Mooring Line Types
Chain
The most common product used for mooring lines is chain which is available in different diameters and grades. Two different designs of chain are used frequently, studlink and studless chain. The studlink chain is most commonly used for moorings that have to be reset numerous times during their lifetime, for instance semi-submersibles, while studless link chainis often used for permanent moorings (FPSOs, buoys, FSOs). A chain mooring line can be terminated in either a common link or an end link.
In almost all mooring systems chain is one of the components, both ends of a mooring line mostly consist of chain. The reason to use chain as termination at the vessel side is that chain is relatively easy to hande in case of pull-in or pay out. Furthermore chain is not very sensitive to abrasion which could be a consideration when handling of a mooring line is necessary. The reason to use chain as a termination at the anchor end is again its resistance against abrasion. Especially in a catenary shape mooring line , a significant part of the mooring line lies on the seabed. This piece of chain will be lifted off the seafloor and will touchdown, every time the vessel experiences an offset. Also chain realizes the highest friction with soil, which reduces the horizontal loads at the anchor. The fact that chain is heavy both has a positive and negative aspect. In caternary mooring systems the restoring force is generated by the weight of the suspended mooring line length, therefor using chain ensures a high restoring force. However in deeper waters chain mooring lines become very heavy. This causes very high loads at the top of the mooring lines, which reduces the maximum payload of the vessel.
Steel wire rope
When compared to chain, steel wire rope has a lower weight than chain, for the same breaking load and a higher elasticity. Common wire ropes used in offshore mooring lines are six strand and spiral strand. The wire rope is terminated with a socket for connection to the other components in the mooring system. Gernerally wire rope is more prone to damage and corrosion than chain. Steel wire rope does suffer from fatigue more than chain, which results in the need for thorough maintenance plans during the lifetime of a project. It is less sensitive to abrasion than synthetic fiber rope, but more than chain.
Synthetic fibre rope
A recent development is the use of synthetic fibre ropes as mooring line. Typical materials that can be used are polyester, high modulus polyethylene (HMPE) and aramids. The difference between these materials can be found in the resistance against abrasion, strength-weight ratio, creep properties, and tension-compression properties.
The major advantage of synthetic fibre rope is the light weight of the material, the excellent fatigue properties and the high elasticity and strength. The synthetic fibre rope is gernerally terminaterd with a special spool and shackle for connection to the other components in the mooring system.
Some disadvantages are that the ropes are relatively sensitive to abrasion, which makes it important to handle them with care and little is known on the long-term application of fiber ropes. Also the axial siffness of synthetic fiber ropes is non-linear which makes it difficult to model these ropes. Another issue related to the use of synthesic fiber ropes is creep. Creep is permanent deformation of the line when it is under tension, which means that the line lengthens during the lifetime of the mooring system.
