Fundamental Study on Elastic Response of Offshore Structures under Collision (2nd Report)

Abstract

When offshore structures are collided with supply boats, the damages or the plastic deformations are limited within the collided membrers in most cases. The supporting structures which support the collided member only oscillate elastically. Though no damage is caused in themselves, the dynamic characteristics of the supporting structures are very important to study the motion of the collided members and to predict the degree of the 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 shows the procedure to model or to idealize the given collision problem is proposed and its validity is demonstrated through numerical examples.