Time Simulations of Ocean-Going Structures in Extreme Waves

Abstract

A summary is presented on calculation procedures to determine extreme responses of ocean-going structures using nonlinear time domain simulations. As such nonlinear simulations are time consuming full scatter diagram assessments are not feasible. Advanced techniques are therefore required to assist in the selection of response critical sea states. Of particular importance is to what extend a few or even a single design sea state can be used to determine a lifetime design value. An overview of methods, which can be used to determine design sea states, is given and discussed. Secondly, if a design sea state is specified different approaches are possible to assess a response in that sea state. Regular, random irregular and conditioned irregular waves can be modeled, which affect both the accuracy and the computation time. Alternative extreme wave conditions are considered, in perspective to existing techniques, and the application of freak waves for response simulation is discussed. An example simulation is shown of a cruise-ship in a freak wave. In this example different phases are identified and the associated physical phenomena are discussed. Based on this study and the calculations, requirements for future research are formulated in order to use extreme/freak waves in response analyses and secondly to consider the use of freak waves in design procedures.

Modeling freak waves from the North Sea

“…This phenomenon has become evident in recent years mainly due to the increasing number of instrumental measurements of abnormally high waves appearing at the background of a typical wave motion. Determination of the extreme values of waveinduced loads and the design of safe ships and offshore structures are being considered as knowledge of possible extreme and freak waves increases, as described in [14,19,31]. The increasing attention to freak waves led to the development of different approaches for understanding and describing the phenomenon; they take into consideration strong currents, variable bathymetry, atmospheric fronts, dispersive and nonlinear properties of water waves as described, for instance, in the review [24].…”

Extreme Wave Generation, Breaking, and Impact Simulations Using Wave Packets in REEF3D

“…Several methods in the time-domain for determining the design extreme wave for offshore structures have been developed over time such as a large regular wave Soares (1993), random irregular waves Wang (2001), identification of critical wave episodes Torhaug (1996) and the most likely extreme response (MLER) Tromans et al (1991) Hansen and Nielsen (1995) where an irregular wave train is conditioned to produce a linear extreme wave with a prescribed response profile at a prescribed time. The MLER method can be considered a generalisation of the critical wave episodes method Pastoor et al (2003).…”

Numerical and experimental analysis of extreme wave induced vertical bending moments on a FPSO

“…SWAN, LAMP) codes can only be used together with simplified wave models (e.g. [8,9]) since they are computationally intensive.…”