Stiffness of mooring lines
The stiffness of a mooring line is a measure of its ability to stretch under load. Under a given load, a low stiffness cable will stretch more than a higher stiffness cable. Stiffness plays an important role in mooring systems. The stiffness of mooring lines can affect the mooring system in a number of ways, including:
ropes with low stiffness can absorb higher dynamic loads. For this reason, such cables can be used in ship-to-ship operations and in berths that are susceptible to swells, passing ships.
Very low stiffness cables (like all nylon cables) also mean that the vessel will move around its berth extensively, which can cause problems with the range of motion of the boom or hose.
Movement also generates additional energy in the mooring system. The energy stored in a low stiffness cable, if released in the event of an unexpected failure, will "bounce back" unpredictably and could result in personal injury and/or equipment damage.
The different stiffness of various cable materials will affect the force distribution of the mooring system.
Ensuring that cables are properly stressed can reduce cable accidents caused by dynamic effects such as passing ships, standing waves and gusts of wind.
The stiffness of the mooring rope mainly depends on the following factors:The type of material,structure,length.
The steel cable is very rigid. The typical elongation of a wire rope under load is about 1% of its length. Under an equivalent load, the tensile capacity of polypropylene rope is ten times that of steel wire rope. Therefore, if the steel cable and the conventional fiber cable run parallel, the steel cable bears almost all the load, while the fiber cable hardly bears any load.
Fiber ropes are used in a wide variety and new products are constantly being introduced. These fiber lines include lines made from high modulus synthetic fibers (HMSF), which, like steel lines, are also very stiff compared to traditional fiber mooring lines.
There are also differences in stiffness between different types of fiber ropes, although this difference is usually not as pronounced as between fiber and steel wire ropes, this difference can affect the load distribution. For example, HMSF ropes have higher stiffness compared to other synthetic fiber ropes, and if run parallel to conventional synthetic ropes, HMSF ropes will carry most of the load.
Therefore, the effect of the material on the load distribution is critical, and mixed materials should be avoided for cables that serve a similar role.
When hybrid mooring lines are used for a similar role, the low stiffness fiber lines bear almost no load, while the high stiffness lines can be heavily loaded. The same applies to hybrid fiber ropes of different stiffness, although unless the mooring also includes HMSF ropes, the difference between the two will not be as great.
For cables of similar diameter and construction, age also affects the stiffness of the cable. Usually this is not an important consideration because the load relative to the strength of the cable is the controlling factor, not the absolute load.
A synthetic tail rope is usually used at the end of the wire rope to facilitate handling and reduce the stiffness of the wire rope. Tail ropes may also be used to reduce the stiffness of less elastic lines made of HMSF material, and all lines serving the same function in a mooring arrangement should use the same size and type of tail rope.