Addressing Challenges Of Bar Wrapped Pipe
Renowned for its reliability, cost effectiveness and durability, bar-wrapped pipe (BWP) has been used in large-diameter transmission and sewer force mains since its introduction in 1942. Since then, thousands of miles of BWP have been installed in the western and southwestern regions of United States as well as Canada.
Typically, BWP is manufactured with mild steel for the welded steel cylinder and reinforcing bars. While BWP looks much like prestressed concrete cylinder pipe (PCCP) in a cross section comparison, the pipe materials differ, as do the rates of deterioration. Although covered by an inner and outer mortar coating, BWP performs much like steel pipe, and the cylinder plays a much larger role in the structural integrity of the pipe.
Despite its historic acceptance with pipeline operators, the downside to BWP has been the difficulty to effectively assess the pipe’s condition, where failures are often precipitated by deterioration of the reinforcing bars and long periods of leakage that often go undetected. Failure can also result from transient pressure or other sudden catastrophic events.
Technologies to Assess Condition of BWP are Relatively Recent
For BWP, inspection methods that locate cylinder corrosion have only been recently developed and commercialized.
Pure Technologies leads the way with a suite of technology tools that can identify both bar breaks and broad areas of corrosion on the steel cylinder. Based on data analysis, pipe sections with broken bars or cylinder corrosion may warrant a more thorough condition assessment to better understand the pipe integrity or can be immediately repaired or replaced. This approach allows operators to only renew pipe sections that could be at risk of pipe failure. In one specific case with the Trinity River Authority of Texas, 8.5 miles of a 30- and 54-inch (750- and 1350-millimeter) BWP transmission main was assessed and renewed for roughly 4 percent of the US$25 million capital replacement estimate. This inspection found that the majority of the BWP transmission main was in good condition, with only 3 pipe sections being repaired immediately.
In general, the condition of metallic pipe – and in particular BWP – is typically first accessed with leak detection technology, followed by some level of pipe wall assessment (PWA) via medium or high resolution methods. This multi-tool approach provides the operator with a variety of condition information that can help inform renewal decisions.
Low Resolution Assessment Methods
The first tool designed for live inspection of large-diameter water mains, the Sahara® Pipeline Inspection Systemis one of the most accurate tools available for detecting leaks, pockets of trapped gas, and pipe wall stress in large-diameter water mains.
Sahara pipeline inspections are conducted while the main remains in service by inserting a sensor into any standard tap. A small parachute uses the flow of water to draw the sensor through the pipeline. The sensor is tethered to the surface, allowing for real-time results, and maximum control and sensitivity.
Pure’s SmartBall® platform can complete long assessment surveys in a single deployment without disruption to regular pipeline service. The tool is inserted into a live pipeline and travels with the product flow for up to 12 hours. It can collect both pipe wall stress data and acoustic leak data. It requires only two access points; one for insertion and one for extraction and is tracked throughout the inspection as it passes available access points on the pipeline.
Both the Sahara and Smartball prescreening tools can accurately identify leaks and gas pockets, as well as pipe wall stress. Areas of the pipe wall that are under more stress could potentially be damaged. These sections warrant a further investigation via excavation or higher resolution assessment.
Medium Resolution Assessment Methods
For structural inspection of BWP, operators can use PipeDiver® technology, a free-swimming electromagnetic tool that identifies bar breaks and broad areas of cylinder corrosion using PureEM™ technology. The tool operates while the pipeline remains in service.
Another option for locating BWP bar breaks and cylinder corrosion is to use PureRobotics® technology. This powerful robotic system, equipped with PureEM™ technology, can be configured to inspect a variety of pipelines and materials with different operational conditions.
Using PureEM technology, both PipeDiver and PureRobotics are capable of identifying broad areas of corrosion on the pipe wall and bar breaks. These areas should be investigated further or repaired to restore pipeline reliability.
Good News for BWP Owners
In reviewing data gleaned from more than 8,000 miles (12,000 kilometers) of pressure pipe condition assessments, Pure has found that pipe distress is often localized and that the majority of pipelines can be safely managed.
For operators of BWP pipelines, the news is comforting, and confirms their trust in this reliable and enduring pipe material, as there is now a well-established method for safe management of BWP.