An Evaluation of Design Wave Selection Methods for Floating Offshore Structure Design

For floating offshore structures, the structure design should consider site-specific environmental loads in a target return period, typically 100-year (annual probability of occurrence of 10−2). The equivalent design wave (EDW) approach has been a popular method to represent long-term extreme responses in a site-specific environment. Although the method is convenient and efficient, the EDW approach is a great simplification that represents extreme responses with a number of wave scenarios. Therefore, the EDWs have to be chosen with great care. The conventional method to choose EDWs is based on the heading angles and the wave frequencies at the peak of the response amplitude operators (RAOs). This approach works well for trading vessels but may not be the case for offshore applications where equal probability of occurrences cannot be assumed for every wave heading angle. The EDW selection determines the load combinations which are applied not only to structural analysis but also to prescriptive local scantling calculations, therefore, an appropriate method should be adopted so that realistic wave scenarios can be applied to the structural design of offshore structures. In this study, several EDW selection methods are reviewed and evaluated for the rational design of floating offshore units.