Abstract
New policies driven by climate change concerns have made the construction of new pipelines more difficult with uncertain prospects in the years ahead. With expected increasing demand in hydrocarbon transportation, pipelines operators have no alternative but to plan for further use of existing vintage pipelines. These pipelines have inborn flaws such as longitudinal seam weld defects or environmentally assisted defects such as stress corrosion cracking (SCC). Therefore, the need to proactively maintain pipeline integrity and mitigate failure probability has become essential to meeting customers’ demands and protecting the public. This paper discusses Saudi Aramco’s experience in monitoring pressure fluctuation for vintage pipelines that are susceptible to SCC.
Pressure fluctuation contributes to an increase in crack-like longitudinal defects that are common in vintage pipelines. Therefore, a pipeline integrity management program should consider pressure fluctuation-induced fatigue damage. In addition, a crack growth prediction model can help determine an appropriate re-inspection interval. This paper discusses an innovative approach to evaluating, categorizing, real time monitoring, and reducing pressure cycle aggressiveness by reviewing the pressure cycle history in a complex pipeline network and analyzing several factors contributing to pressure fluctuation.
There are several pipeline operation modes; the transmission pipeline pressure cycling index is mostly critical at the pump or compressor stations and is dependent mainly on the equipment operations. As upstream gathering pipeline systems most often have multiple tie-ins at several wellheads, each wellhead becomes a source of pressure in the pipeline network. Moreover, pressure fluctuation is a result of different operation activities; for instance, pigging operations contribute in some cases to up to 20% pressure fluctuation during each pigging activity.
In this paper, an innovative real-time monitoring method is utilized to proactively determine which pipelines are subjected to pressure fluctuation. Meanwhile, cyclic index monitoring was conducted with a digital tool to facilitate large data processing utilizing introduced methodologies that are dependent on fatigue damage rate instead of material strength. More than half a million data points were processed to identify the cyclic index for each pipeline segment for one year. A cyclic index categorization was introduced; therefore, when the pressure cycling index is moderate or higher, pipeline operators should investigate the operational factors to reduce the fluctuation effects. In addition, a fatigue assessment with the recently developed PRCI MAT-8 fracture mechanics model was conducted to predict remaining lifetime and determine appropriate inspection intervals. The technique for assuming the initial flaw size was based on API 1176, which is critical for pipelines with no hydrotesting history.
