and research was that waves in a major hurricane could cause sufficient bottom pressures to create soil instability. This situation, however, could only be developed in severe hurricanes occurring about once in a hundred years or greater, and would not occur under normal sea conditions or moderate storms. Accordingly, the decision was made to design the structure using the once-in-a-hundred-years wind and wave. Because of the very weak near-surface soils, the bottom of the structure was extended into the soil (see Fig. 2). This extension provided for increased stability during installation and reduced lateral deflections under wave loading. Hurricane Camille formed in the western Caribbean during the morning of Aug. 15, 1969. The eye of the storm, following the path shown in Fig. 1, crossed the Mississippi Gulf Coast on the evening of Aug. 17. Shell's South Pass 70 Platform B was destroyed in this storm, as was the Gulf Oil Co. South Pass 61 platform. Pass 61 platform. Investigation Program After Hurricane Camille, an intensive investigation was initiated to determine the cause of failure of the South Pass 70 Platform B. The investigation included above-water inspection of several adjacent platforms, the use of side-scan sonar to get a post-Camille picture of the South Pass 70 Platform B structure, a series of topographical surveys to compare with pre-camille conditions, several post-Camille soil borings to compare with pre-camille post-Camille soil borings to compare with pre-camille conditions, and a detailed diving and underwater television inspection of the fallen Platform B. Results and Interpretations Above-Water Inspection Seven major platforms in the general area were visited by a team of offshore-platform design engineers. Their main purpose was to ascertain the above-water damage at each site and to establish, where possible, the maximum wave crest reached on each platform. JPT P. 263