Stochastic and deterministic combinations of still water and wave bending moments in ships

Abstract

Probabilistic models are presented for the still water bending moment (SWBM) and vertical wave bending moment (VWBM). Different stochastic and deterministic methods are applied for the combination of the long-term SWBM and VWBM. It is found that commonly applicable deterministic load combination methods, namely, the peak coincidence method, the Turkstra's rule and the SRSS rule, are either very conservative or nonconservative. Using stochastic methods, including the exact point-crossing method, approximate load coincidence method and Ferry Borges method, leads to identical predictions for the current load combination problem. For code application, load combination factors are introduced for both sagging and hogging conditions to reduce the conservatism inherent in the existing ship rules. Case studies are performed for an offshore production ship. A considerable reduction of the total design bending moment is found when the load coincidence is properly considered instead of combining the maximum of each load.