Abstract:
A apparatus for pipeline integrity monitoring comprising a magnetically permeable backing bar and at least three magnets comprising a relatively medium-strength magnet positioned at one end of the backing bar, a relatively low-strength magnet positioned at the other end of the backing bar, and a relative high-strength magnet positioned between the medium-strength and the low-strength magnet. The at least three magnets are adapted and positioned to induce a plurality of resultant fields within the pipeline wall comprising a first resultant field suitable for detecting a reduced metal-related anomaly and a second resultant field suitable for detecting a mechanically worked-related anomaly. Preferably, the first resultant field has a strength greater than 120 Oersted and the second resultant field has a strength between 40 and 80 Oersted.

Description:
This invention was made with Government support under Agreement No. DTRS56-02-T-0002 awarded by the Department of Transportation, RSPA. The Government may have certain rights to this invention. 

   FIELD OF THE INVENTION 
   This invention relates generally to apparatus designed and operated to inspect magnetically permeable objects, pipeline 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 utilize magnetic field leakage (“MFL”) in pipeline integrity management programs. 
   BACKGROUND OF THE INVENTION 
   Much of the lifeblood of the world economy flows through pipeline transportation 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 12 inches and 60 inches in diameter and extend to thousands of miles in length. In addition, there are curves and bends along the pipeline with radii of curvature of generally about three times the pipeline diameter, though tighter bends are possible. 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. One inspection method utilizes 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. Such pipeline inspection apparatus may comprise magnetic components to induce magnetic flux (commonly illustrated by lines) 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-based apparatus have the capability of addressing, with gas pipelines for example, nearly all the threats listed in ASME B31.8S except incorrect operation and incorrect equipment. 
   Other inspection methods include, for example, inducing eddy 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 (“mph”) relative to the pipeline wall to generate measurable and reliable eddy current signals. In addition, inspection apparatus which induce and rely upon eddy currents must necessarily induce only magnetically saturated states to reduce permeability and allow the eddy currents to penetrate the entire pipeline wall thickness. Sensors are placed to detect maximum eddy current. In contrast, MFL-based apparatus rely upon MFL for detecting changes in the magnetic field associated with corrosion and mechanical damage. Eddy currents, which can interfere with the MFL signals, are minimized by selecting a suitable velocity for the apparatus and by positioning sensors where any spurious eddy currents are at a minimum and the magnetic field is most constant. 
   Each implementation of MFL technology typically focuses on a subset of pipeline wall anomalies that affect pipeline integrity. And, there are varying levels of success, or sensitivity, for each implementation and not all implementations will provide sufficient information for detailed defect assessments. 
   MFL-based apparatus for detecting corrosion commonly use high magnetic fields to saturate the pipeline material. Such high magnetic field-based apparatus help suppress noise due to local stress variations and changes in the microstructure of the metal. At metal-loss defects, such as those caused by corrosion, an increased amount of magnetic flux attempts to flow through the remaining material, but some flux leaks from the pipeline wall. In addition, a second phenomenon causes even more flux to leak. In magnetically saturated materials, an increase in flux causes the flux-carrying capability (permeability) to decrease. This double effect of increased flux and decreased flux-carrying capacity results in significant flux leakage at such defects. 
   Stress and material variations can also change the flux-carrying capacity of magnetic materials such as metal pipe. A local decrease in flux-carrying capacity causes leakage similar to that resulting from metal-loss defects. A local increase in flux-carrying capacity causes a decrease in flux leakage relative to the nominal, magnetic field level. For example, for tensile stresses, the overall flux levels in the pipeline increase. For compressive stresses, such as cold-worked areas, the flux levels decrease. It is known, for example, that such flux density variations between tensile stresses and compressive stresses are small for magnetic field levels greater that about 80 Oersted and particularly for magnetic field levels greater than about 120 Oersted. As will be appreciated by one skilled in the art, however, these values may vary by up to 20 percent with pipeline wall chemical composition, grain structure, and fabrication methods. As discussed above, most MFL-based apparatus for corrosion are designed to operate above these levels to reduce stress noise. To detect stress changes in the pipeline wall, however, the magnetic field must be at lower, unsaturated levels, typically about between 50 and 70 Oersted. Unfortunately, field levels in this range can produce results that are difficult to interpret because they can be affected by corrosion, stresses, and changes in material composition. For example, stress damage is often accompanied by metal loss due to corrosion. While current commercially-available low field strength MFL-based apparatus can be used to detect stresses and material variations using fields in the range of about 50 to 70 Oersted, noise and signal processing and interpretation to properly detect anomalies is difficult. 
   Thus, two magnetic field levels can improve the detection and assessment of pipeline anomalies. The high magnetic field employed in most inspection apparatus detects and sizes metal loss such as corrosion. A low magnetic field must also be applied to detect the metallurgical changes caused by mechanical damage (e.g., from excavation equipment). It is known, therefore, to utilize an approach using more than one apparatus sent separately through the pipeline. This approach, however, is expensive and disruptive to the operation of the pipeline. A single apparatus having two separate sets of magnetizers, while a technically feasible way to apply dual magnetization technology, results in an increase in the size of the apparatus to unacceptable lengths. In addition, when the two magnetizers are placed in close proximity to one another, a magnetic interaction occurs which distorts the constant magnetizing fluxes. In this case, the lower field slightly increases the high field, but, more importantly, the high field distorts the zone of constant magnetization for low magnetization levels. Furthermore, this effect is more pronounced at higher apparatus velocities. 
   There is, therefore, a need for a single MFL-based apparatus and method which is capable of detecting metal loss such as corrosion as well as stresses such as those caused by mechanical damage. 
   BRIEF DESCRIPTION OF THE INVENTION 
   An improved design for an MFL-based pipeline inspection apparatus has been developed. Both high and low fields are attained with a three-pole configuration. Preferably, the strongest pole is at the center of the apparatus and tuned, weaker poles at the leading and trailing edges. The weaker poles have the same magnetic polarity and the strong pole at the center has an opposite magnetic polarity. The design may also be implemented with a backing bar that pivots at bends and obstructions and which, when the pivot point is placed at a magnetic null point in the center pole, will not significantly interfere with the magnetic performance of the apparatus. 
   In one embodiment of the present invention, an apparatus for pipeline integrity monitoring comprises a magnetically permeable backing bar, a first magnet having a relatively moderate magnetizing field strength positioned proximate to a first end of the backing bar, a second magnet having a relatively strong magnetizing field strength positioned proximate to the central portion of the backing bar, and a third magnet having a relatively moderate magnetizing field strength positioned proximate to a second end of the backing bar. Preferably, the polarity of the second magnet is opposite the polarity of the first magnet and the polarity of the third magnet is the same as the polarity of the first magnet. 
   In a more general embodiment of the present invention, an apparatus for detecting anomalies in a magnetically permeable object comprises a magnetically permeable backing bar and a least three magnets wherein the at least three magnets are positioned to induce a plurality of resultant fields. 
   In yet another embodiment of the present invention, a method comprises the steps of inserting an apparatus according to the present invention into a pipeline, traversing the apparatus through at least a portion of the pipeline, inducing at least a first resultant field having a strength greater than 120 Oersted, and inducing at least a second resultant field having strength between 40 and 80 Oersted. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cutaway view of a portion of a conventional two-pole, single-field MFL-based high magnetic field pipeline inspection apparatus showing the effects on magnetic flux caused by a mechanical (stress) defect and a corrosion (metal loss) defect. 
       FIG. 2  is a cutaway view of a portion of a conventional two-pole, single-field MFL-based low magnetic field pipeline inspection apparatus showing the effects on magnetic flux caused by a mechanical (stress) defect and a corrosion (metal loss) defect. 
       FIG. 3  is a cutaway view of a portion of a three-pole, two-field MFL-based pipeline inspection apparatus according to the present invention showing a static baseline flux pattern. 
       FIG. 4  is a cutaway view of a portion of a three-pole, two-field MFL-based pipeline inspection apparatus according to the present invention showing the effects of magnet configuration on a static baseline flux pattern. 
       FIG. 5  is a cutaway view of a portion of a three-pole, two-field MFL-based pipeline inspection apparatus according to the present invention showing the effects of varying magnetic configuration on a static baseline flux pattern. 
       FIG. 6  is a cutaway view of a portion of a three-pole, two-field MFL-based pipeline inspection apparatus according to the present invention showing the effects of varying magnetic strength on a static baseline flux pattern. 
       FIG. 7  is a cutaway view of a portion of a three-pole, two-field MFL-based pipeline inspection apparatus according to the present invention showing the effects of varying pipeline material magnetic properties on a static baseline flux pattern. 
       FIG. 8  is a cutaway view of a portion of a three-pole, two-field MFL-based pipeline inspection apparatus according to the present invention showing the effects of varying pipeline wall thickness on a static baseline flux pattern. 
       FIG. 9  is a cutaway view of a portion of a three-pole, two-field MFL-based pipeline inspection apparatus according to the present invention showing the effects of varying apparatus velocity on the dynamic flux pattern for various pipeline wall thicknesses. 
       FIG. 10  is a cutaway view of a portion of a three-pole, two-field MFL-based pipeline inspection apparatus according to the present invention showing the effects of varying apparatus length on the static baseline flux pattern. 
       FIG. 11  is a cutaway view of a portion of a three-pole, two-field MFL-based pipeline inspection apparatus showing the effects of encountering a bend in the pipeline. 
       FIG. 12  is a cutaway end view of a pipeline inspection apparatus showing the effects of modifying the profile of the backing bar(s) to provide an improved ability to negotiate bends and improved obstruction clearance. 
       FIG. 13  is a partial cutaway elevation view and a partial plan view of a portion of a three-pole, two-field MFL-based pipeline inspection apparatus showing a split backing bar configuration. 
       FIG. 14  is a cutaway end view of a pipeline inspection apparatus showing the effects of a split backing bar configuration to provide an improved ability to negotiate bends and improved obstruction clearance. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIG. 1 , a portion of a conventional high-strength, two-pole, single-field MFL-based apparatus  10  is shown which is adapted to sense anomalies in a pipeline wall  40 . Included are a first high-strength magnet  30  and a second high-strength magnet  30 ′ in magnetic communication with each other through a backing bar  20 . The first high-strength magnet  30  and the second high-strength magnet  30 ′ produce a generally high strength field (e.g., 120 to 200 Oersted) in the pipeline wall  40 . Also included are a first means  32  for enabling magnetic communication between the first high-strength magnet  30  and the pipeline wall  40  and a second means  32 ′ for enabling magnetic communication between the second high-strength magnet  30 ′ and the pipeline wall  40 . As will be appreciated by one skilled in the art, first means  32  and second means  32 ′, while conventionally wire brush-like elements, may be any equivalent structure which enables a high magnetic flux  60  to flow through the pipeline wall  40  and dynamically maintain magnetic communication between the first high-strength magnet  30 , the second high-strength magnet  30 ′, and the pipeline wall  40  as the apparatus  10  travels through along the pipeline wall  40 . Other examples include rollers, leaf springs, and a thin friction-reducing coating. A suitable sensor  50  is provided to detect the leakage or other disturbances in the magnetic flux  60  flowing through the pipeline wall  40 . 
   Shown in graphical inset in  FIG. 1  is an area of stress  70 , caused, for example, by mechanical damage. As will be appreciated by one skilled in the art, the magnetic flux  60  will tend be distorted toward the area of stress  70 . Such distortion may then be detected by the sensor  50 . Shown in related graphical inset in  FIG. 1  is a typical plot of Gauss versus Axial Position for an area of stress  70  as detected by the sensor  50 . As will be appreciated by one skilled in the art, the amplitude  90  of such distortion is small (e.g., generally on the order of about 10 Gauss) and, thus difficult for the sensor  50  to detect and distinguish from background noise. 
   Also shown in graphical inset in  FIG. 1  is an area of metal loss  80 . As will be appreciated by one skilled in the art, the reduced volume of metal caused by the area of metal loss  80  will produce a distortion and leakage of the magnetic flux  60  which may then be detected by the sensor  50 . Shown in related graphical inset in  FIG. 1  is a typical plot of Gauss versus Axial Position for an area of metal loss  80  as detected by the sensor  50 . As will be appreciated by one skilled in the art, the amplitude  92  of such leakage is relatively large (e.g., generally on the order of 100 Gauss or more). The amplitude  92  produced by an area of metal loss  80  is, therefore, more readily distinguished from background noise than the amplitude  90  produced by an area of stress  70 . 
   Referring next to  FIG. 2 , a portion of a conventional low-strength, two-pole, single-field MFL-based apparatus  110  is shown which is adapted to sense anomalies in a pipeline wall  40 . Included are a first low-strength magnet  34  and a second low-strength magnet  34 ′ in magnetic communication with each other through a backing bar  20 . The first low-strength magnet  34  and the second low-strength magnet  34 ′ are sized to produce a resultant field strength in the pipeline wall  40  of 40 to 80 Oersted. Also included are a first means  36  for enabling magnetic communication between the first low-strength magnet  34  and the pipeline wall  40  and a second means  36 ′ for enabling magnetic communication between the second low-strength magnet  34 ′ and the pipeline wall  40 . Again, as will be appreciated by one skilled in the art, first means  36  and second means  36 ′, while conventionally wire brush-like elements, may be any equivalent structure which enables the magnetic flux  160  to flow through the pipeline wall  40  and dynamically maintain magnetic communication between the first low-strength magnet  34 , the second low-strength magnet  34 ′, and the pipeline wall  40  as the apparatus  110  travels through along the pipeline wall  40 . A sensor  50  is provided to detect the leakage or other disturbances in the magnetic flux  160  flowing through the pipeline wall  40 . 
   Shown in graphical inset in  FIG. 2  is an area of stress  70  caused, for example, by mechanical damage. As will be appreciated by one skilled in the art, the magnetic flux  160  will be distorted toward the area of stress  70 . Such distortion may then be detected by the sensor  50 . Shown in related graphical inset in  FIG. 2  is a typical plot of Gauss versus Axial Position for an area of stress  70  as detected by sensor  50 . As will be appreciated by one skilled in the art, the amplitude  94  of such distortion (e.g., generally on the order of about 30 Gauss) is somewhat larger than the amplitude  90  in the high-field case shown in  FIG. 1  and, thus, more easily detected by the sensor  50  and distinguished from background noise. 
   Also shown in graphical inset in  FIG. 2  is an area of metal loss  80 . As will be appreciated by one skilled in the art, the reduced volume of metal caused by the area of metal loss  80  will produce a distortion and leakage of the magnetic flux  160  which may then be detected by the sensor  50 . Shown in related graphical inset in  FIG. 2  is a typical plot of Gauss versus Axial Position for an area of metal loss  80  as detected by the sensor  50 . As will be appreciated by one skilled in the art, the amplitude  96  of such leakage is relatively equal to that produced by the stress  70  (e.g., generally on the order of about 30 Gauss) but of opposite direction. 
     FIG. 3  illustrates a portion of a multi-strength, three-pole, two-field MFL-based apparatus  210  according to the present invention. The portion of the apparatus  210  comprises a first magnet  230 , a second magnet  234 , and a third magnet  231  in magnetic communication with each other through a backing bar  220 . The first magnet  230  is preferably of generally relatively low strength, the second magnet  234  is preferably of generally relatively high strength, and the third magnet  231  is preferably of generally relatively moderate strength compared with the first magnet  230  and the second magnet  231 . Both the absolute and relative strengths of the three magnets  230 ,  234 ,  231  may vary to accommodate differing pipeline wall  40  materials and thicknesses, the type of anomalies detected, and the velocity of the apparatus  210  along the pipeline wall  40 . The strength of the various magnets  230 ,  234 ,  231  is selected to produce a relatively high resultant field  260  of greater that 120 Oersted and a relatively low resultant field  261  of between 40 and 80 Oersted. Applicants have found, however, that a preferred resultant field is effected with the second (strongest) magnet  234  being positioned between the first magnet  230  of generally low strength and the third magnet  231  of generally relatively moderate strength. Furthermore, and importantly, each of the first magnet  230 , the second magnet  234 , and the third magnet  231  may comprise a plurality of magnets together to produce the desired strength. As shown, the first magnet  230  and the second magnet  231  are of like polarity while the third magnet  234  is of opposite polarity. Also included are a first means  232 , a second means  236 , and a third means  233  for enabling magnetic communication between the first magnet  230 , the second magnet  234 , and the third magnet  231 , respectively, and the pipeline wall  40 . As with  FIGS. 1 and 2 , appropriate sensors (not shown in  FIG. 3 ) are included to detect the flux leakage. 
   Also shown in  FIG. 3  is a finite-element analysis (FEA) representation of the nominal field strength in the pipeline wall  40 . A low resultant field is indicated by a curve  202  having a relatively low amplitude but having a relatively longer axial length. A high resultant field is indicated by a curve  204  having a relatively high amplitude and a relatively shorter axial length. As will be appreciated by one skilled in the art, a relatively long section of a relatively flat field is desirable. This is particularly true with the low field strength curve  202  where velocity effects can cause distortion and interference. Also shown in  FIG. 3  is an FEA representation of a null point  206  where the field strength crosses the zero value on the ordinate showing magnetizing field. 
     FIG. 4  illustrates a portion of a multi-strength, three-pole, two-field MFL-based apparatus  310  according to the present invention showing the effects of the position of a second magnet  334 ,  334 ′,  334 ″ on magnetization levels. As the FEA representation of  FIG. 4  shows, the greater the distance between a first magnet  230  and the second magnet  334 ,  334 ′,  334 ″ the longer (and lower amplitude) a resultant field  334   b ,  334   b ′,  334   b ″, respectively. Conversely, the smaller the distance between the second magnet  334 ,  334 ′,  334 ″ and a third magnet  231 , the shorter (and higher amplitude) the resultant field  334   c ,  334   c ′,  334   c ″, respectively. Also shown in  FIG. 4  is an FEA representation of a first null point  334   d , a second null point  334   d ′, and a third null point  334   d ″, corresponding to the position of the second magnet  334 ,  334 ′,  334 ″, respectively. Thus, the null point  334   d ,  334   d ′,  334   d ″ is determined primarily by the position of the second magnet  334 ,  334 ′,  334 ″, respectively, the ratio of the strengths of the high resultant field  334   c ,  334   c ′,  334   c ″, respectively, and low resultant field  334   b ,  334   b ′,  334   b ″, respectively, and the inspection velocity. (The latter effect causes the null point (e.g.,  334   d ) to shift upstream and is discussed below.) 
     FIG. 5  illustrates a portion of a multi-strength, three-pole, two-field MFL-based apparatus  410  according to the present invention showing the effects of the strength of a third magnet  431  on magnetization levels. As the FEA representation of  FIG. 5  shows, the greater the strength of the third magnet  431 , the greater the amplitude of the high-strength resultant field. Curves  431   a ,  431   b ,  431   c ,  431   d , and  431   e  of the FEA representation represent a third magnet  431  strength of 28, 30, 32, 35, and 42 megaGauss-Oersted, respectively. While the low-field curve  402  and the null point  406  show no appreciable change, the high-field amplitude increases and the shape becomes more distorted as the strength of the third magnet  431  increases. 
     FIG. 6  illustrates a portion of a multi-strength, three-pole, two-field MFL-based apparatus  510  according to the present invention showing the effects of varying the strength of each magnet simultaneously. As the FEA representation of  FIG. 6  shows, the greater the strengths of the magnets  530 ,  534 ,  531 , the greater the amplitude of both the low-strength and the high-strength resultant fields. Curves  540   a ,  540   b ,  540   c ,  540   d  of the FEA representation represent magnet strengths that produce a range of resultant field strengths in the pipeline wall  40 . Again, while the null point  560  does not change significantly, the amplitude increases and the shape becomes more distorted as the strength of the magnets  530 ,  534 ,  531  increases. 
     FIG. 7  illustrates a portion of a multi-strength, three-pole, two-field MFL-based apparatus ( FIG. 8 ,  610 ) according to the present invention showing the effects on varying pipeline wall  40  magnetic properties (B-H Curve). As the FEA representation of  FIG. 7  shows, as the B-H Curve becomes more pronounced (greater “knee”), there is little change to the low-strength resultant field, somewhat more pronounced change to the high-strength resultant field  650 ,  652 ,  654  and the null point  660  does appear to not change. 
     FIG. 8  illustrates a portion of a multi-strength, three-pole, two field MFL-based apparatus  610  according to the present invention showing the effects of the thickness of the pipeline wall  40 . As the FEA representation of  FIG. 8  shows, wall thickness can be a significant variable. In the FEA graphic shown, a wall thickness of 0.30 inches produces curve  651 , a wall thickness of 0.50 inches produces curve  653 , and a wall thickness of 0.75 inches produces curve  655 . The magnet configuration was held constant. Various combinations of magnet strength and pole lengths can be used to induce an optimum field level for pipe up to about 0.5 inches thick. While the signal processing technique to detect cold work-material works best when the high resultant field is above 150 Oersted, reasonable results may be obtained if this field exceeds 110 Oersted. Again, the null point  660  does not appear to change. 
   The optimum speed for operation of most MFL-based in-line inspection apparatus is between one and six miles per hour. The lower value is constrained by inspection time (e.g., battery life) and, in some instances, sensor type. The upper end value is determined by the velocity effect of the MFL magnetizer. At higher speeds, MFL signals become distorted by eddy currents generated in the pipeline wall  40 . The speed at which the distortion becomes significant depends to a very great extent upon resultant field level, the thickness of the pipeline wall  40 , and magnetic pole spacing. For a static or very slow-moving magnetizer, the resultant field is uniform across the thickness of the pipeline wall  40 . As the velocity increases, however, changes in the distribution of the resultant field occur as illustrated in  FIGS. 9   a – 9   d.    
     FIG. 9   b  is an FEA representation of the effect of a 30-inch long magnetizer (backing bar  220 ) according to the present invention traveling at 5.0 mph through a pipeline having a wall thickness of 0.3 inches. A zero velocity (static) curve  718  is shown as a reference. As shown in  FIG. 9   b , the resultant field near the inner wall (0.01 inches from the inner wall) (curve  712 ) increases and near the outer wall (0.01 inches from the outer wall) (curve  716 ) decreases. Also, the fields at the inner surface increase most at the center pole. In the high magnetization zone, the field levels are initially decreased and then increased. In the low magnetization zone, the field is decreased at the leading edge, but attains a more-desirable level approximately midway between the poles. 
   Inspection for mechanical damage is focused on the outer surface.  FIG. 9   c  is an FEA representation showing that the onset of velocity effects is not immediate. The configuration shown is for a 30-inch long magnetizer (backing bar  220 ) and a pipeline wall  40  thickness of 0.3 inches with a typical magnetic permeability. At 2.5 mph, the velocity effect is minimal (curve  722 ). At 5.0 mph, there is still a relatively large zone of nearly constant magnetization level (curve  724 ) for sensor placement. (For comparison, curve  720  shows a static condition. As illustrated in  FIG. 9   d , however, while still acceptable, the inspection of a pipeline wall  40  with a thickness of 0.5 inches shows significantly more velocity effects. Curve  726  shows a static condition, curve  728  shows the effects at 2.5 mph, and curve  730  shows the effects at 5.0 mph. Note, too, at higher velocities, the null point  726   a ,  728   a ,  730   a  shifts upstream. 
   To increase the size of the zones of nearly constant magnetization level, a longer magnetizer would be required with appropriate increases in magnet strength. As shown in  FIG. 10 , a comparatively longer apparatus  810  provides a significantly longer zone of constant magnetization. Velocity will also have less of an effect. The effects of length  820   a  (e.g., 36 inches) is shown in the FEA graphic as curve  840   a , the effects of length  820   b  (e.g., 30 inches) as curve  840   b , and the effects of length  820   c  (e.g., 27 inches) as curve  840   c . A longer apparatus  810 , however, presents its own problems since bends and other pipeline features limit the length of the apparatus  810 . 
   Turning now to  FIG. 11   a , a longer apparatus  910  with a proportionally longer backing bar  920  may present problems when negotiating a curve in the pipeline wall  40 . It has been surprisingly found, however, that the magnetic null point  660  (See,  FIG. 8 ) between the high resultant field and the low resultant field enables the placement of an articulated connection  1050  in the backing bar  1020  thus enabling the inspection apparatus  1010  to pass through bends while minimizing interference with the magnetic field. See,  FIGS. 11   b  and  11   c . As will be appreciated by one skilled in the art, the articulated connection  1050  may be any suitable element which enables radial and circumferential relative movement between the multiple segments of the backing bar  1020 . Examples would include, but not be limited to, a complex hinge, a ball-and-socket joint, a universal joint, and a flexible material such as rubber or plastic. Preferably such articulated connection  1050  is magnetic. By properly balancing the relative strengths of the first magnet  1030 , the second magnet  1033 , and a combination third magnet  1034 ,  1034 ′, the desired profile for the field strength  202 ,  204  (e.g.,  FIG. 3 ) may be achieved and there will be no significant change in the magnetic performance of the apparatus. In addition, since, as shown in  FIGS. 9   c  and  9   d , the null point shifts upstream, the two-pole combination third magnet  1034 ,  1034 ′ will be balanced to effect placement of the articulated connection  1050  at the desired dynamic null point (e.g.,  FIG. 9   d ,  728   a ). Applicants have found that the combination third magnet  1034 ,  1034 ′ performs like a single magnet whose magnetizing strength is approximately the sum of the combination  1034 ,  1034 ′. Thus, first pole  1034  of the combination third magnet may be a multiple of a second pole  1034 ′. A so-called inter-pole gap  1040  must exist between the first pole  1034  and the second pole  1034 ′. An inter-pole gap  1040  of zero means that the combination third magnet  1034 ,  1034 ′ collapses to the case of a one-pole magnet. Depending upon the size and configuration of the backing bars  1020 ,  1020 ′ and the expected pipeline bend radius of curvature, the inter-pole gap  1040  should be as small is practicable. Preferably, about one inch or less, more preferably about one-half inch or less. This will allow the combination third magnet  1034 ,  1034 ′ to perform as a single pole magnet while still enabling the apparatus  1010  to negotiate the bends in the pipeline. 
   Even with the articulated connection  1050 , the three-pole magnetizer apparatus  1010  may have to be further adapted to pass bends and obstructions while maintaining its magnetic performance.  FIG. 12  shows an eight-segment magnetizer apparatus  1110  designed to pass a ten percent obstruction  42 . The segments  1112  can be forced together until the widest parts touch. The widest part is typically the backing bar(s)  1120  which may be tapered to facilitate sufficient collapse. Each magnetizer segment  1112  is free to collapse to the center. 
   As shown in  FIG. 13 , since less flux must be carried by the low-field backing bar  1220 , the low-field backing bar  1220  can have a smaller cross-sectional area than the high-field backing bar  1220 ′. The reduction in cross-sectional area can result from reducing the circumferential extent, the radial extent, or a combination. As shown in  FIGS. 13 and 14 , the collapse of the segments  1212  of the magnetizer apparatus  1210  can be enhanced in the low-magnetizer side by splitting the low-field backing bar  1220  into a first sub-segment  1220   a  and a second sub-segment  1220   b , in effect reducing the circumferential extent of the backing bar  1220 . As will be appreciated by one skilled in the art, the combined cross-sectional area of the backing bar  1220  should be about twice the cross-sectional area of the pipeline wall  40 . However, in the low-field backing bar  1220 , the flux density is approximately one-half of the high-field density. Therefore, the cross-section of the low-field backing bar  1220  may be further reduced. Reducing the radial dimension would not provide significant benefit, since this dimension does not constrain the collapse. Keeping the same radial thickness, however, and reducing the circumferential extent will enhance collapse. At the same time, however, the low-field magnetizer must provide a uniform magnetic filed within the pipeline wall. (Not shown in  FIG. 13 .) This can be accomplished by removing a portion of the middle of the low-field backing bar  1220  to form the first sub-segment  1220   a  and the second sub-segment  1220   b . This split backing bar  1220   a ,  1220   b , if designed to carry the flux without saturating, will produce a uniform magnetic field at the desired field level. As shown in  FIG. 13 , the magnet configuration would include a, for example, moderate-strength magnet  1233 , two relatively weaker-strength magnets  1230 ,  1230 ′, and magnets  1234 ,  1234 ′,  1234 ″ which combine into a higher-strength magnet. Looking then at  FIG. 14 , the collapse of the sub-segments  1220   a ,  1220   b  is further enhanced. Now, a tighter bend or an obstruction  44  greater than the ten percent obstruction  42  can be negotiated by the apparatus  1210 . 
   While the present invention has been described in several embodiments, it will be understood that numerous modifications and substitutions may be made without departing from the spirit of the invention. Accordingly, the present invention has been described in several preferred embodiments by way of illustration, rather than limitation.