离岸直升机起落甲板设计指南 OFFSHORE HELIDECK DESIGN GUIDELINES 2(17)

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5:1 gradient (over at-least 180o), this assessment should consider the effect of the turbulent wake from one platform impinging on the helideck of the other, and any hot gas exhausts from one influencing the approach to the other helideck.  The helideck on a mobile unit is likely to be at a much lower level than the bulk of the fixed platform it is alongside, and is therefore likely to experience severe turbulence when downwind.
These considerations are likely to determine that, under certain wind conditions, helideck operations to the mobile unit need to be curtailed.  Where the combined operations have more than one helideck available and a gangway platform for personnel, it may be possible to switch from using one helideck to the other depending on the conditions.  All such limitations need to be fully investigated, documented, and communicated to the helicopter operators to ensure that the various management procedures to control the use of the helidecks are put into place.
10.8  EXAMPLES OF GOOD AND BAD PRACTICE IN PLATFORM HELIDECK LOCATION
This section contains sketches of the main types of offshore installation (fixed jacket, semi-submersible, jack-up, tension leg platform and FPSO) with examples of each, illustrating good and poor practice in helideck location.
10.8.1 Fixed Installations

Good: Helideck is above the level of the surrounding main modules.
Bad: Two large clad derricks present a major solid obstruction to the wind flow and the helideck will experience serious downdraft and turbulence when downwind. A set of four gas turbine exhaust stacks is also located close to the helideck. They are not high enough to prevent problems with hot gas exhausts, and are also a significant obstruction to the wind flow over the helideck at certain wind headings. The helideck has insufficient overhang and air gap.
Good: The helideck is mounted significantly above the level of all the platform modules, and with an appropriate air gap beneath. The only structure above helideck level is the derrick, which is unclad, and therefore likely to produce little significant turbulence at the helideck.
Bad: The installation has downward facing gas turbine exhausts, which may cause clouds of rising hot gas to envelop the helideck and helicopter approach path. This is particularly likely to happen in light wind conditions (when helicopter performance is inherently poor).

Good: Being mounted on the top of a separate accommodation platform and with a significant air gap and overhang, the helideck is unlikely to suffer from any significant turbulence problems. 
10.8.2 Semi-submersible and jack-up drilling units
Good: All semi-submersible drilling units are good from a wave motion point of view unless they are floating at a very shallow transit draft. At operating or survival draft, motions are generally of low amplitude and low frequency.
Bad: Helideck is in close proximity to a partially clad drilling derrick and other adjacent solid structures, which all extend to a height significantly above the height of the helideck. The helideck will experience significant shear turbulence and downdraft when the derrick is upwind. (The lack of an air gap is likely to be less significant due to the relatively shallow deck on which it is mounted.)
　
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