TESTING OF BURIED PIPES BY SLOFEC TECHNIQUE IN COMBINATION WITH A MOTOR-DRIVEN CRAWLER SYSTEM. W. Kelb, KontrollTechnik, Germany

Transcription

1 More Info at Open Access Database Introduction TESTING OF BURIED PIPES BY SLOFEC TECHNIQUE IN COMBINATION WITH A MOTOR-DRIVEN CRAWLER SYSTEM W. Kelb, KontrollTechnik, Germany In 2001 Kontrolltechnik applied the SLOFEC - technique for the inspection of storage tankfloors and above ground pipes for the first time. In the meantime scanners, inspection procedures and signal evaluation have undergone several improvements to meet client requirements and to expand the area of applications. In this sense development of the SLOFEC - technique for the inspection of buried pipes is a continuous improvement process. Nuclear plants have a large number of buried pipes for cooling and process water supply and disposal. Due to the age of those pipes, the demand for inspection of such pipework is rising. These pipeworks generally were not designed with piggability in mind. Inspection systems therefore have to fulfill multifarious requirements to be applicable to the inspection of non-piggable pipes. The requirements can be classified as follows: Accessibility requirements to the inspection tool are: In many cases pipelines are not foreseen to be piggable, meaning that launching/receiving stations are nonexistent. Therefore for insertion of the tool only a short pipe section or an armature can be removed or the tool has to be inserted through a dome. In many cases access into the pipe is possible only from one end. The tool must allow bidirectional movement. Therefore the inspection tool must be connectable to a crawler system. Fig. 1: Examples for tool installation situations for testing of buried pipes 172

2 Requirements to the inspection tool due to pipe conditions are: Most of the pipes have short bends with radiuses down to 1.0 x D. The inspection tool must be able to pass such bends. Pipes can be coated on the inner surface by rubber, GRP or concrete. Testing through coatings shall be possible. Comprehensive cleaning prior to the inspection is not possible. The surface can be covered by residue from product, debris, scaling, etc Testing shall also be possible if rest of product, scale or debris remain in the pipe. Fig. 2: Examples for conditions of pipes Requirements to the testing technique are: In many cases the use of foreign products or the use of any liquid is unwanted or even not allowed. Additionally, the use and feeding of coupling media can be difficult. For that reasons inspection techniques working without coupling media are preferred. The testing technique shall reliably identify signals from defects and discriminate those from noise signals e.g. generated by grinded-off transport eyes, external pipe supports, ground anchors etc The testing technique shall evaluate the depths and dimensions of wall thickness reductions with a defined accuracy. The testing technique shall reliably determine and document the position of defects in axial and circumferential directions. Fig. 3:Re-examination of the defect depth detected by the SLOFEC - technique The Inspection Tools Kontrolltechnik has developed three types of inspection tools for the inspection of non- piggable buried pipes, meeting the above stated requirements. By the modular design, each inspection tool can be configured according to the situation on site. 173

3 Fig. 4 Internal Pipe Scanner Type PLM with a motor-driven crawler system from Inspector Systems Fig. 5 Internal Pipe Scanner Type PLS with a motor-driven crawler system from Inspector Systems Fig. 6 Internal Pipe Scanner Type Pegasus during the testing of a drinking water pipe 174

4 The design features of the SLOFEC TM pipe inspection tools are as follows: The SLOFEC TM pipe inspection tool consists of single modules, which are connected by flexible joints. Each module is minimized in length allowing the tool to pass narrow bends down to 1.5 x D. The modules of the tools can be assembled on site if necessary. Even if due to limited access only modules or parts of the tool can be inserted in the pipe in many cases the assembly inside the pipe is possible. Each SLOFEC TM pipe inspection tool can be moved bi-directionally using a crawler system or can be pulled through the pipe by means of a winch. The SLOFEC TM pipe inspection tool type PLS has retractable centering devices and a retractable sensor head to pass sections with a reduced diameter. All inspection tools are equipped with inspection cameras and illumination for para llel video inspection. SLOFEC Internal Pipe Scanner Type PLM The SLOFEC Internal Pipe Scanner Type PLM is designed as a drive-through unit either connected to a crawler system or pulled by a winch. The sensors are spring loaded and arranged between the poles of the magnetization unit. The complete sensor head is guided on wheels for low friction and bi-directional operation. Advantages of the PLM Type Disadvantages of the PLM Type applicable to relative small pipe diameter reaches a relative high inspection speed the net weight of the scanner increases due to the necessary magnetization field strength disproportionate to an increase of the pipe diameter. In consequence, this concept is practically not applicable to pipes with larger diameters. due to the limited number of sensor channels, the resolution decreases with increasing pipe diameter Inspection camera sensor module and centering crawler modules (Inspector Systems) electronic module 175

5 Technical Data : Diameter range : max. wall thickness : 16 mm (depending on the coating thickness and the detection limit) max. coating thickness : 10 mm smallest passable radius max. inspection length max. inspection speed drive systems : 1,5 x pipe diameter (for 10 2,5 x pipe diameter) : 300 m : 6 m/min : electr. crawler / pulling winch SLOFEC - Internal-Pipe-Scanner Type PLS The SLOFEC TM Internal Pipe Scanner Type PLS is a scanner unit with a rotating sensor head. The scanner unit is designed to be connected either to a crawler system or pulled by a winch. For data acquisition the scanner will be positioned by the crawler or winch in axial direction. In position, the sensor head will be pressed in radial direction against the pipe wall by means of pneumatic or hydraulic cylinders and moved in circumferential directio n. After completion of a complete circumferential scan, the sensor heads will be retracted and the complete pipe testing unit will be moved by one sensor head width in axial direction. In this position, the data acquisition procedure will be started again. The scanning procedure is fully automated to reach reasonable inspection velocities. The complete sensor head is guided on wheels for low friction and bi-directional operation. The centering devices can be expanded and folded to pass sections with reduced diameter. Advantages of the PLS Type Disadvantages of the PLS Type the net weight of the scanner compared to the PLM type is drastically reduced. it has a higher resolution than the PLM type. The resolution is independent of the pipe diameter. the centering is variable in diameter and can compensate pipe diameter changes in both directions. dead zones on either side of butt welds and pipe collars are minimized. pipe sections with heavy debris or contamination can be passed without the need to doze the debris in front of the scanner. inspection speed is lower than for the PLM type tools inspection camera pan & tilt sensor module with centering crawler modules (Inspector Systems) electronic module 176

6 Technical Data : Diameter range : (planned up to 48 ) max. wall thickness : 19 mm (depending on the coating thickness and the detection limit) max. coating thickness : 10 mm smallest passable radius : 1,5 x pipe diameter max. inspection length : 300 m max. inspection speed : 0,2 m/min drive systems : electr. crawler / pulling winch SLOFEC TM Internal Pipe Scanner Type Pegasus The SLOFEC TM Internal Pipe Scanner Type Pegasus is a scanner unit with a rotating sensor head. The scanner unit is designed for the inspection of big diameter pipes were a crawler system cannot be applied. With the Pegasus unit inspections with pipe diameters of up to 2m were realized. The unit can either be pulled by a winch or moved manually in humanly accessible pipes. The main advantage of the unit is that it can be completely assembled inside the pipe, allowing a high flexibility for the inspection of pipes, which are accessible only through a dome. Advantages of the Pegasus Type Disadvantages of the Pegasus Type low net weight of the scanner has a higher resolution than the PLM type. The resolution is independent of the pipe diameter fast adaption to different pipe diameter the centering is variable in diameter and can compensate pipe diameter changes in both directions. dead zones on either side of butt welds, pipe collars are minimized. Pipe sections with heavy debris or contamination can be passed without the need to doze the debris in front of the scanner Installation through manholes is possible the inspection speed is lower than for the PLM type tools the scanner cannot be operated in combination with the crawler system Fig. 7 SLOFEC -Internal-Pipe-Scanner Type Pegasus, Installation of the scanner through a manhole, operation of the scanner during inspection of a drinking water pipe with manual feeding 177

7 Technical Data : Diameter range max. wall thickness max. coating thickness smallest passable radius max. inspection length max. inspection speed drive systems : 30 (realized up to 3 m pipe diameter) : 19 mm (depends on the coating thickness and the detection limit) : 10 mm : 5 x pipe diameter : 300 m : 0,2 m/min : pulling winch / manual The SLOFEC - Technique The SLOFEC inspection system is based on the eddy current method with superimposed DC- magnetization. Fig. 8:field distribution in a defect- free sample Fig. 9:field distribution in a sample with wall thickness reduction Figure 8 shows schematically the working principle of the SLOFEC technique at a defect- free sample. A magnetic yoke containing a permanent or electro-magnet is used to generate a strong magnetic field in the material to be tested. The magnetic DC field has an effect to the material properties of the test sample. In a defect-free sample the magnetic field leads to a homogenization of the material properties. Between the poles of the yoke an eddy current sensor is located. This sensor also generates a small alternating magnetic field in the material under test, super-imposed to the magnetic field of the yoke. The eddy current field is sensitive to changes of the material properties of the test sample. In a defect free sample the DC magnetization and therefore the material properties do not change. As a result the eddy current signal remains unchanged. Figure 9 shows the field distribution at a sample with a wall thickness reduction. A reduction of the wall thickness, e.g. by a corrosion pit will result in a concentration of the magnetic fields in the remaining wall, resulting in an increased magnetic field strength above and around the defect. This results in a change of the material properties at this location. This local change of the material properties will be detected by the eddy current sensor. The eddy current signal amplitude of the defect indication is a measure for the volume of metal loss. Figure 10 and 11 show the typical SLOFEC signal responses at the example of 40% deep flat bottom holes placed at the inner and outer surface of the pipe. Characteristic is the phase difference of signals from external and internal defects. 178

8 Fig. 10 S i g n a l response from a flat bottom hole at the inner surface of the pipe Fig. 11 S i g n a l response from a flat bottom hole at the outer surface of the pipe Influence parameter and signal analysis The signal response of the SLOFEC technique depends on several influence parameters as shown in figure 12. Fig. 12 Influence parameters to SLOFEC signal response Like all other electromagnetic testing techniques, SLOFEC is a comparative technique, meaning that the signal response from a defect will be compared to signal responses from known defects and evaluated accordingly. In other applications like e.g. SLOFEC tankfloor testing, follow-up tests e.g. by ultrasonic testing can be performed to confirm the evaluated depths. In the field of buried pipe inspection due to the limited access to the pipe, follow-up tests are not possible or are time and costs intensive. Therefore more comprehensive data analyzing procedures under consideration of the influent parameters have to be applied than it would be necessary for other applications. The defect volume has essential influence on the SLOFEC signal response and is therefore a major influence factor. Following the SLOFEC depth evaluation procedure under consideration of the defect volume is shown. The defect depth evaluation will be performed using so-called amplitude defect depth calibration curves, showing the relation between defect depth and signal amplitude. For generation of the amplitude defect depth calibration curves flat bottom holes and half-round shaped holes with known depths are used. Unique amplitude defect depth calibration curves can be generated for different hole diameters (see figure 13). Several of these calibration curves are stored in the data analysis software. 179

9 Fig. 13 amplitude defect depth calibration curves for flat bottom and half-round holes at the outer surface of the pipe The signal analysis software evaluates besides the signal amplitude and signal phase also the distance between the amplitude maxima, which is proportional to the diameter of the defect (figure 14). The signal phase determines, if the signal is generated by an internal or external defect. Fig. 14 Evaluation of the amplitude maxima distance All of the amplitude defect depth calibration curves represent a plane as shown in figure 15. Using the values of the signal amplitude and the distance of the amplitude maxima a point in the plane will be determined, which is used for the depth evaluation. Points between two calibration curves are calculated by interpolation. Fig. 15 Evaluation of the defect depth under consideration of the defect volume 180

10 By application of the above described and additional measures a satisfying accuracy in defect depth determination even for a large spectrum of different defects is achieved. Figure 16 shows the result of a blind test with more than 60 defects with different dimensions and forms. The tolerance of ±20% in defect depth determination corresponds to the accuracy of intelligent pigs. Fig. 16 Accuracy in defect depth determination (result of a blind test) Detection limit for various types of defects The detection limit for particular defects depend on the magnetization field strength the tool can generate, the orientation of the magnetic field relative to the defect, the distance of the sensors to the surface and the condition of the pipe surface. Figure 17 gives a general overview about the detection limit for the SLOFEC internal pipe scanner type PLS and type Pegasus. Due to the fact that for both scanner types the magnetic field is oriented in circumferential direction of the pipe, all defects with an extent in axial direction have a good detectability. Such defects are slots and notches with axial orientation as well as pitting and trough shaped wall thickness reductions. With an increasing ratio between circumferential length to axial width the detection limit will decrease. The same is valid for big planar defects with small gradients. The detection limit for planar defects decreases with increasing areal extent and decreasing gradient. Fig. 17 detection limit for various types of defects for the SLOFEC Internal Pipe Scanner Type PLS and Type Pegasus 181

11 The SLOFEC internal pipe scanner type PLM has qualitative the same detection limit to various defect types. However due to the orientation of magnetic field lines in axial direction of the pipe for the PLM types, the detection limit for defects with circumferential orientation is higher and the detection limit for defects with axial orientation lower. SLOFEC - data acquisition and result documentation During the SLOFEC data acquisition the signals of all sensor channels are displayed online. By this function the operator can overview the signal data quality and repeat the data acquisition if necessary. Figures 18 and 19 show the typical signal data of all 8 sensor channels of the PLS type scanner for one turn of the sensor head. Directly after completion of the data acquisition the software analyses the data and shows the operator the results. The operator can now confirm, delete, edit the result data or select from the chart display sections to be analyzed. For documentation the raw signal data as well as the result data will be stored. Figure 18 shows the screen of the data acquisition software for a pipe section without wall thickness reductions. Only signals caused by surface roughness and lift-off of the sensor head can be seen. Fig. 18 screen of the SLOFEC data acquisition software for a pipe section without wall thickness reductions Figure 19 shows the screen of the data acquisition software for a pipe section with three typical indications caused by pitting shaped defects. The big, red colored indication was caused by a 100% deep through the wall pit and both small, green colored indications were caused by 20% deep pits. Fig. 19 screen of the SLOFEC data acquisition software for a pipe section with indications from a 100% deep pit and two 20% deep pits at the outer surface of the pipe 182

12 After confirmation by the operator the evaluated defect depths are converted into an easy to read colored C-Scan with defined colors representing defect depths. Besides the inspection report, the documentation consists of a colored Pipe-Scan-Report for each tested pipe section (see figure 20) and an indication list (see figure 21). The Pipe-Scan-Report shows a schematic picture of the pipe consisting of single pipe sections. For each tested pipe section the inspection result in colored C-scan mode is shown. Wall thickness reductions are displayed in different colors according to the evaluated depths and the colors defined in the color palette. The indication list contains for each evaluated defect its depth, position and classification as internal or external defect. Fig. 20: Example of a Pipe-Scan- Report Fig. 21: Example of an indication list 183