Transient motions of a semsubmersible are induced by the damages such as a breaking of mooring lines or a flooding into compartments The possible symptom of the hazardous causes of a disaster due to the motions of a semisubmersible after damages is investigated. The transient behaviour after damages observed in the process of the experiment is described and its physical meanings are scrutinized by the theoretical modelling of the behaviour. In particular, the sudden release of forces and the gradual cumulative effect of mass are considered, which are related to actual damages of a semisubmersible.


It is important to know the response of a semisubmersible to the sudden change of Force/moment equllibrium, because it is considered to be the response to the damages which may lead to a catastrophic disaster. The disaster is apt to be caused during a transient motion before a semisubmersible will reach a static equilibrium state. However the rules for the stability of semisubmersibles of the classification societies at present account for only the static state after damage. Although the stability criterion m the rule tries to include the dynamic behaviour of a semisubmersible, it is not thought to be enough to cope with the various aspects of the dynamic behaviour. So several proposals for the stability regulation have been proposed based on the analysis of dynamic motions of a semisubmersible1,2 However, so far, there exist only a few research works which deal with directly the motions of semisubmersibles after damages3. Therefore the transient motions of a semisubmersible after damages are our interest m this paper

For a semisubmersible at the site of deep water depth, the change of the equilibrium state of overturning moments will be caused by the sudden change of exerting forces, the addition or the loss of weight and the loss of buoyant parts. And at the site of shallow water area, when the submerged part of a semisubmersible touches with the sea bottom due to a large inclination, the drastic change of overturning moment also may occur due to the change of a moment lever. In these occasions, when the force/moment equilibrium is broken, a semisubmersible moves into a new equilibrium state after experiencing the transient motion. Due to the dynamic effects, the transient state usually gives larger displacements or inclinations than the final equilibrium state and may trigger further damages as schematically shown in Fig. 1(Fig 1 is available in full paper) The change of forces due to the gradual addition of weight is another important modelling of the damage such as a flooding. When the rate of weight increase is small enough for the dynamic effect to be neglected the prominent transient behaviour will not be observed. On the contrary, when the increase rate is large, the phenomenon may be simulated by the motion which resulted from an initial Impact. In this case the transient motion becomes prominent.

However, since the transient motion usually involves a large number of parameters which vary with geometries and dynamic conditions of a semisubmersible, the theoretical analysis is difficult and the experimental work needs a lot of tune to cover the wide aspect of the transient behaviour of a semisubmersible. As typical examples of the damages caused by the sudden change of exerting Forces/moments and the gradual addition of weight, we consider in this paper the breaking of a mooring line and the flooding Transient motions of a semisubmersible after the damages are observed m the experiments and a simple theoretical model is made In order to grasp the essential features of the behaviour after the damages.

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