MeRGWT on Pipe Trunnion Dead Legs for Hidden Corrosion

Transcription

1 MeRGWT on Pipe Trunnion Dead Legs for Hidden Corrosion R. Jenkins, I. Pettigrew, and S. Kenny Specialist Inspection Services, Oceaneering, Unit 5 Castell Close, Swansea, U.K. rjenkins@oceaneering.com, ipettigrew@oceaneering.com, skenny@oceaneering.com More info about this article: Abstract Corrosion initiating from entrapment of water and deposits in crevices in dead legs is a significant threat to the integrity of the upstream petroleum sector, with corrosion rates > 2 mm per year reported on lines with previously minimal historical wall loss. Conventional ultrasonic and radiographic examination methods are not generally possible due to the complex geometry and obstructions posed by a dead leg. Small-diameter dead legs can be interrogated with the use of profile radiography. However, this technique is unsuitable for larger-diameter dead legs. Medium-range guided wave testing (MeRGWT) is an in-service rapid-screening tool used to detect corrosion and erosion in areas that are inaccessible for direct contact methods, and it is particularly valuable where there is only single-sided access to the component. This paper presents a state-of-the-art qualitative solution for inspection of dead legs through the application of MeRGWT. Evidence is provided in the form of a feasibility study investigating simulated external damage on a pipe bend under a trunnion dead leg. The paper includes the use of reference plates for verification, semi-automated MeRGWT inspection of the pipe bend, and a comparison of results with visual and radiographic inspections. The inspection system incorporates an encoded chain scanner that can be used with rope access to allow greater flexibility of deployment without the need for scaffolding. This paper also demonstrates that MeRGWT is an effective tool for the detection and qualitative classification of corrosion, and, given the complex geometry of the test subject, that it is capable of determining the circumferential and axial extents of a corrosion patch within reasonable tolerances. Keywords: NDT, medium-range guided wave testing (MeRGWT), trunnion dead legs, localized corrosion 109 1

2 1.0 Background Industry standards identify critical design flaws that trap water or moisture in crevices, such as dead legs, as being more prone to attack from corrosion [1]. In a similar manner to corrosion under insulation, corrosion inside a dead leg is not visible, and, therefore, a structured and systematic approach to inspection is required. Many examples of dramatic loss in wall thickness have been documented resulting from stagnant conditions: Step Change in Safety [2] describes a case of a wall loss of 2.2 mm per year; the U.S. Department of Transportation [3] explained a case of a natural gas release where a 83.6% wall loss due to microbiologically induced corrosion (MIC) caused a pipeline rupture at a dead leg; and the API Advisory Committee [4] defined internal corrosion in facilities as being almost exclusively confined to facility piping with intermittent or no flow, such as dead legs. Conventional inspection techniques, such as direct visual inspection and manual ultrasonic testing, are not practicable solutions for dead legs attached to pipework, particularly on bends. The application of profile radiography is limited to smaller-bore pipework normally less than 150 mm in diameter. 2.0 Medium-Range Guided Wave Testing (MeRGWT) Medium-range guided wave testing (MeRGWT) is a qualitative screening tool also referred to as short-range ultrasonic testing [5], long-range ultrasonics [6], or higher-order modes cluster (HOMC) guided wave testing [7], among others. The technique has been used extensively for many years, particularly for corrosion detection at locations with limited access, such as pipe supports, air to ground interfaces, clamp supports, and buried pipework. MeRGWT uses an ultrasonic transducer to pulse low-frequency torsional waves into the medium being inspected. The torsional wave mode converts into laminar waves and reflects back to the same transducer when an anomaly or feature is detected. The time of flight from the reflected signals determines the distance of the feature from the transducer exit position (Figure 1)

3 Figure 1 Simple dead leg with MeRGWT probe and corrosion patch Both the circumferential and axial position of responses can be established when scanning is carried out with an encoder tracking circumferential position. Benefits of the technique include being able to perform inspections in a limited space or in inaccessible areas and with close proximity, full data recording, and portability of the equipment. Capabilities of the technique are being able to inspect at high temperatures (up to 350 C/662 F), testing up to a range of 1 meter (3 feet) from the transducer, low-cost screening with full coverage, and flexibility of the inspection from small-bore pipe to flatplat geometries. Limitations of the method include the inability to differentiate between top and bottom side anomalies, no quantitative readings for wall thickness, and transducer surface placement cleanliness and access. Although MeRGWT relies on higher-order mode cluster guided waves, it is essentially a pulse-echo ultrasonic method. Therefore, in the case of trunnion supports on pipe bends, issues of complex geometry can be overcome by differentiating between the weld position of the trunnion on the bend and any corrosion indications located within that area. This paper details a fresh application of MeRGWT for the inspection of trunnion supports on pipe bends. The purpose of the inspection was to determine the ability of MeRGWT to detect artificial external surface flaws representative of corrosion under insulation (Figure 4), along with corrosion positioned underneath the trunnion supports on the extrados of the pipe bends, and then to compare the experimental results with radiography and visual testing. For most ultrasonic inspection methods, the amplitude response of indications is highly reliant on the morphology and orientation of the reflector [8]. Further variables, such as coupling, surface 109 3

4 conditions, mode conversions, and general scattering and attenuation of ultrasound can have an effect on the signal amplitude received [9]. As such, this paper also discusses the variation in MeRGWT responses obtained from different types of reflectors to show that there is no linear correlation in signal amplitude for the severity of all types of corrosion when performing MeRGWT, as discussed in Section Experimental Technique A low-frequency MeRGWT probe with an integrated wedge driven by a phased array flaw detector was used to inspect two test specimens with 90 bends and welded trunnion supports, as shown in Figure 2. The test specimens consisted of an 8-inch sch 40 (8.18-mm nominal wall) 90 elbow, butt-welded between two straight pipe sections. A trunnion support was fillet-welded onto the 90 elbow. Before attaching the trunnion support, external surface gouges were machined (Figures 4 and 5) to simulate localized corrosion found at dead leg locations, as illustrated in Figure 3. Figure 2 MeRGWT probe configuration for inspection of a pipe bend trunnion dead leg 109 4

5 Figure 3 Example of corrosion under insulation (CUI) a type of corrosion [1] Figure 4 Test Specimen One with external gouges on the extrados of the elbow 109 5

6 Figure 5 Test Specimen Two with external gouges on the extrados of the elbow Table 1 details the physical dimension measurements of the external gouges. Table 1 External physical measurements of gouged areas Test specimen no. Axial extents (mm) Radial extents (mm) One Two The maximum wall loss, or severity, for each test specimen was different. Profile radiography was performed to determine the maximum wall loss for the gouged areas, as shown in Figures 6 and 7 and in Table

7 Figure 6 Profile radiography for Test Specimen One Figure 7 Profile radiography for Test Specimen Two Table 2 details the measured wall loss for each specimen. Table 2 Wall loss for each test specimen, as measured by profile radiography Test specimen no. Wall loss (mm) Wall loss as a percentage of measured full wall (%) One Two

8 Figure 8 Test specimen pipe end showing probe positioning for the TCG The MeRGWT probe was calibrated for velocity and wedge delay by using standard industry calibration blocks, and the velocity of sound was found to be that of a standard shear wave in steel. A time corrected gain (TCG) calibration was carried out on the test specimen pipe ends with a six-point TCG created over a range of 100 mm to 600 mm, at 100-mm increments. The sensitivity for the test was also set in this way, with an additional 6 db added for scanning. Figure 8 illustrates the pipe end and probe positioning for the creation of the TCG. A reference test plate was manufactured with several round bottom holes of different depths to confirm that the system would detect localized corrosion in discrete locations that were artificially replicated in the test specimens. Ultrasound was directed perpendicularly to the plate edge toward the center of the plate at a fixed stand-off distance from the center line of the reference reflector. The deepest round bottom hole was the first reference reflector in the positive encoded direction. Figure 9 demonstrates the reference plate, round bottom holes (external), and probe positioning for the system calibration Figure 9 Reference test plate with round bottom holes showing probe positioning 109 8

9 Figure 10 is a data image from the MeRGWT data collected from the reference plate. The screenshot shows B-scan data on the right side of the image, along with A-scan data on the left side of the image. The A-scan shows the pulse echo response in relation to distance from the probe (vertical scale) and the amplitude of the response (horizontal scale). The B-scan is created from the A-scan raw data. The B-scan horizontal X-axis represents probe travel, and the vertical Y-axis represents distance from the probe. An A-scan was collected for each 1 mm of probe travel. A color palette was used to represent amplitude of the A-scan; red represents greater than 100% full screen height, while the large band of red at the top of the B-scan is the initial pulse and the large band of red at the bottom of the B-scan is the response from the far edge of the test plate. The variation in color is representative of a variation in wall thickness. The rainbow color palette represents blue at the top end of the thickness, decreasing in thickness through the color spectrum (green to yellow to orange), with the red area representing the thinnest of the wall thickness measurements. The four reference reflectors are annotated through the center of the B-scan. The deepest round bottom hole produces the greatest amplitude response, while the shallowest round bottom hole produces the lowest amplitude response. 1 [Gra 2 3 b 4 Figure 10 Reference plate MeRGWT data screenshot with deepest to shallowest hole depths (1 4) For scanning of the trunnion dead leg specimens, the probe configuration was mounted onto an encoded chain scanner and attached to the pipe (Figure 2). The probe was positioned on the bend side of the weld on both test specimens to eliminate the loss of energy encountered at welds. 4.0 Experimental Results The elbow trunnion extents were scanned with MeRGWT, which does not differentiate between wall loss and attachments to the wall, and, therefore, responses were obtained from the fillet weld of the 109 9

10 trunnion support. These geometric reflections on the B-scan data allowed definition of the trunnion weld position and any reflected responses within that area to be recorded. Figures 11 and 12 show the MeRGWT data for Test Specimen One and Test Specimen Two, with the fillet weld position annotated on each B-scan image. In the absence of corrosion, only material noise is expected. However, during these trials, the machined area representing localized hidden corrosion was detected on both specimens. Figures 11 and 12 show the cursor positions for sizing the indications from inside the fillet weld boundary on each specimen. In these figures, the A-scan data cursor is positioned through the indication of highest amplitude within the fillet weld boundary. Figure 11 Test Specimen One: MeRGWT data with the geometric reflection from the fillet weld (dashed line) and showing the additional reflections within the fillet weld corresponding to the position of the simulated localized corrosion (black box)

11 Figure 12 Test Specimen Two: MeRGWT data with the geometric reflection from the fillet weld (dashed line) and showing the additional reflections within the fillet weld corresponding to the position of the simulated localized corrosion (black box) Pulse-echo ultrasound relies on the reflection of energy, meaning that the morphology of corrosion has an effect on the amount of energy reflected back to the probe. Therefore, signal amplitude is an unreliable method for determining corrosion severity. However, the likely severity of corrosion may be inferred from the amplitude and extent of indications; for example, where geometry signals are present from welds, their amplitude may be compared to the signal amplitude of any other suspect indications. In this paper, a traffic light system is considered for the likely severity of corrosion responses when using MeRGWT. The density of indications and the amplitude response from the area are also considered when sentencing the welds. Based upon the amplitude of indications, a decision is presented made on the likely severity of corrosion indications as per the following traffic light system: No Corrosion Detected: No indications of corrosion are present. Minor Corrosion Likely: Indications of corrosion are present. However, the amplitude response is low compared to the geometry signals and is not considered a significant wall loss. Monitoring with MeRGWT or an alternative technique is recommended

12 Severe Corrosion Likely: Indications of corrosion are present. The amplitude response is high compared to the geometry signals and the wall loss must be considered to be significant. Immediate further investigation is recommended. A qualitative decision was made on the MeRGWT data for corrosion severity based upon this traffic light system. Figures 11 and 12 show that MeRGWT detected indications beneath the trunnion supports, suggesting that severe corrosion was likely in these locations due to the amplitude and density of these indications (Table 3). The trunnion support dead legs would be classified as such in this instance, and immediate further investigation would be recommended. Table 3 details the MeRGWT dimension measurements of the external gouges for each test specimen taken from the data shown in Figures 11 and 12. Table 3 MeRGWT dimension measurements of gouged areas Test specimen no. Axial extents (mm) Radial extents (mm) One Two Comparison of Results For Test Specimens One and Two, the visual inspection (used for measuring the axial and radial extents of the artificial corrosion), profile radiography (used for measuring the wall loss of the artificial corrosion), and MeRGWT results are provided in Tables 4 and 5. In comparison with visual inspection, the worst-case results show that the MeRGWT data measured axially within ±20% and radially within ±5% of the physical measurements. To correlate the traffic light system of likely severity with the profile-radiography wall loss percentages, the measured values of 24.3% and 52.9% were classified as Severe Corrosion meaning, in practical terms, that severe corrosion likely is indicative of a wall loss of approximately 25%

13 Table 4 Comparison of results for Test Specimen One Technique Axial extents (mm) Radial extents (mm) Maximum wall loss (mm) Corrosion severity classification MeRGWT Severe Corrosion Likely Visual Radiography 2.1 (24%) Table 5 Comparison of results for Test Specimen Two Technique Axial extents (mm) Radial extents (mm) Maximum wall loss (mm) Corrosion severity classification MeRGWT Severe Corrosion Likely Visual Radiography 4.6 (53%) 6.0 Variation of MeRGWT Responses to Reflector Shape MeRGWT was used to interrogate two reflectors of known dimensions in machined calibration plates of similar materials and a thickness of 10 mm to demonstrate the variation in amplitude response and, hence, illustrate the difficulty in making direct correlations between signal amplitude and corrosion severity if the morphology of the expected corrosion type is an unknown. One plate contained a 1- mm-deep notch to simulate an ultrasonic response nominally encountered from cone-shaped reflectors (Figure 13), and the second contained a 5-mm-deep-radius round bottom hole to simulate an ultrasonic response from a dome-shaped reflector (Figure 15), both examined at a standoff of 100 mm from the targets. The MeRGWT data shows that the 1-mm-deep notch produces a maximum amplitude response of 30% full screen height (FSH) (Figure 14). At the same sensitivity, the 5-mm round bottom hole gave a maximum amplitude response of 15% FSH (Figure 16), and, interestingly, there was a reduction in amplitude response by 14 db from the geometric response from the end of the plate

14 Figure 13 MeRGWT setup for examining the 1-mm-deep notch Figure 14 MeRGWT data from the 1-mm notch showing a maximum amplitude of 30% FSH

15 Figure 15 MeRGWT setup for examining the 5-mm-deep-radius round bottom hole Figure 16 MeRGWT data from the 5-mm-deep-radius round bottom hole showing a maximum amplitude of 30% FSH

16 7.0 Conclusion This paper has shown that MeRGWT is able to detect corrosion under trunnion dead leg supports on bend pipework for localized corrosion typically prevalent on this type of geometry. In practice, visual inspection is only achievable from the costly exercise of the removal of the dead leg. A comparison of the MeRGWT results with visual inspection has shown a good correlation of axial and radial extents, providing confidence that the extent of corrosion present can be sufficiently measured. The wall thickness measurements from profile radiography showed the worst wall loss areas to be 24.3% and 52.9% for Test Specimens One and Two, respectively. These areas were both detected on the MeRGWT data sets, with high amplitude responses providing confidence that wall loss 25% can be detected. Akin to conventional ultrasonic testing, the shape of a reflector, and, by extension, the morphology of corrosion, have a critical effect upon the ultrasonic amplitude received when inspecting with MeRGWT. Variation in geometric responses from the fillet weld may be used as additional information to assist in recognizing the severity and type of corrosion in the absence of high amplitude responses. 8.0 Future Work Further trials are in progress with a greater number of test pieces to qualify the accuracy of axial and radial sizing with varying types of corrosion severity from no corrosion or minor corrosion, to severe corrosion with different morphology to have a greater understanding of the limitations of the technique. As an alternative to relying on traditional geometry signals, it would also be beneficial to have a formal system of determining test sensitivity that would allow a more suitable method of categorizing signal amplitude as being low or high. References [1] Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, API 571, 2011 [2] Step Change in Safety/Hydrocarbon release from operational dead leg, July 2017 <

17 [3] Roberson, G., Failure Investigation Report Tennessee Gas Pipeline Co. - Internal Corrosion, U.S. Department of Transportation, 2011 [4] PPTS Operator Advisory: The ins and outs of corrosion releases, API, USA, , 2014 [5] SRUT Short Range Ultrasonic Testing, July 2017, < [6] Hoppenbrouwers, M. B. and J. A. de Raad, The LORUS (Long Range UltraSonic) technique for corrosion detection in hard-to-access locations, 5th BANT/KINT, Liege, 1997 [7] Chandrasekaran, J., I. Anto, K. Balasubramaniam, K. S. Venkataraman, Higher order modes cluster (HOMC) guided waves for online defect detection in annular plate region of above-ground storage tanks, Insight, vol. 51, no. 11, pp , November 2009 [8] Gajdacsi, A. and F. Cegla, The effect of corrosion-induced surface morphology changes on ultrasonically monitored corrosion rates, Smart Mater. Struc., vol. 25, pp. 1-12, 2016 [9] Grosser, P., Ultrasonic detection and measuring of isolated or pitting corrosion, 12th A- PCNDT, Auckland, New Zealand,