In this paper a simple numerical procedure has been proposed to predict the probable extent of damage to tubular members due to collisions. In the procedure a tubular member is reduced to a spring-mass system having two degrees-of-freedom, one for local denting damage and the other for overall bending damage. Results of impact tests have been correlated with those of numerical analysis in order to achieve an empirical representation of the strain-rate sensitivity and other dynamic effects upon the spring coefficient for bending deformation. Reasonably good correlation between the theoretical estimates of damage by the proposed procedure and the test results is realized. A rigorous parametric study has been conducted to calculate local denting damage, overall bending damage, maximum spring forces and plastically dissipated energy for various values of geometric and material property parameters and speed and mass of the striker. Simple design equations are then derived based upon the parametric study results.

1. Introduction

For more efficient design of offshore structures considering offshore collisions it is necessary to be able to predict the probability of occurrence of minor collisions, the consequential extent of damage and the residual strengths of the damaged structures. In this paper, however, only the extent of damage of offshore tubulars due to minor collisions will be considered. A question may be raised whether offshore minor collisions can be considered as quasi-static phenomena or need to be treated as dynamic ones. An experimental study was carried out by Arochiasamy et al.[l] on the response of a hydro-elastic semi-submersible to bergy-bit impacts. It was observed from the experiments that the rebound velocity of the bergy-bits after impact were approximately 70 to 75% of the impact velocity. A series of lateral impact tests on tubulars having simply supported roller conditions were conducted by the authors[2].

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