April 2010 Vol. 237 No. 4

Features

BG&Es Experience With Cased Carriers On Transmission Lines

The Pipeline Safety Improvement Act of 2002 is federally mandated legislation that addresses risk analysis and integrity management programs for pipeline operators. It also directs the U.S. Department of Transportation (DOT) to adopt regulations relating to integrity management. DOT finalized these regulations on Dec. 17, 2004.

Natural gas transmission pipeline operators were required to begin conducting assessment by June 17, 2004, have a management program in place by Dec. 17, 2004, and to complete baseline assessments of pipe in high consequence areas (HCAs) by 2012.

This included challenging External Corrosion Direct Assessment (ECDA) situations, such as: river, road, and expressway crossings; casings; bare pipe; and AC interference. These ECDA areas are difficult to assess for most LDCs with transmission lines.

As a result, “What are we going to do about casings?” became an oft-repeated phase for Baltimore Gas & Electric’s (BG&E’s) pipeline integrity team back in 2007. There were approximately 70 cased crossings identified on the company’s transmission pipe in HCAs regulated by the transmission integrity rule.

BG&E’s assessment method for transmission lines is ECDA. Pipe inside of casing is covered by the rule, but cannot be indirectly inspected or directly examined.

In 2007, the 2012 deadline for finishing the baseline assessment seemed far off; however, experience suggested the deadline was right around the corner.

The Pipeline Integrity Team had four options for dealing with casings. The first option was to see whether the cased carriers really were in HCAs, by definition. When the transmission system was initially examined for delineation of HCAs, in some instance two HCAs were joined into one if the distance between them was a few hundred feet. Occasionally a cased carrier was found to be in that area. The second option was to use alternative technology to assess the condition of the carrier pipe inside the casing. Guided wave (GW) was selected as the assessment method. The third option was to remove the casing completely. It was unclear how that would be accomplished, but was worth investigating. The fourth option was to completely replace the cased carrier pipe.

The team made an initial assessment of which cased carriers in the system were not in HCAs, which would be assessed by guided-waved, which could be removed and which would probably be replaced. An initial schedule and plan for all cased carriers was developed, but first a pilot program would be developed and executed in order to determine costs and operational concerns – procedures, permitting, traffic considerations, coordination of excavation contractors and GW contractors.

Pilot Program
A pilot program was conducted in 2008. Five cased carriers were selected. All were under secondary roads. Four of the casings were between 45-50 feet long. One casing was 84 feet long. All were GW tested. Two of the casings were completely removed.
This pilot program identified several areas of concern:

• Locating casing ends – cased carriers were installed in the 1960s. Many roads had been widened since then. It was not obvious where to start digging to find the end.
• Holding excavations open – a single mobilization of the GW contractor was planned. Weather was wet and the holes tended to slough in.
• Guided wave traversal – waves made it through the casing for all shots, but did not have the overlap at the required sensitivity.
• Removing the casing – done with hand-held grinders and took longer than anticipated.
• Cost – complete removal of the casing was only slightly more than guided-wave assessment.

Two casings were removed, making it possible to compare the GW test results to actual conditions under the casing. No corrosion pitting was indicated by GW and none was found. Girth welds were within two feet of GW indications. Spacers and centralizers were also within two feet of GW indications. Indications missed on the shot taken from one side were picked up on the GW shot taken from the opposite side. The two casings removed showed extensive corrosion pitting. One of the casings was found to be completely corroded through the half-inch wall.

2009 Program
With information gained from the pilot program, BG&E re-evaluated its initial plan for cased-carrier assessment/removal and made some revisions. Additionally, the contractor selected by BG&E, Mears Group Inc., had spent the winter refining casing removal equipment and techniques and introduced a rail-mounted saw that greatly reduced cutting time (relative to the pilot).

Eight casings were scheduled for removal starting on March 3, 2009. All casings were on a single transmission line that had been installed in 1949. Two casings were on an approach road to Oriole Park at Camden Yards, and Ravens Stadium. It was important to get them removed before the home opener on April 6. One casing removal was postponed due to permitting issues. The other five casings were removed without incident. On one of the casings removed, a small corrosion pit was found on the carrier pipe, well inside the casing. The presence of corrosion under the casing was unusual.

Nine casings were scheduled for GW testing. The GW contractor would mobilize three times to inspect three cased carriers per trip. On most excavations, casing ends were trimmed as much as two feet in order to visually inspect the carrier pipe immediately inside the casing and under the end seal. Given the age of the pipe, various mechanisms had been used to seal the ends. Split boots, link seals and asphalt-impregnated fibers were all found. Some corrosion pitting was found under the asphalt-impregnated fibers (presumably jute).

Of the 18 GW shots taken (2 shots * 9 casings = 18) no overlap was achieved at the prescribed 5% CSA at 2:1 S/N ratio. The signal attenuation is expected to have been caused by heavy field-applied coal tar enamel coating on the carrier pipe. Originally all nine casings were to be filled with wax following the GW testing. After the first GW mobilization, hot wax was pumped into the annular spaces of the three casings tested. When the final GW report was received, showing no overlap at the required sensitivity, future hot wax filling was canceled pending better GW results on the second and third mobilization.

Lesson Learned And Future Plans
With the new rail-mounted saw, complete removal of the casing proved to cost essentially the same as excavating the ends and taking GW shots. The pending work schedule was adjusted to reflect a “removal bias”. This eliminates the casing and allows the carrier pipe to be indirectly inspected by normal ECDA methods.
Short casings that can’t be removed will still be assessed with GW every seven years. There will be far fewer casings after 2012. Long casings which can’t be removed due to limited access such as highways or railroads will be the last to be addressed. The current plan is to directionally drill a parallel pipeline, and configure the tie-in in such a way that both lines can be hydro-tested for future assessment.
New technology is also being considered for long cased crossings. BG&E is in the pre-assessment phase of a project to insert a polyethylene composite sleeve into an existing transmission main. If cost-effective and accepted by regulators, the technique holds the promise of solving many cased-carrier problems.