Trans RINA, Vol 152, Part A2, Intl J Maritime Eng, Apr-Jun 2010
validation, nor indicate that this would be achieved by further development.”
5 COLLATION OF DATA
To put all of these ship and model data into perspective, a combined graph is presented in Figure 6. Twelve fishing
vessel casualties, which have been well
documented, have been collected by the Wolfson Unit and added to the data already discussed. It should be understood that, although they are plotted as points for the sake of clarity, the wave heights for
capsizes cannot be known precisely and a vertical error bar might be more appropriate. Only capsize data are presented, because the survival of a vessel is not a reliable measure of the formula unless, by some means, one could be certain that the vessel had encountered waves at the most vulnerable heading and wave period.
It is apparent from this graph that the HARDER model test data fall into a similar envelope to the capsize data derived by the Wolfson Unit in Research Project 509. The HARDER data represent vessels with extremely low residual stability. At the other extreme, some of the other model capsizes and ship casualties have very high stability and lie well outside the range of stability values tested in Research Project 509. Correlation with the Wolfson model
test data requires considerable
extrapolation by the proposed formula, from x-axis values of less than 1.2 in the model test database to values of 3 or more for some ships.
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50
Fishing Vessel Casualties RP 583 Other Ship Casualties RP 583 Other Model Capsizes
real vessel
With the exception of the model which capsized as a result of water trapped on deck, and therefore had less stability than that presented, all of these model and ship casualties are close to the line or to the unsafe side of it.
6 POTENTIAL APPLICATIONS Unsafe zone
Water trapped on deck
Figure 6 can be considered as truly non-dimensional, and the fact that model and full scale data can be presented together is evidence of this. It also appears to be applicable to all types of vessel, whether intact or damaged, upright or heeled. Whilst it is not claimed to give an accurate or even reliable prediction of capsize, it does offer an extremely simple means of estimating the minimum level of safety of a vessel, assuming that the external or internal heeling moments can be anticipated. It might be useful in a regulatory environment, but it may be more valuable if used as the basis of safety guidance, to inform masters whether a proposed operation has a reasonable level of safety. It could be used, for example, to assess a heavy lift over the side, or the carriage of an unusual cargo, and set an appropriate maximum seastate for the operation. This is not something that conventional criteria address very well, because they are limited to a pass or fail judgement, regardless of the vessel size or seastate. An operation that should not be contemplated in bad
weather might be safe to undertake in calm
conditions, and the definition of bad weather is very different for a 300 metre cargo vessel compared to a 12 metre fishing vessel. On most vessels, the master will have no such guidance on his level of safety on a day to day basis.
Safe zone
Envelope of Wolfson model test data Envelope of HARDER model test data
0.0 0.5 1.0 1.5 2.0
Range(RMmax) LB
2.5 0.5
3.0 3.5 4.0
Figure 6 Correlation of casualty and model data with the proposed formula
The casualty with the greatest stability by this measure was Meridian, a 22.6 metre UK registered fishing vessel. It had very good stability characteristics, well in excess of the minimum requirements and therefore considered safe by all current methods of assessment. It was on
Casualty statistics indicate that the vessels most at risk from capsize are fishing vessels. There is no requirement for UK registered fishing vessels to assess the stability when lifting their catch, or indeed their gear, over the side. Many capsizes have occurred as a result of very heavy lifts, or in attempting to free gear fastened on a seabed obstruction, because fishermen have
no
information on when a particular operation might become hazardous in the prevailing conditions. With safety
guidance based on this method, related to
information from a load cell to monitor the lift, or an inclinometer to monitor the heel angle, the crew could be made aware of the level of hazard and take appropriate precautions, or abandon the lift. This philosophy was followed in MCA Research Project 560, [9], to develop a simplified presentation of stability information for
A - 90 ©2010: The Royal Institution of Naval Architect
guard duty in storm force 10 conditions and apparently capsized undamaged. Another vessel nearby reported the wave conditions and the Marine Accident Investigation Branch considered it most likely that Meridian, which had a relatively high GM, suffered as a result
of
synchronous rolling in beam seas that had a mean period close to the vessel’s natural roll period. [8]. This hypothesis correlates well with the model test findings, and represents an extrapolation to much higher stability values although, of course, the capsize wave height is an estimate.
Probable wave height in accident seastate/L
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