This paper summarizes a simplified method with which strength-level earthquake analyses of jacket-type platforms can be performed. By examining the primary bending, shear, and foundation rotation responses, estimates of platform vibration characteristics can be obtained from which earthquake forces can be estimated by the response spectrum method. This process is referred to as SRSA (Simplified Response Spectrum Analysis). These forces can then be taken together with capacities derived from ULSLEA (Ultimate Limit State Limit Equilibrium Analysis) to develop an evaluation of the demand-capacity behavior of the platform. The ULSLEA-SRSA method is applied to the assessment of two platforms. Results from 3- D frame analyses of the two platforms are used for validation of the simple approach. Agreement between the ULSLEA-SRSA and detailed 3-D analyses is excellent. Several studies related to the simplified assessment of platforms subject to earthquakes are documented in this paper. In the first, a design code approach to earthquake forces based on that contained with the Uniform Building Code is demonstrated and compared to more detailed earthquake force estimates. Next, common simple approximations to pile-head stiffnesses are reviewed, and the impact of foundation flexibility on platform response examined. Last, the impact of local inertia forces on brace axial capacity is studied.
During the past five years there has been growing interest in the development of simplified structural analysis methods which are inexpensive to apply yet provide sufficiently accurate results to help make timely and economic engineering assessments. A major reason for this development is the re-assessment of aging infrastructure. As many structures (buildings, bridges, offshore platforms, etc.) approach the end of their original service lives, many owner/operators desire to keep these structures in service. As many of the structures in existence today were designed for much less stringent load criteria than current code recommendations, some form of analysis must be performed.
