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Jack-ups
The marine operating environment for a jack-up on station is the same as a fixed installation. However when under tow, the helideck conditions are similar to a vessel under way.
Vessels
The marine operating environment for a drilling vessel ‘on station’ is similar to a semi-submersible insofar as the helideck heading is generally fixed as a result of the anchoring arrangement or, if fitted, the dynamic positioning (DP) system. Similarly, the helideck has a dynamic movement in roll and pitch axes, heave, surge and sway due to the vessels dynamic characteristics.
7.2.3.2 Anchoring
Anchoring arrangements should be taken into account when locating a helideck. The locations of windlasses and anchor chains (above sea surface) should be checked to ensure they do not encroach into the obstruction free sectors and 5:1 falling gradient.
7.2.3.3 Dynamic positioning
Care should be taken to ensure correct positioning of the DP system heading control sensors, etc. that may be adversely affected by disturbances from helicopter rotor downwash during landing and take-off. In particular, anemometers can be greatly affected by spurious windflows.
Apart from spurious DP changes potentially causing severe problems for the vessel whilst performing its primary operating task, even small changes to vessel heading, etc. can induce increased helideck motions. These increased helideck motions may suddenly put the helideck out-of-limits for a safe helicopter landing and / or create serious stability problems for a helicopter parked on the helideck.
7.2.3.4 Drainage
Semi-Submersibles
Despite the best efforts of the builders, steel plated helidecks rarely remain flat enough to be reliably drained by rig motion alone. Normal practice on such helidecks is to camber the deck about a centreline knuckle. Camber is generally between 1:100 and 1:50 (0.57° and 1.15°).
Vessels
As a result of vessel trim and motions the helideck should drain naturally toward the drain holes and scuppers.
Therefore the helideck does not need a built-in ‘fall’, as is the case with fixed installations.
If a ‘fall’ is built into a vessel helideck this effectively introduces a pre-fixed ‘list’ onto the helideck relative to the vessel datum. This ‘list’ has the effect of reducing motion tolerance at the helideck for helicopter landings and the degree of ‘list’ must therefore be added to the recorded motion measurement in the relevant axis (e.g. roll or pitch).
7.2.3.5 Helideck Location, Size and Obstacle Environment
Helideck location on MODUs will largely depend upon the type of hull structure employed (e.g. Semi-submersible, Jack-up or Ship). Size is not normally a problem other than the structural and weight considerations associated for example with very large, elevated helidecks that may have an adverse effect on vessel stability.
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离岸直升机起落甲板设计指南 OFFSHORE HELIDECK DESIGN GUIDELINES 1(33)
