In seismically active offshore regions of the world subsea facilities could potentially be subjected to significant earthquake excitation. The consequences of failure of such facilities carrying hydrocarbon products at high pressure and high temperature can be catastrophic to the environment.
In current practice these structures are typically designed to meet the requirements of ISO 19901-2 against both the extreme level and abnormal level earthquakes using response spectrum analysis methods.
Subsea 7 has recently analysed subsea facilities in an integrated manner in which all nonlinear effects are included and all system effects are captured. In these models all the components of the system are represented. These may include pipelines, flexibles, PLEMs on suction piles or sliding PLETs on skirted mudmats, often joined together with rigid spools.
In direct integration non-linear dynamic FE analyses the system is simultaneously subjected to free field time history accelerations in three orthogonal directions at appropriate depths through soil-structure interfaces. These interfaces are modelled with special kinematically hardening elements with the hysteretic damping characteristics calibrated to the site-specific soil properties.
General purpose post-processing scripts extract the vital data at multiple selected time increments from the large amount of output generated and produce, for example, contours of the envelopes of utilisation factors of various components to their respective codes.
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