Damage Evolution and Failure Prediction in Rubber Marine Cone Fenders Subjected to Cyclic Loading

Abstract

Marine rubber cone fenders are critical components in port infrastructure designed to absorb the kinetic energy of a berthing vessel. However, when a vessel is permanently moored in harsh wave and wind environments, the fenders are subjected to cyclic loading due to resonant vessel motions. This can result in 500.000 fender deflections per year, which can cause complete rupture of the cone fender due to fatigue. The PIANC 2024 mentioned that within current guidelines, there is insufficient information available to provide limits for fenders subjected to cyclic loading. This thesis addresses the gap to characterize damage evolution and predict the service life of rubber marine cone fenders subjected to cyclic loading. In this study, the Super Cone fender of Trelleborg with a height of 400 millimeters (SCN 400) was used as a reference model. First, experimental uniaxial tension and compression tests were performed. Three different engineering strain rates were tested to capture the rate dependence of the rubber. This provided input for the determination of the stress strain relationships that were subsequently used to calibrate the hysteresis material model in Abaqus. The Bergström–Boyce model showed the best correlation with the experimental data among the models investigated with a total normalized mean absolute difference of 9.4%. To investigate the most suitable damage parameter, a data set from the literature containing uniaxial fatigue data on natural rubber has been used. To gain insight into the corresponding stresses and strains, the fatigue specimen was modeled in FEA. The maximum principal stress, shown to be effective in multiaxial fatigue and exhibiting the highest correlation across all loading ratios in the data set studied, was selected as the most suitable damage parameter. The cone fender was implemented in FEA using axisymmetric four-node quadrilateral hybrid elements (CAX4RH) within an implicit quasi-static dynamic framework in Abaqus. The FEA model was found to have a maximum reaction force of approximately 14% higher than the experimental performance, while matching the overall shape of the force deflection curve. The steepest decrease in predicted service life was observed for deflections up to the buckling point. The influence of loading velocity on stress states was analyzed by selecting three critical elements with relatively high maximum principal stress development. The average increase in stress among critical elements was found to be the highest in the buckling phase during compression. When the loading velocity increased by a factor of eight, the average stress in this phase increased by 38%, corresponding to a reduction of 57% of its predicted service life. In addition, the average increase in the maximum principal stress among the critical elements was closely related to the increase in stress in the uniaxial tension experiments at a strain of 0.3 when the strain rate was increased by a factor of ten. Finally, modifications have been made to the example cone fender to enhance its suitability for handling a large amount of small energies. A geometric variation analysis had been conducted that resulted in the selection of two important dimensional parameters. Focusing on the inner radius and the angle of the inner axis led to a design that achieved a 32% increase in energy absorption up to the defined maximum principal stress limit.