Structural Resistance and Loading Case Analysis for Buckling Collapse Prevention in Thermoplastic Liners for Oil Applications

The buckling collapse induced by external pressure is the main failure mechanism in thermoplastic liners for oil applications. This phenomenon is caused by permeation of dissolved gases in oil through the polymer pipe thickness. This results in gas trapped into the annular cavity defined by the metallic host pipe and the external liner surface. During operation, the pressures of the annular and inner cavities remain in equilibrium. The problem may arise when depressurizing the pipeline, due to the resulting pressure difference between both cavities. If the liner structure is not capable to bear the radial load, collapse buckling occurs. The damage grade depends on many factors that can be divided in two main aspects: (i) the liner resistance, related with its geometry, boundary conditions and material behavior; (ii) and the actual loading case as a consequence of the compressible fluid in the annulus between liner and host pipe. Although both must be properly determined to predict structure behavior, works on this subject are often concerned only with liner resistance. In this work, we propose a simple experimental method to quantify the annular resulting space after liner fitting. This information is then used to estimate the actual loading case. Finally, by using previously verified numerical methods to predict the liner resistance, we establish safe operation parameters in order to prevent buckling collapse.