In-depth | SHIP STABILITY
(International Association of Classification Societies) Unified Interpretations. IACS’ agreed interpretations, arising from queries from industry relating to IMO Regulations and Explanatory Notes, are submitted to IMO for consideration. From the point of view of practical application of these probabilistic rules to design, Mr Jóannes Gullaksen provided a detailed outline of the probabilistic damage stability calculation steps as applied to cargo vessels. He also provided a Microsoſt Excel spreadsheet to evaluate the probabilistic damage factors (though a suitable hydrostatic analysis programme would be required to evaluate the survivability indices). Following the comments of the papers by
Prof Vassalos and Mr Hutchinson, it appears likely that there will be at least some revision of the SOLAS 2009 regulations in the not too distant future. Te two authors highlighted several key
areas of difficulty: 1. Prof. Vassalos showed that the SOLAS 2009 A-index can be interpreted as “a value that reflects the average survivability of a vessel following collision damage” [Vassalos and Jasionowski 2007] and this value can be used to provide an analytical inference model for the cumulative probability that the vessel will capsize in a given time. When this analytical model is compared with a performance-based assessment, using for example time-domain simulations of damage scenarios, good correlation was obtained for RoPax vessels whilst cruise ships were found to be an order of magnitude safer than the SOLAS 2009 A-index would imply [Vassalos et al. 2008]. Tis is because the SOLAS 2009 A-index is unrelated to cruise ships; there were effectively no cruise ships in the casualty sample used to develop the SOLAS 2009 rules. Tus designing a cruise ship to the R-index actually results in a much safer ship than might be expected.
2. Te probabilistic method in SOLAS 2009 is designed to give an “equivalent level of safety” to the deterministic method in SOLAS 90. Mr Hutchinson, in particular, noted that there is no unified interpretation as to whether SOLAS 2009 is equivalent to SOLAS 90 plus WoD (water on deck), as required by the Stockholm Agreement and EU directives. Tis is an area of ongoing investigation by IMO.
3. Several speakers raised the issue of LLH 28
(long lower hold) RoPax vessels which can be designed to pass SOLAS 2009 but fail SOLAS 90 criteria. Also “the loss of the margin line non-immersion criterion is of much more significance for RoPax ships than conventional passenger ships so it is difficult to understand how, in future, these two, very different, designs of passenger ships can both be covered by the same probabilistic damage stability formulae.” [Hutchinson]
From a practical point of view, the
probabilistic rules present some difficult challenges. These were elucidated by Mr Hutchinson: 1. From a designer’s point of view there is essentially no guidance on how a vessel’s compartmentalisation should be laid out. With the older prescriptive rules, the damage conditions examined were directly specified, essentially determining the general compartment layout. However, in the probabilistic approach there is no such guidance.
2. The probabilistic rules also create a considerable challenge to provide useful information to the Master in a clear, concise, consistent and above all usable form. Tis is because, from a design perspective, the rules provide a probability of the vessel surviving a given damage; however, in the operational context, it is not immediately apparent to the Master if the vessel should be safe given a damage event. Te older rules gave a black and white pass/fail status; the new probabilistic rules give a “shade of grey” in the form of a probability of survivability.
Addressing this second point, Mike
Simpson’s paper showed a non-computer- based method of providing decision support to the Master in the event of damage to the ship. An example “Damage Decision Support” book was provided for a high-speed catamaran ferry. Te contents/index of this book is a colour-coded “damage condition” vs. “incident severity” matrix. Te damage matrix consists of single- and multiple- adjacent compartment damage; for each damage condition the incident severity is indicated by its colour. Full details of the vessel under a particular damage scenario may then be found on the relevant page. Te key outcome of this system is that the incident severity is immediately apparent to
the ship’s Master. Te proposed “Damage Decision Support” book provides a means of presenting the vast array of damage stability data generated during a probabilistic damage analysis in a format that is more accessible to the Master should an incident occur. A point that was raised from the floor on a
• Onboard computer software systems become statutory instruments requiring
• Information provided to the Master needs to be clear, concise, consistent and usable
official approval
• Due to the volume and type of data generated, especially for probabilistic
• Condition monitoring
damage stability analysis, fulfilling the above requirements is a non-trivial task
• How can the progress of floodwater be tracked
• How can initial structural damage be assessed and its propagation tracked
resulting in deformation of hull plating) which may then lead to plate buckling.
This review has focused on issues that
were raised relating to SOLAS 2009. However during the conference, some interesting fundamental research and some enthralling accounts of real-life incidents were also presented. NA
Additional references: Vassalos, D. and Jasionowski, A., 2007, “SOLAS 2009 – Raising the Alarm,” Proc. 9th ISSW, Hamburg, Germany Vassalos D., Jasionowski A. and Guarin L., 2008, “Risk-Based Design: A Bridge too far?”, OC 2008 Seakeeping and Stability, Osaka, Japan.
Please contact the RINA staff to obtain the conference proceedings by email at
publications@rina.org.uk or by phone on +44 (0)207 235 4622.
The Naval Architect April 2011
• Residual strength can depend significantly on previous incidents (e.g. grounding
number of occasions was that in many cases research and regulation developments seem to have a primary focus on design aspects with operational issues being less well covered. Te primary instrument/document for the Master is still the SIB (Stability Information Booklet). Tools for decision support based on assessment of stability and structural viability under damage were the focus of several papers. Some of the key points raised were as follows:
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