Advanced Inspections methods of Submerged Underwater Tunnels Using ROV

Tunnels are engineering marvels, which are technically demanding to design, and even challenging to construct and maintain. When such tunnels are underwater or fully submerged in water, their inspections and maintenance become even tougher and often impossible. This leaves the structures to the mercy of elements and they stand by the sheer design and factor of safety of the design. There are many such tunnels often employed in Hydro power dams and various water storage dams, diversion tunnels, aqueducts, etc…. All submerged tunnels are continuously undergoing varying hydrostatic and thermal stresses throughout their lifetime, leading to various structural integrity problems. Until a decade ago, submerged tunnel inspections were rare and these tunnels were often never seen after they were once flooded and put into operation. The integrity of such critical structures is of high importance and their failure can be extremely catastrophic creating extensive financial losses, and even human losses. These structures, though static, need timely inspections to determine their long-term viability and condition and to reduce any mishaps.

Underwater Remotely Operated Vehicles(ROV’s) or Underwater Drones have proven to be an invaluable tool for assessing these kinds of tunnels to collect invaluable data on the structural health conditions and make timely decisions and corrective actions. They can be used to assess the conditions of tunnels to determine structural conditions, rock trap conditions, cave in’s,  sediment build-up, etc… One such ROV, EyeROV TUNA, an Observation Class underwater drone, is a high-performance portable system. Advanced technology and strong experience enable ROVs like TUNA to conduct inspections in dam inter-connection tunnels(Headrace RaceTunnels /TailRace Tunnels), flooded tunnels, etc… without emptying the infrastructure, and with minimal interruption of electricity generation or water supply for which the tunnels function. These interventions prevent the development of stresses that are induced on tunnel linings when they are dewatered.  

Recently TUNA ROV completed a 4km long tunnel inspection in one of the major hydropower plants in central India. The TUNA ROV was equipped with visual and non-contact assessments of the tunnel by using specific payloads like Imaging SONAR, 3D Profiling SONAR, and Laser Scalar. Further, the endurance of the TUNA ROV was increased by five times by extra batteries in order to complete the inspection within the limited shutdown period of the power plant. This was very crucial since the hydropower plant was continuously working and a limited shutdown period was granted for this inspection. The ROV was Equipped with a Fiber Optic Tether which enabled the ROV to work in long tunnels of up to 5 km and more. Each of the entry points or deployment positions into the tunnel was about a 40-50m long shaft. Special arrangements were done at each deployment position for safely deploying the ROV and proper guidance of the tether or cable through the shaft opening to inside the tunnel. The field execution was done in a systematic and accurate manner so that the project was completed within the given shutdown time frame.

ROV-SONAR-based inspections gave a crystal clear picture of the condition of the tunnel. The Imaging Sonar helped to get a general real-time sonar visual in murky water and helped in navigation through the tunnel. Debris such as rock deposits, silt deposits, etc was clearly identified from the Imaging SONAR output. The Profiling SONAR gave the cross-sectional shape of the tunnel throughout the inspection. The tunnel cross sections and 3D model of the tunnel were created using the profiling SONAR data. HD cameras, with high-intensity underwater lighting to light up tunnels in pitch black, captured and saved HD Videos, Further, these videos are live streamed to user monitors at the control station for piloting the ROV as per the Client's interest. The system had high-accuracy navigation and positioning capabilities to geo-reference the place of interest inside the tunnel to sub-meter accuracies. This highly reduced the time to mitigate faults or plan repairs. Further, Laser-based measurement tools (Laser Scalar) was used to estimate the defect sizes. Imaging SONAR, and Profiling SONAR help in the volumetric estimation of silt deposits inside the tunnel.

The tunnel condition i.e. the deformation, type of defects and its size, and any prevailing sediment buildup or obstructions inside the tunnel can be easily assessed. The result of such a survey provides reliable and repeatable means of assessment of the state of the hydraulic structures. 3D model of the tunnel can be generated from the survey enabling owners to be able to understand the general tunnel condition, and deformation like cracks, displacements, separations in the tunnel lining, etc…

The defect mapping and outputs from tunnel inspection enable repair decision-making to happen. Not all defects require immediate attention. These are the areas for later evaluation. In case of severity or the defect gets worsen repair actions need to be done immediately. Timely planned interventions of modern technology like ROV can prolong and make Tunnels safer, and further reduce the financial burdens of unplanned maintenance and production outages.