Fendering is basically the interface between a vessel and the berth facility. This medium serves to absorb a certain portion of the kinetic energy of a vessel without damage to the vessel and the waterfront structure. In the case of rubber fenders, which are generally relatively soft, the majority of the energy is absorbed through elastic deflection of the fender. But, possibly also the deflection of the berth facility and/or the vessel’s hull will contribute to the absorption of the kinetic energy. On the other hand, when a vessel berths against a single vertical pile the majority of the energy will be absorbed by the deflection of the relatively flexible pile.
The deflection multiplied by the reaction force which is generated and a certain efficiency factor equals the kinetic energy.
For a rubber fender this relation can be expressed mathematically as follows, whereas it is assumed that only the rubber fender will absorb the kinetic energy (hence neglecting e.g. the energy absorption through deformation of the berth structure and the vessel’s hull):
Ef = f * Rm * dm
- Ef = the vessel’s kinetic energy which is to be absorbed by the fender (in kNm)
- f = factor representing the energy absorbing efficiency of the fender system (between 0 and 1)
- Rm = maximum fender reaction force (in kN)
- dm = maximum fender deflection (in m)
The factor f is depending entirely on the fender characteristics, viz. the relation between deflection and reaction force.
The R/ Ef –ratio (fender factor) provides knowledge of a fender system, whereas the R and Ef values shall be taken at the design (or rated) deflection of the fender. A low R/ Ef –ratio indicates that low reaction forces are generated to absorb the required energy which is often considered favorable. In some cases, however, it is not required that fenders absorb energy and then a high R/ Ef is advantageous, e.g. for surface-protecting fenders.
Then energy that is absorbed by the fender system during compression is partially returned to the vessel (the vessel is pushed back) and partially dissipated in the form of heat within the material (hysteresis). See also figures below:
Figure 2.1: the shaded area represents the energy absorption; factor f is equal to the shaded area divided by the rectangular area O-Rm-A-dm
Figure 2.2: Curve 1 represents the compression of the fender, Curve 2 the decompression of the fender, whereas the area between those two curves is the energy dissipated (warmth generated) as a result of hysteresisis.
The selection of a fender or fender system should be tuned to the following stages of usage:
a) during the berthing process (initial contact between vessel and berth facility);
The berthing process consists of a vessel approaching the berth facility, generally under an angle with a certain approach velocity defined as the velocity perpendicular to the face of the facility. The impact of the vessel in motion on the facility must be absorbed in such way that no damage occurs to vessel or facility.
b) while the vessel is moored;
With respect to situation around the berthed vessel along the berth facility, a distinction can be made between the operational regime and the safety regime.
The operational regime is the regime in which it is still possible to load and unload the vessel, the safety regime is the regime in which it is still possible to allow the vessel alongside the berth without endangering the vessel, the berth or the fendering.
In both regimes the fender should be able to absorb the energy generated by the vessel. The energy is partially transformed by the fender through elastic deformation into heat and into a reaction force. This reaction force acts in two directions, leading to a concentrated load on the berth facility and to a load on the vessel’s hull. This reaction force is especially of importance when:
1. the berth facility is sensitive to horizontal forces (structure on piles);
2. the vessel is moored and moves due to waves.
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