Real-time 6-DOF Vessel-to-Vessel Motion Compensation using laser tracker

The industry state-of-the-art offshore motion compensation is today carried out by offshore cranes equipped with Active Heave Compensation (AHC) systems, meaning that a hanging load is positioned at a certain height above the seafloor, even though the floating vessel carrying both the crane and the load is moving due to wave disturbances. Lately, there has been an increased interest among the industry also to compensate for the movement of a secondary vessel, which indicates that a more detailed investigation of the Vessel-to-Vessel Motion Compensation (VVMC) problem is required. The primary motivation for such compensation technique is influenced by the demand for load transfer between ships during harsh weather conditions, service on floating wind turbines and fish farms, and the transportation of personnel and equipment safely between two floating vessels, for instance. In this paper, the capability of the Leica AT960 laser tracker to measure the relative position and orientation between the two moving Stewart platforms simulating the vessels movement at sea is investigated. The acquired measurements are used to carry out a real-time VVMC using the experimental lab setup in the Norwegian Motion Laboratory at the University of Agder. The obtained experimental results and motion compensation effectiveness are quantified in terms of the accuracy and real-time performance.