TOFD-Based Ultrasonic Inspection of Pipeline Welds Using Multifrequency Array Scanning and AlexNet-SVM Classification

The quality of girth welds in long-distance oil and gas pipelines is critical for the safe operation of pipeline systems. This study investigates time-of-flight diffraction (TOFD) ultrasonic inspection of X70-grade pipeline steel butt welds containing typical defects such as porosity, lack of fusion, cracks, slag inclusions, and lack of penetration. A multifrequency dual-probe array scanning strategy was developed to increase blind-zone coverage to 95%. A multimodal joint filtering algorithm was applied to suppress impulse noise while preserving high-frequency details, resulting in a peak signal-to-noise ratio of 38.6 dB and improved defect contrast. For defect classification, an AlexNet-SVM hybrid model was constructed by replacing the Softmax classifier with an RBF-kernel SVM and optimizing hyperparameters. The proposed method achieved a classification accuracy of 98.67%, which is 9.97% higher than that of the baseline AlexNet model and meets the requirements of ASME B31.8S-2021. The results demonstrate that the combined scanning strategy, image preprocessing, and optimized classifier can significantly improve the reliability and automation of TOFD-based weld defect detection.