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.

Issue Section:
Research Papers
Topics:
Design, Offshore structures, Waves, Wind, Stress, Databases, Errors, Shorelines, Engineers, Locations, Ocean engineering, Statistical distributions, Wind velocity
1.
Baird, W. F., and Associates, Coastal Engineering Ltd., Hydrotek Water Resource Consultants Inc., and Lawless, J. F., 1986, “Review and Assessment of Procedures for Extreme Value Analysis for Selected Geophysical Data,” Canadian Climate Centre, Report No. 89-7, Downsview, Ontario, Canada.
2.
Baird, W. F., and Associates, Coastal Engineering Ltd., Hydrotek Water Resource Consultants Inc. and Lawless J. F., 1989, “Review and Assessment of Procedures for Extreme Value Analysis for Selected Geophysical Data, Phase III Guidelines and Recommended Methodology for Undertaking Extreme Value Analysis,” Canadian Climate Centre, Report No. 89-7, Downsview, Ontario.
3.
Brown
R. D.
, and
Swail
V. R.
,
1988
, “
Spatial Correlation of Marine Wind Speed Observation
,”
Atmosphere-Ocean
, Vol.
26
(
4
), pp.
524
–
540
.
4.
Canadian Standards Association, 1989, “Canadian Standard Association S471, General Requirements, Design Criteria, the Environment, and Loads,” Rexdale, Ontario, Canada.
5.
Cardone
V. J.
,
Greenwood
J. G.
, and
Cane
M. A.
,
1990
, “
On Trends in Historical Marine Wind Data
,”
Journal of Climate
, Vol.
3
, pp.
113
–
127
.
6.
Chakrabarti, S. K., 1987, Hydrodynamics of Offshore Structures, Computational Mechanics Publications and Springer-Verlag, London, U.K.
7.
Cramer, H., and Leadbetter, M. R., 1967, “Stationary and Related Stochastic Processes; Sample Function Properties and Their Applications, Wiley, New York, NY.
8.
Castillo, E., 1988, Extreme Value Theory in Engineering, Academic Press, Inc., London, U.K.
9.
DnV (Det norske Veritas), 1989, “Rules for Classification of Fixed Offshore Installations,” Norway.
10.
Dobson, F. W., 1983, “Review of Reference Height for and Averaging Time of Surface Wind Measurements at Sea,” Geneva, Switzerland: World Meteorological Organization, Marine Meteorological and Related Oceanographic Activities, Report No. 3, p. 64.
11.
Galambos, J., 1978, The Asymptotic Theory of Extreme Order Statistics, John Wiley and Sons, New York, NY.
12.
Gu, G. Z., Berek, E. P., Dello Stritto, F. J., Szabo, D., and Leder, H. V., 1992, “Extremal Analysis of Hibernia Wave Hindcast Data,” 3rd International Workshop on Wave Hindcasting and Forecasting, Montreal, Quebec, Canada.
13.
Gumbel, E. J., 1958, Statistics of Extremes, Columbia University Press, New York, NY.
14.
Laing
A. K.
,
1985
, “
An Assessment of Wave Observations from Ships in Southern Oceans
,”
Journal of Climatology and Applied Meteorology
, Vol.
24
, pp.
481
–
494
.
15.
Larrabee
R. D.
, and
Cornell
C. A.
,
1978
, “
Combination of Various Load Processes
,”
Journal of Structural Division
, ASCE, Vol.
107
, pp.
223
–
239
.
16.
Larrabee, R. D., and Cornell, C. A., 1979, “A Combination Procedure for a Wide Class of Loading Processes,” Proceeding of Speciality Conference on Probabilistic Mechanics and Structural Reliability, ASCE, Tuscon, Az.
17.
Leadbetter, M. R., Lindgren, G., and Rootzen, H., 1983, Extremes and Related Properties of Random Sequences and Processes, Springer-Verlag, New York, NY.
18.
Madsen, H. O., Krenk, S., and Lind, N. C., 1986, Methods of Structural Safety, Prentice-Hall, New York, NY.
19.
Maes, M., and Nessim, M. A., 1989, “Examination of Load Combinations for CSA,” Report to the Canadian Standards Association, DnV (Canada) Ltd., Calgary, Alberta, Canada.
20.
MAST, 1985, Marine Statistics Software Package User’s Manual, Hydrometeorology Division of the Canadian Climate Centre, Apr.
21.
Naess, A., 1988, “Prediction of Extreme Values of Linear Combination of Load Effects,” Reliability and Optimization of Structural Systems ’88, Proceedings of the 2nd IFIP WG 7.5 Conference, Springer-Verlag, London, U.K.
22.
Thoft-Christensen, P., and Baker, M., 1982, Structural Reliability Theory and Its Application, Springer-Verlag, Berlin, Germany.
23.
Turkstra, C. J., 1970, Theory of Structural Safety, SM Study No. 2, Solid Mechanics Division, University of Waterloo, Waterloo, Ontario, Canada.
24.
Turkstra, C. J., and Madsen, H. O., 1980, “Load Combinations in Codified Structural Design,” Journal of Structural Division, ASCE, Vol. 106, No. ST 12.
25.
Wen, Y. K., 1990, Structural Load Modeling and Combination for Performance and Safety Evaluation, Elsevier, Amsterdam, The Netherlands.
26.
World Meteorological Organization, 1988, “Guide to Wave Analysis and Forecasting,” Report No. WMO-702, Geneva, Switzerland.
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