Section 3 Intact Stability Criteria

301. General

1. All units are to have sufficient stability (righting ability) to withstand

the overturning effect of the

force produced by a sustained wind from any horizontal direction, in accordance with the stability criteria given in 302. for all afloat modes of operation.

2. Realistic operating conditions are to be evaluated, and the unit is to be capable of maintaining in operating mode with a sustained wind velocity designated by the Owner, but not less than 36 m sec (70 knots).

3. The capability is to be provided to change the mode of operation of the unit to that corresponding to a severe storm condition, with a sustained with velocity of not less than 51.5 m sec (100 knots), in reasonable period of time for the particular unit. In all cases, the limiting wind velocities are to be specified and instructions should be included in the Operating Booklet for changing the mode of operation by redistribution of the variable load and equipment, by changing draughts, or

4. both.

For restricted operations consideration may be given to a reduced sustained wind velocity specified

5. in Par 2 and 3, of not less than 25.8 m sec (50 knots).

For the purpose of calculation, it is to be assumed that the unit is floating free of mooring

6. restraints. However, the possible detrimental effects of mooring restraints are to be considered.

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 re- moval or relocation of solid consumables or other variable load. However, the Society 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 ex- ceeded:

(1) in a geographic location where weather conditions annually or seasonally do not become suffi- ciently severe to require a unit to go to severe storm condition; or

(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

7. should be identified in the operating manual.

Alternative stability criteria may be considered by the Society, provided an equivalent level of safe-

ty is maintained and if they are demonstrated to afford adequate positive initial stability. In de- termining the acceptability of such criteria, the Society should consider at least the following and take into account as appropriate:

(1) environmental conditions representing realistic winds (including gusts) and waves appropriate for world-wide service in various modes of operation;

(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;

(3) potential for flooding taking into account dynamic responses in a seaway;

(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;

302. Stability Criterion under Wind Force

1. Intact condition

(1) Righting moment curves and wind heeling moment curves related to the most critical axis, with supporting calculations, are to be prepared for a sufficient number of conditions covering the full range of drafts corresponding to afloat modes of operation.

(2) Where drilling equipment is of the nature that it can be lowered and stowed, additional wind heeling moment and stability curves may be required, and such data is to clearly indicate the

position of such equipment.

(3) The stability of a unit in each mode of operation should meet the following criteria (see also

Fig 4.1):

(A) 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.

(B) For column-stabilized units the area under the righting moment curve to the angle of down-

flooding should be not less than 30% in excess of the area under the wind heeling moment curve to the same limiting angle. (Refer to An example of alternative intact stability criteria for twin-pontoon column-stabilized semisubmersible units, adopted by the Organization by resolution A.650(16)).

(C) The righting moment curve should be positive over the entire range of angles from upright to the second intercept.

2. Wind heeling moment

(1) The wind heeling moment is to be calculated at several angles of inclination for each mode of operation, based on the wind force obtained from 201.

(2) The calculations are to be performed in a manner to reflect the range of stability about the

most critical axis.

(3) The lever for the heeling force is to be taken vertically from the centre of lateral resistance or, if available, the centre of hydrodynamic pressure, of the underwater body to the centre of pres- sure of the areas subject to wind loading.

(4) In calculating wind heeling moments for ship-shaped hulls, the curve may be assumed to vary as the cosine function of the vessel's heel.

3. Wind tunnel test

(1) Wind heeling moments derived from authoritative wind tunnel tests on a representative model of the unit may be considered as alternatives to the method given in Par 2. Such heeling moment determination is to include lift effects at various applicable heel angles, as well as drag effects.

4. Other stability criteria

(1) The stability may be reviewed and evaluated in accordance with the alternative criteria deemed appropriate by the Society, taking into account the operation modes and environment conditions.