Two design criteria used to calculate the extreme response of steel risers are reviewed in this paper. The first criterion is concerned with the definition of the maximum response throughout the environmental contour of random load parameters. The second one is based on the long-term response statistics, in which all relevant sea states should be simulated. The use of inflated contour is also discussed and an omission sensitivity factor is calibrated. The results from these two methods are then compared in two examples of steel risers.
The design of steel risers must satisfy ultimate, fatigue and accidental limit states. The assessment of such limit states should be carried out using the most relevant sea states in each case. Sea states are described in terms of environmental parameters, for example, wave height and period, and wind velocity. A statistical description is needed due to the random nature of such parameters, so that one can determine the relevant load conditions. For such purpose, scatter diagrams are widely used to describe the joint occurrence of significant wave height and zero up-crossing wave period (DNV, 2007). From this discrete data diagram one can fit a joint distribution probability model, which is very useful in extrapolating measured data and predicting extreme events (Bitner-Gregersen and Haver, 1991). The ultimate limit state is defined as the collapse of the structure, which means the loss of its ability to withstand loads. Steel risers are subjected to the combined effects of effective tension, bending moment, internal and external pressure. Upon combining such effects, a yielding limit criterion can then be defined at any point of the riser cross section. The key task is to select those sea states that will give the worst response in terms of combined load effect. Recent research based on structural reliability is still being conducted, but some practical results have already been introduced to offshore regulations (DNV, 2001; DNV, 2004), and at least three design criteria are available for steel risers: design storm, environmental contours and long-term response statistics. Only the last two will be discussed herein.
