Fluid dynamics analysis of heliports is the focus of this study, which shows how airflow and gas exhaust affect the safety of offshore landing operations. Based on CFD simulations, the following text reveals important risks and explains why understanding wind behavior around helidecks makes a difference in aircraft planning and operation.

1. Impact of gas exhaustion on offshore landing operations

Offshore oil exploration and production operations require transportation by rotary-wing aircraft. In particular, the most critical phase of operation for these aircraft is landing on offshore vessels. For this type of operation, when the helideck is located at the stern, close to the heated air outlet of the vessel's propulsion engines, lift is reduced.

Furthermore, in the case of FPSO (Floating Production Storage and Offloading) production platforms, the existing structures on the deck (topside) are composed of various modules, such as compressors, pumps, generators, and other equipment necessary for the process. Consequently, the forced air outlets of the equipment near the heliport are sources of turbulence that can impact aircraft operations.

Figure 1: Offshore landing operation of AW189 aircraft.

In this context, landing on platforms involves several steps, such as flying over the platform to identify and visually confirm the destination platform, for subsequent entry at the end of the landing. During this flight path, the pilot adjusts the aircraft to the existing turbulence conditions. However, conditions near the helideck may be different, which requires a specific study, as required by NORMAM-223/DPC.

In view of this, this article presents a study of the wind environment on a helideck using CFD (Computational Fluid Dynamics), considering the gas output from a chimney and its influence on the lift of an Agusta Westland AW189 aircraft. Based on the velocity vector field calculated in the simulations, helicopter pilots have valuable information about the conditions, assisting in the guidance for approach and landing operations.

2. CFD simulation of air flow in offshore helidecks

In this specific example, the exhaust stack for the vessel's engines is located near the helideck. However, although it is not recommended to position gas exhausts near the helideck, this situation is still found on offshore exploration vessels.

In addition to the discharge of gases, another important aspect is the interference of the structures on the ship's deck in the flow that reaches the helideck. The relative wind is subject to the effect of the roughness of these structures, forming vortices that can cause turbulence for helicopters. In this study, the interference of the ship's tower was considered.

For this purpose, the CFD model representing the landing operation, considering the offshore vessel with a helideck at the stern, was constructed to represent the flow field, including the influence of the hull.

Thus, in order to develop a computationally viable model, various degrees of refinement were defined for the finite volume mesh.

Figure 2: CFD model for determining the wind environment on a helideck located on the bow of an offshore vessel.

In general, flow on the helideck is influenced by several conditions, such as the flow rate and temperature of the gases exiting the stack, the relative speed of the vessel, and the height of the helicopter in relation to the offshore vessel. The study should be conducted for a variation in wind angle in order to determine the typical conditions to be faced during landing operations.

For certain conditions of relative wind and gas flow in the chimney, there is a strong influence in the helideck region. The typical appearance of the flow in the helideck region for a relative headwind can be seen in the figure below.

Figure 3: Air density and current lines - CFD simulation to visualize the flow field in the helideck under the influence of engine exhaust - headwind.

The results show a downward air current over the helideck, with the formation of two vortices on the port and starboard sides.

In addition, the presence of the helicopter generating lift on the helideck has a significant effect on airflow at the stern of the vessel. In order to study the influence of the helicopter, a steady-state hovering condition over the helideck was simulated.

Gif 1: Simulation with AW189 helicopter hovering over the helideck.

Figure 4: Simulation with AW189 helicopter hovering over the helideck - note the suction of heated gases from the chimney, resulting in a loss of lift.

As a result, the flow from the exhaust gases is altered by the helicopter's main rotor, which has a suction effect and results in a loss of lift. Another effect is related to the loss of engine efficiency through the intake of heated air with lower density. To avoid or minimize the impact of these effects on landing operations, deflectors can be installed to raise the height of the chimney.

A higher flow rate or exit velocity of the gases in the chimney also has the same effect, as it increases the inertia of the exhaust flow. However, the pressure drop in the exhaust increases under these conditions, which can have an impact on machine operation.

3. Contributions of CFD analysis to safe operations on offshore platforms

By analyzing the flow in the helideck region, pilots are able to visualize typical conditions, allowing for better planning of the final approach for landing and ensuring greater safety in operations.

Therefore, the influence of exhaust gases on aircraft lift is significant and should be avoided by distancing the exhaust ducts from the helideck area.

In terms of operational safety, loss of control or trajectory is responsible for approximately 15% of incidents involving offshore aircraft. Therefore, by understanding the turbulence parameters encountered in each specific operation through CFD studies, new accidents can be avoided.

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FAQ

1. Why does the proximity of exhaust stacks compromise helicopter lift on the helideck?

The propulsion engines and generators on offshore vessels expel extremely high-temperature gases. The heated air has a significantly lower density than cold atmospheric air. When the aircraft enters this mass of rarefied air, the aerodynamic lift generated by the blades decreases. Furthermore, the steady-state hover flight simulation revealed a severe dynamic effect: the helicopter’s main rotor itself generates a suction force that pulls the hot exhaust gases from the exhaust stack toward the aircraft, causing the engines to draw in this low-density air and reducing their mechanical efficiency.

 

2. How do a vessel’s superstructures (topside) affect wind flow?

The relative wind hitting the vessel is affected by the geometric roughness of complex topside modules (such as compressors, generators, pumps, and towers). This physical obstruction disrupts the linear flow of the wind, creating turbulent vortices and downdrafts directly over the stern or bow contact area. Without a three-dimensional study of these obstacles, the pilot may adjust the aircraft for one set of wind conditions during the visual flyover and encounter a completely different and unstablewind shear scenario near the helideck.

 

3. What engineering solutions can be evaluated using CFD to mitigate these aerodynamic risks?

Although the ideal recommendation is to maintain physical distance between the exhaust ducts and the heliport, on platforms already in operation this is often not feasible. As an alternative, simulations are conducted to evaluate the installation of aerodynamic deflectors to redirect the flow or to increase the height of the exhaust stacks. Another variable tested is increasing the velocity or flow rate of the exhaust gases to increase the flow’s inertia (propelling it away from the helideck), although this latter solution requires caution, as it increases the pressure drop in the vessel’s engine exhaust system.

 

4. What is the statistical and regulatory impact of a wind study on offshore safety?

Statistics indicate that loss of control or course deviations account for about 15% of aircraft incidents in the offshore environment. For this reason, the maritime authority requires specific studies of the wind environment through NORMAM-223/DPC. The velocity vector fields and temperature gradients calculated by the CFD simulation provide pilots with predictable visual data on actual conditions at the platform, allowing them to plan safer approach routes depending on the relative wind quadrant.