Control of the launch and recovery of small boats to a mothership in high sea states using sliding mode methods

Abstract

The launch and recovery (L&R) of a small vessel from a large mothership occurs in various settings, such as the rescue of stranded personnel, extraction of unmanned autonomous sea monitoring sensors and the deployment of rescue submersibles. L&R of a small vessel, from a large mothership, at high sea states is difficult and is often avoided for safety reasons. This paper proposes sliding mode controllers to automate the process involving a modified davit crane in which the suspension point is movable. This is the first time a robust control strategy has been used to tackle this specific maritime problem. In this work, a second-order sliding mode controller is used to change the length of the cable during the recovery process and ensures that a pre-defined profile for the cable’s length is followed. This controller alone proves to be sufficient for a safe recovery at low sea states. However, for higher sea states, the small vessel and mothership collide under certain circumstances. An integral sliding mode controller is introduced to adjust the crane’s boom, which modifies the position of the suspension point, such that the oscillations of the small vessel are minimized. This controller aims to keep the distance between the mothership’s hull and the centre of the small vessel at a constant value by adjusting the position of the crane’s boom and reducing the swing angle. The overall control algorithm is found to be very effective, compared to the fixed boom case, ensuring that collisions do not occur under any of the tested configurations, and a safe distance is maintained between the mothership and the small vessel. The performance evaluation of the controller predicts that it will safely operate in rough sea states for any initial combination of swing angle and velocity, if a safe recovery can be carried out at a lower sea state for the same initial conditions.

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Identifiers

ORCID for Dominic Taunton: orcid.org/0000-0002-6865-089X

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