The Emscher sewer system will have a length of approximately 51 km with pipe diameters of DN 1,400 to DN 2,800 and will be constructed at a depth of 12 to 40 m. The maximum distance between manholes of the free gradient sewer is 600 m.
The sewer material is reinforced concrete. Even in dry weather, large quantities of water and sewage will be transported in the sewer constantly. At present, optical inspection (TV inspection or walk-through inspection) is usually employed to inspect structural condition. Conventional inspection methods cannot be used to inspect the Emscher sewer system because of its constant partial filling. The automatic inspection and cleaning systems being designed as part of the project will effectively do away with walk-through sewer inspections.
With Emschergenossenschaft, Fraunhofer IFF has developed and built a prototype of the rough inspection system as well as a test prototype of the cleaning system and a test prototype of the inspection system to test them in a comparable, already existing sewer system (with a diameter of 2,300 mm).
Inspection strategy
The strategy for automatically inspecting and cleaning the Emscher sewer system is a three-stage approach. In the first stage, a small swimming system called the damage detection system (SEK) is employed in the sewer for rough inspection. The SEK detects corrosion, obstructions, deposits and cracks. It inspects and measures the entire sewer line and conducts camera inspections, detecting significant abnormalities such as erosion and leaks in the gas space. At the same time, it uses ultrasound scanning sensors to check the water space for deposits and obstructions. The motion allows a rough inspection. The SEK is deployed in the sewer through a manhole and moves cable-guided with the current over the distance between manholes.
Where necessary, in the second stage, a wheel-driven cleaning system (RS) eliminates sediments and cleans the sewer wall before the inspection system is deployed.
In the third stage, the damage measuring system (SVM) inspects the sewer completely, measuring the sewer (joint widths, pipe offsets, cracks) with higher accuracy than the SEK. It also detects infiltration in the water space. Since the inspection system measures only when it is at a standstill, the measurement data is more exact.
Concepts for all three systems were designed and developed for their control, operation, insertion into and extraction from the sewer line, as well as for their energy and water supply and data transfer.
Positioning and pipe axis measurement
In the case of the SVM a sensor system was conceived which uses 15 laser ranging sensors (five aligned vertically and ten horizontally) to constantly record position. The sensor distance data is transformed into the sensor coordinate system.
The system’s position along the sewer line axis is determined by measuring the lengths of uncoiled cable. In addition, the current position is referenced at all joints with the camera system. Every single pipe can be approached this way.
Types of damage and damage detection
Since Emschergenossenschaft is looking to achieve sewer lifecycles of up to 120 years, one focus of the project was to find and/or develop sensor systems which have the required accuracy of measurement under difficult conditions in the sewer. Minimum requirements for sewer inspection in Germany are stipulated in self-monitoring regulations issued by the states. Legal requirements, technical specifications and negotiations with local authorities have produced the demanding inspection tasks displayed in Table 1.
Chemical corrosion
Optical measurement methods detect surface corrosion of the concrete in the gas space and represent possible developments of damage. The option of representing the concrete wall comparatively with previous inspections is important in order to represent any possible development of damage. Several cameras are used to image the sewer wall completely.
Using an image processing algorithm, the appearance of individual structural elements of the surface is inspected for abnormalities. When a variable limit value is exceeded, surface corrosion may be likely.
Obstructions, sediments, incrustations, mechanical corrosion
Newly developed ultrasound scanners with a measurement accuracy of ± 2 mm are being used to detect obstructions, deposits and mechanical erosion in the water space.
Crack detection in concrete pipe
Digital image processing systems are used to detect cracks in gas space. In accordance with the requirements, cracks with a width of 0.5 mm and upward have been positively identified and logged. Since crack widths vary frequently, it is not easy to give one certain value to the crack width. Because of this, comparison with previous inspections gives information about the crack behaviour. Automatic crack detection is an important analysis module, particularly when there are small cracks, which an operator could overlook on the monitor. This automatic system therefore constitutes a considerable advantage and increases the quality of inspection results.
Figure 1 illustrates how different analysis modules identified a crack. The automatic analysis module is able to terminate the processing of the current image and inform the user as soon as any crack segment has been found.
In addition, new acoustic methods were developed to detect cracks in the concrete both in the gas and the water spaces. These acoustic systems are able to provide information on crack depth and spalling.
Deviation of position and joint gaps
Optical sensors, aligned laterally or on the apex of the sewer, are used to detect and log the horizontal and vertical deviations of position. In the gas space, cameras measure the joint gap. Differences in joint width compared with earlier inspections indicate an axial displacement, inconstant joint width along the pipe circumference a deformation. Automatic measurement requires exact identification of the joint edges.
Summary and outlook
Since 2002, Emschergenossenschaft has developed a comprehensive concept for inspection and cleaning systems for the Emscher sewer system. Subsystems have been determined and designed in detail and subjected to all tests necessary for reliably assessing their feasibility. Their feasibility was fully demonstrated. Different sensor systems have been developed and tested successfully. All damage types can be recognised with high accuracy.
Three systems for the favoured inspection and cleaning concept were designed. This involved a damage detection system for rough inspection of the sewer as well as cleaning systems and damage measuring systems. A prototype of the SEK as well as test prototypes of the cleaning system and the inspection system have been built and tested in order to acquire more experience under real conditions in the sewer.
Research and tests have demonstrated automatic inspection and cleaning systems are feasible for the Emscher sewer system and the legal requirements for inspection and cleaning are fulfilled. Current work focus is the extension of the prototypes to Zero-series, which will be finished in 2007.
