Due to its high costs and disruptive nature, emergency pipe rehabilitation is a measure of last resort. In 1992, a 40-ft section of Almeda Road in Houston failed due to poor soils and leaking construction joints, requiring emergency pipe rehabilitation. Shortly thereafter, engineers at the City of Houston Department of Public Works and Engineering prepared a contract to inspect 13 miles of large diameter wastewater infrastructure. Engineers focused on the Northside Sanitary Relief Tunnel (NSRT) section, which carries wastewater from the north through downtown to the largest plant in the city. At that time, only a manual option was available for large diameter pipe inspection.
Wastewater infrastructure is a city’s largest asset and also poses the greatest catastrophic risk to its citizens. Due to the infrastructure’s depth and high flows, manual inspections of large diameter sewer pipes are costly and hazardous. Nevertheless, the City of Houston understood the greater public risk in not doing manual inspections.
Teams of two walked through the pipe using flashlights, a video camera and simple hand tools to check the integrity of the pipe walls. Those teams recorded pipe conditions while others above ground monitored their safety. Flows had to be greatly restricted to allow the technicians to make their way through and expose the maximum amount of the pipe wall. In some pipes, flow could not be re-routed and inspections were not completed. Large fans were used to exhaust hazardous gases.
“It was a painstaking and expensive process,” said Ravi Kaleyatodi, P.E., C.P.M., City of Houston, “but it was the only technology available to us at the time.”
In 2007, an old 78-in. pipe northeast of downtown Houston had rapidly corroded due to excessive H2S gas. The corrosion completely and silently devoured the pipe crown, allowing the 13 ft of cover to slowly fall into the pipe and get washed away, causing the road above to eventually fall in. The City of Houston was once again reminded why it is imperative to be more proactive in taking care of its sewer system.
“We didn’t think that corrosion could proceed so rapidly between visual inspections,” said Kaleyatodi. “As a result, we accelerated our study of wastewater pipe corrosion and concluded that we needed data enabling an early warning of future catastrophic problems.”
Compliance EnviroSystems LLC (CES) — an expert in large diameter pipe inspection — has been studying this same problem. “By the time you have corrosion bad enough to be seen with CCTV, you might as well make plans to reline the pipe,” says CES vice president of marketing Brad Dutruch. “We want to help our clients avoid costly rehabs by providing information that allows them to save money with cheaper preventative maintenance, such as pipe cleaning. It became clear that we needed to synchronize data from multiple sensors to be able to accurately predict problems before they occur.”
After extensive evaluation, CES chose to partner with RedZone Robotics, Pittsburgh, Pa., whose Responder robot can simultaneously collect data from six sensors: sonar, laser, H2S gas, temperature, XYZ position and CCTV. RedZone engineers also analyze the data to reveal problems such as corrosion-causing sediment and even provide cleaning recommendations.
Houston engineers saw the value in RedZone’s multi-sensor inspection technology and planned the inspection of the NSRT for summer 2008. RedZone, CES and the City of Houston worked cooperatively to ensure that the inspection proceeded smoothly. Weighing 600 lbs and propelled by a 3.5-hp hydraulic engine, the Responder is able to make it through a variety of debris and flow conditions. However, large diameter pipe inspections often encounter problems in the field such as excessive sediment, debris or high flow conditions. In some cases, the Responder needed some assistance and was helped along with a “tag line” (cable) and winch.
As the inspection procedure moved downstream, and into successively larger lines, flow levels became higher, eventually reaching near half-pipe level in 12-ft diameter pipe. Needing to stay above the flow line, the heavy CCTV camera, lighting and laser equipment had to be supported 6 ft above the Responder base.
“We have integrated a sophisticated inertial measurement unit (IMU) into our robots that makes very precise positional and directional adjustments to the data to compensate for the robot’s changes in tilt, roll and yaw,” explained RedZone chief technology officer Dr. Scott Thayer. “So the data from the Responder platform will be accurate as it crawls over debris, regardless of how much it oscillates. At high flow conditions in very large pipes, a floating device is simply easier to traverse through the pipe.”
The RedZone floating device is also considered a “robot,” housing all of the same sensors as the track-based Responder and generating the same data. However, because of the turbulent ride, the floating device generates slightly less accurate data than the Responder. Thus, it is generally used when in pipe greater than 10 ft in diameter with high flow or pipe with heavy debris or obstacles. In either circumstance, switching from one robotic platform to the other is straightforward.
Two unique problems arose during the project. First, Hurricane Ike struck in September and inspection had to be suspended. Afterward, using sonar and positioning sensors, RedZone was able to locate a 6-ft by 8-ft flap gate that had broken loose from the Clinton Drive lift station. A team of divers was able to secure the gate for extraction at a later time. Second, an odor problem in a section of pipe not originally on the inspection project had been getting worse over time. The multi-sensor inspection results show an unusual pattern of flow disturbance, debris and H2S concentration. Initial speculation indicates the liner may have buckled or there is a strange obstacle in the pipe. At the time of this article, a definitive answer had not been reached.
One of the key advantages of the multi-sensor inspection will not be realized for a number of years. Together, the raw and results data establish a baseline for comparison with future inspections. For example, municipalities can see trends in corrosion or sediment at specific pipe locations or how one pipe section can affect another over time. With this process, small changes in pipe ovality can be more easily detected.
Initial results from the inspection show corrosion in several pipe sections. Fortunately, no emergency rehab is necessary, but preventative maintenance plans are being developed. “We have better knowledge of what’s going on beneath our streets,” said Kaleyatodi. “This reduces the safety risk to our citizens and allows us to make better cleaning and rehab decisions.” Innovative, proactive technology is now being deployed to increase the overall efficiency and benefit of this public good.
Jason Iken, P.E., is a managing engineer with the City of Houston. Wayne Brown is a large pipe specialist with Compliance EnviroSystems LLC.