Offshore wind farms in the North Sea are being built close to gas and oil platforms. These platforms get visited regularly by helicopters with the goal to pick up staff members or to deliver supplies. Because of the wind parks so called suction/wake effects can occur, which have an influence on the safety of these travels by helicopters. This practice recommends a certain distance for helicopters close to offshore wind farms.
Questions this practice may help answer
- What are the recommended distances for helicopters close to offshore wind energy farms?
- Which factors can be of danger for helicopters working close to offshore wind energy farms?
Wind parks create so called suction effect, which have an influence on the safety of nearby travels by helicopters. At the moment no research has been done on the effects of suction powers of wind turbines on helicopter operations. However, it is known that three phenomena can be of influence. A vortex directly behind the wind turbine, a speed deficit immediately downstream and an increasing turbulence further downstream. Of these, the velocity deficit behind the turbine is the most significant because of the relatively short distance with respect to the wind turbine.
Aspects / Objectives
To find out until which distances from the suctions area of offshore wind farms helicopter operations are negatively influenced.
- The local velocities in the wake of the wind turbine will be determined using a mathematical model.
- To determine the limits, criteria established for fixed wing aircraft and landing with a wind disturbance by a large object nearby, will be used. Consdering these landings, it is recommended to use a maximum allowable (sudden) rate decrease of 6 and 7kt for resp. the lateral and longitudinal components of the velocity difference. These limits is one of the results from previously performed studies (Ref 1).
- In addition, crosswind with 10.5 and 15kts will be used in the testing to investigate trends and bandwidth. These rates will be used to evaluate the influence of a lateral gust of helicopter stability and - controllability.
- A so called lateral gust occurs when the flight path of a helicopter at right angles crosses the wake. This is used as a worst case flight condition.
The approach used in this pracice is to apply the same guidelines for fixed wing aircrafts as for helicopters. However, it is assumed that a helicopter, in varying wind conditions, is more stable then a plane. Therefore this starting point is open for discussion. On the basis of the above-mentioned cross wind gust and criteria, and the computed speeds in the windturbine suctions, saftey distances for helicopters will be determined.
Main Outputs / Results
- A distance of almost 6 times the rotor diameter of the wind turbine has been found in which there occurs a difference in speed of more than 6kts. Therefore, a safe distance of more than six wind turbine rotor diameters is advised for helicopters passing by indivudal wind turbines.
- For the crosswind criteria 10.5 and 15kts no limiting distance has been found. A safety of two wind turbine rotor diameters would be sufficient.
- The recommended six wind turbine rotor diameters should be used with caution with helicopters pass by a series of wind turbines. Based on empirical models, a distance of more than eight times the wind turbine rotor diameter is adviced.
The results of this practice are highly transferable, as the results are not area or site based. Is can therefore be recommended that these results will be spread to helicopter companies all around Europe.
Costs / Funding Source
Dutch Ministry of Infrastructure and the Environment
NLR- Netherlands Aerospace Centre
Anthony Fokkerweg 2
1059 CM Amsterdam
Tel.: +31 88 511 3113