离岸直升机起落甲板设计指南 OFFSHORE HELIDECK DESIGN GUIDELINES 2(9)

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10.4.5.1 Clad Derricks
A fully clad drilling derrick is a tall and solid structure and generates a correspondingly significant wake. The important flow property of the wake is that it is unsteady and so, if it is upwind of the helideck, it subjects the helideck area to large and random variations in wind speed and direction.
A general guide on wake decay from bluff bodies indicates that wake effects largely dissipate within a downwind distance of 10-20 structure widths.  For a clad derrick 10 m wide at helideck level, this would correspond to a decay distance of 100-200 m (see Figure 10.4).

Consequently it is best if the helideck is not placed closer than 10 structure widths from a tall solid structure such as a clad derrick.  However, few offshore installations will be large enough to permit such a clearance to be included in the design, and so the specification of a clad derrick is almost certain to result in a significant operational limitation for helicopters when the derrick is upwind of the helideck. It will be particularly important to try to ensure that the installation is aligned such that this only happens in rarely occurring wind directions (see Section 10.9.4).
10.4.5.2 Unclad Derricks and Cranes
Unclad derricks are relatively porous.  A wake still exists, but the turbulence is of much higher frequency and smaller scale due to the flow being broken by the lattice elements of the structure. An unclad derrick can therefore be safely located closer to the helideck than its clad equivalent.  Ideally the separation between the helideck and an unclad open lattice derrick should be at least 5 times the derrick width at helideck height (see Figure 10.4).  Separations of significantly less than 5 derrick widths may lead to the imposition of operating restrictions in certain wind conditions.
Crane pedestals and crane booms are also usually of lattice construction, and the same approximate rule can be applied as for lattice derricks.  Generally the disturbed flow region will be much less due to the smaller dimensions.
10.4.5.3 Exhaust stacks
Gas turbine and other exhaust stacks, whether operating or not, also represent a physical blockage to the flow and create a turbulent wake (as well as the potential hazard due to the hot exhaust – see Section 10.4.6).
The same guideline as defined for the clad derricks is recommended, namely, a minimum of 10 structure widths between the stacks and the helideck. If there are multiple exhausts and these are located in close proximity to each other, then it is recommended that the structure width be considered to be the overall span of the group of stacks.
10.4.5.4 Other Enclosed Structures
Some offshore drilling rigs include large enclosed structures in close proximity to the drilling derrick (e.g. shaker house).  If the height of these structures extends to helideck elevation, then they may give rise to large-scale turbulent disturbances downwind, and should be treated much as for a clad derrick.
　
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