The Use of Marine Fenders as Energy Absorbing Damper Units in Mooring Systems

Introduction

Engineers and scientists in many countries have pointed out that the mooring conditions o f a ship as well as berthing is affected by fender characteristics.

In this paper, first discuss factors affecting fender characteristics, then, the concept of new simulation methods and the actual simulation results are explained.

The characteristics of rubber fenders

(1) Factors affecting fender characteristics:

It is usually assumed that the reaction force of a fender may only be found from the load-deflection curve for a given deflection. Several external factors, however, are considered to influence the performance of rubber fenders. These factors are simply classified into four main categories as follows.

(2) Mechanical factors:

The compression angles and directions can usually reduce the reaction force of a fender by bending and shearing the fender unit. A series of tests and an extensive research program has been done to establish a set of coefficients which can be used to determine these performance characteristics. Friction, torsion and other mechanical factors often combine with them, which will make the system more complicated.

(3) Dynamic factors:

The deflection speed (frequency), the total number of compressions (fatigue), the creep and the stress relaxation which are called dynamic factors also influence the fender performance.

These dynamic factors are considered to be caused by the visco-elastic properties of rubber. The hysteresis loss of a fender is also considered as one of negative velocity in the recoil cycle. Each of these factors can be independently studies by the relevant model or actual size test.

THIS FIGURE gives an example of results from the berthing simulation studies which gives an indication of the velocity dependency of the reaction force of a cell type fender.

(4) Chemical factors:

The environmental factors such as: temperature, ozone, ultraviolet rays etc., influence the physical properties of rubber and, consequently, change the performance of fenders. Aging can be considered as one of the chemical factors although it is also affected by other factors.

(5) Statistical factors:

The performance deviation resulting from production tolerances should be considered statistically. Usually the range +-10% is adopted according to the record of the pre-delivery tests conducted in the plant.

The range of performance deviations:

When a rubber fender is used as a mooring damper, the fact that some factors are independent and some are not makes it hard to grasp the magnitude of the total deviation.

THIS TABLE shows the range of deviations.