Dynamic Analyses of Offshore Structures

ABSTRACT The collapse of Texas Tower Number 4 radar platform offshore from New Jersey in 1961 with the loss of 28 lives and the destruction of 22 offshore platforms in the northern Gulf of Mexico during the passage of Hurricanes Hilda (1964) and Betsy (1965) emphasize the importance of sound dynamic analyses of offshore installations subject to environmental extremes. This paper presents an analysis of the dynamic response of a Cook Inlet monopod structure subject to severe tidal current-driven ice loads. Dynamic analyses of several offshore structures in the northern Gulf are also presented. Basic dynamic analysis methods are discussed with an emphasis on their range of applicability and limitations. Measurement techniques utilized in gathering pertinent vibration at a from offshore platforms are discussed. Comparisons of measured and computed dynamic response data are presented to establish the validity of the analysis techniques utilized. The importance of developing sound analytical and measured dynamic response data for offshore installations is outlined. INTRODUCTION More than 16 per cent of the free world's petroleum is currently being produced from the continental shelves offshore from some 60 countries. While much of the early development has been concentrated in shallow water areas (under 100 feet), more and more of this production is being developed in water depths greater than 100 feet. Problems associated with the drilling and production of oil in these deeper water areas end to increase exponentially with increase in water depth. This is particularly true with regard to the dynamic response of offshore structures to high wind and wave forces created by environmental extremes such as exist during the passage of a major storm or hurricane. In the early stages, offshore structures installed in shallow water depths were designed with little consideration for the dynamic response of these structures. So long as the structure was composed of a large deck are supported by relatively short piles, the inherent stiffness of the structure resulted in a high natural frequency of response with low amplitude oscillations due to severe win and wave loads. With fixed platforms now installed in mean water depths as great as 340 feet, platform dynamic response has become a major consideration for the safety of these structures which are exposed to high storm induced environmental forces. The devastating effects of hurricane winds, waves, tides, and currents are clearly demonstrated by the destruction of 22 and severe damage to 10 other offshore structures during the passage of hurricanes Hilda (1964) and Betsy (1965) across the Gulf of Mexico and the Louisiana coast (Reference 1). Although 4 of these structures were old, 3 of them had been designed for a "maximum 100-yearstorm". Most structural failures during the passage of hurricanes HiIda and Betsy were probably quite similar to the collapse of the Eugene Island Block 276-A platform shown in Figure (Reference 2). In this artist's drawing (based on divers' reports) it may be noted that the platform jacket and the platform deck section were found essentially intact.