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► CATENARY MOORING SYSTEM ► TAUT LEG MOORING SYSTEM
► SEMI-TAUT MOORING SYSTEM
► SPREAD MOORING SYSTEM
► SINGLE POINT MOORING SYSTEM
► DYNAMIC POSITIONING SYSTEM
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The catenary system is the most common type of mooring system employed in shallow water. The catenary refers to the shape that a free hanging line assumes under the influence of gravity. The catenary system provides restoring forces through the suspended weight of the mooring lines and its change in configuration arising from vessel motion. In other words under environmental loadings the moored vessel tries to lift the mooring lines, which create a restoring force. By catenary system the mooring line terminates at the seabed horizontally, the anchor point is only subjected to horizontal forces at the seabed. This requires that the mooring lines be relatively long compared to the water depth. With the increase of the water depth the weight and the length of the mooring line start to increase rapidly. In deepwater the weight of the mooring lines becomes excessive and the mooring lines tend to hang directly down from the rig. The excessive weight diminishes the working payload of the vessel of floating offshore structure. To overcome this problem synthetic ropes are used in the mooring line.
The taut leg system or taut system is characterized that the mooring lines are pre-tensioned until they are taut. By the taut leg system the mooring line terminates at an angle at the seabed. A taut-leg system will usually have an angle of between 30 and 45 degrees. This means that in a taut leg mooring the anchor point are loaded by horizontal and vertical forces. By the taut leg system the restoring forces are created through axial elastic stretching of the mooring line rather than geometry changes. The restoring forces are determined by the stiffness and elasticity of the mooring line. The taut leg system has a much more linear stiffness than the catenary system that gives the advantage that the offsets under mean load better can be controlled and the total mooring line tensions are smaller. A further advantage of the taut-leg system is the better load sharing between adjacent mooring lines. The disadvantage of the taut-leg system is that the mooring line must have sufficient elasticity to absorb the vessel wave motions without overloading. The mooring lines of polyester rope have good characteristics for the taut leg system. For deepwater the taut leg system is a better cost effective solution than the catenary system.
The semi-taut system is a combination of the taut mooring system and catenary mooring system, wherein some parts of the mooring system are taut and other parts are catenary. The semi-taut and taut systems are better suited for deepwater application than catenary system. The semi-taut system and taut system have shorter mooring lines and require less seafloor space or seafloor spread than the catenary system. The shorter mooring lines result in material saving. In general the taut system and semi-taut system are lighter and cheaper designs than the catenary system for deepwater applications.
The spread mooring system consist of many groupings of mooring lines, normally arranged in a symmetrical pattern, attached to the bow and stern of the vessel. The system keeps the vessel or the floating offshore structure on a predetermined location at a fixed heading at sea. Spread mooring systems can be used in applications requiring long service life, in any water depth, and on any size of vessel. Spread mooring systems are most commonly used in unidirectional environments on floating offshore structure that are insensitive to the direction of environmental loads. As the environmental conditions are multidirectional, this type of mooring system is not ideal. For the spread mooring all types of mooring line configurations can be used with this system. The spread mooring system can consist of an equally spread mooring pattern or a grouped spread mooring. The grouped spread mooring has the advantage that the grouped spread mooring will provide better redundancy against possible progressive failure of a mooring system and the maximum mooring line tension is 20% lower.
In the single point mooring system are the mooring lines connect to a single point. Single point mooring systems are generally used on ships. The single point can consist of an external or internal turret, a floating buoy in a CALM (Catenary Anchor Leg Mooring) or a SALM (Single Anchor Leg Mooring). The single mooring system allow for a ship to weathervane into environmental conditions. The ship is often free to rotate through 360 degrees. By the single point mooring system the mooring line are mainly determined by the bow sea loading condition. The bow sea loading condition is significant smaller than the beam sea loading condition.
The dynamic positioning system is a station keeping system for floating units, which uses thrusters to compensate wind, wave and current forces in a dynamic controlled mode to keep the unit on a predetermined location and heading at sea. The dynamic positioning system does not have mooring lines. The dynamic positioning system can be used in combination with a different type of mooring system to provide additional restoring forces.
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