Ultrasonic algorithms for calculating probe separation distance, combined with full matrix capture with the total focusing method

Abstract

Girth weld inspections play a key part in the maintenance and management of numerous different types of pipe that are mainly used within the oil & gas industry. The most modern technique for girth weld inspection exploits full matrix capture (FMC) with the total focusing method (TFM), utilising two probes in a combined pulse-echo and pitch-catch set-up. This set-up has the advantage of allowing multiple modes to be carried out within a single inspection, with the pitch-catch part requiring a precise measurement of the probe separation distance (PSD) to be determined for accurate TFM image reconstruction. This distance is the lateral measurement between the first elements of each probe. The currently accepted method for calculating this distance is through visual means (with the use of a physical measurement ruler), which suffer from operator error, the ruler not being calibrated and difficulties determining the position of the first element. This paper presents an ultrasonic measurement technique for accurately calculating the PSD by exploiting the lateral wave or the shear wave backwall response within the pitch-catch mode A-scan analysis.