Storage Tank Floor and Wall Defect In-Situ Inspection With Ultrasonic Guided Wave Technique

Abstract

Large storage tanks for oil storage are widely used in petrochemical industry. Corrosion in the tank floor and wall is a serious threat for environmental and economic safety. Owing to their unique potential for long-range, in-plane propagation through plates, Ultrasonic Guided Waves (UGW) offer an obvious solution in the development of an on-board structural health-monitoring (SHM) system, providing assessment of structural integrity for storage tank floor and wall defect in-situ inspection. This paper presents this application by focusing on their propagation through the plate structure. Even very small mechanical discontinuity or geometry change of plate structure, e.g. corrosion defect on tank floor, will influence the propagation characteristic of the guided waves. These effects are measured as mode changes, frequency shifts or filtering, reflection and diffraction of new ultrasonic modes or overall distortion of the original ultrasonic signals. By capturing and analyzing these changes we can deduct the corrosion defect of the tank floor and wall which causes the ultrasonic signal change and interactions. The T/R transducers are required to be attached on the outer edge of the tank floor and outer surface of the tank wall. The technique is developed based on the Lamb wave transmission tomography. Starting from the dispersion curve and choosing the appropriate wave mode, the propagation of the guided waves in the tank floor and wall has been carried out through numerical simulation and the experiment has been conducted for verification using the full-size oil storage tank. The low frequency guided waves can propagate longer distance in planar and tubular structures. The later has been already used in pipeline inspection. The complexity of the application of ultrasonic guided wave in tank floor inspection lies in the object containing multiple lap joint welds along the large diameter of the tank (up to 100 m) and the complicated reconstruction of the two-dimensional defect distribution information. The main scope of the investigation was the application of the ultrasonic transmission tomography for localization of non-uniformities of inside tank floor, taking into account ultrasonic signal losses due to the loading with oil on the top and ground support at the bottom for the tank floor, and the loading with oil inside for the vertical tank wall.