Pressure vessels used by the oil and gas industry have thick, magnetic walls that make non-intrusive inspection (NII) difficult or unreliable. Battelle is refining several NII methods to increase their accuracy and reliability in detecting corrosion, cracks and other flaws inside pressure vessels.
NII methods use electromagnetic currents or radiography to detect differences in the inside surface of a metallic structure that can indicate corrosion or cracking. Much like doctors use x-rays or ultrasound to generate medical images, inspectors can use techniques like micro-focus eddy currents or neutron radiography to “see” inside a metallic object by analyzing changes in a current or reflected radiation. These techniques have long been used to detect corrosion and other flaws in pipelines, airplane hulls and other metallic structures. However, they are generally unreliable when applied to oil and gas pressure vessels, which tend to have thicker, ferrous walls that dissipate the electromagnetic signals.
Finding better non-intrusive inspection methods for pressure vessels would provide significant cost savings for the oil and gas industry. Manual inspection requires pressure vessels to be brought offline for the inspection period, interrupting normal operations. Sending human inspectors into the vessel also presents health and safety risks for personnel. Many NII methods can be conducted without taking the pressure vessel offline, significantly reducing operating disruptions and costs associated with inspection. Rapid, cost-effective NII methods could allow for more frequent screening of pressure vessels, faster identification of emerging problems, and more focused use of offline manual inspection.
Battelle researchers are developing new methods to adapt existing NII technologies to the challenges of oil and gas pressure vessels, building on research originally done for the U.S. Department of Transportation. In one study of micro-focus eddy current techniques, Battelle developed a proprietary method to reduce the magnetic permeability of a ferrous material in order to reduce signal loss and scattering as the current moved through the material. This allows the transmitted pulse to stay focused and retain a large enough amplitude to penetrate the thick magnetic walls of the pressure vessel and generate reliable data about the thin nonmagnetic metallic coating on the inside. A second study examined a neutron backscatter technique that could be used as a rapid screening tool to identify areas of concern. Battelle is also studying a number of other NII methods to determine their applicability for pressure vessels and other oil and gas applications. The methods include multi-level magnetization (MLM) techniques for mapping wall stresses, Barkhausen demodulation for crack-stress mapping, robotic intrusion methods similar to pipeline inspection crawlers, large-scale radiographic inspection, differential stress via magnetic measurement and the synthetic aperture focusing technique (SAFT).