Patent Application
20060164091
Not applicable.
1. Field of the Invention
This invention relates generally to apparatus designed and operated to inspect conductive objects, including electrically-conductive and magnetically-permeable objects, pipeline, heat exchanger tubing, and well casing walls for example, for corrosion and mechanical damage as well as other anomalies and defects such as cracks and pitting. More particularly, this invention relates to pipeline inspection apparatus which “crawl” inside the pipe independent of fluid flow within the pipe which employ rotating permanent magnets to generate uniform low-frequency electric currents within the walls which, in turn, generate magnetic fields detectable with conventional Hall Effect sensors.
2. Description of Related Art
Essential liquids and gases, including water, chemicals, and fossil fuels, flow through pipeline systems. Large volumes of products as diverse as petroleum and liquid hydrocarbons, natural gas, propane, and slurries of solids such as granulated coal and minerals such as copper and iron are constantly being transported between production sites and processing and consumption sites over long distances. These pipelines range generally between a few inches and 60 inches in diameter and extend to thousands of miles in length. Usually constructed of metal, in particular, ferrous metals, pipelines are susceptible to damage and other defects which affect the integrity of the system. The result can be a failure which threatens life and property, serious environmental damage, disruptions to both local and distant economies, and loss of the product being transported. The further result can be reduced public confidence in this efficient and economic means of transporting materials with possible public opposition to the growth of such means.
To minimize the risk of failure, pipelines are closely monitored and inspected. Inspection methods generally involve injecting a uniform energy into an object whereby an anomaly or defect disrupts this uniformity. Sensors then detect changes in the uniformity and, thus, the anomaly or defect. These methods utilize pipeline inspection apparatus which are inserted into the pipeline and move through the pipeline generally, but not exclusively, via the flowing material in the pipeline. In radiography, for example, anomalies are detected when a portion of the incident X-ray energy passes thru the material. A film or charge-coupled device placed on the opposite side of the material from the source can be used to detect this change in absorption. A pipeline inspection apparatus may also comprise magnetic components to induce magnetic flux within the pipeline wall. The magnetic flux naturally enters the metal wall of the pipeline and distributes evenly to produce a full volumetric inspection. Anomalies or defects in the wall of the pipeline tend to disrupt the uniform flow of the flux and create a leakage of magnetic flux which can then be detected by sensors, generally within the apparatus itself. This inspection methodology is known as magnetic flux leakage (“MFL”). MFL, however, can be difficult to implement on autonomous crawler systems because the apparatus are large and heavy. In addition, MFL-based apparatus are not able to detect all defect types and have nearly the same form factor as the pipe, making passage of diameter restrictions difficult. Among alternative approaches, small and lightweight apparatus have shown promise but implementation attempts have been thwarted by high speed and long distance requirements, factors that are not as restrictive for crawler-based inspection systems.
Other inspection methods include, for example, inducing electric currents in the pipeline via the placement of an auxiliary magnetic pole and relative movement of the inspection apparatus and the pipeline wall. See, e.g., U.S. Pat. No. 5,751,144 to Weischedel (“Weischedel”). Such methods generate circumferential currents that are best employed when attempting to detect axial cracks. As described in Weischedel, one of the “necessary conditions” for reliable detection is the induction of “substantial eddy currents so that eddy current changes representative of structural faults can be readily detected.” To properly induce such eddy currents, the inspection apparatus must first include a small, relative to the two main poles, “auxiliary pole” which “has the same magnetic poling as one of the primary poles” and the apparatus must be moving at a rate in excess of, generally, about four miles per hour relative to the pipeline wall to generate measurable and reliable eddy current signals. Thus, these methods rely upon the linear motion of the magnets through the inside of the pipe. In addition, the eddy currents and resultant signals are still generally weak and thus require more sophisticated sensors to detect anomalies.
Methods which utilize the remote field technique (RFT) or the remote field eddy current (RFEC) technique are also known. This technique uses a sinusoidal current flowing in an exciter coil (e.g., electric coils) to induce current in the pipe wall and a remote receiver coil over two pipe diameters away to detect defects such as metal loss and stress corrosion cracks. The resulting signals, however, are very small which necessitates expensive, sensitive lock-in amplifiers to detect such signals. These traditional RFEC techniques also use low-frequency exciters, but the sensitivity of the coil sensors used to detect signals from anomalies decreases with decreasing frequency. In addition, such techniques employ “indirect energy coupling” by sending the field outside the bounds of the pipe wall. See, e.g., U.S. Pat. No. 2,573,799 to MacLean. The use of electric motor concepts (though still coils) has also been developed. See, e.g., Plamen Alexandrov Ivanov, Remote Field Eddy Current Probes for the Detection of Stress Corrosion Cracks In Transmission Pipelines (2002) (unpublished Ph.D. dissertation, Iowa State University). Further discussions of RFEC may be found in T. R. Schmidt, History of the Remote-Field Eddy Current Inspection Technique, 47 Materials Evaluation 14 (1989).
Recent development efforts in the field of internal inspection of pipelines include a new generation of powered inspection platforms that “crawl” slowly inside a pipeline and are capable of maneuvering past physical barriers that can limit inspection. The present invention provides an electromagnetic inspection system for pipeline crawlers which can be used to assess a wide range of pipeline anomalies including corrosion, mechanical damage, and cracks. A rotating permanent magnet-based exciter assembly produces strong, uniform, low-frequency electric currents in the pipe wall. These currents spread in the wall away from the magnets and generate magnetic fields inside and outside the wall. At distances of generally one pipe diameter or more, the currents flow circumferentially and the current and the resulting magnetic fields are deflected by pipeline defects such as corrosion and axially-aligned cracks. Conventional Hall Effect sensors are used to detect these changes.
In one embodiment, a device is provided for detecting an anomaly in an electrically-conductive cylindrical structure, the device comprising a first and a second pole, the poles having opposite polarities, means for rotating the poles about an axis of rotation, the axis of rotation being coaxial with a long axis of the structure, whereby currents are induced within the structure, and at least one current flow sensor, the current flow sensor spaced apart from the first and second magnetic poles along the axis of rotation.
The device of the invention may also include spacing the sensor from the magnetic poles according to the relationship expressed in Eq. 1, below. The sensor of the invention may also include sensors adapted to sense axial magnetic flow and radial magnetic flow. In a preferred embodiment of the invention, the sensor is placed outside a near field and within a far field. In a further preferred embodiment, the first magnetic pole and the second magnetic pole each comprise a strength of greater than or equal to 25 megaGauss-Oersted.
The device of the invention may also include a magnetically-permeable bar to which the first and second poles are attached. In a further embodiment, the bar is adapted to include means to allow the bar to telescope along its long axis, thereby enabling the bar to ride over obstructions on the inside wall of the structure. Alternatively, or in combination, the bar may be hinged to provide the same capability.
In a method for detecting an anomaly in an electrically-conductive wall of a tubular structure such as a pipeline, the device of the invention may be placed within the structure and the bar rotated about its axis of rotation at about 1-10 Hz. The device is then urged through the structure. The induced currents, which themselves induce magnetic fields, are sensed by, for example Hall Effect sensors.
a is a graphical representation of the signals coaxially near the rotating magnets according to the present invention.
b is a graphical representation of the signals coaxially far from the rotating magnets according to the present invention.
a-7d are graphical representations of the signals at the inside surface of the pipe wall at varying coaxial distances according to the present invention.
a-8d are graphical representations of the signals at the inside surface of the pipe wall at varying coaxial distances according to the present invention.
Reference Numerals
Beginning with
Operably connected to the shaft 50 is a sensor array 60, preferably comprising Hall Effect sensors 62. The shaft 50 preferably comprises a connector joint means 40, widely known in the art, which allows the magnet assembly 20 to rotate 30 while enabling the sensor array 60 to remain rotationally stationary. In addition, the connector joint means 40 is preferably adapted to provide universal joint-like articulation between the sensor array 60 and the magnet assembly 20.
Shown in
Turning now to
Turning now to Eq. 1, below, the amplitude of the magnetic field (Bpk) produced by the current 80 decreases as the coaxial separation distance (z) between the magnet assembly 20 and the sensor array 60 increases. To a first order approximation, the rate of decay is nominally exponential and proportional to the ratio of the number (n) of N-S magnet pairs 22, 24 divided by the radius (r) of the cylinder 70.
Eq. 1 implies that the current 80 in the cylinder 70 decays faster for smaller diameter cylinders 70 and magnet assemblies 20 with more poles 22, 24. The physical explanation is that few poles 22, 24 in large cylinders 70 enable the establishment of large current loops 80 that extend a significant distance from the magnet assembly 20. Conversely, many poles 22, 24 in small cylinders 70 produce many small loops 80 that oppose one another and do not extend far from the magnet assembly 20. The initial magnitude of the current 80 (at z=0) is proportional to many variables; the amplitude increases with increasing diameter, magnet strength, magnet coupling, conductivity, permeability, and rotational frequency and decreases with increasing number of pole pairs. From the examination of both the amplitude and decay terms, the signal is strongest for larger diameter tubes with few poles.
A typical test result from an anomaly is shown in
A combination of modeling and prototyping was used to improve the inspection current strength propagating along the pipe 70 used to detect pipeline anomalies. After examining various configurations, the best performance was achieved using a two-pole exciter 20. A prototype for a 12-inch diameter pipeline is shown in
a-7d illustrate the magnetic field at the inside surface of the pipe wall at distances ranging from close to the magnet array (one-sixth pipe diameter) to 2.5 pipe diameters for a two-pole system rotating at 5 Hz in a 12-inch diameter pipe. Indicated are the radial (solid lines) and axial (broken lines) signals.
Similarly,
In another embodiment of the present invention, the magnet configuration comprises a form factor that enables the tool to pass obstructions within the pipeline. For example,
Following from the above description and invention summaries, it will be appreciated by those skilled in the art that, while the apparatus and methods described and illustrated herein constitute exemplary embodiments of the present invention, the invention is not limited to those precise embodiments and that changes and modifications may be made thereto without departing from the scope of the invention as defined by the claims. Likewise, it is to be understood that the invention is defined by the claims and it is not intended that any limitations or elements describing the exemplary embodiments set forth herein are to be incorporated into the claims unless explicitly recited in the claims themselves. Finally, it is to be understood that it is not necessary to meet any or all of the recited advantages or objects of the invention disclosed herein in order to fall within the scope of any claim, since the invention is defined by the claims and since inherent and/or unforeseen advantages of the present invention may exist even though they may not have been explicitly discussed herein.
This application claims priority to U.S. Provisional App. Ser. No. 60/647,123, filed Jan. 26, 2005, the contents of which, to the extent not inconsistent herewith, are hereby incorporated by reference as if fully rewritten herein
This invention was made with Government support under Agreement No. DE-FC26-03NT41881 awarded by the Department of Energy, NETL. The Government may have certain rights to this invention.
| Number | Date | Country | |
|---|---|---|---|
| 60647123 | Jan 2005 | US |
