Inspection of a Water Tunnel by Remote Technology and Manned Entry.

The Great Hill Tunnel (GHT) is owned and operated by the South Central Connecticut Regional Water Authority (RWA). It is a 15,800-cubic-meters/day (100 million gallons per day [MGD]), 1,100m long (3,600 ft long), 1.8 m (6 ft) horseshoe-shaped raw water transmission tunnel that cannot be taken out of service and dewatered without installation of a bypass system. After completion of the tunnel in 1927, the first documented inspection was by divers in 1973. Because of safety failures during the 1973 inspection and the lack of alternative technologies to perform this inspection remotely, no additional inspections were performed until 2011. Since 2011, the tunnel has been inspected by remotely operated vehicle (ROV) three times, most recently in fall 2017. During the 2017 inspection, routine leakage from the tunnel was noted to have increased dramatically. The owner became concerned that known defects observed during the ROV inspections--including a large defect at the transition from the tunnel to a 1,200 mm (48 in.) diameter cast iron pipe--were not only getting larger, but also that the increased leakage was a sign of impending failure of the system. Therefore, an emergency manned inspection and repair program was planned based on the information from the three ROV inspections. This manned inspection and rehabilitation work was completed in spring 2018. Regular inspection of water infrastructure, especially manned, is often challenging because of lack of redundancy, limited shutdown windows, or aging infrastructure that could be compromised by dewatering. ROV inspections have limitations, but with ever changing and improving technological capabilities can be called upon in many situations where they previously were not ideal solutions. This paper discusses some of the logistical and operational limitations of ROVs, including umbilical restrictions, visibility while underway and location tracking, and also provides a comparison between the observations from the ROV and manned inspection and repair work that was performed. [ABSTRACT FROM AUTHOR]