THEORETICAL ANALYSIS ON MECHANICALLY LINED PIPE FABRICATION

Mechanically Lined Pipe (MLP) is a double-walled pipe comprising a carbon steel outer pipe internally coated with a thin layer of Corrosion Resistance Alloy (CRA). This CRA layer serves to safeguard the outer pipe against corrosive substances presented in the production fluid from offshore O&G fields, while the high strength low carbon steel outer pipe bears majority of external loads. The process of hydroforming expansion represents the predominant technique employed in affixing the liner into the carrier pipe. Precise and dependable theoretical models have been consistently sought for enhancing the fabrication efficiency and serving as a benchmark for subsequent failure analysis. The Tresca criterion has been adopted in stress-strain analysis of MLP due to its mathematical simplicity. Nevertheless, it is worth noting that the Tresca criterion is widely acknowledged for its conservative approach in predicting the yielding of metallic material. Consequently, the accuracy of employing the Tresca criterion in analyzing the fabrication of MLP is arguable. To further enhance accuracy, the infinitesimal strain theory and von Mises yield criterion are applied in conducting theoretical analysis for the MLP processing. The stress state within liner after being fully-plastified and loaded with external interfacial contact pressure is derived with radial displacement compatibility. And residual contact pressure is derived analytically through the circumferential strain compatibility method. The interfacial contact pressure, residual contact pressure and hydraulic expansion pressure working range of one set MLP liner and carrier pipe is predicted using the improved model with von Mises criterion.