December 2017 Vol. 25, No. 4

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Drones offer advantages for bridge inspection

Drone flying by bridge

Photo: MnDOT

The use of drones—also known as unmanned aircraft systems (UAS)—has been shown to improve the quality of bridge inspections, reduce costs, and increase safety. The Minnesota Department of Transportation (MnDOT) and the Federal Highway Administration (FHWA) are both exploring the wider use of drones for bridge inspection. The third phase of a MnDOT research project into the use of drones for bridge inspection is now under way. This phase focuses on how the devices work in confined spaces.

Why is MnDOT studying drones?

MnDOT and local bridge owners have 600 bridge inspectors who monitor more than 20,000 bridges in Minnesota. Each bridge must be inspected once every 24 months. Bridges in poor condition and those considered fracture-critical (where failure of a single component could cause collapse) must be inspected every 12 months.

Large bridges can take weeks to fully inspect and often require inspectors to dangle from ropes or stand in buckets on the end of “snoopers,” cranes that reach from the bridge deck to below-deck level to put inspectors within sight of under-deck elements. Snoopers are expensive and require traffic lane closures, presenting safety risks to the traveling public and inspectors.

Phase 1 completed in 2015

Drone flying above

Photo: MnDOT

MnDOT completed a small research project in 2015 to study the effectiveness of UAS technology applied to bridge safety inspections. The project team inspected four bridges at various locations throughout Minnesota. Results indicated that drones significantly augment inspection findings while reducing safety risks and inconvenience to inspectors and the public. The project earned a 2016 Minnesota State Government Innovation Award as well as awards and recognition from such groups as the American Public Works Association.

Drones designed specifically for structure inspections were unavailable during Phase I. The UAS used in that phase also had key operational limitations, including the inability to proceed when concrete and steel bridge components blocked Global Positioning System (GPS) signals. When that happened, the drone simply returned to base automatically.

Phase 2 examined more challenging bridges

In the second phase, MnDOT tested the use of an upgraded drone to examine larger and more challenging bridges. The new drone, which was specially designed for structure inspections, featured more robust imaging and infrared data-gathering capabilities, and was more flexible to control. Its operational capabilities also were not diminished by the loss of GPS signals.

Investigators selected a prototype senseFly albris UAS to inspect four bridges. One was the Blatnik Bridge over the St. Louis River between Duluth, Minnesota, and Superior, Wisconsin, a 7,980-footlong steel through-arch bridge with steel deck trusses. Researchers also investigated UAS use for infrared deck surveys.

The senseFly albris UAS offered a clear operational upgrade over the Phase I unit. It can operate without GPS; the camera lens can turn up and down at 90-degree angles; and protective shrouds and ultrasonic sensors prevent the propellers from striking bridge elements.

The drone worked well in the high, confined spaces of the Blatnik Bridge. It identifies and measures clearances, rope access anchor points, and other pre-inspection conditions for planning largescale or emergency inspections. Photogrammetry software can be used with the UAS to develop three-dimensional models of bridges and bridge sites. Using infrared thermal sensors, the UAS can detect delamination of concrete while flying adjacent to lanes of traffic.

For smaller, confined spaces on bridges and culverts, the senseFly albris may not be ideal, according to the study. Despite its protective shrouds, it is not as collision-tolerant as needed for very tight spaces.

Costs were significantly lower with UAS inspections. Conventional inspection of the bridge would have required four snoopers, an 80-foot lift, and eight days of inspection, at a cost of about $59,000 (without the cost of mobilizing equipment and traveling). The UAS Blatnik Bridge inspection would contract as a five-day, $20,000 project.

Phase 3 to inspect confined spaces

Phase 3 will allow MnDOT to use a new drone specific to confined-space inspections. “This new drone is meant to reach places the prior drones could not, which will supplement our efforts nicely,” says Jennifer Wells, MnDOT maintenance bridge engineer. “Also, Phase 3 will include more bridge inspections in order to get a more comprehensive feel for cost and time savings.”

Phase 3, which began in the summer of 2017, uses the senseFly albris and the Flyability Elios, a collision-tolerant drone more suited to confined spaces such as box girders or culverts. During this phase, researchers will identify which situations are best suited for drone use, what parameters should govern drone use in bridge inspections, and how UAS can be integrated into standard inspection operations at a county and district level. The Phase 3 project is scheduled to be complete by July 2018.

Based on the third phase of the research, MnDOT’s goal is to implement a statewide UAS bridge inspection plan, which will identify overall cost-effectiveness, improvements in quality and safety, and future funding sources for both state and local bridges.

(Adapted from the Crossroads blog and MnDOT Technical Summary 2017-18TS.)

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