Laser scanning (profilometry) is a relatively new technique used for nondestructive testing and evaluation of numerous structures. It involves using laser technology to capture the three-dimensional shape and physical characteristics of objects. Laser scanning works by sweeping the surface area of an object with laser beam pulses; these pulses are then bounced back to a sensor that measures specific properties of the reflected beam to map out a 3D image of the object. (For more information on how laser scanning works, see Using 3D Laser Analysis for Nondestructive Testing and Evaluation of Pipeline Corrosion.)
Corrosion under insulation (CUI) is one of the most difficult types of corrosion to detect and prevent. This is attributed to the fact that the corrosion is obscured by an opaque material (the insulation). Water seeps into the insulation where it is hidden from external view and reacts with the metal substrate. As a result, CUI problems are only detected when the corrosion has reached an advanced state. While removing the insulation for routine inspection is the best way to detect CUI, it can be a relatively time-consuming and expensive process. Nondestructive testing is, therefore, a popular tool in the corrosion inspection industry.
While laser scanning is an effective nondestructive visual inspection technique in many situations, it depends heavily on the data reflected off of an object’s surface. Therefore, laser scanning is only useful for mapping objects within the scanner’s field of view. As a result, it is best suited for measuring external geometrical deviations, such as those caused by corrosion, mechanical damage and other physical distortions.
Since CUI occurs under thick, fibrous insulation (e.g., mineral wool insulation), the corrosion damage is shielded from interaction with the laser. The insulation surrounding the object is likely to reflect any incoming laser beams, resulting in inaccurate corrosion readings.
Other methods such as real-time radiography (RTR) and computed radiography (CR) are the preferred imaging methods for detecting and assessing CUI. These techniques use either X-rays or radioactive gamma waves (both of which are capable of penetrating the thick outer insulation) to create a three-dimensional profile of the pipe’s outer diameter.