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10.4.5.5 Lay-down Areas
A lay-down area in the vicinity of a helideck poses a number of potential problems to helicopter operations. Bulky or tall items placed in a lay-down area close to a helideck may result in turbulence and or downdraft. The temporary nature of such lay-down areas increases the potential hazard because the helicopter pilots, though perhaps familiar with the installation, may not be expecting turbulence.
The platform design should seek to ensure that lay-down areas are significantly below helideck level or sufficiently remote from the helideck to avoid such problems. If this cannot be achieved then it is essential that management procedures are in place to ensure that appropriate limitations are placed on flight operations.
10.4.6 Temperature Rise due to Hot Exhausts
Increases in ambient air temperature are a potential hazard to helicopters. Increased air temperature means less rotor lift and less engine power margin. Rapid temperature changes can also induce engine surge and even compressor stall or flameout.
It is therefore extremely important that helicopters avoid these conditions, or that the occurrence of higher than ambient conditions is foreseen and steps taken to reduce payload to provide an appropriate performance margin.
Gas turbine power generation systems are usually the most important source of hot exhaust gases on offshore production platforms, but diesel propulsion or auxiliary power system exhausts on mobile units could also be significant.
For certain wind directions the hot gas plumes from the exhausts will be carried by the wind directly across the helideck. The hot gas plume mixes with the ambient air, and the mixing increases the size of the plume, and reduces the temperature (by dilution).
Evaluations of likely temperature rise, based on a Gaussian dispersion model and supported by wind tunnel tests, indicate that for gas turbine exhausts with release temperatures up to 500°C and flow rates of 50 -100 kg/s, the minimum distance required before the temperature rise drops to 2oC rise above ambient is in range 130-190 m (see Figure 10.5). Some gas turbine power generation systems may include waste heat recovery systems that have lower exhaust gas temperatures of about 250oC, resulting in reduced minimum distances in the range 90 -130m.
Except for very large platforms, this implies that regardless of design, there will always be a wind condition where temperature rise above the helideck exceeds 2oC. It is likely to be impossible, therefore, to design a helideck that is compliant with the criteria under all conditions. The design aim becomes one of minimising the occurrence of high temperatures over the helideck rather than eliminating them. This can be achieved by trying to ensure that platform layout and alignment direction are such that these conditions are only experienced rarely (see Section 10.9.4).
Many offshore installations have the power generation modules and exhausts located close to the accommodation modules and helideck. This is because the power generation is regarded as significantly less hazardous than drilling or production modules. This can be a good location provided that the stacks are high enough, are not wide enough to cause large amounts of turbulence, and do not impinge on the ‘obstacle protected surfaces’.
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