When offshore structures are collided with supply boats, in most cases, the damages or p" l1astic deformations are limited to a single member. The supporting structures to which the collided, member is, attached only oscillate elastically. Even though no damage is caused to the supporting structures" its dynamic characteristics are very important to study the motion of the collided members and to predict the extent of the local damages. To clarify the dynamic characteristics of the supporting structures, the authors employed the modal synthesis. The types of the dynamic characteristics are classified using the number of the dominant vibration modes as an index. Further, it is shown that the index can be related to direct physical parameters such as collision time and vibration period of the structure. It is also shown that these parameters can be estimated using the static finite element analysis and a simple dynamic analysis of a spring-mass model with three degree of freedom. Based on the above study a flowchart which describes the procedure to model or to idealize a given collision problem is proposed and its validity is demonstrated through numerical examples.
A large number of studies have been reported on the subject of collision between ships or dropped objects and offshore structures (Ellinas, C.P. and Valsgard, S., 1986). Various authors developed methods for predicting the extent of damage to a structure or estimating the residual strength of damaged structures. In spite of a rather large bulk of information, their scope it limited because these theoretical and experimental research are restricted to a single collided member and loads are assumed to be applied statically. For the purpose of predicting the collision damage of an offshore structure, a research program is carried out by the authors (1987,1989).