Seismic Soil-Structure Interaction Design Considerations for Offshore Platforms

Abstract

Offshore platforms for oil and gas production in seismic regions around the world are often required to be designed for seismic hazards according to International Standards (e.g., ISO 19901-2 [1] and ISO 19902 [2]). This paper discusses three important aspects of the nonlinear dynamic time history analysis commonly used to design for Abnormal Level Earthquakes (ALE) in light of findings from recent centrifuge modeling and numerical simulation of the response of offshore structures under earthquake excitations. First, greater-than-expected ground motion de-amplification has been observed in a recent seismic soil-structure interaction centrifuge program for typical “soft” marine clays with undrained shear strength up to 100 kPa per API RP 2GEO [3]. Second, the current industry practice of using uniform down-pile ground motions in the time history analysis tends to underestimate pile bending moments. Use of depth-varying ground motions is strongly recommended to better characterize pile bending moments. Alternatively, a simplified design approach is proposed to account for the higher bending moments from the use of more realistic depth-varying ground motions. This approach is illustrated with a design example. Lastly, hysteretic and radiation damping in soil-structure interaction is discussed. Modeling of hysteretic damping is achieved using nonlinear elasto-plastic soil springs with unload-reload behavior following Masing’s rule, whereas modeling of radiation damping is achieved using viscous dashpots in a parallel or series arrangement with the axial and lateral soil springs and with dashpot coefficients based on O’Rourke and Dobry [4]. The centrifuge data show that proper modeling of radiation damping is important to accurately predict pile load and settlement.