Unpiggable suggests one of several perceptions. The simplest explanation considers the obvious barriers to pigging a single pipeline:
- Access – a free swimming tool can’t be introduced or removed;
- Low pressure low flow – there is insufficient flow to overcome friction and drive a pig;
- Multi-diameter – making high velocity excursions and the accompanying loss of data normal;
- Physical barriers – the tool can’t navigate past internal obstacles or barriers; and,
- Inconvenience – the customers can’t tolerate any reduced product flow and access is curtailed.
Most operators also recognise that almost three-quarters of the gas pipelines in the United States were never built with in-line inspection (ILI) in mind and considerable preplanning and capital expense will be required to remove these normal built in barriers.
However the most realistic reason for being unpiggable, that there might also be a range of objections due to preconceived perceptions and these objections need to be addressed, internally, with a realistic plan.
In its discussions, the Pipeline Research Council International (PRCI) decided to discourage the use of the word unpiggable. Its engineers agree there is no such thing as unpiggable; it just takes ingenuity, planning, and money to conduct integrity assessments when either the safety or operational risks warrant it. Unpiggable lines do require considerable preplanning. They might require the novel assembly of tool components such as sensors and software onto several linked body joints. This single-use pig train may be driven in and out of the section by hydraulically pumping a bi-directional tool, or by adapting tractor or robotic joint sections to the pig train, or sometimes as in older days just pulled through by a cable. The pre-planning, test runs, the finding of an operational window can easily triple the cost of a comparable a free swimming pig run and analysis.
Recent public perception
At the San Bruno incident hearing on 30 August 2011, conducted after the gas pipeline explosion in September 2010, the National Transportation Safety Board (NTSB) concluded that owner PG&E was solely responsible for the blast, and this occurred on an unpiggable line. Regulators have the impression that ILI will address all threats, while actually only corrosion threats are well managed. The regulators feel that ILI technology progress has remained dormant, and no real progress has been made especially to address unpiggable lines. This lack of progress was felt to be unacceptable and that maybe more prescriptive regulations were the answer to improving safety performance.
Reality
In spite of two recent downturns – Y2K and Wall Street in 2008 – constricting the world economy, considerable ILI progress has been made. The industry now has improved and closely-paced rings of sensors which provide high resolution for corrosion and deformation. Reduced field magnetic flux leakage (MFL) permits a better assessment of mechanical damage. There are ultrasonic pigs and electromagnetic acoustic transducer’s (EMAt), which is looking promising for finding small cracks like stress corrosion cracking (SCC) in gas lines. Pigs are no longer a fully-customised design, but are assembled from “tool boxes” of proven components. Sensors and recorders have been miniaturised to fit into a wide range of carriers. Magnets are smaller and more powerful. The carriers range in size from 6 inches to 48 inches. More power is available from compact batteries or even generated by turbines driven by the product flow through the pig. Tractor and robotic solutions have been added to cable and tethered insertions.
The gas transmission industry discussed some of its unpiggable concerns in a Southern Gas Association Webinar in September 2011. Generally, telescoping or multi-diameter lines are the major reason pipelines are considered unpiggable. These large diameter changes are due to many reasons, but typically occur in new construction such as river crossings. The original pipe was inserted as a redundant crossing. When the line was looped, the larger new diameter lines either side of the crossing were welded to the “˜spare’ river crossing.
The second largest problem seems to be the lack of launchers and receivers. The third largest was the number of small diameter pipelines for which few if any tools were available for conducting internal inspections. Flow pressure, bends, dents, small diameters, fittings, ageing weld designs such as mitered welds, debris, valve diameter differences and variable fittings made up this larger unspecified concern in smaller diameter lines.
Ageing pipe records sometimes suggest seam weld concerns that could make pigging difficult:
- Considerable concern was indicated by only one operator:
- Early stick welds
- Furnace weld long seam
- Butt-welded long seam.
- Significant concern was indicated by one operator:
- Acetylene welds
- Lap seam-welded.
- Some concern was indicated by about three or four operators:
- Expansion girth welds, bell and spigot explosion welds
- Hammer-welded long seams.
- No concerns, approximately 50-70 per cent of the operators had none of these concerns.
A second set of design considerations that could present ILI problems had more interest:
- Considerable:
- Congested underground facilities seem to be a concern for all.
- Significant:
- Dead legs and stranded pipe
- Bridge crossings
- Horizontally directionally drilled (HDD) crossings and/or deeply buried pipe
- Cased roads and rail crossings
- River crossings.
- Some of the operators expected these design concerns may represent ILI problems.
- None/only a few had no problems with these design concerns.
All the attendees agreed that they might consider temporarily adding a single point entry system for the introduction of internal inspection tools whether robotic, wire line, or conventional. Obviously flow control will be essential to return conventional free swimming pigs back to the launch site. Most operators indicated that they would consider using robotic or tethered ILI technology over short distances. These distances were not defined, but river crossings, meter runs, valve cross overs, all come to mind.
A summary of successes with ILI tools:
- Can be launched and received by hand directly into/out of nominal pipe;
- Navigate continuous 1.0 diameter, back-to-back 180 degree bends including miters bends;
- Run bi-directional combo tools – providing fully-aligned metal-loss and deformation data including odometer referencing;
- Accurately record continuous 0.050″ through 1.0″ wall thickness in a free-swimming platform in small diameter pipe;
- Navigate small multi-diameter pipelines as well as drips, dresser couplings, unbarred offtakes in the 6 o’clock position and even through heavy weld penetrations; and,
- Run metal-loss tools at only 50 psi.
In addition, the Clarion Unpiggable Solutions Forum provide a comprehensive update of commercially available solutions: tools that work in smaller diameters, more choices in tractor or robotic based equipment, and more successful examples. It offers papers on a very wide variety of inspection tools:
- Typically internal but also novel external inspections;
- Alternate access to the interior when launchers and receivers are not available;
- Robotic and crawler tractor bodies, as well as pressure differential free swimming tools;
- New sensor arrangements – MFL, eddy current, remote field, ultrasonic;
- Considerable fore thought and predesign needed to avoid operational constraints;
- Multiple capabilities in assembly of a combination of sensors on a tool; and,
- Above-ground techniques such as magnetic tomography to detect wall loss from aboveground.
This article is based on Mr Leewis’s presentation at the Tiratsoo Technical and Clarion Unpiggable Pipeline Solution Forum, held as part of the Pipeline Pigging & Integrity Management Conference (PPIM) held in Houston in February 2012.
The upcoming Unpiggable Pipeline Solution Forum, is to be held in May 2013 in Houston.
For more information visit www.clarion.org
