Common Considerations in Fender Design
A fender is the interface between a ship and the shore facilities. Generally, its main objective is to protect the ship’s hull from damage. In some cases it’s the shore facilities that require protection against the impact of the ship.
There are many types of fender systems available ranging in complexity from a simple bolt-on timber whaler to a very sophisticated arrangement of frames, chains and buckling components.
A proper berth design will include a comprehensive analysis of two or three fendering alternatives, as the choice of fendering system could have a significant impact on the berth design. Beside all main factors in fender design, such as calculating berthing energy, vessel dimensions and type energy requirements, there are a few common considerations should be also concerned.
1. Fender Performance Characteristics
Not only must the fender design absorb the required berthing energy. but also the designer must also consider the reaction loads that this system will impart to the structure. The reaction loads and their location may have a significant impact on the structure design. Generally the reaction loads are not a problem with gravity structures, however, with pile supported piers, the reaction loads may become critical to the design and may influence such things as batter pile locations and the rebar design.
2. Fender Spacing
Fender spacing along the pier face is an important design consideration. Here the designer is trying to maximize protective pier coverage while minimizing the fendering costs. Three methods are standardly used.
1) Fender spacing of not more than 1/10 the length of the design vessel.
2) From the design vessel’s geometry
r = the bent radius of the ship’s hull at the contact line.
h = the compressed height of the fenders at their rated deflection.
Some typical bow bent radius values are shown below. Exact values from the design vessel should be used.
3) Site conditions
The fender spacing can be determined using the wind and current forces and equating them to the fender reaction forces. Use the following formula:
N=(Ra + Rc)/R
N = number of fenders required
Ra = load due to wind
Rc = load due to current
R = fender reaction at rated deflection
3. Normal Operations
1) Stand Off Distance
The allowable standoff distance will be governed by the loading/unloading activities and the normal operating procedures of the ship and pier while berthed. Operating constraints such as crane reach, roll, yaw and freeboard are major considerations in the design. The fenders must provide adequate protection yet accommodate the design.
2) Vertical vs Horizontal Mounting
There is an ongoing concern as to when the fenders should be mounted horizontally and when vertically. In general, vertically mounted fenders provide the best coverage for piers which experience tidal fluctuations. Where the operating procedures require that the vessel slide along the pier face, horizontal bolt-on fenders provide good protection. A combination of horizontal and vertical arrangements are often used.
3) Tidal Variation
The change in water level due to tides will have a significant impact on the operation of the pier and consequently the pier design and the fender design as well. Protection in all cases must be achieved for both the largest and smallest ships.
4) Range of Ship Sizes
While the energy absorption capacity of the fender system is chosen for the design vessel, the fender system should be suitable for the full range of ships expected to use the facility. Fender stiffness on the smaller vessels may have an influence on the arrangement of the fenders. Also, if barges are to use the facility, special attention must be given to their fender requirements.
5) Frequency of Berthing
A high frequency of berthings normally justifies greater capital expenditures for the fender system.
4. Accidental Impact
The fender system is less expensive than the dock structure and it should be recognized that the damage to the fenders is less critical than to the vessel or the structure. The design should incorporate a reasonable level of energy absorbing capacity. The mode of failure of a fender and its effect on the dock structure should be considered.
5. Ongoing Maintenance Costs
Maintenance costs can be an important factor and should be considered when analyzing the overall costs of the various fender options. Maintenance costs will vary with fender type.
6. Ease of Installation
A well designed fender system will be as easy to install as possible. This will minimize initial capital costs and reduce down the road maintenance costs.
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