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► MOORING LINE CONFIGURATION
► CHAIN
► WIRE ROPE
► SYNTHETIC FIBER ROPE
► SUBMERSIBLE BUOYS
► REGULATION
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A mooring line can consist of one or different components like chain, (metal) wire rope, synthetic fiber rope. With the different components are different mooring line configurations possible. The mooring line configuration depends on the mooring system, water depth and the required strength. The most common mooring configurations are:
► All Chain ► Chain & Wire Rope ► Chain & Synthetic Fiber Rope ► Chain & Wire Rope & Synthetic Fiber Rope
The all chain mooring configuration is used in shallow water till the water depth of 100 m. The chain & wire rope mooring configuration is generally known as the conventional mooring line. The chain & wire rope combination is used till the water depth of 2000 m. The chain is replaced by wire rope to save cost and weight. By a conventional mooring line submersible buoys and/or clump weights can be placed along the mooring line for design considerations. In shallow water long mooring line length is required to provide the elasticity for stretching, otherwise the mooring will be too stiff to cope with large dynamics loads from the ship motions. In water depths greater than 300 m wire rope is used to reduce the top tension of the mooring line and to increase the stiffness of the mooring line. In those water depths the conventional mooring line consists for the most part of wire rope. In water depths greater than 2000 m the conventional mooring line of chain & wire rope will become too heavy. The mooring line combination of chain and wire rope will also become too soft in horizontal stiffness to resist environmental forces. In ultra deep water (water depths greater than 2000 m) the mooring configuration of chain & synthetic fiber rope or chain & wire rope & synthetic fiber rope are used. The wire rope is replaced by synthetic fiber rope, because the relative light weight of submerged synthetic fiber rope reduces the weight of the mooring line and synthetic fiber ropes have good line elasticity. The synthetic fiber rope of polyester is generally used for those mooring line configurations. About the application of synthetic fiber ropes in mooring lines are concerns. A recent guideline for the design of a mooring system is API RP 2SK (API 1995).
The chain also known as anchor chain is a flexible connection through multiple rigid metal links. The 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 offshore mooring lines. Two chain constructions are stud link chain and studless link chain. 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 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.
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 strength. The wire ropes in common use for offshore mooring lines are spiral strand and six strands. The disadvantage of wire rope is that wire rope is more vulnerable to corrosion and soil ingression than chain. In general wire rope is used in combination with 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 Loads.
The advantages of synthetic fiber ropes are that synthetic fiber ropes are lighter and synthetic fiber ropes have higher elasticity. They are almost neutrally buoyant in seawater. The synthetic fiber ropes have also a greater strength to weight ratio than steel. The weight savings is very important for deep water mooring. The synthetic fiber ropes are often made of polyester and high modulus polyester (HMPE), but synthetic fiber ropes are available in different synthetic materials and rope constructions. To select a synthetic fiber rope you can visit the website http://www.tensiontech.com/tools_guides/rope_selection_calculator.php The synthetic fiber ropes are extensively used for short duration mooring, but synthetic fiber ropes are not often used in permanent mooring. The disadvantages of synthetic fiber ropes are that synthetic fiber ropes are relative expensive and synthetic fiber ropes have also larger diameters than wire ropes. The synthetic fiber ropes are not as abrasion resistant as wire ropes. Synthetic fiber rope has strength reduction through hydrolysis. An important issue for the synthetic fiber mooring line is the possible failure of the synthetic fiber rope by creep rupture and fatigue. Creep is the elongation over time of a line under tension. The possible failure by creep rupture and fatigue is a very important design consideration for permanent mooring. A recent guideline for the use of synthetic fiber rope is API RP 2SM (API 2001). The cost of synthetic fiber rope can range from the cost function 0.2 Euro/kN/m to the cost function 0.02 Euro/kN/m based on Minimum Breaking Loads. Most common synthetic fiber rope in offshore mooring lines is polyester rope. The cost of polyester rope can range from the cost function 0.06 Euro/kN/m to the cost function 0.02 Euro/kN/m based on Minimum Breaking Loads.
Pressure-resistant floats also known as submersible buoys and sub surface buoys on a mooring line are an alternative solution for synthetic fiber rope to overcome deep water mooring problems. The submersible buoys reduce the vertical forces on the floating offshore structure due to the heavy weight of suspended mooring lines in deep water and increase the stiffness of the mooring system. The use of submersible buoys on a mooring line will reduce also the line dynamic effects on line tensions due to large wave frequency vessel motions, because the submersible buoys will keep the mooring line in a more perpendicular direction. The advantages of submersible buoys are that the installation and operations of mooring lines are easier and buoys can create clearance from risers or any other obstacles. The disadvantages of submersible buoys are the additional cost of the submersible buoys and the submerged depth of the submersible should be deep enough to alleviate wave forces acting on the buoys. There are two types of pressure-resistant floats like buoys with two ears with holes through them and buoys with a hole through the centre. By the attachment of the float to the mooring line a 2 m length rope is required. On the mooring line the submersible buoys must position at least 2 m apart.
The classification societies and institutes have developed over time guidelines, rules and regulation for offshore mooring systems, offshore mooring lines. The following classification societies and institutes have developed guidelines:
► American Petroleum Institute API ► American Bureau of Shipping ABS ► Lloyd’s Register of Shipping
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