Dynamic Response to Wet Deck Slamming

Abstract

In offshore construction, there is a demand for vessels with ample deck space. Large catamarans could provide larger deck space for the same tonnage. However, one operational concern for catamarans is wet deck slamming. Wave impact with the wet deck can excite the global flexible vibration modes of the vessel. While extensively studied for high-speed catamarans, the dynamic response induced by slamming in large, slow-moving, heavy-lift catamarans remains poorly understood.

This thesis presents a fast numerical method for predicting the dynamic response of a heavy-lift catamaran subjected to wet deck slamming in head seas. The method combines Capytaine, a BEM potential flow solver, with ANSYS Mechanical, a finite element software package. ANSYS calculates the dry modal properties, and Capytaine includes the water effects to obtain the wet modal properties. Capytaine also finds the rigid body motions resulting from wave loading. A simplified slamming pressure formulation based on immersion velocity and wetted length is applied. Both monochromatic and bichromatic wave conditions are investigated to assess their influence on the immersion behaviour.

It was found that monochromatic waves produce immersion of long duration. The proposed method cannot accurately determine forces and responses due to the varying properties during a slamming event. Additionally, the limited impulse can not effectively excite the eigenmodes due to the long wetting time compared to the natural periods. In contrast, bichromatic wave conditions lead to shorter and more severe immersion events. For the bichromatic wave loading, the modal dynamic oscillatory response was found as a result of wet deck immersion.

To assess the accuracy of the proposed numerical method, a validation against experimental measurements is performed. The comparison focused on accelerations resulting from slamming events and the intervals between such events. The resulting accelerations of the proposed numerical method slightly underestimated the measured accelerations. One problematic finding was that significant slamming events could disrupt the vessel's oscillatory motions and that resolving the disruption cannot be achieved in the frequency domain. The proposed numerical method can be used to identify conditions where slamming can be problematic quickly. However, to obtain accurate displacements, accelerations, and stresses, further analysis by another method is required.

The thesis provided insight into the dynamic behaviour of a heavy-lift catamaran induced by slamming. An indication of slamming effects can be calculated in the early design stages. More importantly, identifying problematic wave loading cases can be done at a relatively low numerical cost. The exact magnitude of the slamming effects requires a more detailed time-domain analysis.