The oil and gas industry relies heavily on helicopters for transporting personnel and cargo to and from offshore installations and support vessels. A growing number of offshore helicopter operations are to moving helidecks, which include large vessels such as FPSOs, drill ships, and semi-submersibles, as well as smaller service vessels. Landing a helicopter on a moving helideck presents additional challenges to those faced on fixed helidecks, not only at the point of touchdown but also for the entire period the helicopter remains on the helideck.

The UK Civil Aviation Authority (CAA), on behalf of the joint CAA/industry Helicopter Safety Research Management Committee has led a comprehensive programme of research over a number of years, aimed at improving the operational safety of helicopters landing on moving helidecks. The work focused on the aspect of the stability of helicopters once landed on a moving helideck, this being the main source of in-service incidents and accidents as evidenced in the Mandatory Occurrence Reports. The project culminated in the development of a new standard for Helideck Monitoring Systems (HMS), which was published by the Helideck Certification Agency in April 2018 with an implementation compliance date of 31 March 2021. Operations to moving helidecks not equipped with HMS meeting the new standard will be restricted to stable deck conditions from this date. The research underpinning the new standard is presented in two papers.

This paper (Part A), presents the analytical approach that has been developed to model the Reserve of Stability (ROS) for all modes of failure of a helicopter on a moving offshore helideck.

The analytical model covers all modes of on-deck failure (roll-over and sliding), for any nose wheel tricycle undercarriage helicopter. The mathematical expressions that have been derived are remarkably simple, physically intuitive, and make the relative contribution of all the destabilising factors easy to understand and assess. These analytical expressions can be used to calculate the ROS of any helicopter in real time, as well as for calculating an envelope of safe operating limits.

This approach has many advantages compared to conventional ‘black box’ modelling methods. The main advantage is that it simplifies the modelling of the destabilising effect of helideck motion and allows the most salient parameters governing on-deck ROS to be defined, namely the Measure of Motion Severity, the instantaneous wind speed as the Measure of Wind Severity, and the wind direction relative to the helicopter (Relative Wind Direction).

The main rotor lift generated during the time the helicopter remains on-deck (at Minimum Pitch on Ground, MPOG) has been one of the most important unknowns that this research programme has sought to address. An empirical model for estimating the lift at MPOG has been developed, based on experimental and field data. Practical methods for quantifying fuselage wind drag and the vertical position of the centre of gravity were also developed, which allow different helicopter types to be assessed without recourse to helicopter Original Equipment Manufacturer (OEM) proprietary information or models. Finally, a comparison and evaluation of the model against dedicated field trial measurements is presented, together with a discussion of the modelling strengths and weaknesses, and recommendations for further work.