This information is sometimes presented graphically in the form of a wind rose (see Figure 10.12).
10.9.4 Prevailing Wind Direction
The wind frequency table or the wind rose can be used to identify the prevailing wind directions. These may be defined as the highest probability directions with a combined probability of occurrence of approximately 50%. For example taking the data in Figure 10.11 we can rank the directional sectors as follows:
Wind Direction (from) Probability Cumulative
South (157.5° – 202.5°) 16.5% 16.5%
South-West (202.5° – 247.5°) 16.1% 32.6%
West (247.5° – 292.5°) 14.9% 47.5%
North-West (292.5° – 337.5°) 14.9% 62.4%
Figure 10.13 - Prevailing wind directions.
The prevailing wind directions are therefore defined to be in the range 157.5° to 292.5° with a cumulative probability of 47.5% (or 173 days in the year).
NOTE: It should be noted that wind directions are invariably defined in terms of the direction that the wind blows FROM. However, occasionally such data may be presented as directions TO (often to be consistent with wave direction data, which is usually presented in this way). If there is any doubt about the direction definition then it is essential that the data be checked with the authority that generated or published it. An error of 180o in determining the prevailing wind directions is likely to be disastrous for helideck operability.
10.9.5 Upwind Helideck Location
When a pilot selects his approach direction to an offshore helideck he will take into account a number of considerations such as:
.
Direct approach wherever possible
.
Clear overshoot available
.
Sideways / backwards manoeuvring minimised
.
Turbulence effects
.
Right versus left seat pilot.
The balance (or relative weighting) between these considerations will change depending on the wind speed. For example, if the turbulence is low, a pilot could prefer to make a straight-in approach downstream of an obstacle rather than fly a sideways manoeuvre. Hence there could be a trade-off between turbulence and sideways and backwards manoeuvring, related to wind strength.
However, generally the helideck should be located such that winds from the prevailing directions carry turbulent wakes and exhaust plumes away from the helicopter approach path. To assess if this is likely to be the case, overlay the prevailing wind direction sectors onto the centre of the helideck. Figure 10.14 to Figure 10.17 give examples ranging respectively from most to least favourable helideck locations for a platform with prevailing winds from the southwest.
Major items of obstruction, including drilling derricks and exhaust stacks should be outside the areas embraced by these sectors as shown in the figures. If they are, then conditions at the helideck are likely to be compliant for 50% of the time. If obstructions are located within the prevailing wind sectors, then the following options should be explored:
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