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Mitigating Risks and Providing Safe Heli Operation at Sea
Cdr Balakrishnan G Nair (Indian Navy Retd), (Naval Architecture & Structural), Chennai Design Centre, GL Noble Denton, Chennai Helicopters are being increasingly deployed for various marine operations involving ships. A safe landing of helicopter on the helideck of the ship can be chaotic unless certain deliberate attempts have been made during the design of the platform and also during heli operation at sea. A Ships Helicopter Operational Limit (SHOL) study throws a lot of light into how the operation can be made safe and less hazardous. Th e article takes a look at SHOL studies and the potential they off er in mitigating the problems encountered in the above field.

Ship and aircraft operators are evincing an increasing need for ship-based helicopter capabilities. For the military, the helicopter is often regarded as a force multiplier, by extending the surveillance and strike capabilities of the ship’s weapon system. Other roles such as Medical Evacuation, Personnel Transport, and Supply Support are common to both military and civil operators. Helicopters often need to land on ships in extreme environmental conditions, day and night. When coupled with combined effects of ship motion, turbulence, and visual cues, such operations can be both demanding and potentially hazardous.

The picture above is a clipping from the video recording of trials of Lynx Mark 90B helicopter onboard Knud Ramussen class Offshore Patrol Vessel of Royal Danish Navy. Here the vessel is in high seas experiencing combined rolling and pitching and certain amount of heaving (Heli Deck of vessel seen). The helicopter is facing strong winds. The scenario renders the landing potentially hazardous.

The Environment
The behavior of a helicopter while landing on/taking off from a ship depends on the complex environment consisting of the Helicpoter itself, the ship, the weather, crew and operators. In accordance with RTO-AG-300 Vol 22 AC/323(SCI-038)TP/53 of NATO, (Part 1 of 'Dutch / British Clearance Process' by R Fang, Cdr H W Krijns, R S Finch), the factors affecting ship-borne helicopter operations can be grouped as can be seen in the table in the adjoining page.

SHOL Studies
The objective of Ship Helicopter Operating Limit (SHOL) studies is to develop operating limits that maximise the operational capability whilst maintaining safety to an acceptance level. The major focus while conducting First of Class (FoC) flight trials, also referred to as Dynamic Interface Trials is to ensure that safe and practical helicopter operations can be conducted on a given vessel. For this, the ship’s flight deck and aviation facilities must meet a range of certification standards, national guidelines and operator requirements. The embarked environment has unique characteristics, which must be considered prior to a helicopter operating at sea, particularly if the aircraft type has had little embarked operating history.

Turbulence aspect of the embarked environment can vary significantly between ship types. During the ship design phase effort should be made to reduce adverse aerodynamic effects over the flight deck. One of the major analysis tasks that is conducted prior to embarking a helicopter onboard a ship involves dynamic modeling of the aircraft on the flight helideck, to determine when the unrestrained helicopter begins to slide or topple, or when the restrained helicopter exceeds a structural limit. This activity is commonly referred to as “Toppling and Sliding” analysis, also referred to as Ship Motion Limits Analysis. Extensive instrumentation is done for the FoC trials to provide critical safety information for the test crew, to measure trials data to produce well-defined limits and provide a telemetry link to enhance safety and trials efficiency.

SHOL are a group of defined operating limits for safe operation of a particular helicopter type with a given class of vessel. The limits are a function of the vessel’s motion and superstructure turbulence characteristics, and also the performance and configuration of the helicopter type. SHOL development involves operating the helicopter in a wide range of environmental conditions, commencing in benign conditions, with an incremental build-up. Proven processes are followed to determine the operational flight envelope and any unique procedures required to safely operate from the vessel. Other aspects evaluated during FoC Flight Trials include night lighting, landing aids, hangar facilities, aviation environmental sensors and displays, communication equipment, etc Ship’s course and speed may create limiting factors for helicopter operations, in combination with sea-state, wave/swell direction and true wind.

The major factor limiting helicopter operations on ships compared to land-based operations is the small flight deck for take-off and landing, which is moving (pitch, roll and heave) and obstructed by obstacles (mainly the hangar in front of the flight deck). This generates, apart from collision risk, distorted air flow, complicated turbulence field (in addition to natural turbulence), significant amount of spray, causing a reduced view over the flight deck and possibly resulting in engine surging or even flame out. Where stacks and funnels generating exhaust gas are in the near vicinity, additional turbulence may be caused besides a rise of ambient temperature and reduced view over the flight deck. Another factor that affects helicopter performance is hot exhaust gas ingestion. The helicopter may ingest hot gases from the ship propulsion; or it may re-ingest its turbine exhaust due to recirculation or a combination of the two may occur, thereby decreasing the helicopter’s available power.

Volker Bertram of Germanischer Lloyd brings out that Computational Fluid Dynamics can be effectively used to simulate the flow pattern around the superstructure and aft region of a ship to study the effect on helicopter landing. This is an area wherein vast amount of study is being undertaken the world over. Such studies significantly reduce the effort involved in SHOL studies and enhance its accuracy levels.

Summary
Helicopter operations onboard a ship is vastly different from those on land due to complexity in operating parameters and environment. SHOL studies are essential for determining safe limits for these operations. These are highly involved full scale experimentation and therefore costly also. Modern tools such as CFD analysis offer some help in drawing preliminary conclusions, particularly in the early stages of ship design, for enhancing the helicopter operability onboard ships, by minimising adverse vortex formation and hot exhaust gas ingestion by the helicopter.