Berthing Reactions and Load Distribution

Berthing reactions are a function of the berthing energy and the deformation characteristics of the fender system.

1. Berthing loads

should be distributed in such a manner that:

a) contact pressures on the ship’s hull are kept within acceptable limits;
b) direct contact between hull and berth structure is prevented;
c) the capacity of the fender is not exceeded.

2. Hull pressures

The maximum acceptable contact pressure between the hull and fender is influenced by many factors, including the type and size of ship, nature of the fender bearing surface (i.e. rigid or flexible) and the position of the contact area relative to the hull frames.

For LNG/LPG tankers and very large crude carriers (VLCC’s), acceptable contact pressures will generally be between 15 t/m2 and 20 t/m2.

3. Fender reaction due to angular berthing

Unless the point of impact is on the straight run of the hull and the vessel is parallel to the berth at impact the fender unit will receive an angular loading. The hull geometry over the impact area should therefore be considered in both horizontal and vertical planes (see Figure 4 and Figure 5) to establish:

a) the angle of application of load to individual fenders;

b) deflection of individual energy absorbing units within the fender and hence the aggregate amount of energy absorbed by the complete fender;

c) clearance between hull and berth structure. Most manufacturers of proprietary elastomeric units and pneumatic fenders supply correction factors to the performance data of their units for use under angular berthing conditions. For flexible elastic dolphins, gravity fenders, etc., the effects of angular berthings should be analysed from first principles.

Where circumstances dictate that angled approaches will be the general practice at a particular berth, consideration may be given to angling the individual fender elements relative to the berth in order to create a closer approximation to parallel berthing conditions and hence more efficient performance of the fender

4. Fender frames

Where a fender frame is used to reduce the contact pressure, or to couple a series of units into a single fender, the fender frame should be one of the following:

a) a proprietary type as produced by the fender manufacturer and demonstrated by him as being adequate to carry the applied loads;

b) a purpose designed steel frame with stresses and constructional details in accordance with BS 449-2 or BS 5950-1;

c) a purpose designed timber frame with stresses in accordance with BS 5268-2 treating berthing impacts as short term loadings.

Steel frames should be faced with suitable materials to minimize abrasive contact with the hull of the vessel. Facing materials may be of timber or polymers. In cases of particularly high wear the use of steel plate facing may be considered. Fixings should be such that worn or damaged facing panels can be easily and rapidly replaced. The heads of fixing bolts, set crews, etc. should be recessed into the wearing face to avoid direct contact with the hull plating, with an allowance for wear of the facing panels.

5. Shear capacity of fenders

Vessels moving longitudinally or vertically induce friction forces at the contact surface between fender and hull. These forces will induce shear deformations in the fender and these should be kept within acceptable limits. (Shear deflections may be limited by chains connecting appropriate parts of the fender assembly.) In the absence of information from a fender manufacturer, shear forces can be calculated using the relevant coefficient of friction, m, multiplied by the normal force at the fender face. Typical values of m are given in Table 4.

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Table 4 — Coefficients of friction of fender facing materials in dry conditions

NOTE: The above coefficients of friction only apply where smooth contact surfaces are present. Hence they will not apply where the ships using the berth have rusty hulls or protuberances on their hulls as is the case, for example, with the rivets and plate ends of older vessels. In the latter cases the designer should consider higher values.