By Brian J. Mergelas, President, Pressure Pipe Inspection Company
As the age of our infrastructure increases, non-destructive evaluation becomes a very important tool for cost-effective decisions, integrity management and overall maintenance of pipeline systems. Pre-stressed concrete cylinder pipe (PCCP), typically used in the water industry, is no exception. The Remote Field Eddy Current/Transformer Coupling technique (RFEC/TC) was pioneered by the Pressure Pipe Inspection Co. and Queen's University. It has been utilized and accepted for non-destructive evaluation by nearly all PCCP users in North America. To date, over 300,000 pipes have been inspected.
The strength and function of PCCP pipes are dependent on a set of pre-stressed wires that are wound around a cylindrical, concrete core. If these wires snap or break, then the pressure-capacity of the pipe is severely weakened or distressed. Should sufficient numbers of wires break, then the pipe itself has the potential to fail under typical operating conditions, unbeknownst to an operator.
Using the RFEC/TC non-destructive technique, the number and location of breaks can be quantified, so that an informed assessment of the condition of the pipes can be made. The inspection is typically conducted at a normal walking pace within the pipeline but is, in fact, quite rapid compared to most other non-destructive evaluation methods. The operation requires de-watering of the majority of the pipeline, which many operators have traditionally done anyway for visual inspections and other qualitative techniques.
Essentially, the system is a radio transmitter and receiver, with the pre-stressed wires in the pipe acting as a secondary antenna, boosting the signal. When the pre-stressed wires are broken, the received signal will be distorted. Wire-breaks can be detected anywhere along the length of a pipe, and it is possible to resolve multiple regions of wire-breaks.
The major advantage of RFEC/TC is its ability to locate and quantify the number of broken wires in individual sections of PCCP. Breaks can be detected anywhere around the circumference or along the length of the pipe. A secondary characteristic of the method, is its ability to determine which pipes are not distressed, which provides security and relief to many operators.
Re-inspection of a pipeline system using RFEC/TC gives the operator another level of information, in that it is possible to determine the rate of deterioration of individual pipe sections. The growth in the number of wire-breaks per identified region within each pipe can be tracked. Trends and projections can then be made and referenced to scheduling and planning of outages, as well as to when the next inspection should be performed.
Many verification assessments have been performed over a wide range of different pipe configurations and in diameters ranging from 36" to nearly 160". It is often difficult to obtain an accurate count of the actual number of broken wires because of pipe corrosion and the fact that to expose all of the wire-breaks would often mean full removal of the mortar by destructive means. Field verifications however, show that there is excellent correlation, proving that the technology and data are viable and effective.
Acoustic monitoring systems may be utilized in conjunction with periodic RFEC/TC inspections to provide ongoing condition-information. Acoustic methods rely on the characteristic sound made when a wire breaks, which can be captured by a well-placed hydrophone (underwater microphone). The RFEC/TC system can set a baseline and reference during later inspection, while the acoustics can monitor the pipeline in operation to indicate relative growth and change in troublesome locations.
The result of inspection is that each individual pipe exhibits its own characteristic signature or fingerprint. By collecting these fingerprints before a pipeline goes into operation, and by keeping continual track of changes in the signal, it is feasible for the method to be extended over time to include detection and quantification of wire-corrosion as well as wire-breaks.
The typical output of an inspection is a report with various tables and figures presenting distressed pipe, as well as a total observed list of pipes to cross-reference the lay-schedule. Distressed areas can be presented and organized into a GIS-based pipeline management system, which can show the location of the individual distressed pipes and interact with various hydraulic and failure models, and can include other sources of information such as land usage, soils and topography.
While this baseline information is essential for knowing the present-day condition of the pipeline, it is important to realize that many factors have to be taken into account to understand the impact of wire-breaks on the structural integrity of a pipeline system.
The capability of RFEC/TC to detect and quantify broken pre-stressed wires in many different configurations of PCCP gives operators a detailed picture of the present-day condition of their pipeline. It lets them know if there are distressed pipes, to what extent, and where they might be located. Such data are fundamental for forming a management plan for a PCCP pipeline.
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