Abstract: Weld defects can be classified as major potential safety hazards that may negatively impact operation of aged pipelines. And for instance, in an old pipeline, the defects, misalignment and evident surface in its girth weld and spiral weld seams are found. Tensile and fracture toughness tests have been performed for parent materials and weld seams of the pipeline, respectively. Based on these test data and with consideration to basic parameters and operation of these pipelines, FAD has been deployed to perform Class Ⅰ and Class Ⅱ assessments of girth weld and spiral weld defects. In this way, safety margin around defected pipes can be analyzed quantitatively. Through iteration, sizes of allowable surface cracks can be determined. Furthermore, quantitative studies have been performed over crack tolerance sizes of girth weld and spiral welding lines. Relevant research results show that Class ⅠS assessments may underestimate bearing capacity of the pipeline, whereas the crack tolerance size determined through calculation is lower than those determined through Class Ⅱ assessments; under the operating pressure of 4 MPa, spiral weld defects of the pipeline displayed minor safety margin with hazards higher than those of girth weld defects, whereas crack tolerance size around welding lines is lower than that of girth weld; compared with crack length, crack depth displayed even higher impacts to assessment results. Relevant researches may provide solid criterions for acceptance of pipeline defects. In addition, these researches may provide valuable references to rational and scientific assessments of weld seams.