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Around 85% of the mooring systems of semisubs consist of all chain mooring lines. As the development goes to deeper water, the use of the combination of chain & wire becomes the more popular choice for catenary mooring systems. For taut mooring systems synthetic fibre rope lines are used. In selecting a material for a mooring line there are three key parameters: - (submerged) weight per unit length - breaking strength - elastic modulus (Young’s modulus) The following picture shows these three parameters for the three considered options. This picture shows that the submerged weight of the three options varies significantly whereas the break strength decreases more gradually. Also in this picture can be seen that the elasticity of fibre rope is significantly less than that of chain or steel wire. This shows why fibre rope is applicable for taut mooring systems. In the following the main characteristics of the three options are mentioned. The synthetic fibre rope is given extra attention, since this option encompasses many possibilities.
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 chain is often used for permanent moorings (FPSOs, buoys, FSOs).
Steel wire The steel wire, or wire rope, is also a well-known option for mooring lines. Wire rope is relatively light compared to chain, but has higher elasticity for the same breaking load. The wire consists of a number of steel strands wound up around a steel core. One disadvantage is the higher sensitivity for corrosion. Also the wire rope can be sensitive to torsion loading, resulting in failure in the wire rope. Steel wire is often combined with chains to have a good combination between weight and strength.
Synthetic fibre rope Synthetic fibre ropes have not been used very often as mooring lines. However with the development of mooring in deeper water, they offer a very good alternative to the steel wire and chain combinations. In a taut mooring system fibre ropes are mostly preferable. There are four main types of fibre rope. Each of these has a different balance of properties. It is equally important to select the right construction as well as the right yarn.
Parallel Yarn Parallel fibres with an extruded polyethylene jacket
Parallel Strand Parallel strands (braided or laid) with a braided fibre jacket (typically polyester)
Wire Rope Constructions Strands laid as in a wire rope. Torque balanced constructions also available
Plaited 8 and 12 Plait Braids
Braided Single Braid or Braid on Braid
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