Like most cities across the U.S., the City of Tulsa is dealing with the maintenance and operation of an aging sanitary sewer infrastructure. One facet of the aging infrastructure of significant concern is the City’s vast array of large diameter concrete sanitary sewer interceptors. The City was quickly finding it difficult to determine the remaining lifespan of a concrete interceptor from traditional close-circuit television (CCTV) methods alone. This is when they reached out to RJN Group, Inc. for assistance with advanced pipe inspection technologies and RJN’s knowledge of utilizing these technologies to determine the Remaining Useful Life (RUL) of each section of concrete interceptor.
In 2013, the City of Tulsa completed a pilot project with RJN for the condition assessment of a 60-inch and 66-inch diameter concrete sanitary sewer interceptor traversing the City’s Mohawk Park where it outfalls into the Northside Lift Station. RJN worked with a sub-consultant to complete the inspection utilizing a Multi Sensor Inspection (MSI) Platform System. The system includes the use of High-Definition Closed-Circuit Television (HDCCTV), Light Detection and Ranging (LiDAR), and Sound Navigation and Ranging (Sonar).
- HDCCTV provides a much higher quality image making it easier for the inspector to determine the extent of pipe defects.
- The LiDAR scanner develops a point cloud of the inside of the pipe above the water surface to determine pipe ovality and wall loss.
- The Sonar sensor gathers information of what is happening below the water surface including determination of pipe ovality, wall loss, and sediment volume and depth.
HDCCTV/Sonar/LiDAR Typical Inspection Output
Together, the entire system provides a clean and clear video with a 360-degree pipe profile. The inspector inputs the original inside pipe diameter into whichever pipe inspection software is utilized to determine the extent of pipe ovality and wall loss. The entire system is mounted to a float or tracked and transported. It can be used in pipe sizes of 18 inches in diameter or larger.
Successful Pilot Program
The pipe selected for inspection during the pilot project was selected due its age of construction and criticality to the overall sanitary sewer network. It was constructed in 1974 and serves a majority of the sewer flow from the north side of Tulsa. 12,025 linear feet of 66-inch diameter reinforced concrete pipe (RCP) and 10,943 linear feet of 60-inch diameter RCP were inspected for the project.
Based on the inspection and RJN’s experienced team, a remaining useful life (RUL) score was given for each pipe segment between two manholes. This is the expected time frame before a complete deterioration and potential failure of the pipe. Only one segment, 819 linear feet of 60-inch diameter pipe, was rated with an RUL score of 5, meaning that the expected remaining useful life of the pipe is less than five years. This came as a surprise since the City of Tulsa expected the pipe to have more significant deterioration. RJN recommended the installation of a cured-in-place pipe (CIPP) liner in this pipe segment immediately to extend the life of the pipe.
Based on the success of the pilot program, RJN was selected to complete condition assessment of an additional five sanitary sewer concrete interceptors throughout the City of Tulsa. During the follow-up study, RJN discovered a concrete interceptor advanced deterioration that warranted a majority of the pipe segment to be rated with an RUL score of 4, and five segments out of 32 were rated with an RUL score of 5.
PROJECT AT A GLANCE TO DATE
Condition assessment of:
The interesting thing is that the pipe was installed in 1998, so one would think the pipe would be in better condition than pipes that have been in operation for longer than 50 years. However, due to the low flow conditions observed during inspection (majority of the pipe segments had flow less than ¼ pipe) and location directly downstream of a force main outfall, it is very likely that this particular interceptor experienced faster rates of corrosion due to high H2S gases. More than 50% of pipe wall loss was determined from the LiDAR scanner on several sections of pipe. After confirmation of projected flows for future build-out, it was determined that the entire interceptor had enough capacity for rehabilitation by slip-lining. The installation of slip-line pipe, which will be installed over the course of three construction projects, will extend the life of the pipe for many years to come.
When a municipality is deciding on how to tackle their network of concrete sanitary sewer interceptors, utilizing the Multi Sensor Inspection Platform System is an efficient way to assess and prioritize rehabilitation of existing concrete interceptors. Keep in mind, it is not always the oldest pipe in the system that needs rehabilitation first. Many factors, such as low flow conditions, inline siphons, and force main outfalls, can cause highly corrosive H2S gas environments that lead to faster deterioration of concrete pipes.