In July 2010, the City of Rockville, Md., experienced a pipeline failure within its water transmission system that halted water distribution.

The effected 24-in., prestressed concrete cylinder pipe (PCCP) is critical to the City’s water distribution to 46,000 customers. While the City does have interconnections with a neighboring utility, quickly returning the transmission main to service was a high priority. Damaged sections of pipe were removed and replaced with steel pipe before the line was brought back into service.

Within 24 hours of putting the pipeline back into operation, another section of PCCP failed, causing great concern for the City. The City of Rockville’s Department of Public Works (DPW) wanted assurances that additional portions of the line would not fail if the line were, once again, brought back into service. DPW needed to assess the condition and develop a management plan for the transmission main.

The City’s transmission main system is comprised of a single, 10-mile long, 24-in. PCCP that traverses from the Potomac River to the Rockville city limits, before branching off into the distribution network. The PCCP transmission main was built under two separate contracts in 1957 and 1969.

In late July 2010, Pure Technologies was contracted to provide a robotic inspection of the 24-in. transmission
line. Approximately 52,000 ft were inspected in three phases between mid-August and late-October, using closed-circuit television (CCTV), electromagnetic and acoustic inspection technologies. Assessing the condition of a PCCP transmission main is best performed using a combination of non-destructive testing technologies, internal visual inspection, engineering science and experiential judgment.

The primary goal of an inspection is to provide an understanding of the condition of the structural component that provides the pipe’s strength — the prestressing wire. An electromagnetic inspection provides a non-destructive method of evaluating the baseline condition of the prestressing wire and establishes a baseline of the current condition of a pipeline. Electromagnetic inspections ascertain a magnetic signature for each pipe section to identify anomalies that are produced by zones of wire break damage. This equipment is integrated into a robotic crawler system that traverses the pipe.

The robotic crawler was also outfitted with three high-resolution CCTV cameras that are used for both navigation of the equipment, as well as conducting a visual inspection of the pipe with the goal of identifying cracks, spalls, carbonation, and other defects. The cameras are capable of capturing high-quality visual data even under fully surcharged conditions. At DPW’s request, the CCTV technology was also used during the inspection to verify that all gate valves were functioning properly. As the robotic crawler approached a gate valve while traversing the pipeline, DPW maintenance crews exercised each valve to ensure that it was seated properly in both the open and closed position. This approach allowed the DPW to visually determine if valves on the line were working properly and required additional maintenance.

Analysis of the electromagnetic inspection data revealed that several pipes exhibited prestressing wire break damage. Specifically, three pipe sections were identified near the previous failure locations that exhibited electromagnetic signals consistent with wire break damage. DPW took a proactive approach to managing the transmission main by immediately replacing these sections. Pure Technologies later validated electromagnetic results, finding a pipe section with significant corrosion damage to the steel cylinder and prestressing wires in the predicted location.

In order to execute the project on schedule, significant effort on the part of the DPW was necessary. The transmission main was initially built with few access points large enough to insert the robotic equipment into the pipeline. In order to deliver complete inspection of the 10-mile transmission main, several access ports were constructed by DPW.  The newly constructed access ports can be used for subsequent inspections in the future. DPW engineers also successfully managed contractors completing a myriad of tasks.

  • Robotic Crawler Insertion Ports
  • Removal and Replacement of Damaged Pipe Sections
  • Gate Valve Replacement
  • Butterfly Valve Removal
  • Robotic Inspection Crews
  • Surge Tank Construction
In order to conduct a full inspection of all its PCCP assets, DPW underwent a significant coordination effort to inspect the raw water intake line, located between the Potomac River and the Water Filtration Plant. A portion of the raw water intake line is located inside the Chesapeake and Ohio Canal National Historic Park. Coordination was necessary with the National Park Service to obtain permits and to develop an acceptable inspection schedule. Access points for the robotic inspection were located in remote locations within the historic park and required significant planning for DPW, Pure and additional contractors to ensure the inspection ran smoothly.

After the City placed the 24-in. water main back in service, an acoustic technology called SmartBall was also used to ensure there was no leakage that may have been caused by corrosion at the joints between PCCP pipe sections. SmartBall is an acoustic technology that detects acoustic activity associated with leaks or pockets of trapped gas in pressurized pipes. The reported location of detected leaks is typically accurate to +/- 5 ft. Three leaks were detected during the SmartBall survey and field verification revealed that all three were caused by leaking valves at connections to the transmission main.

In total, 3,310 pipe sections were inspected by Pure Technologies, not including special sections such as gate valves and steel pipes. A total of 21 pipe sections (0.6 percent of the total) were identified as having electromagnetic anomalies consistent with wire break damage. The findings of the electromagnetic and acoustic inspections created a baseline for the City of Rockville to begin management of their critical water distribution asset. The significant costs associated with a replacement or rehabilitation program led the DPW to look for methods to determine the condition of their critical pipeline asset.

By using state-of-the-art inspection technology and engineering analysis, DPW was able to manage a small percentage of damaged pipe sections rather than incurring the substantial capital costs of a pipeline replacement. This asset management approach provided DPW with the condition assessment data necessary and the confidence to continue operating and managing this critical pipeline asset for years to come.

Jonathan Watts, EITS, is project manager for Pure Technologies. Ilene Lish, P.E., is a civil engineer with the City of Rockville, Md., Department of Public Works.

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