Offshore helideck design considerations - environmental effects
Offshore Information Sheet No. 5/2011
(Issued June 2011)
This sheet reminds duty holders of the need to assess environmental effects including turbulence, thermal effects and wave motion effects in the vicinity of helidecks on offshore fixed and floating installations
The Civil Aviation Authority (CAA) has recorded an increase in the number of reports derived from both helicopter manufacturers and operators, which indicate that helicopter engine compressor surges and stalls are occurring during take-off and landing at a small number of offshore helidecks.
Environmental factors, such as turbulence and thermal effects, can have a significant detrimental effect on safety of helicopters landing and taking off at offshore installations, so it is important to create topside designs and helideck locations that are safe and ‘friendly’ to helicopter operations. To help the industry achieve this, a manual promoting good design practice was published by the CAA in CAA Paper 2008/03 Helideck Design Considerations – Environmental Effects1, produced in co-operation with the Helicopter Liaison Group of HSE’s Offshore Industry Advisory Committee.
The manual describes both air flow turbulence due to structural features and temperature effects due to hot exhaust gas plumes. The latter is a significant hazard as the rate of change of temperature in the plume can cause engine compressor surge and stall to occur (often associated with an audible ‘pop’) which can result in loss of engine power, damage to engines and/or aircraft components and, ultimately, engine flame out. This can obviously put the helicopter and its occupants at significant risk.
Duty holders should ensure, as far as practicable, that the installation topside and design and helideck location are arranged to promote safe and friendly helicopter operations by minimising exposure to environmental effects and that in their Safety Cases they have described and recorded the means of doing so. CAA Paper 2008/031 describes in detail methods of design assessment including the principal tools available for predicting the flow field around a helideck, wind tunnel testing and CFD analysis.
HSE inspectors will be seeking to satisfy themselves that the problems are recognised, reviewed and addressed in Design Notifications, Safety Cases and Thorough Review Summaries. Where hazards are identified, but the modification of topside is impractical to fully ‘design out’ an issue (e.g. hot gas exhaust plumes from offshore platform power generation turbines presenting a hazard to helicopter operations), operating (pay-load) restrictions will need to be applied potentially throughout the life of a platform. Further tools may be available to a duty holder to minimise any such restrictions, for example, implementing a system to ensure effective visualisation of otherwise unseen exhaust gas plumes by the introduction of smoke into exhaust emissions to make them visible to the pilot. These particular systems are described in detail in CAA Paper 2007/022
Relevant legal requirements
- CAA Paper 2008/03 Helideck Design Considerations - Environmental Effects
- CAA Paper 2007/02 Visualisation of Offshore Gas Turbine Plumes
- CAA Paper 2008/02 Offshore Helideck Environmental Research – Part 1 Validation of the Helicopter Turbulence Criterion for Operations to Offshore Platforms
- Civil Aviation Authority (CAA) CAP 437 Offshore Helicopter Landing Areas: Guidance on Standards, Sixth Edition
This information sheet contains notes on good practice which are not compulsory but which you may find helpful in considering what you need to do.