Analysis of heave motions of a truss spar platform with semi-closed moon pool

This paper presents the heave motion of a truss Spar platform in regular waves considering the dynamic coupling between the motions of the platform and water in the moon pool. For a spar platform with semi-closed moon pool, water can flow freely through the guide plate at bottom of the moon pool. The mass of water inside the moon pool is comparable to the platform mass itself if the top tension riser system is used. Then the effect of the moon pool water on the motions of the platform should not be ignored. In the study presented in the paper, a 2-DOF (Degree of Freedom) dynamic coupling equations of the heave motions of the spar platform and the vertical motions of water in the moon pool were derived considering that the moon pool is totally closed, or 30% or 70% open. The results show that motions of water in the moon pool significantly affect the heave motions of the spar platform. Parametric study was also carried out to find the effect of the ratio of opening of the bottom plate on the coupled motions. Finally the model experiments were carried out to validate the numerical calculations.