Failure analysis of lube oil feed tube of a gas turbine operating in oil fields

Tube collapse due to heating of a trapped fluid in confined annuli has become a major concern in production wells and lubrication systems in gas turbines. This is due to the fact that tubes subjected to external pressure developed by fluid heating can generate catastrophic consequences that may lead to the damage of major equipment in the field, which in turn can have a great impact on the production. This paper, therefore, presents a case study of lube oil feed tube collapse of a gas turbine due to pressure build-up caused by fluid heat-up in trapped annulus of the failed tube. A conduction/convection thermal analysis of the tube and its surrounding environment was performed using MATLAB where the obtained results were used in finite element modeling to determine the required buckling pressure of the tube. ANSYS finite element package was used to conduct the stress analysis of the tube subjected to external pressure mimicking the real situation from which the required collapse pressure of the tube was obtained. In addition, an analytical thermal analysis was performed to acquire the build-up pressure at various typical temperatures of the trapped oil in the annulus. The collapse pressure obtained from the finite element analysis was used to estimate the temperature causing failure from the pressure–temperature chart obtained analytically. It was found that the collapse pressure is 125 MPa (18 ksi) which corresponds to 160 °C temperature rise in the annulus. This temperature represents a typical operating condition in the field.