Control Methods for Launch and Recovery at Sea using Overhead Cranes

Abstract

Launch and recovery (L&R) of a small vessel, from a large mothership, at high sea states is difficult and is usually avoided for safety reasons. This thesis deals with the study of the L&R mechanism using a crane onboard the ship. A novel feedback control system automating the recovery process of a small vessel at high sea states has been developed for the first time. An Euler-Lagrange approach is used to develop a two-dimensional dynamic model to determine the behaviour of the mothership-crane-small vessel system under different sea conditions. This model forms the basis for the design of a second-order sliding mode controller (SMC), which is used to change the length of the cable during the recovery process and ensures that a pre-defined profile is followed. At low sea states, this controller works robustly with safe recovery of the small vessel. However, at high sea states it is not sufficient in preventing collisions between the small vessel and the hull of the mothership. To provide more degrees of freedom, the crane has been modified to enable the movement of the suspension point. An Integral Sliding Mode Control (ISMC) approach is employed to adjust the crane’s boom such that the separation distance between the small vessel and the hull of the mothership are maintained at a safe level. This model is upgraded by the development of a three-dimensional mothership-crane-small vessel representation which allows the implementation of the previously designed controllers with only minor adjustments. As only two control actions are applied, one swing angle remains unconstrained resulting in large pendulations. This problem is tackled with the use of tethering lines to connect the mothership to the small vessel resulting in a damping effect. The recommended approach for a safe L&R process made here consists of the use of an adjustable boom crane with tethering lines. These physical tools, combined with the robust control algorithm will help enable the safe and successful L&R of the small vessel even in rough sea states.