Stochastic Still Water Response Model

In this study a stochastic field model for the still water loading is formulated where the statistics (mean value, standard deviation, and correlation) of the sectional forces are obtained by integration of the load field over the relevant part of the ship structure. The objective of the model is to establish the stochastic load field conditional on a given draft and trim of the vessel.

The model contributes to a realistic modelling of the stochastic load processes to be used in a reliability evaluation of the ship hull. Emphasis is given to container vessels. The formulation of the model for obtaining the stochastic cargo container load field is based on a queuing and loading policy that assumes containers are handled by a first-come-first-serve policy.

The load field is assumed to be Gaussian. The ballast system is imposed to counteract the angle of heel and to regulate both the draft and the trim caused by the possible uneven distribution of the cargo load and the bunker load over the system. Stability is not explicitly accounted for. Finally the calculated second moment statistics of the sectional forces in a container vessel in a full load condition are reported.

The obtained statistics for the maximum still water bending moment is compared to statistics from available regression formulas. It is found that the suggested model predicts a coefficient of variation of the maximum still water bending moment that is a factor of two to three times lower than that obtained by use of the regression formula.

It turns out that an important parameter of the stochastic cargo field model is the mean number of containers delivered by each customer.