Introduction
Wing in Ground Effect (WIG) is a niche type of transportation nominally offering significant increases in cost-efficiency over mainstream in-service types. The promise of rose-tinted revolution has thus far been dissipated by real-world practicalities. However, as advances in modern technologies mitigates known negativities, the WIG concept rears its pretty head again and the dream cycle recommences. The aim of this text is to review its potential in the offshore logistic support service context and in comparison with existing core support vessel and helicopter service provision.
Development.
WIG development dates back to the early 1930s with the earliest designs originating in Sweden and Germany, then latterly, post war, in the USA. While effective over any flat surface, the need for a long take-off runs and surface skimming operating heights, has tended WIG designs to amphibian variants. With large internal seas and massive frozen, sparsely inhabited plains, it is not surprising that Soviet Russia became the centre for WIG development during the 60s/70s Cold War period (pun intended!), the focus being on fast surface attack craft of up to 600 tonnes – the so-called Caspian Sea Monster – primarily for use in the Black Sea and the Baltic. With a weakening economy and the fizzling-out of the Cold War, continued research has now almost ceased in Russia.
Seemed like a good idea at the time – 250 tonne, 300 kt. Lun class & Beriev VVA-14 – then & now, a dream in decay
However, interest in commercial applications has continued in developed regions, particularly the USA and various countries in the Australasia locality, resulting in protypes with capacity for up to a dozen passengers. Principal players (as illustrated below) now include:-
- Hoverwing – a late ‘90s development by Universal Hovercraft in Florida. A two-seat prototype for an 80 passenger production vehicle is flying, combining Hovercraft and WIG technologies.
- Airfish – an 8 Pax amphibian WIG vessel now manufactured & operated by Wigetworks in Singapore first flew in 2011. It has a maximum speed of 100 kts and a 180 Nm range. Now on the Lloyd’s Register of Shipping.
- Seaglider – a 12 Pax hydrofoil WIG vessel under design by Regent in the USA with eight electric engines providing a maximum speed of 150 kts and a 180 Nm range. Quarter-size prototype flew in 2022 with a production prototype is scheduled for 2025.
- Liberty Lifter – a DARPA program launched in 2023 for a turbo-prop., amphibian WIG heavy lifter, also capable of operating as a normal aircraft up to 10,000 ft. with a target payload of 90 tonnes and 6000 Nm range (in WIG mode). Seen as a replacement for the C-17 Globemaster.
Good ideas for the future: small Pax. types – Hoverwing, Airfish, Seaglider & return of the heavies (Liberty Lifter)
Technology Overview
The concept is very simple and similar to the phenomenon of a helicopter hovering in ground effect and out of ground effect (IGE /OGE). When air passes over any aerofoil with its axis at a positive angle with respect to that airflow, a pressure differential is generated with a higher pressure below the aerofoil (wing) than above: the faster the airflow, the higher that pressure differential which causes an upwards ‘Lift’ force. So when you catch a flight to go anywhere, the fixed-wing airliner trundles along a straight road (the runway) accelerating like a racing drag truck until after a couple of kilometres it reaches a reckless speed at which the pressure differential creates a lift force equal to the aircraft weight and one staggers into the air just before hitting the perimeter fence at the edge of the airfield. Helicopters have a more suave approach by rotating the aerofoil achieving the same effect in a much less risky and reckless fashion. A WIG aircraft (or vessel – after some 50 years, the regulatory jury is still out on whether they are slow aircraft or very fast ships) modifies the aerofoil so that, as it skims over a flat surface, air is trapped beneath it, thus substantially increasing that pressure differential. The good news is it reaches flight speed a lot quicker (ie at a lower speed): the bad news is, to maintain that IGE air cushion, it cannot go higher than a (very) few feet above the ground and so must operate from airfields without perimeter fences – hence, mostly they operate like amphibians, from marine bases.
The WIG concept – comparing the operation of a standard Aerofoil (left) with one ‘in ground effect’(right)
Note that for the same speed (V), the Lift (L) achieved is greatly increased while Induced Drag remains unchanged
Advantages
As the above diagram shows, the principal advantage comes from the increase in lift for any given airspeed generated by the in-ground effect (IGE) and that, without any increase in induced drag (which is a rearwards vector caused by the lift force being at 90º to the aerofoil axis: hence the greater the aerofoil angle to the airflow, the more the induced drag). More lift at less speed means less power required and also less body friction drag. All these combine to make the Wig aircraft significantly more fuel efficient than equivalent sized aeroplanes and nominally, with the potential to carry larger payloads.
Disadvantages
These are legion.
- Firstly, like all seaplanes, the body needs to be structurally more robust to operate off water which thus results in some of the increased lift being lost due to an increase in aircraft weight
Mitigation – to maximise the use of composites.
- Secondly, the increase in WIG vehicle efficiency only applies IGE. When OGE these vessels are significantly less efficient than an equivalent aircraft.
Note – IGE operations limit WIG vessels to less than 20ft above the sea surface: in moderate seas the ground effect is reduced by waves, both decreasing this platform’s efficiency and comfort due to vibration
- Operation close to the sea surface results in salt ingestion into power plants requiring a significant increase in maintenance and additional corrosion protective measures.
Mitigation – none needed except giving more time for maintenance
- Aero engines are designed to operate at altitude, so are less efficient at sea level thus negatively impacting the fuel efficiency element.
Mitigation – use electrical power plants (however, still early days)
- Certification – still an open question whether this should be marine (cheaper) or aviation (more challenging): given that most WiG aircraft can operate OGE in hops (Class-B) or prolonged (Class-C), aviation standards are relevant.
Note – Class-A WIG platforms are limited to IGE operations: as such, the Singapore-based ‘Airfish’ has just been certified on the Lloyds register of Shipping
- WIG craft are thus weather sensitive being limited to sea-state-4 (2.5m waves) which equates to Beaufort Force-5 winds (ie. up to 20 knots) and somewhat less than that for take-off/landing.
Mitigation – nil, but Hydrofoil types will fare better during take-off/landing in other than calm seas
- IGE WIG vessels are not manoeuvrable with a large skidding turning circle. So at a typical 100-150kt cruising speed, they are a hazard to small, less visible vessels like fishing boats and recreational vessels.
Mitigation – improved digital radar processing and automated collision avoidance software should be able to overcome this issue in the near future (say in a 5-year timeframe).
- Speed lanes – due to this lack of manoeuvrability and the associated risk of fatal collisions with small vessels, in the opinion of this author, as part of the certification process, specific designated routes marked with illuminated buoys with radar reflectors will be required. This is believed to be a ‘killer issue’ and so a separate section has been dedicated to this issue below.
The need for traffic lanes
As stated above, operating IGE just a few feet above the sea, these aircraft-like vehicles are more like surface vessels – but very fast and not very manoeuvrable. With radar, medium and large shipping are detectable at long range and thus easily avoided. Offshore fishing vessels in calm seas will similarly be detected at ranges allowing sufficient time for avoidance. But in less clement weather such detection ranges will be significantly reduced and to assure such detection will require a radar operator dedicated to that task (ie. an additional crew member). Smaller recreational vessels and wooden coastal fishing boats do not show up on radar until very close, if at all, so cannot be avoided. As such, in terms of Risk Management, WIG operations represent an unacceptable hazard. To mitigate this, routes from onshore bases to offshore platforms would need specific designation. Because such small vessels are not in receipt of NOTAMs (Notices to Mariners), such routes will have to be physically designated with illuminated buoy markers equipped with radar reflectors perhaps at some 500m separation. In addition, since any collision between a WIG craft at 100kts and a small surface craft will surely be fatal, at least for those on the latter, such designated ‘Speed Lanes’ will likely require national legislation and perhaps, will have to be patrolled to ensure enforcement. Such additional infrastructure costs will more than eliminate any of the anticipated operating cost benefits.
The final nail in the coffin of WIG OGP sector service provision is their weather sensitivity due to the limiting sea-state (SS) for take-off (ie. getting into ground effect) and landing (ie. returning to normal displacement operations) of SS.4-5 – such ‘moderate’ seas are a very common, even normal, condition in an offshore environment. The resulting high frequency of cancellations due to weather will surely be unacceptable.
Summary & Conclusion
WIG vehicles have long been hyped as a more cost-effective replacement for the helicopters and supply vessels typically used in offshore support operations. The potential is surely there. To support an average production platform requires at least two offshore supply vessels (one in the field, one loading – exchanging (say) fortnightly) and two medium helicopters doing some 10 trips per week (two are required to guarantee a 24/7 emergency response capability). A single 50-tonne payload WIG craft nominally could replace all of these, requiring only a single medium helicopter on standby for medevac, the cost of which can be kept low by farming into an existing operation. At typical 100+kt cruising speeds, a turn-round (outbound-offload-reload-inshore) should be achievable in a single long day. The ability of such a vessel to also carry some 50+ Pax. means such trips would only be required, say, every 3-days. Such would offer less flexibility and redundancy, but by amending SOPs is eminently doable. The cost savings potentially would be significant.
A Sea State 4 limitation kills this potential. However, this applies to the exiting small (<12 seat) craft. Larger, heavier craft, also using hydrofoil types like the Seaglider, can cope with higher sea states thus reducing this limitation . But, as vessels get larger the collision risk remains and may even grow. So nominally, this concept warrants further investigation, except for one thing – such a platform does not yet exist! While all the technologies to create one are available now and well understood, it will take a lot of money and time to develop. The DARPA Sea-Lifter program is gobbling up some $30m p.a. just pushing paper. Designing, building and certifying such craft will thus surely need a few billion dollars. The commercial market is likely too small to amortise such cost. However, if such a vessel is developed under military funding, then the development cost in the commercial sector would, in principle, become more manageable. At that time, one of the players identified above will surely run with it at which point the offshore industries in particular, would be wise to study this application again in more depth.
Meanwhile, the existing 12 seater craft are not fit-for-purpose in the OGP sector. While the procurement cost is about one-third of an equivalent medium helicopter, operating costs, while nominally significantly less, will be increased by corrosion control requirements and additional operating infrastructure requirements and particularly the suggested speed lanes. The increased safety offered by these craft is a risky claim, not only because of the collision risk which still has to be properly addressed, but also the fact that every trial / demonstration in the OGP sector has ended ‘in tears’ with the craft incurring serious damage. That these accidents were a function, in the main, of a lack of familiarity with that environment, if nothing else, this shows that an extensive (and expensive) sea trials are still required to move this forward. But, even if successful, there remains the issue of weather sensitivity. That these existing craft are limited to sea-state 4, an approximately a 50 percentile weather condition, is likely the final nail in the current WIG operational coffin!
Notwithstanding, the use of WIG craft is an interesting concept with some potential. Accordingly, the recommendation of this paper is for the OGP sector to sit on their hands and let the US Government spend the necessary billions to bring a high payload, therefore high potential, craft into service.