Fendering Systems Design Theory|Eversafe Technology

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Fendering Systems Design

Type of Fenders
The purpose of installing fenders on the pier is to protect the vessel and the pier from damage when the vessel berths alongside the pier. The fenders absorb the berthing energy of the vessel and soften the berthing impact on the pier. Fenders can be classified into timber fenders, rubber fenders and plastic fenders, according to the type of materials used.

Timber Fenders
Timber fenders are mainly produced from tropical hardwoods and such application is not environmentally friendly. There is also a high damage rate for the timber fenders due to wear and tear by vessels and this has resulted in substantial maintenance requirement. Hence, further use of timber fenders is not recommended.

Rubber Fenders
Rubber fenders are produced from natural or synthetic rubber. Rubber fenders are available in a wide range of forms and dimensions designed to meet the requirements of a variety of operating conditions. They are designed to absorb the berthing energy by means of their deflection. Advice should always be sought from one or more of the major reputable suppliers regarding the energy absorption characteristics, fender reaction, deflection and other installation details.

Rubber fenders can be classified into two categories based on their performance. Buckling or constant reaction type fenders have a reaction load that increases more or less linearly in the initial stage of deflection as a result of elastic compressive deformation. However, when the reaction load reaches a certain magnitude, it tends to remain almost constant regardless of the increase in deflection due to elastic buckling, until the hollow portion of the fender is closed. After closure of the hollow portion of the fender, elastic compressive deformation is restored,resulting in further increase in the reaction load.

Another category is the constant elastic modulus type fenders for which the reaction load will increase approximately in proportion to the increase in deflection. As soon as the hollow portion of the fender is closed, the behaviour of the fender is similar to the buckling type fender.Different types of rubber fenders and their general performance are shown in Figure 1.

(1) Cell Fenders
Cell fenders and other large unit fenders are usually mounted individually in a series. Each of the fenders is designed to absorb the berthing vessel’s energy on its own.Steel frontal frames may be installed for load spreading to keep contact pressures within acceptable limits. In some fender systems, these unit fenders are connected together to provide a larger energy absorption capacity or to cover a larger operational range of vessels.

(2) Pneumatic and Foam-filled Fenders
Pneumatic and foam-filled fenders are characterized by their high energy absorption and low reaction force. Pneumatic fenders comprise a hollow rubber bag filled with air whereas foam-filled fenders comprise a resilient closed cell block covered by a reinforced rubber skin. These fenders are designed to absorb berthing energy by the work required to compress the air or foam cells.They are generally in the form of cylinders with domed ends floating in front of the berth structure. Chains or wire ropes with swivels and shackle connections are used to anchor the fenders to the berth structure.

(3) Long Strip Rubber Fenders
Long strip rubber fenders are more suitable for installation on piers designed for use by smaller vessels. They are generally more economical than the larger cell fenders and pneumatic or foam-filled fenders. Common forms of long strip fenders include cylindrical,D, arch and turtle fenders.Cylindrical fenders are usually supported on chains but can also be fixed in position.The energy absorption capacity is among the lowest of the long strip fenders for a given section height.D fenders differ from cylindrical fenders by having a flat surface to facilitate installation and therefore they can be directly fixed on the vertical surfaces of the piers. The curved contact surface provides a soft initial contact between the vessel and the berth structure. The area of contact increases with the energy absorbed during the berthing operation. Installation of arch fenders is relatively simple. The fenders can be fixed directly on the vertical surfaces of the piers.The energy absorption capacity provided is relatively high compared to other long strip fenders since the energy is absorbed mainly through compression deformation rather than deflection. The reaction force is also large,resulting in a relatively high contact pressure.

The turtle fender is a relatively new type of rubber fender designed for small craft port facilities, and is characterized by a wide protective area and a relatively large energy absorption capacity for a given section height. It possesses a large breadth to height ratio to minimize damage in severe mooring conditions. The large area of contact results in lower contact pressure for a given reaction force. Similar to the arch fender, each turtle fender can be fixed directly on the pier with two rows of fixing bolts.

Plastic Fenders

Plastic fenders are produced from recycled plastic, which consists of a mixture of high density polyethylene, low density polyethylene and polypropylene. Antioxidants and ultraviolet inhibitors are added to retard the effects of ultraviolet light on the plastic. Fibre glass or steel reinforcing bars are normally provided to enhance the flexural and tensile resistance. The reinforcing bars are arranged in a regular pattern within the core of the fender and extend over the entire length of the fender. The type, size and number of reinforcing bars depend on the structural requirements of the application. Current products of plastic fenders are produced to American Society of Testing and Materials Standards (ASTM) for plastics. Plastic fenders can sustain and transfer the berthing load to the pier. However, these fenders possess less energy absorption capacity and hence result in higher berthing load to the piers compared to rubber fenders. Hence, rubber fenders are normally combined with plastic fenders to improve the energy absorption capacity and berthing comfort. In such an arrangement, the plastic fender acts as a beam to transfer the berthing energy and berthing load by bending to the supporting rubber buffer. Information on the flexural strength, moment of inertia and modulus of elasticity of plastic fenders is required in the design, and advice should be sought from one or more of the major reputable suppliers for detailed design information.

Arrangement of Fenders

The dimensions, deformation and fixing details of a selected type of fender will affect how the fenders are arranged to perform effectively during vessel berthing. In particular, when designing the layout of the fenders,the following points should be noted :

1. The design of the vertical position of the fenders should take into account the range of freeboard of the vessels so that all vessels using the pier can be in contact with the fenders during berthing under all possible tidal levels (see Figure a).
2. The horizontal interval of the fenders should be determined to avoid the vessel from making direct contact with the pier under the designed berthing angle and deflection of the fenders, due to the horizontal curvature of the vessel’s hull (see Figure b).
3. The vessel may contact the pier directly due to its vertical curvature. For example, if the fenders are installed at a low level, the upper part of the vessel may strike the pier before the fender is compressed to the designed deflection (see Figure c).
4. Many small vessels have projections like belting to protect the hull. When the projections contact a fender directly, the fender may be compressed locally and damage such as cracking and cutting may occur. A steel frame may be installed on the face of the fender to avoid such damage (see Figure d).
5. The distance between the edge of the pier and the side of the hull when the vessel is moored should be checked (see Figure e).
6. The pressure on the vessel’s hull when berthing energy under the worst conceivable approach conditions should not exceed the acceptable contact pressure on the vessel’s hull.

For public piers designed for pleasure yachts, trading vessels, motor launches, fishing vessels, kaitos and sampans, from past experience fenders extending from about +0.15 mPD to +3.65 mPD are considered appropriate in order to cater for a wide range of vessel sizes.However, if public piers are to accommodate larger ferries, barges or tug boats, fenders should be to a specific design.

Fenders should in principle be installed on the portion of the pier where berthing will take place. However, it may sometimes be necessary to install fenders at the remaining pier portion to allow for accidental berthing, berthing under emergency circumstances or increase in future demand. A comparison of the benefits gained should be made against the increased cost of installing additional fenders.