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The helideck should be located such that winds from the prevailing wind directions carry the plume away from the helicopter approach path. To minimise the effects for other wind directions, the exhausts should be sufficiently high to ensure that the plumes are above the helicopter approach path. To achieve this, it is recommended that the exhaust outlets be no less than 20-30 m above the helideck, depending on the gas turbine flow rates and temperatures.
In the past, some platforms were fitted with downward facing exhausts so that the hot exhaust gases were initially directed down towards the sea surface. This arrangement is not recommended because the hot plume can rise and disperse in an unpredictable way, particularly in light wind conditions.
NOTE: Where it is considered necessary to extend the gas turbine exhaust outlets, it is important for the design project team to consider early on in the project how the installation of extended outlets can reasonably be achieved. Ideally, the engineering requirement should be established before firming up the gas turbine prime mover specification(s). It is important to consider the potential effects on operating performance and extra maintenance requirements caused by extending the gas turbine prime mover exhaust ducts, particularly when they are used in conjunction with some waste heat recovery systems (it may result in an increase in back pressure on the turbine). A complete picture of the exhaust / flare plume and its potential extremities (i.e. under normal operating and maximum output conditions) for a full range of wind conditions is required. Test Houses will require project teams and manufacturers to furnish them with full details for the varying load conditions, mass flows and exhaust temperatures for all possible operating conditions.
10.4.7 Cold Flaring and Rapid Blow-down Systems
Hydrocarbon gas can be released from the production platform process or from drilling rigs at various times. It is important to ensure that a helicopter cannot fly into a cloud of hydrocarbon gas because:
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concentrations above 10% of Lower Flammable Limit (LFL) might cause the helicopter engine to surge or flameout with consequent risk to the helicopter, and
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the helicopter poses a risk to the offshore installation because it is a potential ignition source for the hydrocarbon gas.
Consideration therefore needs to be given to ensuring that gas release points are as remote as possible from the helideck and helicopter flight path, and that any unforeseen gas releases trigger the automatic activation of the helideck status lights (flashing red). Planned gas releases should only occur when helicopters are not in the area.
The blowdown system on a production platform depressurises the process system releasing the hydrocarbon gas. It will normally be designed to reduce the pressure to half, or to 7 bar, in 15 minutes (the API standard). For a large offshore installation this might require the release of 50 tonnes or more of gas. Once down to this target pressure in 15 minutes or less, the remainder of the gas will continue to be released from the system. A blow-down may be automatically triggered by the detection of a dangerous condition in the production process. Alternatively it may be triggered manually.
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