Fenders as part of mooring system
Vessel motion shall be considered against winds and waves, swells, and long period waves entering from the ocean. Especially, in the design of mooring systems of offshore deep water terminals, forces due to vessel motions are also important items to be considered as well as vessel berthing forces. As for permanent mooring facilities such as vessels used for restaurants, hotels, museums, and floating terminals, the mooring systems including fendering systems shall be designed taking into account the motions of vessels or floating bodies due to winds and waves. To evaluate or improve the working ratio of mooring facilities constructed at the sites, adverse environmental conditions such as winds and waves, the motions of moored vessels have to take into account load-deflection characteristics of mooring ropes and fenders. To estimate mooring forces due to vessel motions, computer simulations or hydraulic model tests are commonly used. Items to be considered in restraining vessel motions:
1. General
The type and the size of mooring systems consisting of fenders and mooring ropes should be decided taking into account the following items:
–deflection and reaction of rubber fenders
–tension of mooring ropes
–strength of vessel’s hulls
–shear force of rubber fenders
–vessel motions
–tidal current and suction by passing vessels
2. Marine fender deflection and reaction
The maximum rubber fender deflection due to wave and wind forces during the mooring process must not exceed the allowable value of deflection as well as the maximum deflection during berthing. Maximum reaction is one of the largest design forces for mooring facilities. For flexible dolphins, maximum fender reaction is usually the dominant design force. The maximum reaction force needs to be limited, taking into account the strength of a vessel’s hull.
The load-deflection characteristics of mooring systems shall be considered to reduce sub-harmonic motions which are due to the asymmetry of load-deflection characteristics of the fenders and mooring ropes.
3. Tension of mooring ropes
The maximum tension of mooring ropes due to wave and wind forces shall be smaller than the allowable value of tension of mooring ropes. The safety factors of tension for synthetic ropes and wire ropes shall be properly decided. To reduce sub-harmonic motions in swaying and long period oscillations in surging, the type and the size of mooring ropes should be properly selected taking into account the external forces.
4. Strength of vessel hull
To prevent excessive concentration of the vessel mooring forces as well as berthing forces on the fendering systems and the vessel, protection panels should be provided as required to reduce the load on the unit area (face pressure).
5. Shear force on fender
In general, vessel motions are likely to generate shear forces between the hull of the vessel and the fender face, due to swaying.
A resin pad (e.g. polyethylene) in front of the protector panel is recommended to reduce shearing forces acting on the fenders.
Sometimes the shear forces are favorable or desirable, as they can limit the motion of the vessel. The facing should be chosen to suit the situation.
6. Allowable vessel motions
For the effective operation of cargo handling, vessel motions should be smaller than the allowable values. Allowable vessel motion varies by vessel type, cargo handling equipment and component of motions.
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