We present case studies of two approaches to efficiently obtain ship response extremes from time domain simulations. These approaches, namely, the so-called coefficient of contribution method and response-conditioned wave episodes, allow long-term statistical predictions based on a combination of frequency domain analyses with a limited number of time-domain simulations with computationally expensive methods. In the present case, the time domain code is a RANS solver coupled to a finite element structure solver.
Rational assessment will be perhaps the future standard in ship structural design. The International Association of Classification Societies (IACS) expressly promotes the shift towards goal based standards. This encourages classification societies to revise their rules and establish new procedures for ship structural reliability assessment. One of the key issues is the ultimate structural strength, e.g., of primary members of the hull girder or local structures. Corresponding loads are typically experienced in severe sea states, where full scale measurement data are scarce. Rational assessment of design loads for ultimate strength thus requires appropriate numerical tools to deal with such severe environmental conditions. Nonlinearities of waves, wave-structure interaction and structural response statistics cannot be neglected when dealing with extreme responses. In the past, several authors established computational procedures to avoid prohibitively time-consuming analyses based on time domain methods. Two of such approaches are studied here, namely a contour line method (coefficient of contribution method), Baarholm and Moan (2000), and response-conditioned wave episodes, Dietz et al. (2004). The applied numerical method to determine ship reactions is based on a Finite Volume (FV) RANS (Reynolds-Averaged Navier-Stokes) solver, coupled with a nonlinear six degrees of freedom solid body motion solver and a linear Finite Element (FE) solver to compute the motion response including structural elasticity. The FE solver uses a simple Timoshenko Beam element representation of the ship.