Operating helicopters to moving helidecks

September 1, 2010

For more than 20 years, the UK’s Civil Aviation Authority has led various initiatives to reduce the risks to helicopters operating to moving platforms in the North Sea. Dr Athena Scaperdas and Dr Paul Gallagher of Atkins describe one such project which looks at helicopter on-deck safety.

The offshore oil & gas industry relies on helicopters to transport personnel and cargo to offshore installations and vessels. There are about 540 registered helidecks in the North Sea, of which 63% are not on fixed platforms but are ‘moving’. These include helidecks on large floating vessels (eg FPSOs, drillships and semisubmersibles), and smaller service vessels.

Landing a helicopter on a moving offshore helideck involves four distinct stages: approach, touchdown, on-deck duration, and take-off, each of which involves differing types of potential hazard.

During approach and take-off pilots need to choose a flight path clear of obstacles and/or hot gas plumes. Just before touchdown, pilots observe the motions of the helideck and then choose their moment to land the helicopter smoothly.

Once on-deck, the helicopter wheels are braked and chocked and there will normally be a net on the helideck to resist sliding, but the helicopter is not secured in any other way. However, the helicopter rotors are normally kept turning and, although the rotor blades are set to the minimum possible pitch, the main rotor still generates an appreciable amount of lift that can act to destabilise the helicopter.

And there is not much that pilots can do to protect a helicopter while on-deck. Pilots cannot adjust controls to counteract the effect of a sudden gust of wind or deck acceleration. ‘Flying’ the helicopter while on-deck is too dangerous for many reasons; it can cause the rotor disc to move unpredictably and injure personnel working nearby, or the pilot can potentially over-compensate and cause dynamic rollover. Another issue is the main rotor mast bending moments that this can generate, leading to fatigue damage.

So clearly, although pilots are able to exercise their skills and judgement in approach and touchdown, there are further risks to be managed during the on-deck phase through a mix of operational procedures.

Recent examples of accidents on moving helidecks include those on board the diving support vessel Mayo in April 1992 and the drillship West Navion in November 2001. In the former, a Sikorsky S-76 slid backwards on the helideck when the vessel was struck by a large wave; the resulting dynamic dip in the rotor disc resulted in a fatal strike to the head of the helicopter landing officer. In the second case, an AS332L Super Puma slid and then rolled onto its side, causing serious injury to one of the pilots, damaging the helicopter and the helideck structure, and resulted in long and costly operational downtime.

Current safeguards
The UK’s Civil Aviation Authority’s guidance in CAP437 strongly recommends that moving helidecks be equipped with electronic motion-sensing systems to measure maximum pitch/roll angles and heave (PRH limits).

The limiting PRH values are jointly set by the UK helicopter operators via the Helideck Certification Agency (HCA). They are dependent on vessel type as well as helicopter type, and are published in the Helicopter Limitations List (HLL).

The limiting values are based on service experience and pilot judgement and there is little scientific backing for them. There have been calls by the industry to develop a more objective approach to setting limits.

For example, it has been argued for some time that heave rate, the vertical velocity of the deck, is more appropriate as a touchdown limit than heave. Indeed, the Norwegian authorities have applied such limits successfully for over 30 years. As a result of collaboration between the UK and Norway, joint limits will be operated from 1 November 2010 that both simplify the criteria and use heave rate wherever its measurement is technically feasible.

These current limits aim to protect a helicopter during both the touchdown and on-deck stage. Although pilots are best equipped to judge if a helideck is safe to land onto at the time of touchdown, it is very difficult for pilots to gauge whether a helicopter will be stable enough or remain stable once on-deck. Thus, even though it could be argued that limits for touchdown are best set by pilots, the only rational way of deriving on-deck limits is by reference to a quantifiable on-deck reserve of helicopter stability.

New criteria
In view of the above issues and with safety as the primary concern, the UK’s CAA contracted Atkins to carry out research to develop new limits appropriate for on-deck stability. This research programme started over 15 years ago at a time when deck motions measurement equipment was relatively primitive, and the understanding of aircraft stability while on-deck was not well developed.

The initial objectives were to identify a single measure of deck motion able to capture the upsetting forces and moments acting on the aircraft, its statistical behaviour, and the best way to report these effects to pilots during their approach.

It became clear early on in the work that wind loading also had a significant part to play. The wind not only increases sideways drag but also increases the lift of the main rotor, which normally continues rotating while the helicopter is on-deck. This result contradicts conventional aerodynamic theory; computer modelling conducted by aerodynamics experts in connection with the investigation of the West Navion accident predicted that a rotor rotating at low blade pitch settings would act to force the helicopter down on to the deck – a stabilising effect. We organised trials to measure the lift directly, and used our helicopter stability model to provide a coherent and convincing explanation for the West Navion accident.

Much of the subsequent work has concentrated on developing and validating our model to predict a helicopter’s reserve of stability for a typical 10-20 minute on-deck duration, and using this to derive objective limits of operability.

A new framework for landing limits was therefore developed together with the CAA and industry stakeholders. This retains existing HCA limits as ‘touchdown’ only limits and introduces two new on-deck safety parameters to cover on-deck stability:
the motion severity index (MSI); and
the wind severity index (WSI).

The MSI is a prediction of how severe the helideck motion will be for the entire on-deck duration, before a pilot decides to land. This advance warning is important because once on-deck the only real option available to the pilot if conditions prove to be too dangerous is to take off and this cannot happen instantly. It takes at least 2-3 minutes to clear the helicopter for take-off.

The WSI is a representative measure of the wind speed at the helideck, which allows the effect of gusts to be accounted for in the limits.

Our research has demonstrated that the severity of deck motion can be quantified using the following ratio:

lateral accelerations in the plane of the helideck

acceleration normal to the helideck

We call this the ‘Measure of Motion Severity’ (MoMS). It combines the effect of both the static deck inclination (ie the roll and pitch currently measured), and the accelerations of the deck, and varies continuously due to the vessel’s motions. The MSI is simply a forward prediction of the MoMS maximum during a typical 20 minute landing duration. It is a single number, derived statistically from measurements of the MoMS prior to landing.

MoMS can be expressed as an angle (by taking the arctangent), and this is very useful because it tells us what the deck inclination really feels like. The roll and pitch angles measured to comply with current limits are actually just the static part of a deck’s inclination. In reality a deck feels a lot steeper than its nominal roll or pitch because of its lateral and vertical accelerations. This is exactly what MoMS represents.

As MoMS increases, the deck gets steeper in terms of both static and dynamic inclination, and a point is reached where helicopters, as well as other objects on the deck, might tip over or slide.

The wind also acts to destabilise the helicopter; as the wind strength increases, the MoMS needed to make a helicopter tip over or slide reduces. On the other hand, when wind conditions are benign, a helicopter can tolerate a larger MoMS. Therefore a ‘safe’ envelope of MSI/WSI values looks something like the curve illustrated in Figure 2.

Modelling helicopter stability
A helicopter can lose its stability by either tipping over or sliding across the helideck.

If all forces and moments acting on the helicopter are known, then it is relatively straightforward to calculate whether the helicopter is in danger of tipping over.

However, it is not trivial to work out when and how a helicopter with a three-wheel undercarriage will slide. The stability equations in 6° of freedom have too many variables for the number of equations available, and hence the problem is indeterminate.

We have managed to get around this problem by identifying all the possible failure modes for sliding, and using this to remove the indeterminacy in the equations. Interestingly, a helicopter will tend to slide in rotation rather than slide bodily down a deck’s incline.

We have also removed a lot of the complexity by deriving a few simple equations to quantify how close a helicopter is to tipping or sliding. A helicopter’s on-deck ‘reserve of stability’ can now be calculated relatively simply if the wind drag, the main rotor lift and other forces acting on the helicopter are known.

Quantifying these forces and their moments has, nonetheless, been an ongoing challenge. As mentioned, the main rotor lift has been the main unknown. The lift for the on-deck condition is not one that helicopter manufacturers design for and existing rotor aerodynamic models break down at very low collective pitch angles.

To overcome this problem we carried out our own helicopter trials, both on- and offshore, measuring the reactions at each wheel using load cells. We also mined data from full-scale rotor tests carried out at NASA Ames (Figure 3). On the basis of all the above data, we have now derived a quantitative model of how the main rotor lift increases with wind speed and as a function of the tilt of the rotor relative to the on-coming wind.

Other important information such as drag coefficients and the vertical location of the centre of gravity were also needed. These are known to helicopter manufacturers, but are commercially confidential. We have worked with helicopter manufacturers to obtain this data for two helicopter types.

Many further challenges remained. The total weight and weight distribution of a helicopter, the deck coefficient of friction, even the way pilots choose to centre their controls while on deck are all variables for the model that have a significant effect on helicopter stability. Simply choosing worst case values for these variables would lead to over-conservative limits on operations.

To account for the variability and the uncertainties involved in the model input parameters we have used a probabilistic spread of values, based on data gathered in operation and based on the judgement of the CAA and industry stakeholders. This has allowed us to simulate a comprehensive range of realistic operational scenarios, and to use this to derive MSI/WSI curves for a given level of probability or risk.

An initial set of MSI/WSI curves have been calculated for the Eurocopter Super Puma and the Sikorsky S-76, and work is ongoing to develop the model methodology further and refine the curves.

Method of application
So, once the MSI/WSI curves are finalised, how will all this be used in practice?

The development of new operational procedures for the MSI/WSI has been a matter of ongoing discussions with vessel and helicopter operators, pilots, and motion sensing equipment providers. The aim is to ensure that any new system operates effectively within known safe procedures, adding to safety but not complexity.

Traditionally, a vessel’s radio officer would be in contact with the shore and an approaching helicopter in order to advise on deck motions for the purposes of landing. Pilots have asked for status lights on the helideck, similar to traffic lights and visible on approach, to confirm that the helideck is within limits. Once on-deck, these lights can also be used to alert pilots to sudden changes in relative wind (eg due to changes in the weather or the vessel losing its heading as was the case with the West Navion accident).

A draft specification for the new system has been prepared and circulated to monitoring equipment providers. It is due to be tested aboard the Global Producer III FPSO, with the help of Maersk, Bond Helicopters and MIROS. These trials will test a full prototype system of motion and wind measurement and reporting equipment, including motion status lights fitted on the deck. Central to this exercise will be pilots’ and radio officers’ views and suggestions for further improvements.

Future implementation
A significant amount of research has been channelled into improving helicopter on-deck safety and providing pilots with reliable and accurate information.

The new MSI/WSI limits, used in combination with current HCA landing limits, will lead to improved safety and will provide an objective way of assessing the safety of offshore helidecks. The MSI/ WSI limits are supported by both UK and Norwegian regulatory bodies, and are to be incorporated in CAA’s CAP437 once work has been completed.

A new technical specification for implementing the new MSI/WSI limits has been prepared. This includes several additional improvements to helideck monitoring systems, such as a helideck motion status light warning system for pilots and better, more intuitive displays for the radio officers. OE

The authors would like to thank all of those that have contributed to this research over the last 15 years or more – offshore operators, equipment manufacturers, helicopter operators and pilots – too numerous to mention but whose help and advice has been invaluable. Particular thanks should also go to the CAA and David Howson for permission to write this article and without whom none of these developments towards improved helicopter safety would have been possible.

Dr Athena Scaperdas is a senior engineer within the oil and gas business at Atkins. She studied engineering at Cambridge University, followed by an MSc and PhD at Imperial College. Since 2002, she has managed and led the research work on behalf of the CAA, developing new criteria for safe helicopter operations to moving offshore platforms.

Dr Paul Gallagher is a director within the oil and gas business at Atkins. A chartered engineer, he graduated in naval architecture and ocean engineering at Glasgow University, where he also completed his PhD in computational fluid dynamics. He has been the project director for all CAA helicopter stability and operations work, and related studies into ship air interface for naval vessels for the UK Ministry of Defence.

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