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The threat of pressure cycle induced fatigue cracking of flaws associated with the longitudinal seam weld continues to be a primary concern for pipeline operators. Cyclic pressure loading can cause initial manufacturing flaws in a seam weld to sharpen and grow over time. While this behavior is most prevalent in pre-1979 electric resistance welds (ERW) and electric flash welds (EFW), historical data also shows that submerged arc welds (SAW) have been observed to develop cracks at the toe of the weld, and those cracks have exhibited fatigue growth from transit fatigue, operating pressure cycles, or both. When managing a large pipeline network, it is important to understand which pipelines exhibit higher priority with respect to seam weld fatigue cracking. While there are industry-accepted methodologies used to prioritize pipelines with respect to seam weld integrity (TTO-5 [1] and API RP 1176 [2] being the most well-known), these methodologies can be improved upon when specifically considering fatigue. Saudi Aramco and Quest Integrity developed a detailed methodology to determine a prioritization for a group of pipelines specifically with respect to seam weld fatigue cracking. This improved methodology was specially tailored to consider additional data available in Saudi Aramco’s records to rank the likelihood for a fatigue failure to occur. This initial prioritization will be used to implement a more rigorous program to manage their assets. Additional data gathered in subsequent assessments can be included to refine the prioritization. The primary metrics used to determine the prioritization are pressure cycle aggressiveness, predicted remaining life with respect to recent hydrostatic testing, and the API 1176 Annex B prioritization classification.

The application of emerging fitness-for-service standards in conjunction with advanced modeling and ultrasonic thickness (UT) inspection is demonstrated with the recent assessment and repair of a CO2 absorber vessel. UT inspection discovered four regions of localized metal loss on the internal surface of a CO2 absorber vessel shell. Of the four regions, two were directly adjacent to major structural discontinuities, including two nozzles, one of which contained a reinforcing plate or repad. In order to define the critical locations of metal loss and estimate a corrosion rate, thickness data for the regions of metal loss was provided in the form of 1 inch by 1 inch equally-spaced, rectangular thickness grid from two separate inspection dates. Based on the estimated corrosion rate, and the specified operation interval, the rate of metal loss was determined to be significant enough to require repair. This conclusion was based on the fact that in certain locations, the metal loss was estimated to grow through-wall before the end of the specified operation interval. Computational analysis using guidelines per API 579-1/ASME FFS-1 [1] was used to evaluate an appropriate repair procedure. This included evaluation of repair plate placement and sizing using advanced modeling techniques including elastic-plastic material behavior and contact interaction. The effect of future metal loss was included based the estimated corrosion rate. The result of this assessment was a repair design that provided sufficient protection against excessive plastic deformation and allowed for continued operation through the specified operating interval. Thus, repairing the vessel based on fitness-for-service (FFS) criteria allowed for continued use of the vessel and avoided costly replacement. The lessons learned provide insight into the improved design of vessel repairs.

This case study exhibits how groundbreaking inspection methodologies combined with innovative computational analysis practices demonstrate the value of conducting fitness-for-service (FFS) assessments on sectional piping. In this instance, a fitness-for-service assessment was performed on two sections of piping experiencing external corrosion at the pipe-to-elbow seam welds. A full external scan and spot ultrasonic thickness (UT) readings were used to create the corroded geometry and verify accurate measurement of the remaining thicknesses in various corroded locations. This allowed for the actual corroded profiles to be accurately modeled using finite element analysis (FEA). Complications were present when modeling the observed metal loss. Through the use of innovative finite element mesh generation practices, the actual measured corroded geometry was modeled without the need for over-conservative geometric simplification. A Level 3 FFS assessment was then performed in addition to a remaining life assessment based on observed corrosion rates. The result of this analysis was that the piping could remain in service for at least two additional years before needing repair.

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