►    CHAIN

 

►    WIRE ROPE

 

 

         

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CHAIN

 

 

The chain is a flexible connection through multiple rigid metal links.

The chain also known as anchor chain is the most common component used for mooring lines of ships and offshore floating structures. The chain is available in different chain construction. Two chain constructions are in common use for mooring line is stud link chain and studless link chain. The stud link chain has studs to stiffen the links and to avoid the tangling of the chains. The stud in the link ensures that the links come in the correct position into the gypsy.

The stud link chain is used for mooring lines that have to be moored numerous times during their lifetime. The studless link chain is often used for permanent mooring. The advantage of the studless link chain is the weight saving, but studless chain is not strong enough for gypsy use. The studless link chain can consist of short open links or long open links.

A long open link has a longer length than a short open link. A mooring chain consists of different links, common link, enlarged link and end link. The links are made of steel or alloy steel. The metal links must have galvanized surface against the saltwater environment. The links are available in different grades and diameters. The higher the grade the higher the strength of the link. The high strength of links can provide for lower chain weight, but in catenary mooring system there is a small advantage to be gained. The catenary mooring system needs chain weight to create restoring forces.

The cost of chain can range from the cost function 1.0 Euro/kN/m to the cost function 0.7 Euro/kN/m based on Proof Loads.

 

 

 

Common Link

Enlarged Link

         End Link 

 

 

 

Stud Link Chain

 

Studless Link Chain

 

 

 

WIRE ROPE

 

 

Wire rope is a metal wire used for the mooring line. Wire rope has a relative lower weight than chain, but a higher elasticity for the same breaking load. Wire rope has a relative lower weight than chain, but a higher elasticity for the same breaking load, but the disadvantage of wire rope is that wire rope is more vulnerable to corrosion and soil ingression than chain.

The cost of wire rope can range from the cost function 0.09 Euro/kN/m to the cost function 0.02 Euro/kN/m based on Minimum Breaking Load.

Wire rope consists of multi-wire strands laid helically around a core. The classification of wire rope consists of the number of strands in the wire rope and the number of wires and the strand configuration (number of strands x number of wires per strand). The wire ropes in common use for offshore mooring lines are spiral strand and six strand.

 

 

 

The following strand configuration of wire rope are well-known:

 

►     Filler Wire, main strands have equal diameters and small diameters are added

►     Seale, strands in any layer have equal diameters

►     Warrington, strands of the outer layer have two different diameters

►     Warrington Seale, the inward has two different diameters and outer layer has equal diameter wires

 

                                       

 

 

 

The wire rope properties are determined by the wire rope construction. The wire rope properties can be estimated with an X-chart. The X-chart gives the relationship of abrasion resistance and resistance to bending fatigue and the wire rope classification. The lay of a wire rope is determined by type and direction. The direction is right laid or left laid around the core. The right laid rope is the standard. The type is regular or lang.

The regular type is laid as the same direction as the strands.The lang type is laid in the opposite direction of the strands.

The advantages of regular type are that the regular type has a better resistance to crushing and rotates less under severe loads. The advantages of lang type are better resistance to abrasive wear and fatigue. Between the lay type and the regular type is no difference in breaking strength or minimum breaking load MBL.

 

 

 

 

 

The strands of wire rope are available with special shaping of strands. The special shaping of the strands changes the rope property of the wire rope. The special shaping of ropes are flattened strand (triangular) wire rope, compacted strand wire rope and swaged (compacted) wire rope. The special shaped rope must be consistent with the application of the wire rope. The wire ropes are made of different materials like stainless steels, carbon steels, brass, bronze, monel and kevlar.

The stainless steels are better corrosion resistant than carbon steels. The carbon steels is stronger than stainless steels and has better resistance to abrasive wear. The materials brass, bronze, monel has better corrosion resistance in salt water.

The material kevlar is a good solution for very corrosive water. The material kevlar is not a metal. Kevlar has the same breaking strength as stainless steels, but kevlar has a bad abrasion resistance and has a higher elasticity than stainless steels. Besides the strands has a wire rope a core. The core is important for the wire rope properties. There are two core types. The two core types are Fiber Core (FC) and Independent Wire Rope Core (IWRC). The advantage of a Fiber Core is that the Fiber Core damps out vibration and the Fiber Core is more flexible than Independent Wire Rope Core.

The advantage of an Independent Wire Rope Core is that the Independent Wire Rope Core has more strength in tension and more resistance to crushing. The disadvantage of an Independent Wire Rope Core is the decreased flexibility.

To protect the wire ropes, wire ropes can have coating, filling, sheathing and plating. The wire ropes are protected with the following solutions:

 

►     Plastic Filling

►     Plastic Coating

►     Galvanized

 

The advantage of plastic filling is that it prevents abrasion. The disadvantages of plastic filling are that it is not resistant against corrosion and inspection is difficult. Plastic filling is not recommended for offshore mooring lines.

An alternative for plastic filling is plastic coating. The disadvantages of plastic coating are that it is not resistant against corrosion and plastic coating wears off and inspection is difficult. Plastic filling is not recommended for offshore mooring lines. The advantages of galvanized wire rope are that it is very good resistant against corrosion and it is cheaper than other solutions. For example, galvanized carbon steel is cheaper than stainless steel. Galvanized wire rope can be manufactured in different ways. Galvanized wire ropes manufactured by hot dip process and by electro-deposition process after weaving have good properties. The galvanized wire rope is zinc coated.

The galvanized wire rope manufactured by other ways has less good properties like less strength. Galvanized wire ropes or stainless wire ropes are recommended for offshore mooring lines.

 

 

 

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