ABSTRACT

The stochastic process model of the long-term statistics of ship responses is proposed. We have investigated the correlation between the wave height and the standard deviations of the ship responses at sea. Analysing the monitored ship response data on board of a container ship and a bulk carrier, we have found that these standard deviations have three stochastic properties: the Markov property, regression to equilibrium, and linearity of fluctuation. The stochastic process model of the ship responses is constructed based on these properties. This model is given by the stochastic differential equations, and the probability density function is obtained by the Fokker-Planck equation. Generally, the stochastic differential equation is difficult but this model is not difficult because it has only three parameters. Furthermore, we explain long-term prediction and simulation of ship responses by this stochastic model and introduce a method of evaluating the monitoring data of ship responses on board.

INTRODUCTION

Ship response is the general term for a ship's motions, speed, power, fuel consumption, wave loads, structural deformation, and frame stress at sea. Monitoring ship responses at all times on board and analysing them appropriately, can help clarify the long-term seakeeping performance and its reduction. The monitoring system concept was first proposed by Ashcroft et al(1998). The information is also useful for route planning, maintenance, and ship design, and enables the rational evaluation of the life cycle value of ships. To achieve this, it is necessary to develop a monitoring system that continuously measures, records, analyses, and evaluates the ship responses. In this paper, we discuss the stochastic model required for the analysis and evaluation of the ship response data. Long-term prediction is often used in the evaluation of seakeeping performance.

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