Review of minimum requirements and development of guidance for weld spacing

Follow-on work to a recently completed joint industry project pertaining to hot tap branch connections included the development of guidance on minimum spacing between various combinations of adjacent pipeline girth and appurtenance welds. Related guidance was also developed for the maximum recommended length of a Type B repair sleeve. Several factors may contribute to minimum weld spacing requirements, including metallurgical, geometrical, residual stresses, and even aesthetics. It is not uncommon to hear concerns expressed about the metallurgical interaction of adjacent welds, in terms of overlapping heat-affected zones (HAZs). However, overlapping or interacting HAZs do not become a concern unless welds are spaced very closely, as HAZ widths for typical pipeline welds are generally less than 0.25 in. (6.4 mm). Often, minimum weld spacing requirements are based upon geometrical reasons, such as avoiding overlapping stress concentrations at the weld toes or allowing adequate access for inspection equipment following welding. Or, they may be based upon residual stresses, and the avoidance of overlapping residual stress fields and distortion, which are influenced by pipe dimensions and the welding procedure and could approach a few inches in distance. Finally, weld spacing requirements, especially for new construction applications, may simply be based upon aesthetics, where the flexibility exists to separate welds by a distance that makes sense from a visual perspective. Examples of company-imposed and code-based minimum requirements are provided that can used to perform a critical assessment of current company requirements or develop new requirements.