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April 2021

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Luma Bendini

Evaluating helideck safety for air ambulance: a wind analysis case

Here’s a real case of data driven design: in 2019, Ingrid Cloud was commissioned to run a Wind Study analysis to validate helicopter safety on an initial architectural proposal. Based on the CFD simulation and local weather data analysis, our team of engineers made important considerations and the proposed designed had to undergo changes in order to fulfil helicopter safety requirements and grant an efficient and safe operation of air ambulances in most wind conditions.


source-wikipedia

"Currently the hospital uses a ground facility for take-off and landing of helicopter ambulance." 

The project, currently under construction, consists of a completely new hospital building with 14 stories and 33.900 square meters. The new design includes a helideck on top of the new hospital building for air medical services, improving significantly the efficiency of aeromedical operations, that currently use a ground facility in the vicinities of the hospital for take-off and landing.

Site description

Danderyd’s Hospital is located in the municipality of Danderyd, North of Stockholm, Sweden.

site-description

site-description-model
Google Maps view from current site (above). Simulation model of the helideck (below, left) and the hospital (below right).

The analysis – Turbulence Criteria of helideck design

The aerodynamics in the vicinity of the planed helideck is analyzed by measuring the standard deviation (SD) of the vertical component of the airflow velocity. SD should not exceed a 1.75 m/s in order to secure workload on helicopter [1].

The weather data is from the Swedish Meteorological and Hydrological Institute (SMHI) for Bromma airport [2], located 6 kilometers away from the area of analysis. The simulations are run for 8 different wind directions (N, NE, E, SE, S, SW, W, NW) and the SD is measured on 6 different heights above the helideck.

evaluation-height

The evaluation area not only covers the helideck but the entire proposed building, to provide a consistent overall analysis.

The area where the turbulence criteria is fulfilled is marked "blue", while "red" means otherwise. evaluation-area

A turbulent, logarithmic wind velocity profile with roughness category for forest/suburbs/villages is applied [3, 4], with a reference velocity (Uref) both for highest wind speed of the yearly distribution and worst-case scenario, 25 m/s according to Eurocode [1]. Reference height is 10m.

Direction Uref yearly distribution Uref worst case
N 8.5 m/s 25 m/s
NE 8.5 m/s 25 m/s
E 8.5 m/s 25 m/s
SE 10.5 m/s 25 m/s
S 10.5 m/s 25 m/s
SW 10.5 m/s 25 m/s
W 10.5 m/s 25 m/s
NW 10.5 m/s 25 m/s

Results – CFD simulations' vizualization and analysis 

The following shows different visualizations for the 2 dominant wind directions, considering yearly distributions, West and South.

South

Standard Deviation (Uref = 10.5m/s)

sd-south-10_5V2

Standard Deviation (Uref = 25m/s)

south-sd-25

Normalized velocity

normalized-south

Streamlines

south-streamline

West

Standard Deviation (Uref = 10.5m/s)

west-sd-10

Standard Deviation (Uref = 25m/s)

west-sd-25

Normalized velocity

normalized-west

Streamlines

west-streamline

Conclusion – A helideck location change is suggested

The analysis showed that for both, the highest wind speed of the yearly distribution and worst-case scenario, there were regions that exceeded the threshold of 1.75 m/s for the standard deviation. 

 

The proximity of the elevator machine room at the top of the building causes turbulent wakes where the velocity strongly fluctuates. The helideck needs to be far enough away from any structure upstream that can cause any disturbance [1].

 

Given the results, it was therefore not recommended to build the helideck at the planed location. However, the western part of the roof top seems to be far enough from the elevator machine room so that the turbulence criteria is better fulfilled. This location could be considered as an alternative.

 

credit-carlstedt-arkitekter-CF-Møller

The final project for Danderyd Hospital including the helideck in a safe location. The new hospital building will be delivered in 2025. Different than the above illustration, the helideck delivered will be in a square format. Illustration: Carlstedt arkitekter/CF Møller


References

[1] CAA Paper 2008/03 Helideck Design Considerations – Environmental Effects
[2] Översiktlig vindstudie Planprogram Bergs Gård Magnus Asp, SMHI 2017-01-16
[3] EN 1991-1-4
[4] Huang, S.H. et all, A general inflow turbulence generator for large eddy simulation, Journal of Wind Engineering and Industrial Aerodynamics