Electricity: measuring and testing – Magnetic – Magnetic sensor within material
Reexamination Certificate
2000-12-18
2002-06-11
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetic sensor within material
C324S240000, C073S623000, C073S643000
Reexamination Certificate
active
06404189
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to methods and devices for the nondestructive evaluation of materials. The present invention relates more specifically to a magnetostrictive sensor based system for the inspection of pipeline structures from an in-line inspection vehicle.
2. Description of the Related Art
The use of magnetostrictive sensors (MsS) in the nondestructive evaluation (NDE) of materials has proven to be very effective in characterizing defects, inclusions, and corrosion within various types of ferromagnetic and non-ferromagnetic structures. A MsS launches a short duration (or a pulse) of elastic guided waves in the structure under investigation and detects guided wave signals reflected from anomalies such as defects in the structure. Since guided waves can propagate long distances (typically 100 ft or more), the MsS technique can inspect a significant volume of a structure very quickly. In comparison, other conventional NDE techniques such as ultrasonics and eddy current inspect only the local area immediately adjacent to the probes used. Therefore, the use of magnetostrictive sensors offers a very cost effective means for inspecting large areas of steel structures such as strands, cables, pipes, and tubes quickly with minimum support requirements such as surface preparation, scaffolding, and insulation removal. The ability to use magnetostrictive sensors with little preparation of the object under inspection derives from the fact that direct physical contact between the sensors and the material is not required.
Efforts have been made in the past to utilize magnetostrictive sensor technologies in association with the inspection of both ferromagnetic and non-ferromagnetic materials. Included in these efforts are systems described in U.S. Pat. Nos. 5,456,113, 5,457,994 and 5,501,037 which are each commonly owned by the assignee of the present invention. The disclosures of U.S. Pat. Nos. 5,456,113, 5,457,1994 and 5,501,037 provide background on the magnetostrictive effect and its use in NDE and are therefor incorporated herein by reference. These efforts in the past have focused primarily on the external inspection of piping, tubing and steel strands/cables wherein the nature of the structure is such that uninterrupted internal access to the pipe wall is very limited. While these systems and their external application to longitudinal structures find significant applications, there are yet other inspection techniques structures that could benefit from the use of magnetostrictive based NDE.
BACKGROUND OF THE MAGNETOSTRICTIVE EFFECT
The nondestructive evaluation of materials using magnetostrictive sensors is based upon the magnetostrictive effect and its inverse effect. The magnetostrictive effect is a phenomenon that causes the physical dimensions of a ferromagnetic material to change slightly when the material is magnetized or demagnetized or otherwise experiences a changing magnetic field. The inverse effect is a phenomenon that causes a magnetic flux in the material to change when the material is stressed. Systems utilizing magnetostrictive sensors use the magnetostrictive effect and its inverse effect to generate and detect guided waves that travel through the ferromagnetic material.
In general, a magnetostrictive sensor consists of a conductive coil and a means for providing a DC bias magnetic field in the structure under inspection. The means for providing a bias magnetic field can include the use of either permanent magnets or electromagnets. In a transmitting magnetostrictive sensor, an AC electric current pulse is applied to the coil. The resulting AC magnetic field (a changing magnetic field) produces elastic waves (also known as guided waves) in an adjacent ferromagnetic material through the magnetostrictive effect. In the receiving magnetostrictive sensor, a responsive electric voltage signal is produced in the conductive coil when the elastic waves (transmitted or reflected from anomalies within the material) pass the sensor location, through the inverse magnetostrictive effect.
With MsS techniques, defects are typically detected by using the pulse-echo method well known in the field of ultrasonics. Since the sensor relies on the magnetostrictive behavior found in ferromagnetic materials, this technology is primarily applicable to the inspection of ferromagnetic components such as carbon steel piping or steel strands. It is also applicable, however, to the inspection of nonferrous components if a thin layer of ferromagnetic material, such as nickel, is plated or coupled onto the component in the area adjacent to the magnetostrictive sensors.
The magnetostrictive sensor technique has the advantage of being able to inspect a large area of material from a single sensor location. Such sensors have, for example, been used to accurately inspect a length of pipe or cable of significantly more than 100 feet. Further, magnetostrictive sensor techniques are comprehensive in their inspection in that the methods can detect both internal and external defects, thereby providing a 100% volumetric inspection. The techniques are also quite sensitive, being capable of detecting a defect with a cross-section less than 1% of the total metallic cross-section of cylindrical structures such as pipes, tubes, or rods. Finally, as indicated above, magnetostrictive sensor techniques do not require direct physical contact between the component surface and the sensor itself. This eliminates the need for surface preparation and permits the movement of the sensor across the surface without concern for abrasive contact.
APPLICATION TO PIPELINE STRUCTURES
Gas transmission pipelines are typical of tubular structures that regularly require inspection for defects to insure their structural integrity and their safe operation. The primary traditional tool utilized to inspect such pipelines is referred to an in-line inspection (ILI) vehicle or pig that is equipped with an inspection device and travels down the length of the pipeline inside the conduit. The detection of corrosion metal loss is typically accomplished using devices based on the magnetic flux leakage (MFL) technique. Magnetic flux leakage devices work well, although they are heavy and difficult to handle. In most instances, MFL devices lack the flexibility to accommodate different pipe diameters, and as such different devices are needed for each pipeline diameter to be inspected.
For the detection of cracks such as stress corrosion cracking (SCC) that occur in the longitudinal direction of a pipeline, devices based on ultrasonic techniques are frequently used. Ultrasonic devices, such as those developed by British Gas, employ an array of wheel type piezoelectric transducers to couple an ultrasonic wave into and out from the pipe wall without the need of a liquid couplant. Such ultrasonic devices work reasonably well but tend to be very expensive to build and operate. Because of the high inspection costs associated with ultrasonic devices, the gas pipeline industry has devoted much research to finding a more economical approach to pipeline inspection.
One current direction of the active research and development in the gas pipeline industry focuses on the use of electromagnetic acoustic transducers (EMATs) which require no liquid couplant to convey a signal to and from the investigated material. Other research and development efforts are focusing on systems that use the high-pressure gas as a coupling medium to convey the interrogating signal. Recent applications of plate magnetostrictive sensor probes have shown promise in a variety of structural geometrys. In addition to the benefits associated with not requiring a liquid couplant, magnetostrictive sensor probes offer further advantages in that: (1) they can detect both corrosion metal loss and stress-corrosion cracking as well as coating disbond; (2) they are simple in design, lightweight, and easy to handle; (3) they can readily accommodate different pipeline diameters; and (4) they are economical to manufacture and opera
Kim Sang Young
Kwun Hegeon
Gunn, Lee & Hanor P.C.
Snow Walter E.
Southeast Research Institute
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