Under what condition must a Welding Procedure Specification be used and the welder be qualified?

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The main idea is that for safety-critical welds, there must be a documented procedure and proof the welder can perform to that standard. When welding is done on structural components or parts that contain or carry pressure, the welds become critical to the integrity of the system. A Welding Procedure Specification sets and controls the exact welding parameters—process, materials, thickness, positions, preheat and interpass temperatures, heat input, shielding gas, post-weld heat treatment, and more—so every weld can be produced with known, approved conditions. Welder qualification then verifies that the individual can actually apply those approved parameters to produce a sound weld on the specified materials and thicknesses. Together, they ensure consistent quality and reduce the risk of failure in demanding applications.

The other options don’t fit because cosmetic welds don’t enter into the same safety-critical requirements, marine welding still follows applicable codes and typically requires a WPS and qualification when dealing with structural or pressure-containing parts, and certification alone does not substitute for a tested, approved procedure and validated welder performance on structural or pressure-containing work.