Collapse Limit of Thermally Treated Line Pipe Under Combined External Pressure and Bending Deformation

Abstract

This study discusses the collapse criteria for thermally treated line pipe and their bending interaction against collapse based on a full-scale test under external pressure with and without bending loading. The critical collapse strain in the pressure bending test was much higher than that estimated by the DNV-ST-F101 standard because it was calculated based on estimating collapse pressures without bending interaction based on specified minimum yield stress (SMYS) of design pipe in the standard. However, the collapse pressures without bending interaction in full-scale test was significantly higher than that of the estimation according to DNV-ST-F101 standard. The effect of the thermal heat cycle simulated anti-corrosion coating heating on line pipe collapse criteria is also discussed based on the change of yield stress of pre-strained and thermally treated material. As the maximum heat cycle temperature increases, the reduction of the compressive yield stress along circumferential direction by the Baushinger effect due to UOE process (U: U-ing, cold forming from a plate; O: O-ing, cold forming from the U shape; E: cold expansion to meet geometric tolerances) becomes small. It is thought that a DNV equation for estimating the critical bending strain to collapse will provide a more accurate estimation of the critical collapse pressure and strain for thermally treated line pipe when the collapse pressure is calculated considering the change of strength parameters due to the tensile pre-strain level and heat cycle temperature.