SOUNDS GOOD – USING ULTRASOUND FOR SAFER PIPELINE INSPECTIONS
When inspecting pipelines, welds require special attention. Many types of defects can lower the strength of a weld, forming possible locations for pipeline failure. Furthermore, so-called dissimilar-material welds, which are frequently used to create components with improved or tailor-engineered properties, can be hard to inspect.
Radiographic inspection is often used for pipeline inspection and is commonly seen as a gold standard. However, radiography carries important safety concerns due to the use of harmful X-rays. Inspections using ultrasonic fl aw detectors address these concerns and detect weld defects fast, with high precision and without harmful radiation.
Regular inspection of pipelines used for transporting gas, oil or other chemicals is essential to prevent costly leaks and spills, and to ensure compliance with safety standards. When pipelines are welded, these welds are often the weak spots where damage is more likely.
Welds can contain a multitude of different defects. These can be introduced during production, either as a result of incorrect operation of the equipment or due to an incorrect welding setup. Additionally, prolonged use of pipelines can introduce new defects or cause further damage to existing faults. Common examples of defects in welds include porosity, lack of fusion, slag inclusions, root or toe cracks, and incomplete penetration (fi gure 1).
Figure 2: A weld with a different material to the component itself (left) and a weld that joins two different materials together (right). Using X-rays to inspect pipelines
One well established technique to examine damage to components such as pipelines is radiography. In a typical radiography setup, a source of high-energy X-rays is placed on one side of a component and a detector on the other. X-rays of different energies have different levels of attenuation when travelling through a component – an effect that can be used to optimise contrast in the image on the detector.
When inspecting components using radiography however, there are important safety aspects that need to be considered. X-ray radiation is harmful to humans and strict legislation is in place to protect workers. As a result, an area around the inspection site needs to be cleared of people when the inspection is taking place, causing disruption to ongoing work in the area.
Figure 1: Different types of weld defects can signifi cantly weaken the bond between the components that are being joined together.
When it comes to detection of defects each method will have different probabilities of detection depending of the type of defect. It is therefore important to be aware of the most likely and the most critical defects when choosing an NDT inspection technology.
One defect that can signifi cantly reduce the strength of a weld is the lack-of-fusion defect (fi gure 3). Lack of fusion occurs when the melted metal that is applied to join two pipes together fails to melt the parent material and simply solidifi es on top of it. Lack of fusion can take place between beads of weld material or between the weld and the parent material. In both cases it leads to the formation of a bond with low strength.
Due to the fact that lack-of-fusion defects are often thin and fl at, radiographic detection depends highly on the orientation of the defect with respect to the X-ray source. Under certain angles these defects are almost impossible to see with radiography-based imaging.
All these types of defects can signifi cantly affect both the quality and the lifetime of a pipeline, which means that welds require particular attention during the inspection process. Nondestructive testing (NDT) methods are commonly used to inspect in-service pipelines as they can extend the lifetime of pipelines without compromising safety.
When using NDT it is important to consider the material properties of both the weld and the parent material. For example, the oil and gas and petrochemical industries use more and more pipelines consisting of carbon steel coated with a corrosion-resistant alloy (CRA) (fi gure 2). CRA can also serve as a fi ller material in girth welds. Interfaces in these ‘dissimilar-material welds’ can present a problem in NDT and a thorough understanding of the properties of the materials involved is necessary to create the right inspection setup.
Figure 3: When newly applied molten metal does not form a stable bond with the underlying material, a lack-of-fusion defect can form.
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Figure 4: Ultrasonic fl aw detectors control multiple elements to offer versatility with respect to the beam’s orientation.
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