Impact of drafts on the damage survivability of cruise ships

The prevailing probabilistic damage stability concept, as outlined within SOLAS 2009 for passenger ships, calculates the Attained Subdivision Index based upon three loading conditions which combine to form a theoretical draft range for a given vessel. To each of these loading conditions a weighting factor is then applied to account for the probability that a vessel will be operating at or near any of these drafts at the time of collision, should one occur. Currently the weighting factors are applied in a 'one-size-fits-all' manner, with the same weightings to be applied in the case of cargo and passenger vessels despite the fact that these ship types are known to have very different tendencies when it comes to the nature of their operation. This in turn, calls into question the suitability of these weightings concerning what degree they, in fact, reflect the operational profile of the vessels covered by the standard. With this in mind, the present paper aims to investigate the suitability and accuracy of the currently assumed draft weighting factors with regards to cruise vessels. This study is conducted using operational loading condition data sourced from 18 cruise ships and spanning up to a period of two years in some cases. On the basis of this data, draft probability distributions are derived and new weighting factors are formed specifically pertaining to cruise ships and the nature of their operation. Finally, an assessment is conducted looking into the impact of the newly derived weighting factors on the magnitude of the Attained Subdivision Index and recommendations are made on how best to implement them.