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Dual field MFL pig may provide best of both worlds.

Special to Pipeline & Gas Journal

A team of researchers at Battelle in Columbus, OH has designed and developed a new magnetic flux leakage tool that could take pigging capabilities to the next plateau.

The design, which has already won an R&D Award for innovation from R&D Magazine, uses dual magnetic fields, upping the ante of expectations for what pigs can do--and what they can tell you.

Signals from the pig's two magnetic fields work in tandem for routine corrosion detection, while providing the ability to assess mechanical damage, measuring both the depth of a dent and changes to the pipe material, like residual stresses, material property changes and gouges.

"This development elevates the inspection capability of these tools while keeping the features that have made flux leakage technology successful," said Bruce Nestleroth, leader of the design team.

The magnets are placed in a linear arrangement, with the strongest magnet in the center, flanked on each side by a weak and moderate strength magnet. This allows for the two magnetic fields to develop, as well as a magnetic null, so the two fields don't substantially interfere with each other.

In the data assessment process, the high and low magnetic signals are decoupled--or separated--revealing a magnetic deformation signal. This brings into focus anomalies in the pipeline that may have been hidden by corrosion and geometry changes. Stress and material variation effects on flux leakage are complex and lower magnetic field instruments provide more information on pipeline anomalies to help avoid problems. At magnetic fields exceeding about 80 Oersted, flux density differences between tensile and compressive stresses start to diminish. And, by the time a magnetic field reaches 125 Oersted, these differences are almost nonexistent.

"MFL pigs from the 1970s worked at the lower field levels, but the stress effects made interpretation of the metal loss signals difficult," Nestleroth said. "Since then, commercially available magnets are 10 times stronger. This allows current pigs to operate at high field levels to improve corrosion detection and sizing. However, these pigs are now missing the stress signals that are important for detecting mechanical damage."

Equipping a single pig with the dual field design makes it possible to have the best of both worlds: the additional information provided by the low magnetic field combined with the ability to detect corrosion with the high field, all collected in the same run.

Given that the signals are measured using essentially two separate rulers, Rick Davis, a physicist at Battelle, developed a formula to decouple information from high and low fields, producing a clearer version of a picture taken with two different cameras. The decoupling process has three steps:

Step 1: Low magnetization. The low magnetic field signal provides a broad field of data containing information on both the geometric and magnetic deformation of the pipe.

Step 2: High magnetization. The pig records signals essentially due to the defect geometry of the pipeline.

Step 3: Isolation of the magnetic deformation signal. The magnetic deformation signal is the product of the low magnetization signal minus a "scaled down" high magnetization level signal. The scaling down process serves to place the high and low signals within an equal frame of reference, with the high signal essentially becoming a hypothetical magnetic flux leakage signal. "A lot of finite element modeling and analysis went into deriving the decoupling process. But the implementation of the process is quite simple," Davis said.

Figure 1 shows three signals from a mechanical damage anomaly collected by a pig with the dual magnetic field design. The data gathered from the high and low magnetization signals combine to provide the decoupled signal. Evident in the decoupled signal is the reround halo and precise information on the gouge, information made possible through the decoupling process.

[FIGURE 1 OMITTED]

Practical Challenges

Looking beyond the mechanics of the dual magnetization process, the team had to tackle a more practical problem: packing the magnets into a single pig. "One of the biggest challenges we faced was implementing an effective magnetic design within the available space," Nestleroth said. Space concerns are paramount in pigging where getting the job can hinge on differences as small as a one-foot length difference in instruments. For the dual magnetic pig to be feasible for use, the team again had to dip into their bag of tricks. What came out was the design for an articulated backing bar, allowing the pig to hinge at the null point between the two magnetic fields.

The backing bar was designed using finite element modeling and the three-pole magnetization is key to its use. With two-pole magnetization, a pivot point can cause more trouble than it solves. When the pig squeaks through bends, the field produced by two poles becomes distorted as does the data collected; stray magnetic fields degrade the performance of the inspection system. But this new design can eliminate this problem. Since the pivot point is located at the magnetic null, the center magnet--and the two fields it creates on either side--remain in contact with the pipeline through bends. The added flexibility enables the magnetizer to pass tight bends and restrictions.

"Rule No. 1 in pigging is 'make sure the pig comes out of the pipe at the receiver,'" Nestleroth said.

Testing The Device

A stone's throw away from Battelle's headquarters in Columbus is Battelle's Pipeline Simulation Facility. It is here that the mechanical damage pig cut its teeth squeezing through about one mile of underground pipeline and countless trials in pipeline sections above ground. "Proof of concept testing is an important step in gaining industry acceptance," Nestleroth said. "The last place you want to discover a problem is in an operating pipeline."

More than 100 mechanical damage anomalies were created on pipeline at the facility with Battelle's Dent and Gouge Fabrication System which has two hydraulic actuators: one to apply radial compression, the other to push a damage tool along the pipe's axis. The maximum vertical load is 60,000 pounds (27,200 kg), while the maximum horizontal load is 37,600 pounds (17,100 kg). The maximum damage the fabricator can impart mimics loads associated with tracked excavators with bucket capacities on the order of four cubic yards (three cubic meters).

The pipes are pressurized to typical operating pressures when the dents and gouges are made. "We have gotten pretty good at making anomalies that are near failure. The pipe survives the installation but the anomalies fail after a few pressure cycles," Nestleroth said. "Some anomalies have cracked and leaked but did not rupture."

Duplicating Pipe Damage

A Kobelco Mark SK200 track hoe was also used to damage sections of pipe. The hoe can produce a load of nearly 47,000 pounds and its use was intended to mirror actual conditions pipelines experience in the field. Pipes were placed within trenches, then pressurized to 200 psig. The track hoe was used to create damage to the pipe both parallel and perpendicular to the direction of the pipe.

Figure 2 shows low, high and decoupled magnetization data that clearly indicates the importance of the decoupling process in determining intricacies in pipeline damage. In this magnetically noisy pipe, the decoupled signal reveals the gouge signal and the reround halo that were obscured by the nearly vertical stripes.

[FIGURE 2 OMITTED]

Conclusion

The pairing of two magnetic fields--one high, one low--in a single pigging device, then decoupling those signals in order to collect data about underground pipeline damage, provides a clearer characterization of pipeline stresses and anomalies than what is presently available. Through the use of the two magnetic fields, a null point can be created which serves as a prime location for a pivot point, allowing the pig to move through bends in the pipeline. An articulated backing bar provides the appropriate design conducive to getting the maximum performance out of a dual magnetic field pig.

The combination of the added low magnetic field--coupled with the articulated backing bar--has the capability to revamp the current modes of pigging and make usable better and more specific data in pursuit of the ultimate goal: ensuring the safety and proper maintenance of the nation's pipeline system.
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Title Annotation:magnetic flux leakage
Comment:Dual field MFL pig may provide best of both worlds.(magnetic flux leakage)
Publication:Pipeline & Gas Journal
Geographic Code:1USA
Date:Oct 1, 2005
Words:1354
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