Dynamics and control of a two-ship ensemble connected by a massless towline

Abstract

Tugboats are often called upon to rescue disabled ships on open seas. This study represents a fundamental step toward automating the towing mission of disabled vessels. It deals with the control of transient planar dynamics for a two-ship ensemble connected by a massless cable. This is a multibody structure whose components are coupled by non-holonomic inequality constraints. Two control strategies were designed by exploiting the essential temporal scale difference between relative tugboat-ship dynamics induced by the constraints and the relatively slow motion of the mass center of the two-ship ensemble. In the first strategy, the leading ship was controlled to maintain a prescribed heading angle and surge speed for the mass center of the two-ship ensemble while in the second one the leading vessel was controlled to track prescribed set targets for its desired heading angle and surge speed. The first strategy reduces the controller's susceptibility to sharp variations in the towline tension and is suitable for maritime applications involving powered vessels performing cooperative transport of a large floating object. The results demonstrated the viability of the control strategies, proved that the first strategy is as effective as the second one and showed the robustness of controllers to environmental disturbances.