Environmental Load Factors for Offshore Structures

[+] Author and Article Information
H. P. Hong

Department of Civil and Environmental Engineering, University of Western Ontario, London, Ontario N6A 5B9, Canada

M. A. Nessim

Centre for Engineering Research Inc., 200 Karl Clark Road, Edmonton, Alberta T6N 1H2, Canada

I. J. Jordaan

Memorial University of Newfoundland, St. John’s, Newfoundland A1B 3X5, Canada

J. Offshore Mech. Arct. Eng 121(4), 261-267 (Nov 01, 1999) (7 pages) doi:10.1115/1.2829577 History: Received July 18, 1999; Revised August 05, 1999; Online December 17, 2007


An analysis of the impact of model uncertainties on the design factors for environmental loads on offshore structures was carried out. Considering uncertainties in environmental processes, the load effect models and the member resistance, an approach was developed that gives explicit consideration to model uncertainty in codified design. For frequent environmental load effects, a two-factor approach was developed that defines the overall load factor as the product of two components: a basic factor accounting for uncertainty in the environmental process and a separate factor accounting for model uncertainty. The overall load factor is to be applied to the specified load, which is defined as the load corresponding to the environmental process value associated with a specified return period. This load can be calculated from the environmental process without considering model uncertainty. The model uncertainty factor was defined as a linear function of the mean and the standard deviation of the model uncertainty parameter. It can be estimated based on a specific model and reliability analysis. This two-factor approach has two advantages: (a ) it allows for reductions in the load factor if conservative or more accurate models are used; and (b ) it eliminates the need for the designer to consider model uncertainty in estimating the specified load. The approach was used to develop a set of load factors for environmental loads on offshore structures. These factors were calibrated to produce reliability levels consistent with those implied by the load factors in CSA-S471.

Copyright © 1999 by The American Society of Mechanical Engineers
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