3.2.1 Curves of righting moments and of wind heeling
moments similar to figure 3-1 with
supporting calculations should be prepared covering the full range
of operating draughts, including those in transit conditions, taking
into account the maximum deck cargo and equipment in the most unfavourable
position applicable. The righting moment curves and wind heeling moment
curves should be related to the most critical axes. Account should
be taken of the free surface of liquids in tanks.
3.2.2 Where equipment is of such a nature that
it can be lowered and stowed, additional wind heeling moment curves
may be required and such data should clearly indicate the position
of such equipment.
3.2.3 The curves of wind heeling moments should
be drawn for wind forces calculated by the following formula:
where:
|
F |
= |
the wind force
(newtons) |
|
Cs
|
= |
the
shape coefficient depending on the shape of the structural member
exposed to the wind (see table 3-1)
|
|
CH
|
= |
the
height coefficient depending on the height above sea level of the
structural member exposed to wind (see table
3-2)
|
|
P |
= |
the air mass density
(1.222 kg/m3 )
|
|
V |
= |
the wind velocity
(metres per second) |
|
A |
= |
the projected
area of all exposed surfaces in either the upright or the heeled condition
(square metres) |
3.2.4 Wind forces should be considered from any
direction relative to the unit and the value of the wind velocity
should be as follows:
-
.1 In general a minimum wind velocity of 36 m/s
(70 knots) for offshore service should be used for normal operating
conditions and a minimum wind velocity of 51.5 m/s (100 knots) should
be used for the severe storm conditions .
-
.2 Where a unit is to be limited in operation
to sheltered locations (protected inland waters such as lakes, bays,
swamps, rivers, etc.) consideration should be given to a reduced wind
velocity of not less than 25.8 m/s (50 knots) for normal operating
conditions.
3.2.5 In calculating the projected areas to the
vertical plane, the area of surfaces exposed to wind due to heel or
trim, such as under-deck surfaces, etc., should be included using
the appropriate shape factor. Open truss work may be approximated
by taking 30% of the projected block area of both the front and back
section, i.e. 60% of the projected area of one side.
3.2.6 In calculating the wind heeling moments,
the lever of the wind overturning force should be taken vertically
from the centre of pressure of all surfaces exposed to the wind to
the centre of lateral resistance of the underwater body of the unit.
The unit is to be assumed floating free of mooring restraint.
3.2.7 The wind heeling moment curve should be
calculated for a sufficient number of heel angles to define the curve.
For ship-shaped hulls the curve may be assumed to vary as the cosine
function of vessel heel.
3.2.8 Wind heeling moments derived from wind tunnel
tests on a representative model of the unit may be considered as alternatives
to the method given in 3.2.3 to 3.2.7. Such heeling moment determination
should include lift and drag effects at various applicable heel angles.
Table 3-1 Values of the
Coefficient Cs
|
Shape
|
Cs
|
| Spherical
|
0.4
|
| Cylindrical
|
0.5
|
| Large flat surface (hull, deckhouse,
smooth under-deck areas)
|
1.0
|
| Drilling derrick
|
1.25
|
| Wires
|
1.2
|
| Exposed beams and girders under
deck
|
1.3
|
| Small parts
|
1.4
|
| Isolated shapes (crane, beam,
etc.)
|
1.5
|
| Clustered deckhouses or similar
structures
|
1.1
|
Table 3-2 Values of the
Coefficient CH
|
Height above sea level (metres)
|
CH
|
| 0- 15.3
|
1.00
|
| 15.3 - 30.5
|
1.10
|
| 30.5 - 46.0
|
1.20
|
| 46.0 - 61.0
|
1.30
|
| 61.0 - 76.0
|
1.37
|
| 76.0 - 91.5
|
1.43
|
| 91.5 - 106.5
|
1.48
|
| 106.5 - 122.0
|
1.52
|
| 122.0 - 137.0
|
1.56
|
| 137.0 - 152.5
|
1.60
|
| 152.5 - 167.5
|
1.63
|
| 167.5 - 183.0
|
1.67
|
| 183.0 - 198.0
|
1.70
|
| 198.0 - 213.5
|
1.72
|
| 213.5 - 228.5
|
1.75
|
| 228.5 - 244.0
|
1.77
|
| 244.0 - 256.0
|
1.79
|
| above 256
|
1.80
|
Figure 3-1 Righting moment and healing moment curves