Seismic Design of Subsea Spools per ISO: Part II- Seismic Requirements

The ISO requires a two-level seismic qualification, namely Extreme Level Earthquake (ELE) and Accidental Level Earthquake (ALE) where damages that do not lead to leak is acceptable. ISO accepts both the response spectrum method and the time history approach.  Since the spool-soil system behaves non-linearly, the time domain analyses must be performed for both levels. ISO requires 7 real earthquakes scaled for the site to be used for each seismic level and the system must pass at least 50% of the cases.  Best estimate soil models were developed to represent soil conditions at the manifolds locations in Part I. A set of 10 real earthquake time histories were propagated through the soil column for each location. The resulting ground motion at the surface was computed using a nonlinear model. The frequency-dependent ratio of spectra acceleration at the mudline to the stiff soil outcrop spectral acceleration (Spectral Amplification Ratio or SAR) was computed for each time history. The mean SAR was then used to modify the stiff soil hazard results from the PSHA to obtain design response spectra at the mudline. This is the second part of three interlinked papers summarises the state of art for the benefit of practitioners of subsea engineering.