Keeping pipelines safe from internal corrosion can be a challenging and expensive business. Annual costs are estimated by some to run to several billion dollars in the U.S. alone.
Panhandle Energy, a subsidiary of Southern Union, operates approximately 18,000 miles of interstate pipelines. "In 2006, Panhandle performed over 300 anomaly investigations," says David McQuilling, Senior Engineer for Panhandle Energy. "On average, we budget $50,000 per investigation in rural areas, but if you're in an urban area, $50,000 may not even cover permitting; I've heard of operators spending over $250,000 on a single dig."
Internal corrosion can occur when impurities are present within the natural gas, crude and refined products being transported. Bill Shaw is an engineering professor at the University of Calgary and the director of the Pipeline Engineering Centre, which studies corrosion and monitoring. "Moisture is the big thing; if you had no moisture, for the most part, you'd be fine," he says. "It mainly mixes with salts, like chlorine, and sulphur compounds."
Corrosion can typically reduce a pipeline wall thickness at a rate of 2-3 mils/year, but it can happen much more quickly in the upstream area of operations. "Gathering systems are really bad: you have large quantities of hydrogen sulphide and brine," says Shaw. Water can chemically react to form very corrosive liquids that collect in low-lying spots. Corrosion rates in these areas may reach several hundred mils per year.
To mitigate the potential for incidents related to internal corrosion, the pipeline industry works assiduously to reduce risk. The U.S. Department of Transportation's Pipeline & Hazardous Materials Safety Administration (PHMSA) notes that 258 natural gas and liquid pipeline accidents (14% of which were caused by internal corrosion) occurred in 2004.
"It is important to place natural gas pipeline safety data in perspective," says Don Santa, president of the Interstate Natural Gas Association of America (INGAA). "INGAA's members operate a network of approximately 200,000 miles of transmission pipeline, and this total does not include all of the intrastate transmission pipelines and LDC-owned pipelines that also are subject to the DOT's integrity management rules."
The Bush administration enacted the Pipeline Safety Improvement Act (PSIA) in 2002. "Under the PSIA, pipelines have to inspect all High Consequence Areas within a 10 year span, and then, after the initial inspections are done, they have to go back and inspect at seven-year intervals," says Santa. The Office of Pipeline Safety estimates that natural gas pipeline companies alone will spend $4.7 billion over 20 years complying with the legislation.
CORROSION MONITORING
In order to comply, pipeline operators rely on a range of survey methods supplied by third parties. Prominent companies supplying internal corrosion monitoring equipment include Rohrback Cosasco of the U.S., Corrocean in Norway, Cormon in the UK and Caproco in Canada. The international market for corrosion-monitoring equipment is growing rapidly due to the fast pace of oil and gas developments, expansion of pipeline networks and the increasing stringency of regulation.
Rohrback Cosasco Systems saw the potential for a high-speed, high sensitivity device that could be permanently installed and devised the UltraCorr(R) wall thickness monitor, a combination of permanently mounted transducers and a portable data logger. "Problem areas, which are typically located at low spots, are identified by pigging or some other means, then dug up." says Brent Ford, president/CEO Rohrback Cosasco Systems.
"Before the pipe is reburied a series of transducers is permanently attached to the bottom of the pipe and leads are run to a test post at ground level. A dozen or so transducers should provide adequate coverage for most areas to be monitored, but this can vary depending on the actual surface area in question. Unlike normal ultrasonic devices, at a resolution of one tenth of a mil, UltraCorr is capable of monitoring low roles of corrosion," says Ford.
With a permanently installed system, internal corrosion surveys can be incorporated into normal maintenance practice; most pipeline companies have field technicians patrolling the right-of-way on a frequent basis to check catholic protection systems. "The technician takes an UltraCorr Corrosion Data Logger, a hand-held device, and loads the transducer information from the test post," says Ford. "It's easy to use and only takes seconds per reading."
"Readings stored on the portable data logging unit can be uploaded to a PC, where proprietary software is used to organize, store, and graphically display data. Simple plots of thickness vs. time are augmented with a cursor-driven corrosion rate calculator permitting detailed event analysis."
Panhandle Energy has been interested in the system since the start "It's simple, clean and cheap; you don't have to blow down the pipe which really gets expensive. or do a hot tap," says McQuilling, "You just buff off a spot on the pipe and attach it with epoxy, then backfill and set up a post for the transducer wires, and you're done."
Pipeline & Gas Journal 234 no10 90 O 2007
