Unmanned aerial vehicles (UAVs) have assumed an increasing number of functional roles within industry but few are as important as inspecting damaged infrastructure.  Yet, most of the attention to date has focused on outdoor inspections of bridges, electric grids, highways and buildings.  Less well known is the emerging role of drones in deep tunnel inspections of train, subway and underwater power stations.

Deep tunnel inspections pose special challenges for drones.  First, GPS signals function unreliably or not at all indoors, limiting remote communications.  Drone operators need another way to maintain contact with their drones underground.  Second, even when consistent communication is established, deep tunnel inspections are unusually time-consuming;  battery-powered drones often need to return again and again to a docking station to be recharged, which greatly limits their operational efficiency. Finally, most drone cameras are geared to long-distance aerial photography, which makes them unsuited for confined underground spaces where full circumference imaging is needed.

Drone developers are beginning to find ways to overcome these challenges.  A team of researchers in Singapore has developed a smart UAV platform, Surveyor with Intelligent Rotating Lens, or SWIRL, that is customized for autonomous operation in tunnels.  SWIRL employs an innovative rotating camera that allows for undistorted imaging of a tunnel’s inner circumference. The size and location of the camera also facilitates weight and power consumption reductions, making more than 35 minutes of autonomous flight possible, the developers claim.  A separate Singapore-based effort, led by Aero-Lion Technologies (ALT), has developed a custom GPS-less navigation algorithm for a deep tunnel UAV that uses depth cameras instead of pure range sensors or image sensors to capture images inside the country’s DTSS sewage system.  Manholes located throughout the sewage system can provide portals for inserting the drones and periodically extracting them for recharging.  ALT’ drones are fully unmanned and programmed to avoid flowing sewage water.

By contrast, some deep tunnel UAVs are being designed for the express purpose of operating underwater.  Genesis Energy is deploying submersible (or so-called “hydro”) drones to perform once-in-a-decade checks of its tunnels below power stations in Tongariro, Waikaremoana and Tekapo, New Zealand.  One tunnel is 20 kilometers long, the longest in the southern hemisphere.  It used to take a team of human surveyors 13 days to conduct the operation, which first required emptying water from the tunnel to ensure their safety.  A specially designed UAV can do the job in less than half the time, without exposing workers to hazards, including toxic gasses.

Some US cities like Boston are also testing UAVs for use in their subway tunnels.  The drones won’t replace their traditional human inspectors, who are protected with union contracts.  Instead, the drones conduct a preliminary survey of tunnel conditions to make sure that they’re safe enough for more in-depth follow up inspections.