Abstract

The concept of conditional stochastic processes provides a powerful tool for evaluation and estimation of extreme wave and wind loads on ships and offshore structures. The talk focuses on application of the First Order Reliability Method (FORM) for derivation of critical wave episodes for different non-linear wave-induced responses. A coupling with Monte Carlo simulations (MCS) seems to be able to give uniform accuracy for all exceedance levels with moderate computational time even for rather complex non-linear problems. Examples dealing with overturning of jack-up rigs, parametric rolling of ships, and whipping vibrations will illustrate the procedure.

For non-linear systems, the estimation of fatigue damage under stochastic loadings can be rather time-consuming. Usually Monte Carlo simulation (MCS) is applied, but the coefficient-of-variation (COV) can be large if only a small set of simulations can be performed due to otherwise excessive CPU time. The reason is that the fatigue damage estimation is very sensitive to the largest values obtained from the simulations. The talk suggests the additional use of the First Order Reliability Method (FORM) to get a better estimation of the tail in the distribution of the estimated fatigue damage and thereby reducing the COV. For a specific example, dealing with stresses in a tendon in a tension leg platform the COV is thereby reduced by a factor of three.

Finally, a current evaluation of the IMO Weather Criterion using the First Order Reliability Method to define possible critical wave and wind scenarios leading to capsize is outlined. The reason for the investigation is that this very empirical and old criterion has proven to be the governing stability criteria regarding minimum GM for both small ferries and large passenger ships.

Biosketch

Jørgen Juncher Jensen is Professor in Maritime Engineering at the Department of Mechanical Engineering, Section of Fluid Mechanics, Coastal and Maritime Engineering. He is leading a research group dealing with wave induced stochastic load and response processes for marine structures. The current topics include whipping and springing of ships, hydroelastic responses of very large floating structures and floating wind turbine systems and much of the research is done within the theory of conditional stochastic processes. He has been at DTU since 1973 and has published more than 150 papers. He is serving in the editorial board of several journals and conferences and has supervised numerous PhD students at DTU. He has been external examiner at several universities in Europe and is regularly a visiting researcher at UC Berkeley. He has received several rewards (e.g. Bronze Medal from Royal Society of Naval Architects, London, Gold Medal from the Alexander Foss Foundation, the Statoil Prize and SNAME’s Davidson medal) . He has been on several PhD-committees at NTNU and is adjunct professor at AMOS.