Oil and gas pipelines require regular inspection to detect faults that can cause leaks. Welds in particular need careful examination as many types of weld defect can lower the overall strength of a pipeline and form a site for potential pipeline failure.
Defects can occur in a weld, either due to incorrect operation of the equipment or due to an incorrect welding set-up. Examples of common weld defects are porosity, lack of fusion, slag inclusions, root or toe cracks, and incomplete penetration. Defects can have a significant impact on the strength of a weld and therefore on the quality of the pipeline.
The most reliable and cost-effective way to ensure the quality of welding on large scales is by non-destructive testing (NDT). Depending on the weld material and its preparation, certain types of defects can be either easy or difficult to detect.
Lack of fusion can occur, for example when a bead of molten weld material fails to melt the parent material and simply solidifies on top of it. The resulting bond has a low strength. This means that lack of fusion is one of the critical flaws that need to be detected.
Detecting damage with X-rays
Radiography as the current gold standard relies on high-energy photons travelling through metal towards a detector on the other side. Any irregularities inside the component – either in the weld, the parent material or at the interface – show up as brighter or darker regions on the detector.
The main limitation of the use of radiography is the harmful effect of the radiation used. Due to the many risks to human health caused by high-energy X-rays, safeguards are needed to avoid exposure to radiation. This usually means that a large area in the direct vicinity of the inspection has to be evacuated, disrupting the work in the area of inspection.
Probability of detection is likely to be a factor when choosing a method for weld inspections. There is evidence that the contrast of defects such as lack of fusion can be low when using radiography. Contrast can be even lower when dissimilar material welds are used. This means that the probability of detection of lack-of-fusion defects is reduced, for example when a stainless steel pipe is welded with a corrosion-resistant alloy.
Phased array ultrasound
An alternative technique for inspecting welds in search of defects is ultrasonic inspection. Ultrasonic flaw detectors, such as Olympus’ OmniScan MX2 use sound waves rather than radiation to inspect components. Detection is based on the deflection of these sound waves at interfaces within the component. To maximise probability of detection – and to enable imaging and sectorial scans – ultrasonic phased array probes can be used.
Ultrasonic transducers work by detection of high-frequency sound waves, either by the emitting transducer itself (pulse-echo technique) or by a receiving transducer (pitch-catch technique). In difficult components, such as stainless steel or dissimilar-material welds, high levels of noise are generated. In these situations, the pitch–catch technique, also known as the transmit–receive longitudinal (TRL) technique, is preferable.
Pitch-catch inspection can be carried out using either conventional, single-element probes or using phased array probes where each transducer contains multiple elements; these probes are known as dual matrix array (DMA) probes. In phased array inspections a flaw detector controls each element individually.
Among the benefits of phased array are imaging capabilities, sectorial scans and easy coverage of a weld without moving the probe back and forth. These capabilities, combined with better control over the ultrasonic beam, simplify inspection for improved probability of detection.
By eliminating the need for harmful X-ray radiation, phased array ultrasound offers improved safety compared to radiography. Phased array inspections can also provide improved detection of certain types of difficult-to-inspect defects, such as lack of fusion. This means that with ultrasonic flaw detectors, such as the Olympus OmniScan MX2, inspectors can examine large oil and gas pipelines safely and with a high probability of detection.l
Thierry Couturier is with Olympus Europa. www.olympus-europa.com
