Section
1 General
1.1 Introduction
1.1.2 Environmental
conditions include natural phenomena such as wind, wave and currents
and also ice and thermal conditions.
1.1.3 The
environmental conditions given here are derived from hindcast data,
long term measurements and theoretical studies complemented with service
experience.
1.1.4 Alternative
methods of establishing the environmental conditions will be specially
considered, provided that they are based on hindcast data, long term
measurements, global and local environmental theoretical models, or
similar techniques. In such cases, full details of the methods used
are to be provided when plans are submitted for approval.
1.1.5 In order
that an assessment of the design requirements can be made, the following
information is to be submitted:
-
Service area notation
required together with the required extent of the operational area.
-
The wave environmental
parameters for the design.
-
Specification
of the environmental conditions used for the design assessment.
1.2 Definitions and symbols
1.2.1
Beaufort
Number. Beaufort Number is a measure of wind strength. The
wind speed corresponding to each Beaufort number is shown in Table 2.1.1 Wind data.
1.2.2
Sea
state. Sea state is an expression used to categorise wave conditions
and is normally defined by a significant wave height and wave period,
a suitable wave energy distribution may also be defined. A list of
standard sea state definitions is shown in Table 2.1.2 Sea state data.
1.2.3
Wave
period. References to wave period are to be taken as the zero
crossing wave period, i.e. the average time interval between upward
crossings of the mean value, unless otherwise stated.
Table 2.1.1 Wind data
| Beaufort number
|
Wind speed range knots
|
| 0
|
0-1
|
| 1
|
1-3
|
| 2
|
4-6
|
| 3
|
7-10
|
| 4
|
11-16
|
| 5
|
17-21
|
| 6
|
22-27
|
| 7
|
28-33
|
| 8
|
34-40
|
| 9
|
41-47
|
| 10
|
48-55
|
| 11
|
56-63
|
Table 2.1.2 Sea state data
| Sea State
Number
|
Significant Wave height(m)
|
Sustained Wind Speed (Knots) (See Note 1)
|
Percentage probability of sea state
|
Modal Wave Period (sec)
|
| Range
|
Mean
|
Range
|
Mean
|
Range (see Note
3)
|
Most Probable
|
| 0–1
|
0–0,1
|
0,05
|
0–6
|
3
|
0,7
|
–
|
–
|
| 2
|
0,1–0,5
|
0,3
|
7–10
|
8,5
|
6,8
|
3,3
–12,8
|
7,0
|
| 3
|
0,5 –1,25
|
0,88
|
11–16
|
13,5
|
23,7
|
5,0
–14,8
|
7,5
|
| 4
|
1,25
–2,5
|
1,88
|
17–21
|
19
|
27,8
|
6,1–15,2
|
8,8
|
| 5
|
2,5 –4
|
3,25
|
22–27
|
24,5
|
20,64
|
8,3
–15,5
|
9,7
|
| 6
|
4–6
|
5
|
28–47
|
37,5
|
13,15
|
9,8
–16,2
|
12,4
|
| 7
|
6–9
|
7,5
|
48–55
|
51,5
|
6,05
|
11,8
–18,5
|
15,0
|
| 8
|
9–14
|
11,5
|
56–63
|
59,5
|
1,11
|
14,2
–18,6
|
16,4
|
| >8
|
>14
|
>14
|
>63
|
>63
|
0,05
|
18,0
–23,7
|
20,0
|
Note
1. Ambient wind sustained at 19,5 m above
surface to generate fully-developed seas.
Note
2. To convert to another altitude,
H
2, apply V
2 = V
1 (H
2/19,5)(1/7)
Note
3. Minimum is 5 per cent and maximum is
95 per cent for periods given wave height range.
Note
4. The wave period shown here is the
modal or peak period, T
p. The zero crossing period, T
z, may be derived by the expression T
z = T
p /1,4 for fully developed seas.
|
|