An operator of a pipeline network in the North Sea in Norway was experiencing problems with several newly constructed offshore flow lines. These pipelines contain sacrificial anodes that are welded to the pipe wall every 100 meters for cathodic protection. Prior to the commissioning of operations, the operator performed a pressure test and found that the pipeline contained circumferential cracks at the toe of the fillet weld of the anodes, resulting in leakage. After closer investigation, the operator determined that the hydrogen charge from the cathodic protection system was, in fact, causing hydrogen-induced stress cracking on the pipeline wall.
The operator needed to obtain critical information on the condition of the 8-kilometer section of these offshore flowlines, including the state of all the welded anodes and detection and sizing of all crack-like features. Due to the limitations of the pipeline design, a specialized bidirectional in-line inspection (ILI) solution was required.
Different components, technology and carrier elements were considered for this inspection, including ultrasonic technology (UT) shear wave and time of flight diffraction (TOFD) technology, as well as free swimming bidirectional tool carriers and tethered bidirectional tool carriers.
Standard free-swimming tools often utilize UT shear wave technology. Although this technology is able to detect and differentiate between internal and external cracks, and length sizing is possible, it is not sensitive enough to quantitatively measure crack depth. In this case, where the crack depth must be precisely determined, this standard method alone was deemed not suitable.
The TOFD method, on the other hand, is able to quantitatively measure crack depth. This technology works by sending a wideband ultrasonic impulse in a “pitch and catch” arrangement and then measuring the difference in the diffraction signals returning from the crack-like features. Through this method, crack depth and location are quantitatively measured. TOFD technology, however, must be brought to a complete stop in order to scan circumferentially, and this is only possible through the control of a tether¹.
Several tool variations went through a preliminary test, but it was clear that an ideal solution would encompass a combination of both UT shear wave and TOFD technology. Additionally, due to the fact that pipeline length was within the specifications for a tether, a bidirectional tethered carrier was deemed suitable.
In order to fully test the new solution, a pipe was constructed with 64 artificial notches and 80 manufactured fatigue cracks. The depth of the fatigue cracks, however, was completely unknown and could only be measured afterwards through destructive testing. The reason for this “blind test” was to simulate the actual unknown condition of the features within the flow lines to determine the true effectiveness of the technology combination.
The UT shear wave unit collected accurate information, detecting the crack-like features within the technology’s specification. The TOFD technology also performed as expected, with tolerances of the depth measurement at +/-1.0 mm for fatigue cracks and +/-0.3 mm for notches; for measurement of the crack length, tolerances were +/-2.0 mm for notches and +/-9.0 mm for fatigue cracks.
After testing proved this technology combination to be effective, obtaining the data the operator required, the inspection tool was launched into the flow lines. The inbound run, from the platform to the template, was to collect data with the UT shear wave unit. This data, including information on the weld positions, was evaluated and stored in real time. This way, on the return run, the TOFD unit would verify the features that the UT shear wave unit detected by stopping at the previously identified welds and performing a circumferential scan, sizing the defects with more precision.
The inspection went as expected. The tool traveled at a speed of around 300 m/h, the UT shear wave unit successfully identified crack-like features, and the TOFD unit successfully verified those previously detected features for their precise sizing and severity.
Although many features in line with the operator’s expectations were identified, the high precision of the combination of technologies in this solution proved that the line could, in fact, still be safely operated. The ILI data collected additionally enabled a life-time assessment on this asset to help ensure safe future operations as well.
With valuable new ILI data on all crack-like features present in the line, the operator could proceed with the calculation of pipeline repair or further integrity assessments, enabling the next steps in the asset care program.
¹ A KTN tether is an umbilical connection through which the tool can be remotely controlled and real-time information shared.