.3.3.1 The stability of a unit in each mode of
operation should meet the following criteria (see also figure 3-1):
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.1 For surface and self-elevating units the area
under the righting moment curve to the second intercept or downflooding
angle, whichever is less, should be not less than 40% in excess of
the area under the wind heeling moment curve to the same limiting
angle.
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.2 For column-stabilized units the area under
the righting moment curve to the angle of downflooding should be not
less than 30% in excess of the area under the wind heeling moment
curve to the same limiting angle.
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.3 The righting moment curve should be positive
over the entire range of angles from upright to the second intercept.
3.3.2 Each unit should be capable of
attaining a severe storm condition in a period of time consistent
with the meteorological conditions. The procedures recommended and
the approximate length of time required, considering both operating
conditions and transit conditions, should be contained in the operating
manual. It should be possible to achieve the severe storm condition
without the removal or relocation of solid consumables or other variable
load. However, the Administration may permit loading a unit past the
point at which solid consumables would have to be removed or relocated
to go to severe storm condition under the following conditions, provided
the allowable KG requirement is not exceeded:
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.1 in a geographic location where weather conditions
annually or seasonally do not become sufficiently severe to require
a unit to go to severe storm condition, or
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.2 where a unit is required to support extra deckload
for a short period of time that falls well within a period for which
the weather forecast is favourable.
The geographic locations, weather conditions and loading conditions
in which this is permitted should be identified in the operating manual.
3.3.3 Alternative stability criteria may be considered
by the Administration provided an equivalent level of safety is maintained
and if they are demonstrated to afford adequate positive initial stability.
In determining the acceptability of such criteria, the Administration
should consider at least the following and take into account as appropriate:
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.1 environmental conditions representing realistic
winds (including gusts) and waves appropriate for world-wide service
in various modes of operation;
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.2 dynamic response of a unit. Analysis should
include the results of wind tunnel tests, wave tank model tests, and
non-linear simulation, where appropriate. Any wind and wave spectra
used should cover sufficient frequency ranges to ensure that critical
motion responses are obtained;
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.3 potential for flooding taking into account
dynamic responses in a seaway;
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.4 susceptibility to capsizing considering the
unit's restoration energy and the static inclination due to the mean
wind speed and the maximum dynamic response;
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.5 an adequate safety margin to account for uncertainties.