The lack of weathervaning also means has the implication that the environmental loads on the vessel are usually higher as usual. Which results in an increased number of mooring lines and higher tensions in the lines. This makes that this mooring system is commonly used in moderate environmental conditions like in West Africa and Indonesia. The effect on structures which are less sensitive to wave direction off course is lower, which makes this a very suitable mooring solution for these structures.
The spread mooring can be used on almost every vessel, water depth, vessel size and has a very long service life. This mooring type can be used with traditional shipboard mooring equipment. Doesn’t need a turret structure and expensive bearings, swivels for fluids or gas. Another advantage is that there is no need for an electrical power system and the system can very easily accommodate a large number of risers and umbilicals.
Some disadvantages of the system are, as said before larger number of mooring lines, which makes it harder to install in a short amount of time and the disability of weathervaning. Also all the dynamic motions of the vessel are transferred to the steel catenary risers at the hang-off positions. This results in fatigue problems at the touchdown point of the pipes. Another big disadvantage is that with increased water depth, the amount of mooring line rises significantly.
Catenary vs. Taut leg system
The major difference between a catenary mooring and a taut leg mooringis that where the catenary mooring arrives at the seabed horizontally, thetaut leg mooring arrives at the seabed at an angle. This means that in a tautleg mooring the anchor point has to be capable of resisting both horizontaland vertical forces, while in a catenary mooring the anchor point is only subjectedto horizontal forces. In a catenary mooring, most of the restoringforces are generated by the weight of the mooring line. In a taut legmooring, the restoring forces are generated by the elasticity of the mooringline.
An advantage of a taut leg mooring over the catenary mooring is that thefootprint of the taut leg mooring is smaller than the footprint of the catenarymooring, i.e. the mooring radius of the taut leg mooring will be smallerthan the mooring radius of a catenary mooring for a similar application.
Spread Mooring
A very simple way of mooring a vessel is the spread moored layout. The system is built up from multiple mooring lines attached symmetrically to the vessel. These lines are attached to the bow and the stern of the vessel. The position of the vessel is fixed and with its fixed heading it is unable to weathervane. This manner of mooring is used on sites where the environmental conditions are highly directional.
Mooring Line Shape
When the mooring line of a floater is deployed, part of the mooring line will lay on the seabed and part of the mooring line will be suspended in the water. The part of the mooring line that is suspended in the water will take on a catenary shape. Depending on the waterdepth, the weight of the mooring line and the force applied to the mooring line at the fairlead, the length of the suspended mooring line (S in [m]) can be calculated with:
The horizontal distance (X in [m]) between the fairlead and the touchdown point of the mooring line on the seabed can be calculated with:
The weight of the suspended chain (V in [t]) is given by:
See fig. 4-01 for a clarification of the symbols used. The angle is the angle between the mooring line at the fairlead and the horizontal.
Interactive - Catenary Mooring Line Shape Tool
Hint:
Try changing the catenary shape parameter b right away, you can see clear and immediate changes in the curvature of the mooring line.