Ma. Nessim et al., DESIGN CRITERIA FOR OFFSHORE STRUCTURES UNDER COMBINED WIND AND WAVE LOADING, Journal of offshore mechanics and Arctic engineering, 117(1), 1995, pp. 1-11
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.