Determination of the long-term hydrostatic strength of multilayer pipes

In this paper, a new methodology for prediction of the long-term (creep rupture) behavior of multilayer pipes under internal hydrostatic pressure is presented. For this purpose, a procedure using the three dimensional theory of thick-walled multilayer pipes together with a combined quadratic/linear regression analysis is used. The theory of thick walled tubes is used to assess the role of each layer in carrying the internal pressure and also the onset of the creep rupture in the composite pipe. For long-term extrapolation, a combined quadratic and linear regression analysis was used and the contribution of each component to the long-term strength of the composite pipe was quantitatively assessed. Using this procedure, the layer at which the creep rupture under internal pressure first initiates would be identified and the additional capacity of the remaining layer would be quantitatively assessed. This procedure is already incorporated in the pipe software called ADAP dealing with the automated design and analysis of pipelines. An example showing this procedure using ADAP is presented. The proposed methodology overcomes the limitations in the existing testing and extrapolation standards and, thus, can be used as a new extrapolative procedure for prediction of the service life of multilayer plastic pipes and pipe fittings. As particular applications, the procedure can be used for the estimation of the long-term behavior of multilayer pipes with metallic inter-layers and also single layer as well as structured pipes.