FRACTURE MECHANICS APPROACH FOR THE MAINTENANCE OF OFFSHORE OIL AND GAS PIPELINE

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ABSTRACT

Failure of offshore oil and gas pipelines occurs under certain conditions due to some applied mechanical forces. These conditions constitute a potential threat to the integrity of in-service life span of the pipelines which can lead to loss of resources and environmental pollution. Several studies have shown that pipelines fail as a result of Welding, Fatigue Crack Growth, Corrosion Fatigue, Stress Corrosion Cracking, and Erosion due to fluid flow.



This paper presents a model by using fracture mechanics to analyze the allowable applied stresses an in service pipeline needs to withstand in minimizing crack growth. Furthermore, the crack size, crack shape and hole radius with pipe thickness will be modeled. The modeling results will be validated using experimental data. The implications of the results will be discussed for the design or development of a robust oil and gas pipelines.


CHAPTER ONE: Background and Introduction



1.1: Research Background



Oil and Gas Pipelines are used as a medium through which petroleum products are transported from the wells to the tanks. When it is under operation, it fails rarely; meanwhile, it causes extremely serious problems like loss of resources and lives if failure does occur. Over half of all in-service pipelines fail as a result of some externally applied mechanical forces which must be properly analyzed to prevent reoccurrence. Fractographic examination is to determine the causes of failures by studying the characteristics of a fracture surface.


Griffith proposed that cracks that already exist will propagate when the released elastic Strain Energy is at least equal to the energy that is required to create the new crack surface. Life prediction for Fatigue Crack by Paris has showed that range of Stress Intensity Factor, k, might characterize Sub-Critical Crack Growth under fatigue loading. He examined that Crack Growth Rate of Stress Intensity Factor gave straight line.


Also, Rice’s J-integral is a commonly used Elastic Plastic Fracture parameter for the description of the local field in the neighborhood of the Stress Concentration and for the study of crack initiation and propagation. His theory is also interpreted as the potential difference in energy between two specimens that are loaded identically having slightly different crack length. Meanwhile, Irwin proposed the Stress Intensity Factor as crack primary driving force.


Neumann and Raju estimated the Stress Intensity Factor for hollow Cylinder for specific Crack Aspect Ratios, Crack Depth to Thickness and Hole Radius to Thickness. Alexander Aynbinder evaluated the thickness of High Temperature (HT) and High Pressure (HP) pipe walls and combined the inelastic behavior of pipe steels by using iterative computational modeling algorithm. Idriss Malik used Lame’s solution to estimate stresses and Neumann and Raju solutions to calculate Stress Intensity Factor for the combined modeling of the wall thinning and crack propagation in pipelines.


Oil and Gas Pipeline reliability is affected by welding defects, corrosion and stresses that cause cracking. Therefore, the applied stresses, Stress Intensity Factor, Composition, and Temperature etc are highly considered for the prediction of the integrity of offshore pipelines. Additionally, Stresses that are resulted to Sub-Critical Crack Growth is a major challenge to the pipe, while the loss of materials can result from the interaction between erosive fluids flow and corroding pipeline.


However, the codes of most pipelines do not actually give an account on how the combined effect of erosion under corrosive conditions and Crack Growth Rate. Similarly, there had been assumption of the mechanical properties of the surface that is corroded to have the same properties of the uncorroded portion......


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