397005 Best Practice in Application of CAA CAP 437 Guidance for Offshore Helicopter Landing Areas

Monday, April 27, 2015
Exhibit Hall 5 (Austin Convention Center)
Tim Jones, Risk and Safety, MMI Engineering, Warrington, United Kingdom and Chris Robinson, Risk and Safety, MMI Engineering, York, United Kingdom

The UK Civil Aviation Authority publishes a set of guidelines for offshore helidecks – “Offshore Helicopter Landing Areas – Guidance on Standards”. This was first published in 1981 and is now in its 7th edition; as well as being used as the standard for offshore installations in UK waters, the standard is used to provide guidance worldwide by various duty holders. As well as providing guidance on helideck layout and equipment, CAP437 places limits on environmental variables over the helideck for turbulence, temperature and hydrocarbon gas plumes. CAP 437 stipulates that modelling should be used to demonstrate these limits are met and this is usually carried out using computational fluid dynamics (CFD).

The requirement for turbulence over the helideck is that the standard deviation of the vertical airflow velocity should be less than 1.75 m/s. Some practitioners struggle to interpret this requirement properly and give poor quality or incorrect advice to duty holders regarding the ability to meet the criterion in different wind speeds and hence the availability of the helideck for operations. There are several points which are commonly mis-understood. Firstly, this applies to turbulent fluctuations in the wind, not the mean velocity. Secondly, the ambient wind itself will be turbulent with the turbulence intensity dependent on the atmospheric stability conditions and wind speed; this must be taken into account along with any additional turbulence generated over the platform topsides and helideck. Finally, in some instances practitioners have used CFD to determine the degree of turbulence at one or two wind speeds and simply scaled these over a wide range of wind velocities to determine the helideck availability. However, this linear relationship only holds for simple boundary layer conditions and shouldn’t be applied without consideration of wind flow “bending” over the helideck and the wakes created by topsides structures.

In the paper information is provided on the implementation of CAP437 limits for turbulence conditions and appropriate assumptions which may be made. A discussion of turbulence generation, transport and appropriate CFD turbulence models is included; an appropriate methodology for generating results to demonstrate CAP437 compliance is discussed.

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