Locating and Repairing Critical Leaks after Freezing Weather in Houston
Shortly after the freezing weather in Houston in early 2018, leaks were identified on 54-, 60-, and 66-in. steel transmission lines. As these lines served thousands of customers, the City of Houston tasked Lockwood, Andrews and Newnam Inc. (LAN), a planning, engineering and program management firm, to identify and fix these leaks through an existing On-Call Large Diameter Water Line Rehabilitation Contract (On-Call)
After a catastrophic failure in 2002, the first On-Call contract was established and bid in 2003 to allow the City to have a pre-existing low-bid contract in place to address emergency situations and other miscellaneous tasks. Through On-Call contracts, more than 270 requests from the City Drinking Water Operations (DWO) have been addressed.
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Following the early 2018 leaks, LAN began preparing work orders to evaluate and repair them. It was suspected that the colder surface water temperatures caused higher thermal stresses leading to welded joint failures, but the exact location of each leak was not available. Due to the critical nature of the transmission mains, the staff from Houston Water, Capital Projects and LAN began coordinating a shutdown of these lines immediately and determined a priority of the leak repairs.
While this coordination took place, LAN evaluated options for identifying the leaks and performing repairs with the least impact to customers. Due to the population it serves, the 66-in. water line leak was identified as the highest priority. The water line is tunneled underneath a toll road and an energy pipeline corridor. An exploratory excavation was performed at the start of the south end of the tunnel casing to determine if the leak was within the tunnel. Excavation was difficult due to the leak, but the work identified that the leak was within the casing. Due to the critical nature of the line, LAN recommended leak detection technology to identify the exact location of the leak and prepared a repair plan.
Leak detection was performed by inserting a tethered probe into the pipeline that travels downstream recording data before being pulled back out through the insertion point. The probe consists of an acoustic sensor that detects leaks and air pockets, as well as a CCTV camera, which collects video of the internal condition of the pipeline. With this technology, once the leak is found, an operator marks the location on the ground surface and takes GPS coordinates of the leak. The system located the leak approximately 20 ft into the tunnel, underneath the pipeline corridor on the south side of a major freeway. Modeling of the required water line shutdown was performed, and the On-Call contractor prepared to perform the internal repair. An internal repair was recommended to prevent costly, slow, and difficult excavations and avoid damage to private pipelines within the corridor.
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As the mobilization of a specialty leak detection sub-contractor was costly, LAN recommended to the City that the sub-contractor perform leak detection on the 54- and 60-in. water line leaks while the leak detection sub-contractor was in town. The City agreed and acted quickly to provide the leak detection sub-contractor the authorization to mobilize to the other leak sites. The entire process for coordinating, authorizing, and performing leak detection took less than 14 days. All three leaks were identified, and internal repairs were recommended for the 54- and 66-in. leaks that were located near busy roadways to limit impact due to excavations. Due to the remote location of the third 60-in. leak, an external repair was recommended, but because of leak detection, the excavations were precise and the work did not require as large of a footprint.
The 66-in. water line was the first to be repaired. Coordination and modeling efforts determined that a 12-day shut-down would be feasible. Initial plans were to shut-down the line for a 24-hour period to confirm model results and proceed after an all clear was given. On Friday, March 2, City valve crews shut-down the line and the 24-hour waiting period commenced. After an initial four to six hours, officials at George Bush International Airport (IAH) requested the City waive waiting period and proceed with repairs due to no impact to water systems at their facilities. The contractor quickly proceeded with depressurizing and dewatering the line and by Friday night, after already passing the point of no return, IAH began experiencing lower pressures at their facilities.
By Saturday afternoon, City and LAN staff entered the pipe to assess damage and determine the exact method of repair. The leak was found approximately 20 ft into the tunnel where a weld had fractured leaving a 5- to 10-mm crack around the top two-thirds of the pipe. Because the soil was completely saturated, water was re-entering the pipe through the leak making a welded repair difficult. Different repair methods were evaluated for the conditions and an internal welded repair was started with intent to install an internal joint seal if a welded repair could not be completed.
Temporary measures were made to slow the water by using wood shims. After working throughout the night, a majority of the fractured joint was welded leaving a few feet of welding remaining to stop the leak. While system operating pressures continued to drop downstream of the shutdown, completing the internal work was critical to restore the water line to service quickly. As LAN was aware that this line was selected for a condition assessment and rehabilitation project within the next two to three years, direction was given to the welder to continue welding until the leak was stopped or City staff determined the water line had to return to service. By 5 p.m. Sunday evening, the welder had completely stopped the leak and the line was returned to the City to be put back into service.
The 54-in. water line was repaired next. Leak detection had identified two leak locations at the first joint north of a manhole riser. The line was isolated, and an internal welded patch was installed at the leak location. During the shutdown, the area was visually inspected, and cracking was visible in the lining appearing around the edge of where the manhole was placed. It appeared the pipe was damaged during construction when the manhole was placed, and despite internal repairs to the mortar lining, corrosion was present. Shrinking from thermal forces likely caused the damaged portion to separate causing the leak. The cracking and buckling in the mortar lining left several feet of cracked concrete for escaping water to enter and give the illusion of multiple leaks in the pipe.
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The third and final 60-in. water line leak was repaired after a leaking joint was excavated and rewelded. Because the leak was in an area with little impact to the public and minimum pavement restoration was needed, an exterior repair was recommended. An exterior repair method prevented isolating the critical line for an extended duration.
By being prepared with a team and a pre-existing low-bid contract in place, the City quickly addressed the rush of leaks. Flexibility of the contract and leak detection support accurately identified the leaks and allowed the team to minimize impact to customers and efficiently make repairs. With the rush of leaks addressed, work returned to normal and the team prepared for future emergencies.
