Long-Term Hydrostatic Resistance of Multi-Layer Pipes

This contribution deals with the short-term and the long-term behavior of multi-layer pipes subjected to internal hydrostatic pressure. A new methodology for prediction of long-term behavior of multi-layer pipes under internal hydrostatic pressure taking into account the material differences and the composite pipe construction is presented that considers the possibility of the composite polymeric pipe having a metallic inter-layer. The three dimensional theory of thick-walled multi-layer pipes together with a combined quadratic/linear regression analysis is used and the contribution of each component to the long term strength of the composite pipe is quantitatively assessed. This procedure is incorporated in the pipe software ADAP developed by the author that deals with automated design and analysis of pipelines. Short term stress analysis of multi-layer pipe consisting of two polymeric facings and a metallic inter-layer is carried out with three methods; namely the analytical method of multi-layer thick-walled tubes, the Finite Element method, and the computational software ADAP. In addition, an analytical stability analysis was carried out. It has been shown that all three methods produce similar quantitative and qualitative stress results. According to these computations, for multi-layer polymers/metal/polymer pipes, higher stress values are predicted for the metallic inter-layers. Using the new long-term extrapolation procedure, the layer at which the failure under internal pressure would be initiated is identified and the state of stress of each layer is quantitatively assessed. Using this extrapolation methodology for multi-layer polymer pipes at elevated temperatures, the theoretical sharp knee predicted by the existing standards is replaced by a transition zone between the ductile and the brittle behavior of thermoplastic layers. An example of this procedure for a multi-layer pipe consisting of two polymeric facings and a metallic inter-layer is presented. The proposed methodology can be used as an extrapolative procedure for prediction of the long-term resistance of composite and multi-layer pipes as well as for optimization of multi-layer plastics and hybrid pipes.