Public Utilities Board (PUB) maintains the sewerage reticulation system in Singapore with about 3,500km of public sewers and more than 90,000 sewer manholes. PUB contacts systematic checks and carry out repairs on defective sewers to enhance the structural integrity of the sewerage system and to prevent the pollution to waterways.
Environmental conditions in and outside sewers can lead to deterioration of the pipe such as liner delamination, dislocation of joints and even collapsed sections over an extended period. In order to prolong the service life of sewers and to protect surrounding environment, inspecting the structural integrity of sewers is essential. Pipelines maintenance robots are available for smaller diameter sewers. Capabilities for larger sewer of diameter greater than 3m are less well developed. While man entry for inspection of sewer may be an option, extended exposure of humans in these sewers are undesirable due to the inherent risk of working in a hazardous environment. Therefore, robotics promises a more attractive option.
Challenges in deployment of robotics in larger diameter sewers are different from those of smaller diameters. Due to the larger size and varying flow conditions, the vehicle may need to overcome buoyancy effects in partially flooded condition to traction over soft debris in low flow condition. The vehicle should also have lights to provide sufficient illumination to enable the imaging system can produce high fidelity images suitable under real-time inspection condition. These requirements can be very energy intensive. Moreover, entry access into larger diameter sewers is confined to the size of a standard manhole opening which restricts the size of the vehicle.
The robotic system is developed to inspect partially-flooded sewer of diameters greater than 3 and up to 5m, and for traverse length up to 400m. It can be deployed through a standard manhole opening of 60cm x 60cm, and is able to lower pass intermediate platforms in deep manholes to reach the sewers.
Large robot footprint provides stability and the ability to travel over sediment deposited on the bottom of the tunnel. However, the manhole of the sewer may impose constraints on the size of the robot in order to pass through the opening. In addition, the robot must also need to negotiate the available space between obstruction such as intermediate platform and walls of the manhole. To manage both requirements, the wheel frames of the robot are foldable resulting in a compact profile and reducing its envelope during deployment.
Once deployed to the bottom of the manhole, the wheel frames extend to increase its footprint for better stability. The wheel frames are extended to a width that lands the drive wheels above the waterline, at the designed level of fill. This provides better traction and avoids sediment that may be deposited at the floor of the tunnel. The robot is driven by four 200mm diameter hub motors, which provides sufficient torque without the need for additional speed reducer and/or transmission components.
Current implemented payload includes a flammable gas sensor, a laser profiler and a semi-circular imaging array with integrated illumination consisting of three HD cameras and four focused LED light sources. The inspection array is capable of capturing a wide angle view of the sewer's interior surface above the water line and designed such that the coverage of each camera and LED light source overlaps to provide seamless illuminated images. Additional post-processing can be performed to stitch the image into a panoramic HD image of the tunnel surface which can be further processed and flattened into a full map of the tunnel internal surface for reporting purposes. Other payloads include: a front pan-tilt camera and rear camera for navigation purposes, power converters, drive controls and a semi-buoyant umbilical cable carrying both power and 1 GB/s Ethernet communications link.
The umbilical cable and power transfer design are optimized to overcome effects of copper losses over long distances (400m and more). Whilst a battery operated system may appear advantageous at first glance, reliable and latency-free real time updates and control typically still necessitates a communication tether as wireless communications can be challenging in such environments especially with non-direct line-of-sight conditions and multi-path effects. Power transfer over the umbilical cable also offers significant advantage over the limited available mission time of battery powered systems.
The cable itself also serves as an important contingency for manual extraction in the event of robot failure without the need for a manned entry (which may not always be possible).
With the challenges to land use in developing countries and exponential growth of urban population, deep sewerage system is the way ahead. Deep sewerage systems come about with its own set of challenges to perform inspections and repair which this technology targets to address. This technology could be applied not only in the inspection of larger diameter sewers and conduits; the development of the mobile robotic platform forms the foundation for other maintenance capabilities such as desilting and localized repair. An increase in interest amongst the engineering and utility services community is envisaged.
There is no existing “commercially ready” prototype system to perform the inspection of the larger diameter sewers. Thus our robotic system supports the fast growth in pipeline assets not only in Singapore but also in foreign countries.