Nonlinear control algorithms for efficiency-improved course keeping of large tankers under heavy sea state conditions

In order to solve the problem of more energy-saving and safer course keeping control for large oil tanker under heavy sea state conditions, a hyperbolic tangent function was used as a new nonlinear feedback function to re-examine the nonlinear feedback in this paper. The new control algorithm was theoretically proved by describing function, and the stability analysis of the system was carried out through Nyquist curve. Taking "YUKUN" and "New Triumph" oil tanker as examples, systematic comparative simulations of hyperbolic tangent function and sine function were carried out. It was found that sine function was better than the hyperbolic tangent function for the course keeping control of general ship under normal sea state conditions. However, for large oil tanker under heavy sea state conditions, the hyperbolic tangent function could achieve almost the same control effect as sine function while the maximum rudder angle decreased by 39% and the average rudder angle decreased by 36%. Therefore, from the numerical investigations, the nonlinear algorithm modified by hyperbolic tangent has obvious advantages in the course keeping control of large oil tanker under heavy sea state conditions.