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In-depth | DAmAge STAbILITy


damage the accumulation of flood water on the bulkhead deck has an adverse effect on stability which has resulted in previous catastrophic losses. This problem was addressed by the Stockholm Agreement. For those ships designed with LLH to


increase cargo capacity, the ingress of floodwater not only results in a significant free surface effect but can also create an adverse trim. Te ability of a ship to survive is greatly influenced by reserve buoyancy and its distribution along the ship’s length. In the previous deterministic


regulations LLH’s were allowed, particularly when the transverse position of the longitudinal bulkheads was inboard of the B/5 boundary. In SOLAS 2009, this boundary has been moved outboard to B/10 and the ‘s’ factor (which represents survivability) reduced to a minimum value of 0.9. Whilst this change to the transverse location of the longitudinal bulkhead is commercially attractive, there is a risk that future ro-pax ships built with a LLH configuration before sufficient experience has been gained. Internal research by BCTQ in 2006


identified this problem. Te concern we raised at this time was, that although it is possible to demonstrate compliance with both local and global safety standards, certain apparently insignificant damage scenarios can still result in ship loss. Te underlying question is whether or not the level of risk arising from this situation is acceptable.


Which ships are safer the small or the big ones? Ideally all ships should have the same level of safety. However, the standard of survivability required by the probabilistic regulations is a function of the number of passengers exposed to risk and ship’s length. Te research of Euroyards seems to confirm that for smaller ro-pax ships, sailing with a lesser number of passengers, the overall standard of survivability ‘may be less demanding than the previous requirements’. This is a result of the decisions taken by MSC 78 with respect to Required Index. According to the mathematical formulae


used in SOLAS 2009, larger ships require a greater degree of subdivision and, therefore,


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from a stability perspective, it could be argued they are relatively safer.


Solving the ‘ro-pax problem’ Tere have been various discussions at IMO as to whether there is a need to improve safety standards for ro-pax ships and, if so, how to address this potential deficiency in the regulation. Fol lowing SLF51 (the IMO


sub-committee discussing damage stability in ro-ro vessels) an investigation into the impact of damage stability using SOLAS 2009 on ro-pax ships in comparison with SOLAS90 Stockholm Agreement was undertaken. Three major projects were reviewed (Euroyard project, a joint NL/ UK project and the EMSA project) but the results were inconclusive. Two of the studies indicated that the vessels examined, though commercially attractive, could be subjected to high risk of capsize or sinking in virtually calm water if the LLH is penetrated. Tere has been agreement that more research is needed to investigate this problem. Future research on this subject includes


continuation of the EMSA studies (evaluation of a further five SOLAS 2009 compliant ships). There are also two EU-funded research projects: GOALDS (focusing on probability of survival for large passenger vessels) and FLOODSTAND (relationship evacuation time and time of survival) but the results will not be available until 2011-2012. It is a commonly held view that some


amendments to SOLAS 2009 may be necessary and, to date, there have been various proposals to address this issue. Some projects suggested modifications to the survivability formula (‘s’ factor), others proposed an increase to the Required Index. Tere was also a proposal to leave the SOLAS 2009 as it is but develop additional, separate Water-On-Deck criterion for ro-pax ships. More research is needed to draw any final conclusions.


Deterministic or


probabalistic regulations? In SOLAS 90 only damages up to the B/5 line needed to be assessed. In SOLAS 2009 all potential damage cases are to be considered. In one damage zone there may be a number of scenarios depending on the extent of damage inboard and all these


cases have a weighted contribution. Tis leads to increased benefits for longitudinal subdivision. SOLAS 90 had a simplistic approach


(s=1 pass, s=0 fail) and offered a relatively straightforward assessment. In contrast, the SOLAS 2009 regulations are more complex because all damage cases can contribute to the final result (index A). Various factors must now be considered, resulting in the probabilistic regulations being less transparent to designers. With the previous regulations, no


pipe-work, valves or openings that could lead to progressive flooding were allowed within an area 20% of the ship’s breadth adjacent to the hull (B/5). Tis meant that no matter what standard of stability the ship met, there was always some protection against local, minor damage. With SOLAS 2009, the analysis of progressive flooding is handled differently through a more thorough and comprehensive process. In addition, the requirements regarding margin line immersion no longer exist, but there is zero contribution to the attained index when evacuation routes are immersed. In theory, the designer now has more


flexibility as an increase of risk in one area, to some extent, may be offset by an increase in safety elsewhere. However, the calculations for probabilistic damage stability are complex and require an iterative process to optimise a design. Specialist knowledge and a comprehensive understanding of the standards are required to undertake such an analysis. Extensive calculations are necessary and the adjustment of any one element will require all damage cases to be re-visited. Tis can only be undertaken by computer using appropriate soſtware. Unsatisfactory results will require the redefinition of one or all of the following: principal subdivision, openings, initial loading conditions or even the hull geometry. To facilitate the approval process, close cooperation with Regulatory Authorities from early stages of the design, is recommended. As a final observation, many factors will


influence the design of future ro-pax ships. However, the conclusions from various research projects underline the importance of starting with a good arrangement of subdivision. NA


The Naval Architect September 2010


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