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The International Maritime Organization (IMO) has been undertaking a significant effort to develop its so-called Second Generation Ship Stability Criteria. These criteria have the potential to dramatically improve both the accuracy and realism of existing quasistatic approaches, which mainly consider the calm water hydrostatic restoring moment arm curve compared with a steady heeling moment. This new approach, under consideration by the IMO, consists of three levels of accuracy beginning with the first level, which is essentially the current prescriptive criteria of the 2008 Intact Stability Code (IS Code), and ending with the third level, a direct stability assessment using physical model testing and numerical simulation. The sec- ond level is an improvement on the first level, but would not be as expensive or time consuming to apply as the third level. The essential aspect of the second-generation criteria is to identify five vulnerabilities or failure modes: 1. Pure loss of stability, 2. Parametric roll, 3. Surf-riding/broaching, 4. Dead ship condition, and 5. Excessive accelerations. Discussions at the recent sixth session of the IMOâs Committee on Ship Design and Construction (SDC) (IMO SDC 6, February 4â8, 2019) resulted in approval of many of the recommendations of an expert group that were based on a request by the IMOâs Maritime Safety Committee (IMO MSC). Appendix F Current State of Second Generation Stability Criteria 157
158 USCG VESSEL STABILITY REGULATIONS AND GUIDANCE The goal of the next meeting of the seventh SDC will be to draft an interim guideline in the form of an MSC Circular, which will consist of three items: 1. Guidelines of ship vulnerability failure. 2. Specification for direct stability assessment. 3. Operational guidance. These three items will be a summary of the contents of the report of the expert group SDC-6 WP6 and will include three annexes. The mandatory Part A of the existing 2008 IS Code will continue to be mandatory. The new MSC Circular to be submitted to the seventh SDC that describes some of the aspects of the Second Generation Ship Stability Criteria will remain in a trial stage. However, not all five vulnerabilities may be addressed, with only the surf-riding/broaching and parametric rolling to be definitely addressed. The eighth session of the SDC may have a goal of producing explanatory notes, which will address extreme conditions and an assessment of how rare these various vulnerabilities are. Although the three levels of assessment are to be performed sequentially with each yielding a more accurate and less conservative judgment, there has been some concern that this may not be the case. Specifically, in some rare cases, it has been found that a vessel may satisfy level one but fail level two. Moreover, the interim guidelines for direct assessment are being exposed to a three-level Verification, Validation, and Accreditation. To comment on the goals and effectiveness of the IMO Second Gen- eration Ship Stability Criteria may be premature. The U.S. Coast Guard (USCG) and the U.S. Navy are represented as part of the international group of experts and therefore the USCG is closely following the develop- ment of these criteria. In theory, the availability of such a âfirst principlesâ ship stability assessment is long overdue. However, for more than two decades, the specific details have been debated and will continue to be debated. These criteria may be particularly relevant to vessels with unique hull forms and hydrodynamic characteristics. Examples include advanced marine vehicles, non-mono-hull displacement hulls, floating offshore plat- forms with unique geometry, and also possibly smaller vessels such as fishing vessels that may be exposed to extreme environmental conditions. Unfortunately, such a physics-based first principles approach may require a level of training beyond what is the norm for vessel designers, particularly at the smaller firms. Therefore, an effort must be made to simultaneously refine the accuracy and realism of these criteria while making them under- standable and usable to a wider audience without an undue investment in time and cost. It merits noting that implementation of a dynamic response-based cri- teria is not new. The American Bureau of Shipping (ABS) undertook such
APPENDIX F 159 a project in the 1980s to develop response-based criteria for column stabi- lized mobile offshore drilling units. Although not widely accepted by indus- try, the criteria have been applied by some organizations. Moreover, in the 1990s the U.S. Navy, in order to consider the dramatically different motions characteristics of its new destroyer hull form, implemented response-based criteria. Unfortunately, the U.S. Navyâs methodology was both time con- suming and expensive to implement and has only been applied to that single hull. Therefore, a more general approach, especially for the second level analysis, must be relatively simple to implement but also must consider three important characteristics of the response: accurate hydro dynamic modeling, the inherently nonlinear character of the dynamics, and a realistic wave and wind environment and its resulting stochastic character. Such a comprehensive approach would be required as part of further developments of the IMO Second Generation Criteria. Specifically, the second level analysis approach would need to be more accurate than the first level (prescriptive), but more time and cost efficient than the third level (physical model testing and numerical simulation). Moreover, this approach would need to be easily applied and understandable by the practicing naval architect.
