Model uncertainty in the long-term distribution of wave-induced bending moments for fatigue design of ship structures

Abstract

The uncertainty of the long-term distribution of the wave-induced wave bending moments in ships has been assessed, relating it to ocean areas, ship routes, ship speeds and ship types. Weibull distributions have been fitted to the calculated values of long-term distributions and their shape factor has been related to the ship length. The number of load cycles in a ship lifetime has been calculated, and characteristic values were derived for different levels of probability. A two-parameter Weibull distribution defined by a characteristic value and a shape factor has been proposed as a load model to be adopted in design rules for the assessment of fatigue damage in ship structures. The model uncertainty of the two parameters defining the distribution is assessed as a function of the method of fitting the long-term distribution, of ship speed and of the climatological data.