Design Criteria for Offshore Structures Under Combined Wind and Wave Loading

Abstract

Offshore codes do not give sufficient guidance regarding design criteria for loads resulting from combinations of stochastic environmental processes such as wind and waves. To assist design engineers in defining such criteria, a suite of methods that use environmental data to calculate the probability distributions of load effects resulting from combination of stochastic loads were investigated. An approach has been developed for using the results to calculate structure-specific and generalized load combination criteria. Extensive application of this approach in connection with Environment Canada’s wind and wave data bases for the Canadian East Coast region formed the basis for some interesting conclusions regarding the process of estimating combined extreme loads on offshore structures. It was found that data based on actual measurements of wave height and wind speed are preferable to hindcast data, since the latter have artificially high correlations that lead to overly conservative results. External analyses are most reliable when 20 or more years of data are used with analysis methods based on distribution tails. Reasonably good results can be achieved with 10 yr of data. Methods based on the point-in-time data and using mathematically convenient assumptions regarding distribution types and process characteristics can lead to large errors if the assumptions made are not substantiated by appropriate data. Load combination solutions are highly dependent on the geographic location and data base. Therefore, a separate analysis should be carried out for the structure and location being 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, companion action factors of 0.65, 0.60, and 0.55 for return periods of 20, 100, and 1000 yr, may be used for wind and wave loading on slender offshore structures in the Canadian East coast region. For wide structures in the same region, the suggested companion factors for the same return periods are 0.75, 0.70, and 0.65.