DESIGN CRITERIA FOR OFFSHORE STRUCTURES UNDER COMBINED WIND AND WAVE LOADING

Offshore codes do not give sufficient guidance regarding design criter ia for loads resulting from combinations of stochastic environmental p rocesses such as wind and waves. To assist design engineers in definin g such criteria, a suite of methods that use environmental data to cal culate the probability distributions of load effects resulting from co mbination of stochastic loads were investigated. An approach has been developed for using the results to calculate structure-specific and ge neralized load combination criteria. Extensive application of this app roach in connection with Environment Canada's wind and wave data bases for the Canadian East Coast region formed the basis for some interest ing conclusions regarding the process of estimating combined extreme l oads on offshore structures. If was found that data based on actual me asurements of wave height and wind speed are preferable to hindcast da ta, since the latter have artificially high correlations that lead to overly conservative results. Extremal analyses are most reliable when 20 or more years of data are used with analysis methods based on distr ibution tails. Reasonably good results can be achieved with 10 yr of d ata. Methods based on the point-in-time data and using mathematically convenient assumptions regarding distribution types and process charac teristics can lead to large errors if the assumptions made are not sub stantiated by appropriate data. Load combination solutions are highly dependent on the geographic location and data base. Therefore, a separ ate analysis should be carried out for the structure and location bein g considered if possible. Wind and wave load combination solutions are sensitive to correlations and assumed distribution types; closed-form solutions for independent and Gaussian correlated processes can lead to significant errors. If site-specific analyses are not practical, co mpanion action factors of 0.65, 0.60, and 0.55 for return periods of 2 0, 100, and 1000 yr, may be used for wind and wave loading on slender offshore structures in the Canadian East coast region. For wide struct ures in the same region, the suggested companion factors for the same return periods are 0.75, 0.70, and 0.65.