Snap behaviour in the upheaval buckling of subsea pipelines under topographic step imperfection

Abstract

Pipelines exposed to high-temperature and high-pressure (HTHP) with a topographic step imperfection are susceptible to the phenomenon of upheaval buckling potentially leading to a hazard for the structural integrity of the pipeline. To analyse this problem we derive analytical upheaval buckling solutions and obtain the locations of maximum displacement and maximum axial compressive stress. We also analyse the typical post-buckling behaviour and its dependence on step height, axial soil resistance and wall thickness. The difference in behaviour between a pipeline with step imperfection and one with a symmetric prop imperfection is discussed. Our results show that a pipeline with a step imperfection is more prone to upheaval buckling than a perfect pipeline. For sufficiently small step heights the pipeline may suffer a snap-back instability under decreasing thermal loading, raising the possibility of hysteretic snap behaviour under cyclic thermal loading (for instance caused by periodic start-ups and shutdowns). The snap-back buckling disappears for large enough step height and the minimum critical temperature difference decreases with increasing step height and wall thickness or with decreasing axial soil resistance. The maximum compressive stress decreases with increasing step height and axial soil resistance or with decreasing wall thickness. A pipeline with step imperfection is safer than one with a symmetric prop imperfection.

A mechanical model to determine upheaval buckling of buried submarine pipelines

“…The failure occurs when a slip plane is generated inside the material, and a relatively rigid body displacement between particles takes place [20]. In this work, in the absence of own field data, the recommended value of parameters given by [1] and common soil properties given by [20] are used. Table 1 shows the corresponding values for clay soils.…”

“…The objective of [1] in buried pipe design is to prevent upheaval buckling, meaning that the pipeline must remain under an allowable deformation 𝑏𝑏 resting on the floor. Hence, the lateral load must be large enough to prevent vertical displacements for a given effective axial load.…”

“…The upheaval buckling response of buried submarine pipelines has been a topic of great interest during the last decades [1][2][3][4]. Over the last few years, several projects involving the design, fabrication, and installation of submarine pipelines have been developed in the oil & gas industry.…”

“…From these models, [1] recommends the calculation of the soil uplift resistance in function of soil characteristics. In practice, this resistance is given as a function of pipe dimensions and cover height.…”

Line-Element Formulation for Upheaval Buckling Analysis of Buried Subsea Pipelines Due to Thermal Expansion

“…However, it is more likely to accumulate heats along with the lengthening of the pipeline when it is buried by soils that may cause the cumulative thermal expansion to trigger buckling [3]. This phenomenon is named upheaval buckling [4][5][6], as illustrated in Fig. 1, which is critical for designing subsea buried pipelines.…”