Non Linearity of Extreme Vertical Bending Moment: Comparison of Design Wave Approaches and Short Term Approaches

Abstract

The evaluation of extreme bending moment corresponding to a 25 years return period requires very long simulations on a large number of sea states. This long term analysis is easy to do with a linear model of the ship response, but is impractical when using a time consuming model including non linear loads (Froude-Krylov and hydrostatic non linear forces). In that case some simplified methods need to be applied. These methods are often based on equivalent Design Sea States (DSS) or Equivalent Design Waves (EDW). The different methods are applied to large database of ships, containing 19 tankers, 29 bulk carriers, and 9 container-ships, in full load or ballast condition. Linear hydrodynamic calculations are carried out to define the extreme linear vertical bending moment, and to define the DSS and the EDW. Then non linear time domain sea-keeping computations are performed, taking into account Froude-Krylov hydrostatic pressure correction and large motions. The difference between linear and non linear hogging and sagging extremes are computed for each type of DSS and EDW.