Simulation of Ship Motion in Seaway

Accurate modelling and simulation of ship motion in seaway is essential to test applications of ship motion control strategies. In this report, we review the mathematical description of ocean waves, and their effect on the motion of marine vehicles. We present a simple method suitable for accurate simulation that incorporates parameters related to the recommended spectral family and particular vehicle being considered. Based on this method, we also present a simple algorithm to tune the commonly used shaping filters to model wave induced motion. This allows different simulation scenarios to be identified with given sea states and particular sailing conditions. Introduction Automatic control strategies for marine vehicles and structures are designed to improve their functions with adequate reliability and economy. It is essential to the design and development of such strategies that accurate and relatively simple mathematical models be available to describe the loads exerted and the motions of vehicles. On one extreme, the state of the art in maneuvering simulation of marine vehicles incorporates sophisticated models to describe the motion of the ship in different environments. These models are based on free-running model tests data, databases, and numerical simulation based on Computational Fluid Dynamics Methods (CFDM)—see for example (Simonsen, 2000). These models are typically too complex to be used for testing control strategies. On the other extreme, the literature on marine control applications such as autopilot, dynamic positioning and rudder roll stabilization, report very simple models utilized to design the control control algorithms. The latter models usually describe the sea state and the motion of the vessel by filtered white noise, i.e., shaping filters. In this report, we review the description of ocean waves and propose a method to simulate ship motion in seaway that incorporates parameters related to the recommended spectral family and particular vehicle being considered. Then we present a simple algorithm to tune shaping filters that allows different simulation scenarios to be readily identified with given sea states and particular sailing conditions. The rest of the report is organized as follows. In section 1, we introduce the stochastic mathematical models for describing sea states, and their properties. In section 2, we describe the relations between the sea elevation and ship motion components giving a stochastic mathematical description of the phenomena. In section 3, we present a simple algorithm to tune shaping filters and we show some simulation results. Finally, in section 4, we summarize and discuss the presented material.