Earthquake Loads
Offshore structures in seismic regions are typically designed for two levels of earthquake intensity: the strength level and the ductility level earthquake. For the strength level earthquake, defined as having a "reasonable likelihood of not being exceeded during the platform's life" (mean recurrence interval 200 - 500 years), the
structure is designed to respond elastically. For the ductility level earthquake, defined as close to the "maximum credible earthquake" at the site, the structure is designed for inelastic response and to have adequate reserve strength to avoid collapse.
For strength level design, the seismic loading may be specified either by sets of accelerograms (Figure 8) or by means of design response spectra (Figure 9). Use of design spectra has a number of advantages over time history solutions (base cceleration input). For this reason design response spectra are the preferable
approach for strength level designs. If the design spectral intensity, characteristic of the seismic hazard at the site, is denoted by a max , then API-RP2A recommends using a max for the two principal horizontal directions and 0,5a max for the vertical direction.
The DNV rules, on the other hand, recommend a max and 0,7 a max for the two horizontal directions (two different combinations) and 0,5 a max for the vertical. The value of a max and often the spectral shapes are determined by site specific seismological studies.


Designs for ductility level earthquakes will normally require inelastic analyses for which the seismic input must be specified by sets of 3-component accelerograms, real or artificial, representative of the extreme ground motions that could shake the platform site. The characteristics of such motions, however, may still be prescribed by
means of design spectra, which are usually the result of a site specific seismotectonic study. More detail of the analysis of earthquakes is given in the lecture of Seismic Design.