ABSTRACT

Offshore pipelines suffer the risk of local collapse and buckling propagation when installed empty or depressurized for maintenance. This paper deals with the mechanics of local collapse and buckling propagation of long pipelines under external pressure. The innovative vector form intrinsic finite element (VFIFE) method is applied and specific solutions for multiple nonlinearities are developed, aiming at offering a new and useful tool for offshore pipeline design. The whole-process simulation of dynamic buckling and quasi-static buckling are carried out and the calculation of collapse pressure and propagation pressure is achieved. The comparison between the VFIFE results and other methods (DNVGL-ST-F101, API RP 1111, the empirical formula, and parallel ABAQUS simulations) proves that the excellent applicability of the VFIFE method in offshore pipeline collapse and propagation simulation. The results can be used to guide the design and verification of offshore pipelines and the development of its numerical tool.

INTRODUCTION

Local buckling modes driven by external pressure are considered as significant failures for offshore pipelines when installed empty or depressurized for maintenance. The instability in terms of local collapse occurs at the transition from an essentially round pipe to a pipe that starts to ovalize and flatten under external over pressure; moreover, propagating buckles, once initiated, can propagate at a much lower pressure. Such buckling failures due to external pressure are influenced by factors including diameter-to-thickness ratio, material properties and initial geometric imperfections of pipe wall (Kyriadides and Lee, 2020). Therefore, designing against local collapse and buckling propagation involves selecting the appropriate wall thickness and yield stress for a given pipe diameter to ensure that any collapse, should it occur, remains local (Det Norske Veritas, 2021).

Over the past century, substantial collapse experiments and its theory has been conducted systematically, ranging from single-walled pipes to the pipe-in-pipe system (Kyriadides and Lee, 2020; Yu et al., 2022). Different calculating methods for critical buckling pressures of offshore pipelines are proposed and one of the most famous and widely used equations is the DNV method (Det Norske Veritas, 2021). It is a practical method developed from the classic Timoshenko Equation (Timshenko, 1961) and adequately accurate for diameter-to-thickness ratios ranging from 15 to 45. Besides, other calculating methods for local collapse pressure have been also developed, like the thin shell model (Dyau and Kyriakides, 1993), the variable thickness pipe model (Li, 2016), the 2D thick shell model (Yu et al., 2019), and so on. For buckling propagation, analytical models include the three-dimensional (3D) cylindrical shell model (Dyau and Kyriakides, 1993), the moving plastic hinge ring-spline model (Wierzbicki and Bhat, 2004), the ring-truss model (Yu et al., 2014), the pipe unit model (Li, 2017), and the matched asymptotic expansion (Langthjem and Jensen, 2022). More effectively and universally, the powerful finite element (FE) software (ANSYS, ABAQUS, ADINA, and so on) can achieve integrated and continuous simulations of local collapse and propagating buckle behaviors, which is difficult for most aforementioned methods. Based on the FE model, complex pipeline configurations and defects are analyzed, such as thickness eccentricity (Zhang and Pang, 2020), corrosion defects (Chen et al, 2022) and sandwich pipes (Li et al., 2022). One of the major challenges in such FEM simulation is the possible computation failure cause by the structural nonlinearities. Successful solutions to this challenge include gradually relaxing the basic theoretical assumptions according to the needs of specific behaviors or replacement of calculation modules during iterations (Smith et al., 2015). Hereof, the author seeks to develop a newer analytic strategy for buckling simulation of offshore pipelines by introducing the vector form intrinsic finite element (VFIFE).

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