Mechanism Design and Dimensional Optimization of a Low-Degree-of-Freedom Parallel Mooring Unit

Abstract:

The automated mooring system of a vessel is subject to large operational loads and requires fast wave-compensation motions. When a full-degree-of-freedom parallel mechanism is used as the mooring device， a large number of hydraulic cylinders are needed as actuators. Due to the high loads and large quantity of cylinders， stringent requirements are imposed on the flow rate and power capacity of the hydraulic system. To address this issue， a low-degree-of-freedom 2UPU-2SPU parallel unit is proposed. By installing one such unit at both the bow and stern of the ship， a dual parallel mechanism configuration is formed that meets the requirements of automated mooring. A comprehensive analysis and optimization of the workspace， kinematic behavior， and mechanical performance of the proposed mechanism are conducted. Screw theory is employed to analyze the motion characteristics of the 2UPU-2SPU parallel unit. Based on the transmission and constraint characteristics of motion and force， a high-quality workspace is defined to avoid transmission singularities and constraint singularities. The regular high-quality workspace volume index， global transmission/constraint indices， and global stiffness performance indices of the mechanism unit are derived. Taking these three categories of indices as objective functions， dimensional optimization is conducted using the SPEA2 multi-objective optimization algorithm. Furthermore， in accordance with preferences for dynamic and static performance， the analytic hierarchy process （AHP） is applied to select solutions with superior overall performance. The proposed mooring system explicitly considers installation constraints and coordinated combined motion. The unit mechanisms can be installed on the quay primarily through an anchor-bolt system， facilitating practical deployment.