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 unitsfootnote 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 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 deck
load 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.