Radiation Backscatter-Based Nondestructive Technology Detects Corrosion under Insulation on Offshore Oil and Gas Platforms

One of the most common forms of corrosion found in the offshore oil and gas industry is corrosion under insulation (CUI). Many components on offshore platforms, such as piping systems, pressure vessels, tanks, and other equipment, are insulated for personnel protection and/or to keep fluids at appropriate temperatures for process efficiency. When insulated equipment is exposed to the harsh offshore marine environment (salt spray and mist), the ingress of chloride-laden moisture into the insulating material renders the underlying metal substrate vulnerable to accelerated localized corrosion, which often goes undetected.

According to Danny Constantinis, CEO of EM&I Group, BCT allows inspection of the internal and external wall surfaces of any size pipe and vessel with multiple layers of various overlay materials, such as protective cladding and insulation or cementitious passive fire protection coatings, with accuracy that is comparable to ultrasonic testing. The single-side tomographic inspection technology is similar to a medical computerized tomography (CT) scan, where narrow beams of radiation are discharged into an object and captured by a detector that sends the data to a computer to generate an image of the inside of the object. 

Unlike a CT scan, where the object to be imaged is placed inside the scanner and transmitted radiation is measured, a BCT scan creates an image by clamping the scanner onto the exterior of the object to be evaluated and measuring the backscattered (reflected) radiation. The scanner moves a focused beam of gamma radiation across the targeted inspection area, which is about as wide as the scanner and a few centimeters tall. Basically, Constantinis explains, as the beam of radiation passes through the insulation covering the object, the radiation collides with the material’s molecules. 

These collisions throw off tiny photons that bounce back toward a bank of gamma radiation detectors built into the scanner. As the radiation travels further and penetrates the steel, which is denser than the insulation, a greater number of photons are reflected back. The distance of the reflected photons is measured by the scanner, and the resulting measurements are processed by the accompanying computer program, which calculates the density of the materials and constructs a computer-generated, cross-sectional image that depicts the insulation and the outside diameter (OD) and the inside diameter (ID) of the steel wall, and provides a quantitative wall thickness measurement. Any diminishing thickness of the OD or ID surfaces can be identified. This enables any insulated pipe or pressure vessel, regardless of its size, to be inspected for CUI through the insulation material. 

The technology has been tested and validated in several field trials since 2010, including a study at ExxonMobil’s Goldboro natural gas processing facility in Canada, where the BCT system was evaluated for its ability to identify CUI and internal wall thickness variations of large insulated pipes and vessels, Constantinis says. A separate offshore study was also conducted using coupons to assess the technology’s capability to gauge wall thickness. 

Source:http://www.nacemp.com/articles/material-selection-design/2015/08/radiation-backscatter-based-nondestructive-technology-detects-corrosion-under-insulation-on-offshore-oil-and-gas-platforms