Fender systems
Introduction
Fender units have a very good ratio between energy absorption and force. Dolphin piles equipped with fender units can be designed with a smaller diameter and/or wall thickness. The dimensions of panels in front of a fender are designed to protect the hull of the vessel against berthing and mooring reactions to the maximum required hull pressure specifications. Allowable hull pressures for different types of vessels can be found in PIANC 2002 [3.21].
In the case of a single steel dolphin pile in combination with a fender system, the energy absorbed by the dolphin pile can be added to energy absorbed by the fender system.
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Figure 3-27 Examples of a fender system.
Dolphins are often equipped with a buckling type cone or cylindrical fender system made of rubber. On the mooring side of a vessel, a steel panel lined with ultra high molecular weight polyethylene (UHMWPE) is mounted on a fender in order to reduce friction and increase abrasion resistance.
Chains are installed to limit or prevent the rotation of panels due to berthing angles of the vessel. Weight chains are applied to support the weight of fender panels. Reaction chains at the top are required when impact forces are possible below the fender unit.
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Figure 3-28 Buckling fender cone type (left) and Buckling fender cylindrical (right).
Energy absorption and reaction forces
Suppliers of fender systems fabricate and deliver their products with production tolerances and design factors because it is difficult to determine exactly how the natural rubber product will react after processing a fender.
In the case of normal production tolerances, it is recommended to decrease the energy absorption by 10% and increase the reaction force by 10 %. Other design factors are temperature variation, impact speed and berthing angle. The variations in temperature and velocity factors differ per supplier. Typical differences are given in the tables below. Temperatures and velocity factors are given for two suppliers.
Table 3-16 Temperature factor on reaction force.
| Temperature | -30ºC | -20ºC | -10ºC | 0ºC | 10ºC | 23ºC | 30ºC | 40ºC | 50ºC | |
| Correction factor | Suppl. 1 | 1.55 | 1.37 | 1.18 | 1.08 | 1.03 | 1.00 | 0.97 | 0.94 | 0.91 |
| Suppl. 2 | 1.26 | 1.20 | 1.15 | 1.10 | 1.05 | 1.00 | 0.97 | 0.94 | 0.91 | |
Table 3-17 Velocity / impact speed factor on reaction force.
| Compression time | 1 sec | 2 sec | 3 sec | 4 sec | 5 sec | 6 sec | 7 sec | 8 sec | |
| Correction factor | Suppl. 1 | 1.05 | 1.02 | 1.01 | 1.01 | 1.00 | 1.00 | 1.00 | 1.00 |
| Impact speed | 1 m/s | 50 m/s | 100 m/s | 150 m/s | 200 m/s | 250 m/s | 300 m/s | ||
| Correction factor on a unit | Suppl. 2 | 0.97 | 0.98 | 0.99 | 1.00 | 1.01 | 1.02 | 1.03 | |
Table 3-18 Angle factor on energy absorption.
| Angle | 0º | 3º | 6º | 9º | 10º | 12º | 15º | 20º | |
| Correction factor | Suppl. 1 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 0.96 | 0.92 | 0.8 |
| Suppl. 2 | 1.00 | 1.01 | 1.00 | 0.99 | 0.98 | - | 0.93 | 0.87 | |
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Figure 3-29 Performance of buckling type fender system.
As illustrated in the fender performance curve, the reaction force reaches its maximum of 100% at two deflection stages. It should be noted that the performance curve in figure 3-29 is representative of a cone fender and that the curve for a cylindrical fender is slightly different. The maximum reaction force is reached at the moment of 35% and 70% deflection, respectively. The first stage results in 30% energy absorption and the second stage in 100%. When normal and abnormal berthing energy has to be taken into account in the design, the reaction force on the dolphin can reach its maximum already at normal berthing. By adding product tolerances, temperature and velocity factors the reaction force will increase. This situation must therefore be taken into account in the safety philosophy.
Engineering practices for fender design
Attention must be paid to the positions of the freeboard edge in relation to a fender panel for loaded and unloaded vessels, in low and high water level situations, and line loads of belted vessels onto fender panels.
In situations with large tidal differences, a steel dolphin pile can be equipped with two fender systems, one above the other, connected by a steel fender panel. The different energy absorption of the two fenders needs to be determined, taking into account the different compression of the two fenders due to bending of the pile, the vertical angle of the ship's hull and the point of contact.
- The two fenders will be compressed differently, due to bending of the pile and the vertical angle of the hull. Therefore, the full rated energy of the fenders cannot be added in the energy calculation.
- The pile must be able to resist the maximum of the combined fender reaction forces, which in most cases is the sum of the rated reactions.
- The low impact level of a small vessel can affect the position of the bottom fender.
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Figure 3-30 Fender system.
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Figure 3-31 Dolphin equipped with two bulking fender units.