Failure Analysis of a Gas Collection Line Tee Fitting Burst

Literature Overview

This 2014 paper by Li Yan, Shan Hua, Bai Wenlong, Zhang Yang, and Xu Haixia, published in Corrosion & Protection, investigates the cause of a pipe burst at a tee fitting connecting a single well branch line to the gas collection trunk line in a condensate gas field. The tee fitting failed after more than three years of production operation. The root cause analysis concluded that the burst resulted from wall thinning due to corrosion, which rendered the pipe body incapable of withstanding the internal pressure. The corrosion was attributed to the combined action of CO2 corrosion, high-velocity fluid erosion, local turbulent flow erosion, and galvanic effects, with local turbulence and galvanic effects identified as the primary contributors to wall thinning, perforation, and ultimate burst.

Root Cause Analysis and Corrosion Mechanisms

The failure analysis in this paper exemplifies the multi-factorial nature of corrosion damage in gas gathering systems. CO2 corrosion, also known as sweet corrosion, is a well-documented degradation mechanism in natural gas production systems, particularly when free water is present. The electrochemical reaction involving carbonic acid formation and subsequent iron carbonate dissolution leads to uniform and localized wall thinning. In the context of a tee fitting, the flow geometry creates regions of high shear stress and turbulence, which accelerate the corrosion process by continuously removing the protective iron carbonate scale.

The galvanic effect identified in this case is particularly insidious. When dissimilar metals or alloys are in electrical contact in a corrosive environment, the more active metal acts as the anode and corrodes preferentially. In gas gathering systems, this can occur when a carbon steel tee is connected to a stainless steel instrument connection, or when a galvanized fitting is joined to a bare carbon steel pipe. The paper's recommendation to avoid contact between metals of different potentials is a fundamental principle of corrosion engineering that must be rigorously applied in pipeline design and fabrication.

The local turbulent flow erosion identified as a primary contributor is consistent with the fluid dynamics at a tee fitting. When gas flows through a tee, the flow pattern changes abruptly at the junction, creating recirculation zones and high-velocity jets that impinge on the pipe wall. This mechanical erosion removes the protective corrosion product layer, exposing fresh metal to the corrosive environment and creating a synergistic erosion-corrosion mechanism. The combination of turbulent flow and CO2-laden gas creates a particularly aggressive environment for wall thinning.

Engineering Lessons and Preventive Measures

The failure of this tee fitting after more than three years of service underscores the importance of proper material selection, corrosion monitoring, and integrity management in gas gathering systems. For condensate gas fields, where the produced fluid may contain significant quantities of water and dissolved CO2, engineers must implement comprehensive corrosion control strategies. These include the use of corrosion-resistant alloy fittings where feasible, the application of internal coatings or linings, and the installation of corrosion monitoring probes at critical locations such as tees, elbows, and reducers.

The galvanic effect identified in this case highlights the need for careful attention to material compatibility in pipeline design. Engineers should ensure that all connected components are made of materials with similar electrochemical potentials, or that appropriate isolation measures such as dielectric unions or insulating flange gaskets are installed. The selection of fasteners, gaskets, and instrument connections must also be evaluated for galvanic compatibility. In my experience, many field failures attributed to corrosion are actually initiated by galvanic coupling that was overlooked during the design phase.

Quality Control and Integrity Management Implications

From a quality control perspective, this failure case reinforces the need for rigorous incoming inspection and periodic in-service inspection of pipe fittings in corrosive service environments. Fittings should be verified for proper material composition, wall thickness, and surface condition before installation. In-service monitoring programs should include ultrasonic thickness measurements at regular intervals, with particular attention to the downstream face of tees where turbulent flow erosion is most severe. The detection of wall thinning rates exceeding acceptable limits should trigger immediate intervention, such as pressure reduction, chemical injection, or fitting replacement.

The use of corrosion-resistant alloy fittings, such as those specified in ASTM A860 or NACE MR0175, should be considered for critical locations in CO2-bearing gas systems. While the initial cost is higher, the extended service life and reduced maintenance requirements often result in a lower total cost of ownership. Engineers should also consider the impact of welding on the corrosion resistance of alloy fittings, as the heat-affected zone may be more susceptible to localized corrosion than the base metal.

Summary

This failure analysis provides valuable lessons for the design, fabrication, and operation of gas gathering systems in condensate gas fields. The combined action of CO2 corrosion, turbulent flow erosion, and galvanic effects at a tee fitting led to wall thinning and ultimate burst after more than three years of service. Engineers must implement comprehensive corrosion control strategies, including proper material selection, galvanic isolation, corrosion monitoring, and integrity management, to prevent similar failures in the future.