Failure assessment of buried offshore pipelines subjected to upheaval buckling

Subedi, Rabindra (2023) Failure assessment of buried offshore pipelines subjected to upheaval buckling. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (22MB)

Abstract

Offshore pipelines transport hydrocarbons at high operating pressure and temperature (HPHT) conditions. The thermal expansion and internal pressure can create large axial compressive forces in a pipeline, resulting in global buckling. In offshore, proper trenching and backfilling operations are challenging. Therefore, depending on the soil resistance, the pipeline might experience severe vertical movement during HPHT operating conditions. An insufficient burial depth might result in the pipeline moving upwards (upheaval buckling) from the original configuration, leading to local buckling instability and ultimate plastic collapse. The buckled section of the offshore pipeline may undergo local sectional deformation (ovalization), tensile rupture and fracture due to high levels of stress and strain, depending upon the operating conditions, sectional properties and uplift soil resistance. The initial imperfection is another factor that governs the initiation and buckling types for the buried or surface-laid pipeline. Recognizing the limitations in the existing theoretical solutions and design guidelines in modelling initial imperfection and soil resistances, the present study focuses on numerical modelling and strain-based structural assessment for global upheaval buckling, local buckling, and fracture in offshore pipelines. Several soil-spring resistance models based on current design guidelines, e.g., bilinear and trilinear, are used to evaluate the upheaval buckling (UHB) behaviour of pipelines. The nonlinear degradation of uplift resistance for dense and loose sand for modelling large vertical displacements (post-upheaval above ground) of the buried pipeline is also investigated. Besides, the present study explores the effects of initially stressed and unstressed imperfections on the upheaval buckling characteristics of the offshore buried pipelines. The limitation of the one-dimensional finite element model in predicting sectional deformation and local buckling is solved through a hybrid three-dimensional coupled finite element (FE) model. This computationally efficient model integrates a three-dimensional finite element model of the pipe and shell, accounting for operating temperature and pressure effects on a long pipe with nonlinear soil springs. It enables analysis of post-upheaval buckling, local buckling, and wrinkling failure in the pipelines. To investigate the tensile fracture associated with local buckling, this study also presents a numerical modelling technique using the eXtended finite element (XFEM) method to analyze the initiation and propagation of tensile fractures in a post-buckled offshore pipeline. Conventional fracture mechanics commonly employ damage initiation criteria based on maximum principal stress (MAXPS) or maximum principal strain (MAXPE) with fixed values. However, these criteria may be overly conservative, neglecting crack-tip constraints during the numerical analysis. To address this challenge, a modified Mohr-Coulomb (MMC) fracture criterion is proposed and implemented into the FE program Abaqus through a user-defined subroutine. The proposed MMC criterion considers shear slip and ductility, providing a more realistic representation of ductile materials than MAXPS and MAXPE models. This study also discusses the calibration of fracture parameters with different damage degradation models. The proposed modelling techniques for large deformation FE modelling of global upheaval buckling, local buckling (ovalization and wrinkling), and fracture prediction address the design and integrity challenges in the offshore pipeline industry. The outcomes of the present study can play a vital role in assessing the structural integrity of offshore pipelines, leading to improved pipeline design and enhanced operational safety in offshore environments.

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