What are pipeline welding defects?
Pipeline welding defects are imperfections that occur in the welded joints of a pipeline, or in the steel material used to manufacture the pipe and its components. These flaws are typically related to the manufacturing or construction process. Since pipelines are assembled by welding individual pipe sections together, any irregularity in these welds can create weak points. While most significant defects are detected during production or hydrostatic testing prior to commissioning, minor imperfections may remain and become critical over time. There are various types of weld defects, including hook cracks, lack of fusion, seam over-trim, burn marks, lack of sidewall fusion, misalignment, porosity, lack of penetration, offset plate edges, hard spots, slag inclusions, toe cracks, undercut, and hydrogen cracking.2,3,4
Crack originating at a notch and growing into a weld.
Toe cracks are fractures that form at the edges (toes) of fusion welds, where changes in shape and microstructure create stress concentrations. These cracks can remain in the heat-affected zone or spread into the weld or parent metal, and they occur only in fusion welds.
External corrosion around a girth weld
How do pipeline welding defects occur?
Welding defects can occur during pipe manufacturing or pipeline construction. Manufacturing-related defects occur in the steel used for the pipe and its components, as well as in the longitudinal seam welds themselves. Construction-related defects often arise during girth welding, when joining pipe connections on site, or during bending. Common girth weld flaws include arc strikes, burn-through, hydrogen cracking, lack of fusion between weld passes, misalignment, porosity, and incomplete penetration. Even if the welds meet design standards, flaws may still be present within acceptable limits. Over time, these imperfections may increase due to stress cycles, operational loads, or environmental conditions.2,3,4,5
What are the risks of welding defects in pipelines?
Although pipelines operating below their maximum allowable operating pressure (MAOP) are generally safe, welding defects can initiate degradation mechanisms, such as corrosion and fatigue. The risk of corrosion is particularly high at field joints, where protective coatings may fail due to challenging application conditions. Due to differences in material properties, preferential corrosion can occur at welds and in heat-affected zones. Structural risks arise because the tensile strain capacity of a pipeline largely depends on the properties of its girth welds. Defects can grow under repeated stress, and welds with lower toughness or weak spots are more susceptible to cracking. If left unmanaged, these defects can compromise pipeline integrity, leading to leaks or failure over time.1,2,3,4
Would you like to find out more about specific welding defects, such as hard spots? Expand your knowledge by clicking through the 'Related Terms' section at the bottom of the page.
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Source reference
- The Encyclopedia of Pipeline Defects, ROSEN UK, Edition 3, 2017.
- Managing Pipeline Threats, John Tiratsoo, 2019.
- Introduction to Pipeline Defects, Angus Patterson, The Competence Club, ROSEN, 2025.
- Battelle Final Report to U.S. Department of Transportation, ‘ERW and Flash Weld Seam Failures’, Final Report No. 12-139, J F Kiefner and K M Kolovich, September 2012.
- Welding of Pipelines and Related Facilities, API Standard 1104, 2005.