Improved mooring line technology for tankers and gas carriers at exposed berths

Abstract

Fixed-pier berths for tankers and gas carriers are now sometimes installed and operated in locations exposed to waves and swell. In these conditions, the moored vessel can experience large motions which overload and fatigue mooring lines. Tails are short lengths of synthetic fiber rope which are placed in series with the vessel's winch-mounted wires to decrease mooring line stiffness and thus to reduce peak line loads and fatigue due to vessel motions. Past guidelines for conventional tankers and berths recommended the use of 1 lm long nylon tails on mooring lines. Those guidelines were developed many years ago when tankers were relatively small and when fixed-pier berths were located in protected harbors. But when such short nylon tails are used on large vessels at exposed berths, the mooring loads are high and the nylon tails tend to fatigue quickly and fail. Tension Technology International (TTI) recently conduced a study to investigate these problems and make recommendations for mooring lines and tails for use on modern large tankers and gas carriers, especially at exposed locations. The study assessed how waves influence vessel motions and line tensions and how tail length and material influence mooring line loads and fatigue. New recommendations were prepared for tail length and material for use at berths where vessel motions are significant. Large vessels can accommodate longer tail, and longer tail length decreases mooring line stiffness thus and reduces peak mooring line loads. Polyester rope is suffer than nylon, but longer polyester tails can achieve loads similar to short nylon tails. In some situations, polyester tails should be preferred, as that material has much better fatigue performance than nylon, especially in wet condition. Many vessels now use HMPE fiber rope mooring lines instead of wires. These HMPE mooring lines are essentially the same size and as strong as the wires they replace. They are much lighter and easier to handle and thus help crew safety. They are not as stiff as wire rope and thus reduce peak mooring line loads. The recommendations of this study are now published in mooring guidelines and used by tanker and gas-carrier operators.