Development and validation of a mathematical model for control of constrained non-linear oil tanker motion

Manoeuvrability is of great importance for large tankers in the marine industry. Manoeuvring, involving yaw rates and drift velocities, which are not small compared with the forward speed, demands a mathematical model of considerable complexity. A great deal of effort has been devoted to the construction of non-linear mathematical models describing the manoeuvrability of large tankers in deep and confined waters. This study is concerned with the effect of the rudder on the turning dynamics of a specific model for ESSO's 190,000-deadweight-ton (dwt) tankers, whose manoeuvrability performance has been investigated in 1972 using full-scale trial results. The turning or heading response of the tanker depends on the turning moment generated by the rudder, which in turn depends on the flow of water over the surface of the rudder. Unfortunately, the classical model used for representing the flow velocity at the rudder causes the rudder to react too quickly and to a greater magnitude (increased turning moment) than is physically possible for an oil tanker of the size considered. As a result, any control system designed using this “fast” model benefits from the improved speed of reaction and increased turning moment produced by the rudder. Consequently, the controller performance derived from simulation results will be significantly reduced when applied to a real tanker. To compensate for this increase, a new rudder model has recently been proposed in the literature. In this study, a comprehensive validation study of this new model is carried out using the original experimental data presented in 1972. This study provides strong evidence to alert readers interested in marine applications that the standard model used for decades in the literature on ship dynamics is not representative of the motions of a real tanker. The proposed model, on the other hand, is superior over the classical model and provides an accurate description of the physical system.