Summary Expandable profile liner (EPL) technology (EPLT) provides an effective solution for lost circulation trouble control without wellbore diameter loss or sacrificing completion and production performance. However, the reliability and stability of the welding seams determine the overall bearing capacity of the EPL string. The application of EPLT in more complicated conditions requires increasing the overall performance of the EPL string welding seams. In this study, we considered four types of artificial welding defects, including cracks, pores, slag inclusions, and incomplete penetration in EPL string samples, and performed internal pressure limit tests to obtain the ultimate failure strength of EPL strings with different welding defects. Abaqus was used to quantitatively assess the influence of welding defects on the strength performance of the EPL string. The critical dimensions and safety evaluation curves of various defects, under a working pressure of 30 MPa (safety factor 1.5), were obtained using a combination of numerical simulations and experiments. Using the latest welding defect identification and detection methods and relevant evaluation procedures, the type, size, and position of welding defects in EPL string can be identified quickly. The new welding process and integrated automatic welding equipment improved the welding quality and reliability of the EPL string. The new research on EPL welding seam defects and recent advancements in EPLT can bring significant economic benefits, thus promoting the application of EPLT to meet future application challenges. The technical innovations presented in this study provide an effective solution for drilling engineering staff to solve the drilling troubles associated with lost circulation and borehole instabilities and are also conducive for oil companies to address the future challenges of exploitation and development of unconventional oil and gas resources.