Trans RINA, Vol 152, Part A2, Intl J Maritime Eng, Apr-Jun 2010


COLLATING EVIDENCE FOR A UNIVERSAL METHOD OF STABILITY ASSESSMENT OR GUIDANCE


B Deakin, Wolfson Unit MTIA, University of Southampton, UK (DOI No: 10.3940/rina.ijme.2010.a2.175)


SUMMARY


This paper reviews two related projects conducted for the Maritime and Coastguard Agency, and collates their findings with additional casualty data, in an attempt to promote a simple method of safety assessment. The method was developed by the author during research conducted for the Maritime and Coastguard Agency. It was evaluated in a subsequent research project, where the recommendations were that it was not worthy of adoption or further development for regulatory purposes.


Contrary to that evaluation, the method has received supportive comments from a number of naval architects, and this paper is offered as a means of presenting it more widely to the industry. Individuals may wish to use the method to assess their own designs, or to provide some simple safety guidance to operators.


1 INTRODUCTION


Much of the recent and current effort in stability research is striving to refine the calculation of ship motions and dynamics, to predict capsize of vessels


in specific


circumstances. This involves increasingly complex analysis using software developed by some brilliant minds. Whilst the value of such research in advancing our understanding of ship behaviour is undoubted, it is debatable whether the prediction of capsize is likely to become a precise science.


Despite the extensive investment in stability research during the last two decades, much assessment of stability still relies on criteria derived from Rahola’s work, published in 1939. Things are changing now, as the IMO is committed to developing a revised Intact Stability Code, and considerable effort is being directed towards it. Whether the revised Code will provide a more reliable assessment of the level of safety remains to be seen, but it is unlikely to be as simple as the current criteria.


The method described here offers a very simple means of using the statical stability to estimate the safety of an intact or damaged vessel, while recognising that safety also depends on the size of the vessel in relation to the operational seastate.


2 DERIVATION OF THE METHOD


During revision of the IMO Code of Safety for High speed Craft, 2000, a number of research projects were commissioned by the MCA to inform the discussions at IMO. One of those, Research Project 509, was to assess the level of safety provided by the criteria for multihulls, and compare it with that provided by the criteria for monohulls, [1]. That work comprised extensive model tests in a towing tank to determine the minimum wave height


that could capsize model ships. Six models,


including monohulls and multihulls, were tested in a total of fifty three intact and damaged configurations. For each


configuration the minimum wave height to capsize was determined by testing at a range of regular wave frequencies, bringing the number of test cases to over 800. Each of these was tested at all headings to the waves, so the overall number of test scenarios was unusually high. Furthermore, many of the configurations had an initial angle of list, due to offset loading or asymmetric flooding. These were tested heeling towards and away from the approaching waves. All tests were on models floating unrestrained in the


“dead ship”


condition, so the tests did not simulate scenarios that might occur when under power, such as broaching or parametric rolling. It is often assumed that beam seas represent


proved that


the most vulnerable heading, but the tests the most vulnerable heading


unpredictable, and 23% of the capsizes in the minimum wave heights occurred at other headings.


was Figure 1


illustrates a typical example of capsize data for one of the model cases tested, and the derivation of the minimum wave height to capsize; 0.8 metres in this case.


0.0 0.5 1.0 1.5 2.0 2.5 3.0


Capsize No Capsize Capsize Boundary


3


4


5 6789 10 Period - seconds


11


Figure 1 Derivation of minimum wave height to capsize, for one model configuration in Research Project 509.


In order to assess the minimum criteria, it was possible to assume that a model represented a vessel at a particular scale, and ballast it such that it just complied with one or


©2010: The Royal Institution of Naval Architects A - 85


Wave Height - metres


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