Transition welds joining pipes of unequal wall thickness are often used in gas and oil pipelines at road crossings, bends, fittings, and locations of class change. Pipeline operators have successively utilized a counterbore-tapered design for transition of unequal wall thickness. In the counterbore-tapered design, the wall thickness transition is moved away from a girth weld region, resulting in reduction of stress concentration near the girth weld, subsequently reducing the risk of hydrogen assisted cracking (HAC) and the driving force for fatigue crack growth.
The counterbore length (L) is an important design parameter in the design of the counterbore-tapered joint. The aim of specifying the minimum counterbore length is to ensure that any local moments caused by the interaction of longitudinal stress and the dissimilar wall thickness are satisfactorily attenuated when they reach the weld area. While determining the minimum counterbore length, the present counterbore-tapered design criteria considers the attenuation of the bending stress induced by the local moments but does not consider the bending stress magnitude. In certain cases, although the bending stress attenuation length is long, the magnitude of the bending stress is very small. The long counterbore length creates practical challenges for boring the pipe due to the limitation of boring tools. Since the magnitude of the bending stress is very small, it is not necessary to fully attenuate the bending stress. In this work, the minimum required counterbore length (L0) criteria was revised. The revised counterbore length criteria considered the magnitude of the bending stress and the stress attenuation, hence avoided the unnecessary long counterbore length. The revised criteria was based on an analytical solution and supported by detailed finite element analyses. The analyses showed that the minimum counterbore length can be greatly reduced for those cases required unnecessarily long counterbore length with negligible or small increase in the stress concentration in the weld area.
The present design method of counterbore-tapered joints allows only one value of taper angles at thickness transition, i.e., 14°. However, for some situations, creating the smoothness required with the 14° taper angle is difficult and an increase in the taper angle is preferred. In this work, it is shown that the stress concentration near the wall thickness transition and weld area for counterbore-tapered joints changes marginally by increasing taper angle from 14° to 30°.