Assessment of As-Built Pipeline Subject to Static and Dynamic Construction Stresses – A Case Study

Abstract Determining pipe fitness-for-purpose and demonstrating code compliance for pipe subjected to both static and dynamic construction stresses can be complicated. This paper utilized a case study to illustrate a process of engineering evaluation that utilizes both analytical and testing approaches to ensure continuous safe operation of such pipe sections. The case study involved a newly constructed bored crossing which was intended to be a straight auger bore. The post-construction measurement survey, however, showed that the carrier pipe centerline was curved. It was identified that a percussive hammering method was used to dynamically pull the carrier pipe through the bore which induced an approximate but large number of dynamic tension cycles on the carrier pipe. Initial static and cyclic stress evaluations of these circumstances indicated that: i) the crossing carrier pipe stress state due to curvature was found unacceptable and ii) that the imparted cyclic tensile forces were sufficient to render the carrier pipe end of life. A detailed engineering evaluation was therefore performed to assess and demonstrate the effect of these circumstances, including detailed numerical fatigue analysis, fatigue crack growth fracture mechanics, finite element analysis (FEA) and crack growth testing on pipe material from the same heat batch as the installed joint of the crossing, as well as from two test welds prepared on the same pipe material using two welding procedures used on the carrier pipe. As part of the detailed assessment, a static elastic-plastic FEA was performed using the design operating conditions in conjunction with an elastic bending strain analysis. This demonstrated that the carrier pipe with the construction induced bending stresses, operates in an elastic shakedown manner for the design operating loads and as such linear elastic behavior is maintained. A Stress-Life fatigue, and fatigue crack growth assessment was performed which produced the following: i) the variation in cumulative fatigue damaged as a result of both the installed bending stresses and the repetitive cyclic tension stresses was determined; ii) demonstrated that the cumulative fatigue damage did not exceed unity, and as such has a finite remaining life; iii) demonstrated from the information provided that under the most severe installation scenario calculated, the pre-existing girth weld flaws, albethey small, when subjected to the calculated cyclic loads, would not grow to exceed the critical sizes determined using the Engineering Critical Assessment (ECA) method for the design operating conditions; and iv) demonstrated that a hypothetical surface breaking crack in the pipe body at the most damaged section of the carrier is highly unlikely to grow to exceed the critical size for the design operating conditions. As a result of the engineering evaluation, it has been determined that the effect of the circumstances assessed, in and of themselves or combined, would not render the installed crossing pipe non-compliant to design code. Details of the analytical and testing approaches are described in this paper, providing a real-world illustration of advanced assessment methods and workflow required to assess fitness-for-purpose and code compliance of as-built pipelines subjected to complex construction loading.