As an extra safety measure and in collaboration with a third-party testing laboratory, we wanted to ensure that the drone system is not an ignition source during normal operation (no failure modes). This was demonstrated by placing the equipment (drone and ground station) a test chamber where a 4-7 vol% propane atmosphere was introduced (note: the lower explosive concentration (LEL) and upper explosive concentration (UEL) are 2.1 vol% and 9.5 vol%, respectively). The equipment was operated safely, including switching on and off, for an hour without risk of atmospheric ignition.
Testing under such realistic conditions in normal operation allowed us to de-risk the operation prior to the inspection on board Skarv. During the inspection on board Skarv, the drone was able to operate under a hot work permit, in the cleaned and ventilated tank, as the risk of the flight operations had been properly assessed.
There are two key characteristics of our tethered drone system that contributes to safer operation within such environments compared to battery powered drones:
- Since the drone is externally powered, if the system needs to be made inert in an emergency, this can be quickly done by switching off the ground station or input power. Since there is no battery on board, there is no residual risk from the stored energy within the drone system.
- During flight, there can often be a build up of static electricity on the drone, particularly on the propellers as they move through the air. The tether system of the Scout 137 also acts as a ground wire to draw away any static from the drone provide earthing for the system.