Inspection of a culvert pipeline in Koblenz
At the beginning of the 1970s, in the immediate vicinity of the “Deutsches Eck”, one of the most important sewage structures in Koblenz: the 294 m long Mosel culvert. This takes about 70 % of the wastewater beneath the bed of the Mosel to Koblenz-Lützel and from there onwards via a wastewater pumping station to the central treatment plant after Koblenz-Wallersheim. The Mosel culvert consists of two DN 800 and DN 1250 wastewater ductile cast iron pipes, four NW 450 (PE) drinking water pipelines and six NW 125 (PE) cable conduits. In addition the culvert upper head and the culvert lower head also belong to the complex components.
The reasons for the culvert inspection
Apart from the necessary evidence of the tightness of the culvert pipelines to be provided for the supervisory authority, the concrete constructions in the upper head of the culvert were found to be in a critical state. So it was quite obvious that there was a need for renovation here. For Stadtentwässerung Koblenz (SEK), the decisive question arose: Is the investment for the rehabilitation of the adjacent structures culvert upper head and the culvert lower head including the pumping station still economical in view of the lack of knowledge about the condition of the two culvert pipelines that have been in operation for 44 years? Because: Should the culvert be unusable for further use or no longer capable of rehabilitation, completely new planning considerations would definitely have to be made. The condition of the two culvert pipelines had to be assessed as well, and this within the scope of an inspection.
Limiting conditions for the inspection process
In order to capture the entire problem and to record what the inspection procedure has to achieve, it seemed sensible to define the major limiting conditions first. The inspection process should:
- function with the culvert pipelines complete full
- supply information on serviceability
- provide data on the remaining working life
- provide data on the condition of the culvert pipelines
- be able to be performed in an acceptable and calculable period
Requirements for the inspection system
Based on this, a test system with special features had to be found and the necessary requirements were specified, which resulted almost exclusively from the accessibility of the object to be inspected:
- The culvert pipelines are not designed for inspection by pig. There are no pig traps. The insertion of an inspection device has to be done at the lower head of the culvert.
- Access to the culvert pipelines is only possible from one end. The inspection system must allow bi-directional operation.
- The inside surface of the culvert pipelines is coated. The inspection system must be able to be used for checking the pipe wall through the coating.
- Thorough cleaning before the inspection is not possible. The surface may still present residues or deposits. Inspection must also be possible in the presence of low-level residues or deposits.
- The pipeline needs to be inspected in the filled state, whereby complete filling without air bubbles cannot be guaranteed. The inspection system must be able to be used regardless of the coupling medium under water and under atmospheric conditions.
The approach for the inspection
Finally, an inspection system was chosen that examines the metal pipe systems with eddy current according to SLOFECTM (Saturation LOw Frequency Eddy Current) technology for corrosion. It is based on the eddy current technique. The Andernach engineers convinced the officers from the Koblenz municipal drainage service of the application and success of this inspection technology, and the public call for tenders was then carried out for separate lots: culvert cleaning and culvert inspection.
The contract for the cleaning was won by Norand Industrieservice GmbH from Löbnitz, the one for the inspection went to 8SEAS consulting engineers – water + energy from Nackenheim. 8SEAS commissioned the experienced company KontrollTechnik GmbH from Schwarmstedt, which specialises in inspection processes and had developed various types of SLOFEC® internal pipe scanners for inspecting underground pipelines in industrial plants.
For the intensive cleaning of the two culvert pipelines, four working days per pipeline were required in 24-hour operation, and the pipes were then filled with clean water. Then their position in the XYZ position under the river had to be determined. The 8SEAS consulting engineers used the 3D gyroscope measurement with the Ductrunner measuring method for this purpose.
The non-destructive inspection of the pipe walls was then carried out with the PLS type SLOFEC® internal pipe scanner starting in the lower head of the culvert, passing through the culvert pipes (DN 800 cement mortar lining, DN 1250 epoxy lining) and ending at the upper head of the culvert:
The wired scanner was lowered into the culvert opening at a depth of approx. 15 m, inserted into the culvert and pulled through the culvert with a winch. For data recording purposes it was positioned in an axial direction by the winch, the sensor head was “pressed” against the pipe wall and moved circumferentially; the corresponding inspection section was 150 mm. After completion of a full circumference scanning by the rotating sensor unit, the scanner was pulled forward by a further 150 mm in axial direction and a new measurement started until the complete data recording across both culvert pipes was complete.
Even after 44 years of continuous operation, strong currents and flooding, the ductile cast iron culvert pipelines only showed slight corrosion along the whole of their outside: at the “foreshore”, most areas were detected as having only slight local inhomogeneities (weak corrosive attack); it was only at the start of the inspection run, close to the pumping station (lower culvert head) that areas of more severe inhomogeneity had formed. On the Mosel riverbed no damage at all was found on the inside and outside of the pipes, in other words the pipe wall is unchanged after 44 years of operation!
There was (and is) no need for renovation of the culvert as any significant impairment to wall thickness by corrosion and/or other ageing damage was able to be definitively excluded, even in the lower head areas of the culvert. With the results showing the good condition of the culvert pipelines, the way was opened for the investment to renovate the dilapidated concrete structures of the adjacent culvert upper and lower heads and thus further long-term use of the culvert structure as a whole.
Authors: Hans-Jörg Schulz, Eigenbetrieb Stadtentwässerung Koblenz
Wilhelm Kelb, Kontrolltechnik GmbH