Requalification process for existing platforms conclusions depend on understanding of interactive parameters

An American Petroleum Institute developed platform assessment initiators and a comprehensive set of guidelines to determine the fitness for purpose of existing platforms have been in use for more than a decade. The assessment process separates the consequence of failure and safety of life issues and if the platform does not pass the assessment, it requires a design level or an ultimate strength level analysis. Determination of the ultimate strength of a platform requires information not only on the "as built" and "present" characteristics of the platform but also knowledge of many interacting parameters including platform configuration, foundation characteristics and the excitation forces on the platform. This is especially important when the platform is located in an area susceptible to earthquakes as the response spectrum is site specific and the foundation stiffness directly affects the magnitude of seismic forces for the given response spectrum. Other key variables affecting the magnitude and distribution of excitation forces are the deck payload and the marine growth. Two platforms, installed offshore Southern California in the late 1960s, were recently analyzed by subjecting them to a 1000-year recurrence interval seismic event. Platform pushover analyses results for a range of parameters illustrate the significance and interaction of these parameters and their effect on the sequence of component member failures and the residual strengths of these platforms. Conclusions and recommendations presented in this paper state that to ensure validity of the ductility level analyses it is desirable to consider both the upper and lower bound deck payload, marine growth and foundation stiffness.