Seesnake Tool Used For the Rapid Inspection of Intake-Line
Discovering the actual wall thickness of a 12-inch steel pipe in just 22 hours
In November 2012, BC Hydro contacted PICA (Pipeline Inspection and Condition Analysis Corporation) due to concerns regarding the status of their intake-line in Port Moody, B.C. The utility had observed some leaks and were interested how many there were and if they were any “leaks in waiting” so they opted for a “full-line” condition assessment that was initially recommended for the 300 mm pipeline. However, due to budgetary limitations, PICA analyzed only 2.2 kilometres of the 6.8-km line (160 pipe segments). PICA ran the See-Snake tool the entire pipeline because logistically it made sense to insert the tool at Buntzen Lake as opposed to where the inspection started. There was no available access point where the analysis commenced, therefore it was decided to insert at the pumping station to reduce excavation costs.
The inspection was performed over three days: a gauge run was completed on April 16, 2013 and a Remote-Field-Testing (RFT) inspection was completed on April 17 and 18. This paper documents PICA’s RFT condition assessment results for a subset of the inspected pipeline.
Physical parameters measured by RFT tools
RFT technology measures three quantities: wall thickness of ferromagnetic pipes, magnetic permeability electrical conductivity. These three factors are measured simultaneously and convey different, important information. For steel pipes, the electrical conductivity remains fairly constant over the length of a pipe segment, meaning that any RFT signal changes along the length of a pipe are mainly due to wall thickness and permeability changes.
Magnetic permeability is not usually a factor of interest. However, in lines that are subjected to soil load stresses, the permeability variations can be significant. For lines known to be under external stresses (for example due to geological ground movement) the permeability variations measured by an RFT tool can be very valuable. Permeability variations produce signals that generally lie just outside the RFT wall loss reference curve that analysts use to differentiate between wall loss and permeability; while wall loss signals lie inside the reference curve.
In the data from cast and ductile iron water lines, we generally notice significant changes in wall thickness along the length of a pipe segment. This appears to be fairly typical, even for brand new pipes that come straight from the foundry. The variation is believed to be the result of the manufacturing process. To capture the spread in wall thickness, we generally report both the minimum and maximum wall thickness per pipe (measured circumferentially without local defects).
PICA Corp’s SeeSnake is a highly-flexible RFT tool that employs Remote Field Testing (RFT) technology for measuring pipe wall thickness. RFT technology works by detecting changes in an AC electromagnetic field generated by the tool that interacts with the metal in the encompassing pipe, becoming stronger in areas of metal loss.
The SeeSnake tool used in the Burrard Water Line inspection employs an articulated mechanical design that gives it flexibility to negotiate 90-degree short radius elbows. The hard diameter of the tool is significantly smaller than the ID of the pipe to allow for protrusions, lining and scale. Centralizers maintain a uniform annulus between the tool and the pipe.
The tool detects wall thinning caused by corrosion or erosion (both internal and external), as well as line features such as joint couplings, branches and elbows.
Weeks prior to the RFT inspection, BC Hydro contracted a local cleaning company to “swab” the pipeline. This was done in an effort to remove sediment and organic matter that had settled in the line. Cleaning runs were conducted over several days using incrementally increasing diameters of cleaning swabs. Pipeline access was granted via an existing launcher at the Buntzen Lake pump-house.
Due to the length of the 300 mm SeeSnake tool, the existing launcher was insufficient in providing pipeline access for the RFT inspection. PICA provided an appropriate launcher that was installed at the pumphouse. An existing receiver at the storage tank was deemed suitable for tool extraction. Prior to the inspection, PICA’s gauge pig was run through the line to identify any restrictions that could impede or stop the passage of the RFT tool. This tool also cleaned the remaining sediment in the line. The gauge run indicated clear passage for the SeeSnake.
The SeeSnake was propelled through the line using flow induced from the Buntzen pumphouse. Progress was tracked using Above Ground Monitors (AGMs) spaced at regular intervals. After approximately 22 hours, the tool arrived at the storage tank where it was removed from the pipeline. A data download was initiated and the data quality was confirmed to be acceptable. The data was then uploaded from the tool to PICA’s Edmonton office where it was analyzed and a report produced.
A 2,209-metre section of data was analyzed at the request of BC Hydro, spanning from the insertion point up to the flange pair connecting the last 45-degree elbow and the receiver-wye. As the predominant features in the pipeline were below ground (i.e. bends) no additional scaling was performed. Nineteen AGM locations spaced at convenient intervals were used to track tool progress through the line and were used as reference markers for later verification of inspection results.
A total of 19 wall loss indications, spread over 12 pipes, were identified. Five of these indications measured as through holes, with another three measuring less than 25 percent remaining-wall. Most corrosion appeared to be concentrated in the area between air vent/vacuum brake #5 and the receiver.
The presence of through holes was suspected, as surging water and air bubbles were observed at these sites during the inspection.
A total of eight AOIs (areas of interest) were noted during this pipe wall-thickness inspection and each was verified by BC Hydro during dig verifications. John Kohut, senior mechanical engineer commented “the measurement were very accurate.” BC Hydro determined that the defects located to date were due to improper installation of yellow jacket shrink-sleeves. Water got underneath the sleeve and locally attacked the pipe, eventually leading to leaks. A leak detection survey was performed in 2012 and only one leak was detected, where PICA was able to locate five through holes (with active leaks) and other areas where pitting was as deep as 85 percent of the initial wall thickness. These locations represent the potential for future failures and water loss.
Since the pipe wall thickness investigation provided accurate pipe failure locations as well as locations for potential future failures, BC Hydro contracted with PICA to analyze the remaining 4.6 kilometres of pipeline in April 2014.
CA, T6B 2S3