Ultrasonic Corrosion Inspection
Intelligent inline inspections (ILI) are widely used to guarantee a safe operation of pipelines. The inline inspection provides reliable data in an economic way. Ultrasonic (UT) is currently the most accurate and reliable in-line inspection technology available in the market. UT ILI tools record data while travelling through the entire pipeline from launcher to receiver. In most cases, the pipeline operator does not need to make major adjustments to their pipeline. Nevertheless, pipeline operators may have to adjust medium flow rates to accommodate optimum inline inspection conditions, e.g. down to 1 m/s (2.23 mph).
Pipeline operators have to ensure a safe operation of pipelines during the entire lifespan of their asset. Depending on geographic area, country, and operator, different legal boundary conditions might apply for safe operation. These conditions directly define approaches and methods, e.g. intelligent pigging. This method is widely used and accepted to gather accurate and reliable information about the pipeline system. Using highly specialized tools, intelligent inline inspection can detect and size threats like corrosion, cracking or deformation - even combinations can be identified. Recent developments by inline inspection companies overcome operational challenges like thick wall pipe, tight bends or bend combinations, multi diameter pipelines or high pressure & high temperature applications as often faced in offshore deep water assets. Reliability and accuracy of inline inspection results and minimized impact on operational conditions are main aspects considered by pipeline operators. First run success rate (FRS) is another major aspect during the planning phase on an inline inspection project. This paper shows major advantages of ultrasonic inline inspection technologies for metal loss/corrosion and crack detection with Evo Series 1.0 tools.
Piezo-electric transducers mounted on a pigging tool - in most cases within a special sensor carrier - are used for measuring the wall thickness of pipelines. Wall thickness variations, mainly caused by corrosion can therefore be detected and measured. The ultrasonic technique (UT) requires a suitable single-phase liquid coupling medium. For wall thickness measurement application, the sensors have to be arranged perpendicular to the wall.
The transducer is operated in a pulse-echo mode: one transducer emits and receives the signal. A sufficient number of sensors cover the full 360° circumference of the pipe. The transducer emits a short pulse of ultrasonic energy which is reflected by the internal pipe wall. A portion of the signal enters the pipe wall and is reflected at the rear wall. The ultrasonic tool's electronics measure the time of flight. As the speed of sound in the product, e.g. water, jet fuel, oil, or similar, and in the steel is known, the distance from sensor to the inner wall (Stand-off=SO) and the wall thickness (WT) can be calculated. Differentiation between internal and external flaws can be done via the stand-off (SO) signal. If the wall thickness varies and the stand-off remains constant the flaw is external. If the stand of varies with the wall thickness the flaw is internal. In addition, lamination's and inclusions can be detected. This method is a direct measurement which does not require a calibration. It is independent from the wall thickness and works with identical accuracy of ±0.4 mm (±0.02 in) in thin (4 mm (0.16 in) ) and thick wall pipelines (60 mm (2.36 in) ).
Detection of corrosion features with a diameter of 5 mm (0.2 in) reaches a probability of detection level (POD) of more than 90%. Classification and depth sizing is provided for all metal loss defects from 5 mm (0.19 in) on wards. High-resolution river bottom profiles enable accurate defect assessments.