Flexible dolphin with fendering
Flexible dolphin piles can be equipped with fender units to absorb berthing energy. In general, there are 3 methods to design the pile-fender system:
- Fender-only absorption: All berthing energy is absorbed by the fender, the pile primarily serves as foundation to transfer the fender load to the seabed. This is a straightforward solution, but may not be the most economical design.
- Combined pile and fender absorption without fender stop: The berthing energy is absorbed by a combination of the pile and fender.
- Combined pile and fender absorption with fender stop: The berthing energy is absorbed by a combination of the pile and fender until the fender reaches the overload stop. Any remaining berthing energy will be absorbed by the pile alone.
For the second method, and to some extent the third method, the dolphin pile and its fendering must be analysed as an integrated system to ensure the design impact energy can be absorbed under all conditions. When energy is absorbed by a combination of pile and fender, there is potential to optimize dolphin pile design, for instance, by using a smaller pile diameter and/or wall thickness, see § 3.2.6. Fender systems themselves always have to be designed in accordance with PIANC WG211 (2024) [6.7]. However, integrating a fender introduces uncertainties into the analysis, including:
- material manufacturing tolerances;
- temperature;
- angular compression;
- impact speed.
The stiffness of a fender can vary from the rated reaction and energy specified by the manufacturer. An increase in fender stiffness, due to factors such as low temperatures, high berthing velocities, and manufacturing tolerances, allows for greater energy absorption. This is because higher reaction forces are required to achieve the same deflection, resulting in higher energy absorption capacity. Conversely, reduced fender stiffness, which can occur due to high temperatures and manufacturing tolerances, results in softer behaviour. In these circumstances the fender absorbs less berthing energy since lower reaction forces are required for same the deflection, leading to a lower energy absorption capacity.
Buckling-type fenders are commonly used in combination with flexible dolphin piles. These fenders typically reach their maximum reaction force at approximately half their rated deflection, which implies that the maximum pile load is already achieved when the fender has 60% energy absorption capacity left. Alternatively, fenders with a more linear behaviour can also be employed. For instance, in situations involving permanently moored vessel at a jetty, it may be advantageous to use a low reaction force under normal conditions.
The selection of the fender should be based on the situation with the lowest possible energy absorption of the pile and fender (refer to Figures 6-4, Figure 6-5 and Figure 6-6):
- The fender selection [S1] includes, but is not limited to:
- Lower bound fender stiffness (e.g., during high temperature) - Upper bound pile system (e.g., with upper bound soil conditions, no corrosion, no scour)
Once the fender has been selected, the design of the pile (diameter, wall thickness, steel quality) should consider both the upper and lower bound conditions of the pile system:
- Highest bending moment at the upper part of the pile [S2]:
- Upper bound fender stiffness (e.g., during low temperatures) - Upper bound pile system (e.g., with upper bound soil conditions, no corrosion, no scour) - Largest pile displacement and bending moment at the lower part of the pile [S3a, b]:
- Upper bound fender stiffness (e.g., during low temperatures) - Lower bound pile system (e.g., with lower bound soil conditions, with corrosion, with scour) - Pile toe level [S3c]:
- Upper bound fender stiffness (e.g., during low temperatures); note that for a buckling type fender the impact load of S2 and S3 could be comparable. - Lower bound pile system (e.g., with lower bound soil conditions, with corrosion, with scour)
The design situation with lower bound fender stiffness and lower bound pile stiffness [S4] gives the lowest pile-fender system stiffness, but does usually not result in the largest bending moment, pile toe levels, or displacements.
The selection of an appropriate fender and the design of the pile typically involve an iterative process.
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Figure 6-4 Fender with linear load-displacement curve
For buckling type fenders, it is crucial that the berthing impact load does not exceed the rated reaction force, including tolerances. To ensure adequate safety, it is recommended to apply an additional model factor
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Figure 6-5 Buckling type fender
In situations when a fender overload stop is installed, it may be permissible to exceed the maximum rateda reaction force. When the overload stop is integrated in the fender itself, the maximum load is limited by the strength of the fender.
Usually, a load up to 2 times the rated reaction force can be allowed on the overload stop. This should however be verified with the fender supplier. Such a solution, with the pile absorbing a higher percentage of the berthing impact load, could be economically attractive. In these circumstances, it is likely that the fender buckles under normal operating conditions.
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Figure 6-6 Buckling type fender with overload stop
In addition to the typical design recommendations for pile-fender systems, terminal owners may specify extra requirements that influence the design approach, especially concerning buckling type fenders and the use of overload stops:
- the fender should not buckle under normal impact load (e.g. characteristic berthing);
- the fender should not buckle due to the design wind load on the vessel (e.g. lean-on force).
Consideration a. may result in slightly bigger fenders. Whether consideration b. could be a limiting condition depends on the number of dolphins the vessel leans against, and the wind load relative to the impact load.
Finally, the limiting loading condition for the fender panel is often the lowest impact due to berthing of the smallest vessel. This is probably not the limiting condition for the dolphin pile design, so it must be determined separately. This check may also need to include a reduced fender capacity due to angular berthing impact.