Abstract

High-pressure high-temperature (HP/HT) reservoirs contain hydrocarbons at pressures and temperatures in excess of 10,000 psi (690 bar) and 300 °F (149 °C). The high pressure often requires the installation of a high-integrity pressure protection system (HIPPS) to facilitate a workable pipeline concept and to prevent over-pressurisation. Should the HIPPS fail to respond to an overpressure event, the pipeline may be exposed to pressures in excess of a code-allowable design. It is, therefore, important to fully understand the pipeline behaviour, especially the burst limit state, to ensure compliance with the pressure containment philosophy, i.e. “burst critical” or “no burst”, and the associated probabilities of pipeline failure due to bursting. Generally, a probabilistic analysis will be performed using existing models that predict the burst capacity of straight sections of pipe under the effect of internal overpressure.

In reality, pipelines are subjected to additional loads over and above pure pressure loading; for example, thermal loading will induce compressive axial loads in the pipeline due to pipe-soil frictional resistance. Pipelines are also frequently subjected to bending loads due to a variety of causes; for example, freespans, trenching induced bending, or lateral buckles. To understand the burst behaviour of a pipeline, the effects of bending and themally induced axial compression in conjunction with internal overpressure must be understood.

This work shows that finite element analysis (FEA) can accurately predict the occurrence of pipeline failure due to burst. The FEA is validated by subjecting models of a straight pipe to internal overpressure until burst occurs, and comparing the burst pressure and strains at burst to the predictions made by an analytical model. Subsequently, the FEA incorporates various magnitudes of bending (representative of the range of bending that may be induced by prop-type imperfections up to 0.5 m in height) before internal pressure is applied. Both displacement and load controlled bending are investigated, as well as cases that consider thermally induced axial compression. The burst behaviour of the pipe for the various cases is compared to confirm whether the analytical model is still valid in cases when bending and/or temperature are present.

Thermal loading and displacement controlled bending are found to have negligible effect on burst capacity, because the axial compressive and bending loads are “shed” as the pipe becomes plastic and reduces in stiffness. However, it is observed that there is a significant change in burst capacity for load controlled bending, and hence the analytical model is no longer valid. This is because the applied moment is, by definition, maintained and unable to be shed; instead, the imperfection shape changes significantly to satisfy the loading condition as cross-section plasticity and the associated reduction in stiffness are experienced.

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