Seismic Response Spectrum in Lieu of Time History Analysis for Subsea Structure Design

Abstract

With oil and gas exploration venturing into more seismic prone regions for global deep water developments, seismic assessment of subsea structures becomes a fundamental part of the design requirements of regulations, industrial codes and standards. This paper demonstrates a response spectrum analysis approach for the seismic design of subsea structures. Response spectrum analysis using commercially available dynamic software package is a time-saving and preferred method for the design of subsea structures as compared to the time history method for the seismic assessment. The study also intends to contribute to the standardization of the seismic design procedure for subsea structures. The paper selected a typical subsea valve skid with a mudmat foundation for the seismic response spectrum analysis. Upper and lower bound values of soil springs were included to capture the response in relation to varying degrees of soil strength. To benchmark the time-saving advantage of response spectrum analysis, time history seismic analysis is also performed in parallel on the same subsea structure. Results of the two methods are compared to establish a technical position that the response spectrum is a more favored and efficient approach as compared to time history for the design of subsea structures. For the subject structure, the response spectrum analysis results show that the structure remains elastic during ELE (Extreme Level Earthquake) and ALE (Abnormal Level Earthquake) seismic events. This is due to the low-rise geometric profile of subsea structures. The results also indicate that the response spectrum approach enables a faster and relatively accurate estimate of structural response with ELE/ALE loading while still using a linear method. The response spectrum analysis serves to bridge the gap between a rough approach of directly applying a simple additional seismic acceleration on the structure, versus subjecting the structure to a time-consuming time history analysis. This time history analysis is not deemed necessary due to the smaller sizes hence the less substantial inertia loads of the subsea structures as compared to typical offshore platforms. Further studies in the future should be able to provide more "accuracy vs. efficiency" type of trade-off information on a wider range of different subsea structures.