Weipeng Chu

Unbonded flexible pipes are key components of deepwater high-temperature oil and gas transportation systems, and their bending performance directly affects in-place response and fatigue assessment. Interlayer contact and sliding of tensile armor layers govern bending hysteresis; under high-temperature service, incompatible thermal expansion of metallic and polymer layers changes contact pressure and the associated slip conditions. This study develops a thermo-mechanical bending hysteresis model in which thermally induced interlayer contact pressure links the radial temperature field to the bending response. A steady-state multilayer-cylinder heat-transfer model and a thermoelastic compatibility formulation are used to determine temperature distributions and interlayer contact pressures. The contact-pressure variation is then introduced into the tensile-armor slip criterion and the incremental moment-curvature relationship, covering non-slip, partial-slip, and full-slip stages. A sequentially coupled finite element model of a 2.5-inch unbonded flexible pipe is established for validation. The numerical model predicts hysteresis loop area and unloading/reverse-loading stiffness with relative deviations of 6.02% and 5.09% from the finite element results, respectively. Increasing internal temperature increases contact pressure and critical slip curvature, prolongs partial slip, and substantially increases hysteretic energy dissipation. The model provides a basis for high-temperature bending stiffness determination and fatigue-oriented analysis of unbonded flexible pipes.

To improve transportation efficiency, a supercritical CO₂ pipeline is the best choice for large-scale and long-distance transportation inshore and offshore. However, corrosion of the pipe wall will occur as a result of the presence of free water and other impurities present during CO₂ capture. Defects caused by corrosion can reduce pipe strength and result in pipe failure. In this paper, the burst pressure of subsea supercritical CO₂ pipelines under high pressure is investigated. First, a mechanical model of corroded CO₂ pipelines is established. Then, using the unified strength theory (UST), a new burst pressure equation for subsea supercritical CO₂ pipelines is derived. Next, analysis of the material’s intermediate principal stress parameters is conducted. Lastly, the accuracy of the burst pressure equation of subsea supercritical CO₂ pipelines is proven to meet the engineering requirement by experimental data. The results indicate that the parameter b of UST plays a significant role in determining burst pressure of pipelines. The study can provide a theoretical basis and reference for the design of subsea supercritical CO₂ pipelines.