Good point, Don,
which is the extremest value which has to be considered in a reliability analysis of a strategic structure?
Usually the regulations will specifiy that. In the recent probabilistic Italian construction law, return times for environmental loads should be 1000 years for all important buildings. In the case of oil platforms, an extreme values analysis with projections on such timespan would have been necessary (and probably in the oil industry there is some reference hurricane severity against which structures are to be dimensioned. The fact that many rigs have withstood Kathrina and others did not could help in building those calibration factors for LRFD.
And as you rightly say, the foresight of those engineers who designed the survived platforms is undeniable!
