A Numerical Study of a Damaged Ship in Beam Sea Waves

Abstract

An important objective of survivability criteria for passenger vessels is to ensure that there is sufficient time called Time To Capsizing (TTC) to evacuate passengers and crew members in case of emergency. This requires a realistical modeling of the phases following an accident. The ship motions can interact with the ingress/egress flooding, water-on/off-deck and/or sloshing of the floodwater in compartments or on vehicle decks. Each of these subproblems is challenging to handle. There is a large variety of phenomena that are neglected in the models proposed so far to predict passenger ship stability. In the sequence of events leading a passenger ship to capsize, numerous questions can be raised on the importance of nonlinear effects. The present thesis is based on the following three considerations. The first one is that the ship motion equations used in the simulations have six Degrees-of-Freedom (6DoF) in order to represent realistic ship capsizing dynamics. The second is to correctly model the flooding flow through the openings. The final aspect is the three dimensional simulation of the floodwater flow on the deck or in the compartment and the prediction of induced loads on the damaged ship, since the flooding flow can not generally be described adequately by a two-dimensional model.

Potential flow theory is used to study the damaged ship motions in waves with ingress/egress flooding through the damaged opening considered. The whole problem involves the exterior and interior flow domains which must be properly described. Here a floodwater solver (FWS) is used for the interior problem. A shallow water solver (SWS) or a multimodal solver (MMS) is used according to different filling ratios. The FWS can also take into account the interior problem with ingree/egress flooding flow conditions. For the exterior flow, the 6DoF ship motion equation system is solved with the flooding water interaction accounted for. The communication between the exterior and interior is enforced by means of corrections of the exterior and interior boundary conditions, except when the Hull Reshaped Method (HRM) is used because it models the exterior and the interior domains simultaneously so that the communication is automatically handled.

The flooding mechanism has been physically analyzed, categorized and mathematically modeled accordingly. Three different approaches, i.e., shallow water equations, multimodal method and hull-reshaped method, are formulated to model the different flooding scenarios. The former two are of a nonlinear nature and the HRM is a linear approach. Additionally, the adaptive nonlinear roll (viscous) damping is determined from the free decay tests. Some necessary veri_cations are performed. Validations against experiments show that the present formulation is a promising way to simulate the complex damaged ship and flooding flow system. The important parameter, TTC, for a damaged ship can be predicted.

Finally, suggestions are proposed on how to minimize the commercial risk and maximize the safety at an initial design stage, as well as to provide a rational analysis or investigation of a capsizing accident