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In-Production Electromagnetic Testing of Steel Pipes Used in OilYu.C. Fedosenko, A. A. Polevoda
MSIA 'Spectrum', Russia
Common use of this method of examination can be easily explained by the next well known advantages: relatively high sensitivity, possibility to inspect tubes with rough surface, contactless interaction of electromagnetic probes with the tested object, automatic classification of inspected tubes on the basis "accepted - non accepted" criteria, possibility to automatize the calibration and workability tests operations, the high productivity. This method provides inspection of the tubes through their wall thickness and location of defects' positioning (internal or external) . Method is harmonized within many countries, the requirements for its application present in API (USA) & DIN (Germany) Standards, as well as in GOSTs (Russia). The basic standardized parameters are described in the international standard ISO9402. A number of well known firms are manufacturing variety of relative instruments, for example, the Dr. Ferster Institute (Germany) for tubes inspection proposes the "Rotomat" flaw detector, the "Tuboscope" company (USA) - "Amolog" flaw detector and MSIA "Spectrum" (Russia) - VMD-30N defectoscope and some others [2,3].
The main goal of this paper is to give a short description to the technology of magnetic method implementation in case of inspection of oil assortment tubes, comprising drill pipes, oil well tubing and casing pipes. Conditions of pipes operation are rather tough that is why the strict requirements are imposed on their quality and corresponding control procedures. One of such a requirements is a necessity to detect flaws along the total length of the pipe. At the pipe ends, for example of drilling ones, the thread is fabricated, during further assembling the single pipes are joint via sleeves with internal thread into longer pieces, from said above, it is clear how important is it to arrange proper examination of the pipes' thread zones and this can be done only with the help of the modern NDT methods. The flaw detectors commonly used for inspection of prolonged tubes (6 - 12 m long) have dead zone (non-controlled end parts of the tube with length some 100 mm each). The phenomena of dead zone is predetermined by the physical principal of used method (this is a result of the suppression of the signal reflected from the pipe end). There are two practical ways of tubes inspection: one way is to examine tubes with omission of their end zones, the other - to arrange two additional sites for examination of end parts. Commonly, the tube' ring like parts are controlled by means of the magnetic particle method while its body is examined with the help of automatic magnetoinductive or ultrasonic methods of inspection. It is necessary to underline the fact that the magnetic particle method is rather labor consuming, low productive, requires visual examination of inspected area and is poorly combined with the automated methods due to number of the features (productivity, dependence on the state of the operator, high level of materials consumption, etc.). Described above reasons gave a start to the development and practical implementation of the system for automatic tubes inspection. Above system is based on the implementation of three electromagnetic flaw detectors of VMD-30N type (where realized the combination of magnetic and eddy-current methods) and one eddy-current flaw detector of VD-40P type with the pass-through transducer. In the Figure 1 the scheme of equipment positioning is presented. At the first inspection stage with the help of VMD-30N-01 defectoscope the tube' front end part is examined, at the second stage (defectoscope VMD-30N-02) the tube' rear end part is inspected, at the third (defectoscope VMD-30N-03) and forth (defectoscope VD-40P) stages the body of tube is inspected. After inspection the tubes are demagnetized by means of demagnetizer DT-10P. The VD-40P with pass through transducer is used for detection of short defects with length less than 30 mm and the VMD-30N provides the detection of longer defects.
|Fig 1: The scheme of equipment positioning|
On the transportation roller conveyers 1 and 2 (see fig. 1) the tube is rotated around its axe. The examination cycle at this stages of the system consists of tubes loading on the roller conveyer, longitudinal tube positioning relative to the position of the transducers module, transportation by means of special trolley of the magnetizing module to the area where inspection will be carried out, tube rotation and backward return of trolley to the initial position. On the transportation conveyer where the VMD-30N-03 and VD-40P are installed the tube has rotating-forward motion. The marks made due to signals, generated by the flaw detectors, are done by special paint markers (PM) - PM RD and PM ID.
The defects are divided in two groups, one group includes removable defects (RD) (small surface defects with depth within accepted limits), so these are flaws that may be fixed by scrapping, and irremovable defects (ID) (large internal defects and big surface defects with depth exceeding accepted limits), the presence of these defects rejects the tube from the group of finished and certified product. The tubes are sorted between two pockets, one pocket is for flaws of RD type, the other for ID type. The sorting system is controlled by the central computer. The sorting is performed basing on the inspection results obtained from four (4) flaw detectors included in the system. The central computer provides not only sorting of tubes but also forming and printing of Test certificate, save data obtained during the shift, day, month, year or for the period of five years in the Data base.
The defectoscopic inspection system has next technical parameters:
|Diameter of inspected tubes:||1st performance from 42 to 146 mm;|
2nd performance from 89 to 350 mm.
|Tube wall thickness, mm||3.5 to 20|
|Tube length, m||8 to 12|
|Inspection productivity, pcs per hour||60 to 100|
|Types of detected defects*||racks, fine cracks, scabs, laps, etc.|
|Sensitivity||In accordance with the international standard ISO 9402|
Further on, the basic specific features of implemented defectoscopic instruments will be presented.
For flaw detection the classical magnetic method based on the transverse tube magnetization is used. The leakage of magnetic fields is registered by multiple-unit inductive transducers. Nevertheless, to detect the area of flaws location through the wall thickness (external or internal surface), in contrast to commonly used method of inductive transducer output signal filtration, in this instrument is used the method based on the analysis of supplementary signals generated by high frequency (f = 300 kHz) variable field introduced into system. The field is formed by the multiple-unit eddy-current transducers. The signals from external surface defects are generated both in magnetic probes and eddy-current transducers while the signal from internal surface defects only in magnetic probes. This method give the way to avoid the mistakes of common frequency filtration method when wide surface defects (for example deep pits) are classified as internal defects.
The VMD-30N is equipped with two sensors holders: one holder comprises six magnetic probes, the other - six eddy-current transducers. The measuring circuit has 24 parallel channels, 12 magnetic channels and the same number of eddy-current channels. The channels calibration, the workability tests and logic analysis of informative signals is carried out by built-in programmed microprocessor with further information presentation on the graphic display. The special feature of the magnetic defectoscopic inspection is that the amplitudes of signals from external and internal defects varies depending on the wall thickness and these variation may be from parts of percent and up to three-four times. To provide the high inspection reliability mostly often used the procedure of signals amplitudes rectification but this operation requires specialized, rather comprehensive electronic means.
In the VMD-30N very simple , but efficient algorithm for logic signal processing is used. This algorithm is based on use of three threshold levels or acceptance criteria: V1 level corresponds to the defect' depth exceeding preset limit (V = V1), V2 - for defects with depth within preset limits (V1 < V < V2); V3 - for large defects (V > V3). The algorithm itself is presented in the table 1.
|Number of combination||Channel type||Threshold levels for sorting||Type of defect|
|2||MM||1||0||0||Internal with relatively small depth|
|5||MM||1||1||0||Internal with large depth|
The eddy current flaw detector VD-40P with the pass-trough type of transducer also has some special features.
It is known that one of the main disadvantages of pass through transducers implementation is reduction of inspection reliability in conjunction with the inspected tube diameter increase and this is connected with decrease of the coefficient Ks = Us/Un (well known defectoscopic parameter signal Us to noise Un ratio).
|Fig 2: The construction of screen.|
To inspect the pipes with diameter more that 100 mm, as a rule, the applicable sectioned coils are used. Of course, due to use of special coils the positive result is achieved, but important advantage of pass through systems - simplicity and construction reliability - are lost in this case. It was found out that the same results, as in case with sectioned coils, may be achieved with the help of metal screen of special design. In the last case, the simplicity and reliability typical for pass through transducers is kept. In figure 2 the construction of screen is presented. The screen presents thin-walled pipe with four bow-shaped cuts. The cuts are oriented as follows: along the angle coordinate they displaced on 90° relative to each other and along the longitudinal axe - at certain distance l1 providing required metallic ring between the cuts. The width of cuts is approximately equal to double width of differential pair of measuring windings. In the figure 3 the construction of the pass through transducer with above screen is presented. It comprises four differential pairs of measuring winding that are displaced relative to each other along the transducer' axe and displacement equals to the same value l1 as a gap between cuts on the screen. In such a way, each pair of measuring windings provides control of the quarter of the pipe perimeter (the coefficient Ks increases almost four times due to the noise decreasing and the last phenomena is explained by the reduction of inspected area square). With the help of four pairs of measuring windings the total perimeter of the inspected object is examined. In this type of the transducer the structure of electromagnetic field typical for path through systems is kept. With the help of four additional measuring windings of the through pass type the automatic monitoring over the tube positioning is carried out. The signal does not depend on the gap between the inspected object and differential winding pair.
|Fig 3: The construction of the pass through transducer 1-the bobbin,2-the winding for discover defect,3-the winding of measuring the gap,4-current winding.|
Industrial tests of the defectoscopic system at one of the Russian tube producing plants proved the fact that planned at the design stage basic parameters are real ones. The basic detected tube' flaws - cracks, fine cracks, scabs, laps with depth from 0.5 to 2.2.mm. Reliability of the inspection is 0.9 to 0.95 depending on the level of set sensitivity (the depth of sorted defects can be 5%, 10%, 12.5% of wall thickness). The number of rejected tubes due to presence of flaws at the end zones for several fabricated lots of tubes was from 10 to 50% from the total number of rejected tubes. And the volume of products faulty due to defects at the end zones shows the great necessity to arrange examination of these very zones. The flaw detector VD-40P with the pass through transducer and metal screen provided detection of scabs and cracks with length from 5 to 40 mm with high reliability (0.93 to 0.96).
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