Dynamic Modeling of Ship-to-Ship and Ship-to-Pier Mooring Performance

Abstract

AbstractTraditional methods for the design of ship-to-pier moorings in normal and storm conditions and for ship-to-ship moorings under normal conditions are currently based on static calculations. These calculations have served well for many years, first with natural fiber ropes and later with nylon and other low- to medium-modulus synthetics. Key to the success of this simplistic approach is lines that can elongate enough under tension to share the loads between multiple lines. When wire rope mooring lines are introduced, an increased weight catenary and the use of constant tension winches allowed enough compliance for the moorings to load share successfully.Now, we have very lightweight, high-modulus synthetic lines like High Modulus Polyethylene (HMPE), Aramid, and Liquid Crystal Polymer (LCP), where there is almost no stretch and very little weight to form a weight catenary. When used with constant tension winches that allow the mooring load to be shared across multiple lines, these can work well. However, when they are used from bollard to chock to bit with no compliance, they are unable to share the load between multiple lines, and high tension failures occur where a weaker but more compliant mooring line would be fine.This article describes advanced dynamic modeling of ships loaded by wind, waves, and currents in these conditions and the tension sharing between mooring lines of different materials and constructions. The need to share the mooring load between multiple lines is the crux of the issue.