Paper for the PPIM 2015, Feb 11-12, Houston, Texas PPIM 2015 COMBINED ULTRASONIC TETHERED TOOL FOR INSPECTION OF WELD CRACKS IN OFFSHORE PIPELINES: TOOL QUALIFICATION AND FIRST RESULTS FROM OFFSHORE INSPECTION Thor-Ståle Kristiansen KTN Bergen, Norway Hans Petter Bjørgen STATOIL Stjørdal, Norway Herbert Willems NDT Global Stutensee, Germany Guus Wieme KTN Bergen, Norway ABSTRACT Typically inline inspection is carried out for the detection and sizing of geometry defects, corrosion / wall thickness loss and cracking defects. Regarding cracking, most of the time axial cracking (SCC, seam weld cracks...) is observed due to the typical loading condition in pressurized pipes. Since 1994 ultrasonic crack inspection tools are available for the reliable detection of such cracks using conventional ultrasonic shear wave technique. This technique is well suited for detection but has limited depth sizing capabilities as the reflection amplitudes are affected by several other factors apart from depth. Under special conditions (bending forces, poor welding) cracking can also appear in circumferential direction. For this type of cracks, basically the same type of tool can be used with a modified sensor carrier providing similar detection and sizing performance as for axial cracking. In offshore applications, however, accurate sizing in particular of crack depth is an important requirement as offshore verification and repair work is usually very cost-intensive. In order to provide a high POD together with good sizing performance a tethered 10" tool was developed for inspection of circumferential cracks at anode pads welded on the pipes. It contains a pulse - echo (PE) unit for reliable detection and a TOFD (time-of-flight-diffraction) unit for accurate sizing of circumferential cracks. The PE technique is used for fast screening during the forward inspection. The TOFD measurement which is basically stationary is performed on the backward inspection by stopping at every location where a crack indication was detected by the PE inspection. Extensive testing of the new tool was performed at a 10" test pipeline with circumferential cracking at original anode pads. Blind testing took place with 64 artificial weld defects and an unknown number of fatigue cracks at the anode pads which were generated by full scale fatigue testing of the corresponding pipes. After testing the sizes of the fatigue cracks (length, depth) were determined by destructive examination. The comparison of the results from ILI inspection and destructive testing showed that the ILI tethered tool fully met the specification with regard to detection and sizing. After the successful validation the tool was used for a tethered inspection of a 10" offshore pipeline (length 8 km). In the paper the inspection concept and the setup of the new tool will be described. Results from the qualification tests as well as from the first inspection run performed with the new tool will be presented. INTRODUCTION In 2002 Statoil detected prior to start-up at his Åsgard field leaks during pressure testing in two of the 13% Cr 9 flowlines [1]. These leaks were caused by circumferential cracking close to anode pads welded onto the pipelines (Fig. 1). The cracks were found at the superduplex fillet weld toe and initiated in the 13% Cr heat affected zone (HAZ). The initiation mechanism was assumed to be Hydrogen Induced Stress Cracking (HISC) which was caused by hydrogen charging from the cathodic protection system (i.e. the anodes) due to sea water entrance and crevices in the field joint coating adjacent to 1
the anode attachments. Further crack growth and propagation into the base metal pipe wall was created by fatigue loading due to vibrational pipe bending. Several inspections have later indicated many locations with HISC cracks at the different 13% Cr 9 flowlines at Åsgard close to existing anode pads. Leaks and not yet leaking cracks were repaired and investigations were done about the cause and behaviour of the failures. Unfortunately the analysis models used to assess the pipeline lifetime were not certain enough due to not satisfying sizing accuracy of the present inspection pigs used [1]. A more accurate/sensitive smart pig was required. Based on this experience Statoil has initiated a qualification program for a combined Pulse/Echo (PE) and Time-Of-Flight-Diffraction (TOFD) inspection tool for reliable detection and accurate sizing of HISC cracks at the anode pads fillet welds and further crack growth from these initial HISC cracks due to fatigue. Both PE and TOFD inspection methods are well qualified and extensively verified as stand-alone applications. The new approach was based on the idea to combine the particular advantages of either technology in a single inspection tool. This paper presents the qualification activities and results obtained for the combined PE & TOFD inspection tool which should be used later on for internal inspection of the Åsgard 13% Cr flowlines. The combined tool was qualified to be used for reliable detecting and accurate sizing of the possible cracks at the anode pad weld toe for the 13 % Cr Åsgard phase I and II lines. anode pads. Targeted minimum defect size was specified as: min. depth: 1 mm min. length: 10 mm Further important requirements were: Probability of Detection (POD): 90% Depth sizing tolerance: ± 1 mm Wall thickness range (typical): 10 mm 20 mm Acceptable inspection speed INSPECTION TOOL The main idea for the combined inspection tool was to use a PE-unit for screening providing a relatively high inspection speed as well as a high POD and a TOFD unit for precise sizing of any detected crack-like defects afterwards. As the TOFD inspection crucially depends on precise positioning of the ultrasonic probes it has to be applied under stationary conditions. Based on these requirements the following tool setup was obtained. Tool setup Typically, a system for tethered inline inspection comprises three main components (Fig. 2): 1. the inspection vehicle itself with external control station 2. a propulsion device 3. the umbilical winch Figure 1: Anode with anode pad attached to pipe INSPECTION TASK The goal of this validation was to develop and validate an inspection setup suitable for the inline inspection of circumferential cracks located at the fillet welds of the Figure 2: Inspection equipment showing the inspection tool in the launching tray, umbilical winch and control station The 10 tethered tool prepared for the combined inspection consisted of altogether 12 modules (Fig. 3). In the current setup, two different ultrasonic inspection systems where chained together: TOFD-system: consisting of the TOFD scanner, an axial articulation module for precise positioning and an electro-module containing sensor electronics and controllers for probe movement 2
PE-system: consisting of a two-parts sensor carrier for circumferential crack inspection, a frontend module containing the ultrasonic electronics, a data storage module and an odometer module Figure 3: Setup of the 10 tethered inspection tool (top). Lower left: TOFD scanner; Lower right: Sensor carrier for pulse-echo inspection Additionally, the tool can be equipped with a camera module located at the front side. The umbilical attached to the inspection tool vehicle has three main functions in such an operation: it serves as transmission line for electrical power, it includes fibre optical lines for communication and data transmission between the control station and the tool, and in real pipeline operations it also serves as a safety line as it includes a kevlar component with substantial breaking load. For propulsion of the tool a standard BiDi pig was applied (see Fig. 3). Although crawlers are available at KTN this alternative was used during the tests for the sake of simplicity. Pulse-Echo Inspection The principle of the pulse-echo inspection for cracklike defects is shown in Fig. 4. Figure 4: Principle of pulse-echo inspection using 45 shear waves for crack inspection (left) and corresponding ultrasonic signal (A-scan) Shear waves (frequency 4 MHz) propagating under 45 through the pipe wall are used which allow for sensitive detection of radially oriented crack-like defects. For the detection of circumferential (transverse) cracks one half of the sensors is inspecting in the upstream direction while the other half inspects in the downstream direction. In total, the 10 -sensor carrier contains 144 sensors (72 for each direction) providing a sensor spacing of 10 mm. While the sensitivity (POD) of this method is very good, the sizing capability is quite limited as the measured reflection amplitudes depend not only on the crack depth but also on other crack characteristics like orientation and structural details. The issue of depth sizing using ultrasonic methods has been intensively addressed in the past. As an important outcome the TOFD-method [2] was developed a few decades ago which is nowadays well approved for many applications allowing a rather precise sizing of crack-like defects. TOFD Inspection The TOFD method is based on the measurement of the time-of-flight (TOF) of ultrasonic signals (rf signals) that are diffracted by the tips of crack-like defects and 3
thus are mainly independent of the signal amplitude. Fig. 5 below explains the principle showing a typical situation (crack-like defect in a plate). A wideband ultrasonic pulse is send from the transmitter T x to the receiver R x in a pitch-and catch arrangement using mostly longitudinal waves at a transmission angle of 70. The received signal contains several contributions. The lateral wave is a grazing longitudinal wave which is used as reference for the surface location. The backwall signal provides the location of the external surface. Any crack-like defect within the soundfield produces diffraction signals at its tips. In this case the embedded crack causes an upper and a lower crack tip signal showing a phase shift of 180. From the time-of-flights of the tip signals the depth and the height of the reflector can be determined using the time range defined by the lateral wave and the backwall signal. Obviously, only one tip signal will be obtained for surface-breaking cracks. gives an overall image of the pipe wall quality and one setup (setup 1) focused on the external side of the pipewall which is the area where the most critical defects are expected. In order to detect and size as accurate as possible a probe frequency of 10 MHz has been chosen for setup 1 while a frequency of 5 MHZ was used for setup 2. Table 1: Setups used for the TOFD inspection Probe spacing [mm] Probe frequency [MHz] Probe diameter [mm] Wedge angle [ ] Setup 1 58 10 6 70 Setup 2 60 5 6 70 TEST PIPELINE Preparation of test spools For the fabrication of the test spool at Technip`s premises the same pipe dimension and material was used as in the Asgard flowlines: 3 off linepipe 13% Cr 2.5 Mo joints; ID=228,6 mm; WT=12.9 mm; L=12,2 m 3 off linepipe 13% Cr 2,5 Mo joints; ID=228,6 mm; WT=15,6 mm; L=12,2 m The anode pads are identical to the pads welded to the Åsgard flowlines: Anode pad material: 316L Stainless Steel Pad size: Diameter 80 mm, thickness 6.0 mm. Welding Procedure Specification WPS 8A (see [3]) Figure 5: Principle of TODF inspection showing a typical setup (top) and the corresponding signal (bottom) As the time-of-flight can be measured very precisely the depth of crack-like defects is typically determined within a tolerance of ± 1 mm. A further advantage of the TOFD method is the fact that the tip diffraction is widely independent on the orientation of the crack. On the other hand TOFD has some restrictions with regard to reflectors that are located very close to either surface as the signal may be hidden by the much stronger reference signals resulting from the lateral wave and the backwall echo. The width of these "dead zones" is related to the ultrasonic wavelength and can be reduced by using higher frequencies. The TOFD scanner available in the tool can handle two sets of TOFD probes simultaneously. For the tests two different setups were applied (see Table 1). One setup to cover the entire wall thickness (setup 2) which Two types of welding were performed; one type with one weld pass and another type with two weld passes around the pad circumference. Fig. 6 shows the arrangement of the pads on the pipe. Figure 6: Arrangement of anode pads (schematically) Preparation of notch-like defects The spark eroded cracks were initiated at SINTEF during April 2012. The cracks were produced with various 4
depths and lengths according to TQP specifications (see [3], [4]). Eight pipe sections containing altogether 64 pads and 64 EDM notches (one per pad) were prepared. Examples of welded pads are shown in Fig. 7. cracks are taken from the destructive testing results (see next section). As can be seen a wide range of crack sizes was provided. Figure 7: Example showing welded anode pads with EDM notches (a); some notches were prepared with a skew of up to 10 (b) Preparation of fatigue cracks The fatigue cracks were initiated and developed in a rotation rig (Fig. 8) at SINTEF during February-April 2012. The 6 m linepipe including 8 pads was mounted into the resonant rotating rig to obtain real fatigue cracks. As the pipe remains stationary a small weight on the shaft bearing rotates and forces the pipe into a circular resonance mode at around 26 Hz. The fatigue cracks initiate after a certain combination of stress ranges and number of load cycles. To find the requested small initiated cracks, the tests were interrupted regularly and the weld toes were inspected by dye penetrant. Based on earlier experience the measured surface length allows a rough estimate of the cracks depth. Maximum crack depth was estimated to be in the range of 4.0 mm. 6 pipe sections containing altogether 48 pads were prepared. Figure 9: Population of test defects with regard to length and depth Setup of the test pipeline The test loop was assembled by Technip at the Orkanger spool base in Norway. The different joints with fatigue initiated cracks and spark eroded cracks were welded together in one straight test spool according to the joint sequence shown in Fig. 10. Pipe section A, C and H got one weld pass around the pad while sections B, D and E have two weld passes. The total length of the test pipeline was approx. 44 m. For the inspection, the test pipeline was filled with water. Figure 8: Test rig used for the generation of fatigue cracks The population of the test defects (with regard to length and depth) is shown in Fig. 9 for both the EDM notches and the fatigue cracks. The sizes of the fatigue Figure 10: View of the test pipeline. The wall thickness and the sequence of the different sections are shown on top. 5
Figure 11: Procedure used for destructive examination of the fatigue cracks RESULTS Destructive Examination The destruction and examination of the test pieces was performed by Statoil Research lab in Trondheim. After cutting out the test pieces from the test spool (performed by Technip in Orkanger/Norway), the process was divided into 4 steps as follows (Fig. 11): 1 Bending of the test pieces to open the crack 2 Dye penetrant inspection & photographic documentation of cracks 3 Break open all cracks by force 4 Documentation of crack dimensions The results of the destructively measured crack sizes are included in Fig. 9. Ultrasonic Examination The ultrasonic testing took place in the middle of 2012. The inspection runs were performed using water as coupling medium. On the way forward the PE data were recorded. After arrival at the end of the test pipeline the tool was pumped backwards for the TOFD inspection. When arriving at the pad locations the tool was stopped and the TOFD scanner was exactly positioned at the axial ends of each pad. Then the TOFD probes were scanned in circumferential direction along the fillet weld and the TOFD data were recorded. With regard to the PE data the main steps of the data analysis are as follows: 1. Detection of the pads: This task is easily achieved as the pads show up clearly in the C-scans. An example is given in [6]. If no cracks are present the pad indications are caused by reflections from the fillet welds. Cracks will cause additional indications. 2. Checking the pads for crack-like indications: Indications from cracks need to be discriminated against weld indications. Basically, the weld indications are obtained from the opposite inspection direction (upstream vs. downstream) as the crack indications. A distinct discrimination is readily obtained from an examination of the corresponding B- scans. 3. Sizing of detected crack indications: see text below Probability of Detection (POD) The POD results for the PE inspection are summarized in Table 2. Table 2: POD results for EDM notches and fatigue cracks EDMnotches Fatigue Cracks Number (total) Number (in spec) Detected (all) Detected (in spec) 64 56 63 (98%) 56 (100%) 79 64 74 (94%) 63 (98%) All notches that comply with the specification were detected. One notch below spec with a length of 5 mm and a skew of 10 was not detected. With regard to the fatigue cracks out of those within specification only one smaller crack was not detected (l=15 mm, d=1.4 mm). It 6
should be mentioned that all the cracks in section E, which were not considered here, have been detected, too. Length Sizing The length sizing of the PE indications is based on the number of sensors that have detected a defect signal. Here, the distance between two adjacent sensors is 10 mm at a sensor diameter of 15 mm which limits the achievable accuracy. The resolution of the TOFD measurement is much better (approx. 1 mm) allowing for a more precise length measurement. The results of length sizing are shown in Fig. 12. While the TOFD results for the notches are more or less "perfect the scatter is approx. ± 10 mm for the fatigue cracks. One issue came up when the destructive length results were compared with the TOFD results. It turned out that the definition of the "real" crack length was different to some extent when dealing with interrupted cracks. Depending on the size of the gaps the TOFD results were sometimes referring to the length of the individual cracks while the destructive results were referring to the total length. An example is given in Fig. 13 to illustrate the point. After fixing this discrepancy the comparison yielded the (corrected) TOFD results. Examples of TOFD signals in the typical grey scale display are depicted in Fig. 14. Figure 12: Results of ultrasonic length sizing Figure 13: Definition of the crack length (example of interrupted fatigue crack) Figure 14: Examples of TOFD signals. The smallest notch (left arrow) has a depth of 1 mm. For crack lengths > 30 mm a tendency of undersizing is visible for both ultrasonic data sets (PE & TOFD). This can be explained by the fact the cracks are following the curvature of the weld along the edge of the disc-shaped pad (Fig. 16). For longer cracks the skew angle becomes larger than the specified max. skew of 10 and the ultrasonic signals are diminished with increasing length. Depth Sizing In order to emphasize the importance of using the TOFD method for depth sizing the results of the PE inspection are depicted in Fig. 15 showing the maximum reflection amplitude as a function of crack depth. As is well known [5] the amplitudes are leveling off for depths larger than approx. 3 mm. But also for smaller depths the scatter of the data is - even for the EDM notches - too large to ensure a reliable determination solely based on the amplitude information. Furthermore it should be noted that the notch amplitudes are nearly 10 db higher than the amplitudes obtained from the fatigue cracks. This is due to the fact that the artificial notches are much better reflectors than real cracks. 7
Table 3: Sizing tolerances as determined from the inspection results (56 notches & 64 cracks) Tolerances Length-Tol. / mm (90% CF) Depth-Tol. / mm (90% CF) TOFD PE TOFD Fatigue cracks 9 15 1.0 EDM notches 2 10 0.3 INSPECTION STRATEGY Figure 15: Ultrasonic amplitudes from PE measurements vs. crack depth ( EDM notches, fatigue cracks) The results of depth sizing with TOFD are shown in Fig. 16. For the (in terms of geometry) well defined EDM notches the results are again almost perfect. The tolerance band for the fatigue cracks is within ± 1 mm which is in agreement with the expectations. After the successful qualification of the combined inspection tool it was planned to start inspections of offshore flowlines by the middle of 2014. The advantages of the combined inspection regarding POD, inspection time and defect sizing are made clear again in Table 4: Table 4: Advantages of combined inspection PE TOFD Combination POD ++ + ++ Inspection Time ++ -- + Depth Sizing - ++ ++ As the inspection time has a direct impact on the overall costs the advantage of using the PE-method for fast screening is obvious. On the other hand precise sizing of detected crack-like defects as provided by TOFD is important for reliable FFP assessment and for better planning of result-dependent future activities. INSPECTION OF 9" X 8KM FLOWLINE (LIVE RUN) Figure 16: Results of depth sizing using TOFD The measuring tolerances as evaluated from the sizing results are summarized in Table 3 based on a confidence level (CL) of 90 %. All in all the results obtained for the POD as well as for length and depth sizing are considered to be very satisfying. The real inspection of the Asgard B flowline M101 started at October 13th, 2014. Tool settings were done exactly as during the blind test. The combined PE/TOFD tethered tool was pumped from the platform to the template (backward run), the PE module inspected the full circumference with some overlap and stored the data in the tool, at the same time the data were visible online already at site at the control station. The analysis was already carried out during the inspection at the KTN office in Bergen with some time delay. Therefore, the inspection data were copied after every shift from the control PC at the control station to a second PC that was remotely accessible from the KTN office by means of the software tool TeamViewer. After arriving at the end of the pipeline (at the template), a pipe tally was generated from the backward inspection data. From this pipe tally a reversed pipe tally was created for the forward run (from template back to 8
Speed [m/s] Wall thickness [mm] the platform). Furthermore, a reversed feature list (preliminary) was generated containing all the detected anode pads as well as any detected crack-like indications. After completion of the data analysis of this backward inspection, the forward inspection was started. Based on the reversed lists all the pads with crack-like indications found in the backward inspection were approached. At these pads, the TOFD scanner was precisely positioned at the circumferential part of the fillet weld and the TOFD inspection was carried out by scanning around the pipe circumference. The forward movement of the tool was performed by pulling the tool with the umbilical. At the respective pads the fine-positioning is done using the crawler unit. Data analysis of the forward inspection (PE + TOFD) and a comparison of the results were performed after the inspection once the inspection data were available at NDT Global in Stutensee. The speed profile of the first section is shown in Fig. 18. The average speed is about 5 m/min. The axial resolution for the ultrasonic measurement is 0.748 mm. The start distance (0 m) corresponds to the end of the riser. During the run the tool was rather constantly rotating with approx. one rotation per 200 m. The distribution of the wall thickness along the distance is shown in Fig. 19. The majority of the line has a wall thickness of approx. 16 mm. Only a short section after the riser has a wall thickness of approx. 20 mm. 30 25 20 15 10 5 0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 Distance [km] Figure 19: wall thickness distribution vs. distance of the backward run The forward run from template back to the platform took longer because of the additional time needed for the TOFD inspection of the detected crack-like indications (10 stops). Figure 17: vertical launching of the inspection tool Run STAAB14U0: Speed Profile (Min/Max) Odometer Factor [mm]: 0.75 Distance Offset [m]: 0 Average[m/s]: 0.05 Std.Dev[m/s]:0.01 Resolution [m]: 1 0.5 0.4 Fig. 20 shows the odometer profile of the first section of the forward run. Several stops can be recognized (horizontal sections of the profile). At these stops the TOFD inspections were performed which took sometimes a bit longer due to the precise positioning. The receiving of the tool after arriving at the platform is shown in Fig. 4.5. 0.3 0.2 0.29 The data quality achieved was very good for both inspection runs. All the crack sensors worked correctly without any signal loss or sensor failure. 100% of the distance was stored. 0.1 0.0 0 1000 2000 3000 4000 5000 6000 7000 Distance [m] Figure 18: speed profile of the backward run (1st section) Due to the fact that precise sizing (length and depth) of cracks is of utmost importance for the assessment of pipes and the PE method can only detect cracks and crack like features very well, length sizing is less accurate, depth grading even worse (signal amplitude is 9
Distance [m] not only dependent on crack depth but also other properties like orientation, microstructural details and structure of the sound beam) the TOFD results were used mainly for the accurate sizing. Run STAAB14U1: Odometer Profile (Stops) Odometer Factor [mm]: 0.75 Distance Offset [m]: 0 Resolution [m]: 1 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0 00-00:00:00 00-12:00:00 01-00:00:00 01-12:00:00 02-00:00:00 02-12:00:00 Time (dd-hh:mm:ss) Start [m]: 0 End [m]: 4371.9 Figure 20: odometer profile of 1st section forward run (stops shown by horizontal sections) The criteria used for data analysis were based on the results obtained from the Orkanger tests. Due to the fact that the entire inspection consists of a backward and a forward run it is important to define a reference system valid for all subsequent data analysis activities. So the distance "0 m" is defined as the end of the backward run which is the start of the forward run. (1st flange at the template when approaching it in the backward direction, 1st run!). The end distance (forward run 7,359.11m,) is defined as the start of the riser in the forward direction. Total length measured in the backward run was 7,363.47m between "0 m" and the transition to the riser. The difference of 4.36m in the entire distance measurement is less than 10-³ or about 0.05% and is caused by the tolerances of the odometer wheels In order to have a common reference system for both inspection runs, the distances from the backward run were reversed in the result lists (pipe tally, feature list). The numbering of feature IDs (anode pads, crack-like indications, ambiguous indications) as well as the numbering of the girth welds was made consistent for both runs. Figure 21: C-scan indication of anode pads (top: backward run; bottom forward run). The anode pads were easily found in the backward run as it was already known that they are located close to the girth welds. An example is shown in Fig. 21. As can be seen the pads come in pairs separated in the circumferential position by 180. The distance of the pads to the girth weld is about 5 cm at the near side. Altogether, 88 anode pads (44 pairs) were detected in the backward run and included into the feature list. This amount was in agreement with the information obtained from Statoil. In order to detect any crack-like indications, all the B-scans at the anode pads were checked. As a result 10 indications were found which were classified as "crack-like". During the forward run back to the platform, again 88 anode pads were detected and entered into the feature list. An example is shown also in Fig. 21. During data analysis of the forward run the 10 crack-like indications from the backward run were checked and the following results were obtained (results of backward run in brackets): < 1 mm 3 (1) 1 mm 2 (2) 1 mm - 2 mm: 3 (6) 2 mm - 4 mm 1 (1) 10
Furthermore, some indications with amplitudes < 40 db were classified as "ambiguous". The overall reproducibility of the PE data achieved between the two runs was very good. Comparison of Results: Pulse-Echo Inspection / TOFD Inspection The results of the TOFD inspection was reported in a separate part of the combined report. A comparison with the PE results is given in Table 5 which also contains the results for both PE inspection runs. The comparison includes the results of length and depth measurements Table 5: comparison of PE and TOFD results Three of the crack-like 11 indications in Table 5 were not detected by TOFD. In the forward run, these indications have depth grading < 1 mm (including feature 5). Thus, they might be in the depth range that is close to the resolution limit of TOFD. One feature (ID 8) was detected only in the backward run. One crack-like feature from the forward run (feature ID 11) was not yet known at the time of the TOFD inspection. Taking into account the tolerances for length sizing (± 15 mm for PE & ± 9 mm for TOFD) all deviations of the length were in the tolerance band. Also the differences between forward and backward inspection were well within tolerance. Based on the TOFD tolerance for depth sizing, which was determined from the blind tests as ± 1 mm, the differences with regard to the PE-data were quite low. The scatter in the PE depths was in the normal range. In particular, the deepest feature from both PE inspections was also the deepest one obtained by the TOFD inspection. Summarizing, it can be stated that the inspection was very successful proving that the approach of combining the PE-method with the TOFD-method is a big step forward for this specific inspection task. Although cracklike features were detected at approx. 12 % of the anode pads, the crack sizes verified by the TOFD method do not pose any immediate threat to this pipeline. SUMMARY & OUTLOOK Based on blind tests with 64 EDM notches and 80 fatigue cracks a new inspection tool combining ultrasonic PE unit for crack detection and a TOFD unit has been qualified for the detection and sizing of circumferential fatigue cracks at anode pads applied in offshore flowlines. After the ultrasonic inspection the crack sizes were determined destructively and compared with the ultrasonic results. The results proved that the required specification with regard to POD (POD > 90 %) and sizing (tolerance of depth sizing 1 mm) has been successfully met. This new tool which combines the advantages of the PE-method (high POD, fast screening) and the TOFDmethod (precise depth and length sizing) was used first time in an offshore pipeline of Statoil Asgard B in October 2014. The results from the 2 PE inspection data (1 from the backward and 1 from the forward run) and the results from the TOFD (forward) run were adjusted and showed excellent match considering the tolerances of the technologies. The inspection with this new tool was not only successful and helped to deliver precise crack sizing data for the assessment but also helped to save time and money due to the combination of technologies in one tool and one run. This technology can be used for distances up to 10km (12km from 2nd quarter 2015) depending on the capacity of the available umbilical and winch and on the geometrical structure (e.g. type and number of bends) of the pipeline. This type of combined inspection is of course not limited to the application described here. The same approach can be applied for the inspection of pipeline welds. Applications are also inspections of steel catenary risers were fatigue loading may cause girth weld cracking. 11
ACKNOWLEDGMENTS The contributions provided by SINTEF (defect preparation, fatigue testing), STATOIL Research Lab / Trondheim (destructive testing) and TECHNIP (setup of the test pipeline in Orkanger) are gratefully acknowledged. REFERENCES [1] I. Grytdal, T. Huseby, G. Rørvik, P.E. Kvaale, T. Håbrekke, and O. Ørjasæther, "Examination of 13% Cr flowlines in service: HISC sensitivity and NDT techniques for inspection of flowlines in service, Paper Ostend 2009-103, Pipeline Technology Conference, Ostend, 12-14 October 2009. [2] M.G. Silk, "The use of diffraction based time-of-flight measurements to locate and size defects", British Journal of Non-destructive Testing, May 1984, pp. 208-213. [3] SINTEF project 802009-Initiation of fatigue cracks at anode pads on pipes Åsgard, Ver 02 [4] Lab-2012067 NDT_vs Opened cracks, Statoil Research Lab Rotvoll [5] H. Willems, M. Nadler, M. Werle, and O.A. Barbian; "First experiences with ultrasonic in-line detection of circumferential cracks in pipelines, Corrosion 2006, NACE Internationals 61th Annual Conference, San Diego, March 12-16; 2006. [6] H. Willems, H.P. Bjørgen, T.S. Kristiansen, G. Wieme; "Qualification of a combined UT inspection tool for detection and sizing of circumferential weld cracks in offshore pipelines", 10th IPC 2014, Sept. 29 - Oct. 3, Calgary, Canada. 12