Étude de la propagation des ondes guidées basse fréquence au travers de joints de recouvrement soudés dans le cadre d’inspection de réservoirs industriels

Abstract

Large storage tanks are heavily used to hold liquid in the petrochemical industry. The thickness mapping of the storage tank floor is carried out periodically during out of-service inspection. The current inspection method begins with a screening of the whole floor using magnetic flux leakage. The thickness of the hot spots identified during screening is then gauged using ultrasonic testing. The current method requires an inspector to scan the whole floor from inside the tank. The storage tank must therefore be empty and the floor needs to be cleaned ahead of the inspection. There is therefore a need to screen storage tank bottom from the annular chime and identify the tanks requiring further inspection or repair. Low frequency ultrasonic guided waves are nowadays routinely used to screen long sections of pipelines. Under the cutoff frequency of the first high order mode, ultrasonic guided waves have the ability to propagate over long distances with minimal attenuation. However, multiple sources of attenuation are present in tank bottoms: (1) contact with fluid inside the tank, (2) contact with soil outside the tank and (3) the floor assembly. Indeed, the floors of the storage tanks are typically constructed using many overlapping welded steel plates. In the largest storage tanks, a line across the diameter may go through up to 15 welded joints. Minimisation of the ultrasonic guided wave amplitude loss at each joint is therefore paramount. In this paper, the amplitude loss through welded lap joints is studied for the fundamental shear horizontal mode (SHₒ) and the fundamental symmetric Lamb wave mode (Sₒ). The antisymmetric fundamental mode (Aₒ) was rejected as a result of its high attenuation due to fluid loading. Using finite element simulations and experiments the propagation of SHₒ and Sₒ was studied in 12.7 mm steel plates welded in a lap joint configuration. The SHₒ mode can be up to 6 dB less attenuated than the Sₒ mode. Therefore, careful selection of the mode and frequency may result in long propagation distances and may enable screening of the floors using ultrasonic guided waves.

Date	4 Nov 2020
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Pierre Bélanger (Supervisor)