Alternative Design Approach for Ship Damage Stability Enhancement Based on Crashworthiness

Abstract

Damage stability rules and regulations have been continuously tightened up to enhance ship survivability from the deterministic method to the probabilistic approach. Nowadays, environmental conditions such as wind and waves have been considered for dynamic assessment. Nevertheless, the current prescriptive regime still has problems. Fundamentally, it is assumed that ship flooding is taking place up to B/2 penetration. Therefore, transverse bulkheads must be considered to control the amount of flooding to avoid ship loss. Another problem is that the current regulations cannot fully evaluate the contributions for further improvements using innovative risk control options (RCOs) such as crashworthy structures. Therefore, this research suggests a new methodology to address these problems using damage extent estimation derived from crashworthiness analysis of a target vessel. The methodology begins by selecting target subdivision zones where high risk exists deriving from vulnerability assessment. Then, several feasible RCOs are applied to the vulnerable target zones. A collision simulation of the region in question using crashworthiness analysis is followed to determine the damage size in the worst scenario collision case. The collision speed of a striking ship is derived from a series of pre-simulations, and the speed generating B/2 penetration is employed based on the current SOLAS criteria. After that, a cost-benefit analysis is performed for an optimum solution, considering the penetration results and each RCOs’ cost, along with design change effects of each RCO to the target ship. The final RCO can be approved and reflected on the ship design as an alternative solution from the Approval process of Alternative Design and Arrangements (AD&A). In the case of a 65K GT cruise ship collision example against 45K GT Ropax, the most vulnerable subdivision zone is investigated with a series of passive RCOs such as a longitudinal bulkhead implementation, hull thickness increase, and combinations of  RCOs. Interestingly, using an RCO with 50T hull thickness no penetration is observed on the hull of target ship, improving the attained index by 0.043, while it requires an additional 280 tonnes of steel.  Another feasible RCO with 30T hull thickness and additional longitudinal bulkhead at 6.6m from the hull shows 6.58m penetration. In the latter case, the index A is improved by 0.035, with reduction in steel weight of 145 Ton. In conclusion, this research introduces a new methodology on how to evaluate the contribution of crashworthy structures on damage stability assessment, leading to alternative and innovative design solutions from the current regulatory regime. Especially for the RCOs with a longitudinal bulkhead right after the maximum penetration or over 50T hull thickness, the internal spaces are fully protected from collision damages. Therefore, if they were applied to more than two adjacent zones, a transverse bulkhead between zones might not be necessary anymore, offering a more flexible ship internal space. In this context, this research may provide a new gateway for innovative design solutions to ship owners, shipyards and engineering companies.