PSI - Issue 60

Dhanesh N. et al. / Procedia Structural Integrity 60 (2024) 456–470 Dhanesh et al. / StructuralIntegrity Procedia 00 (2019) 000 – 000

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1. Introduction Cross-country underground pipelines are the safest and uninterrupted mode of transportation of hydrocarbon products (gas/liquid) for long distances compared to traditional road/railways/waterways transport modes. However, underground pipelines are susceptible to different types of damages/flaws during installation and operation, such as metal loss (general/local), buckling, dents, gouges, cracks, etc. (Revie 2015). The structural integrity of the pipelines featuring such defects needs to be assured for safe operation and to avoid serious hazards (fire/explosion/fatality/injury etc.) which have both environmental and financial impacts (Belvereresi and Dann 2017). This paper discusses the integrity assessment of an actual case of an in-service Liquefied Petroleum Gas (LPG) pipeline featuring dent damage located at a highway crossing. A dent in a pipeline is defined as a permanent inward or outward plastic deformation of the circular cross-section of the pipe and is characterized by a small local radius (Cosham and Hopkins 2017). Dents that are formed during in-service conditions are of serious concern as these spots will be prone to fatigue damage, increased corrosion activity, obstructing in-line inspection (ILI), etc. ILI of the in-service pipelines is conducted periodically for various reasons such as the detection of defects, prevention of leaks and spills, maintenance planning, optimization of the pipeline operations, etc. Commonly used methods of ILI include caliper (geometry) inspection, magnetic flux leakage (MFL), ultrasonic testing (UT), eddy current testing (ECT), and the combination of MFL and UT sensors in pipeline pigging tools (Cordell and Vanzant 2003). A routine ILI of the considered pipeline using the Electronic Geometric Pigging (EGP) method showed the presence of a dent type of damage. Since the damage is located 7.5 m below a 10-lane expressway with heavy traffic, dig verification of the dent was found to be not feasible. Fitness-for-service assessment of the dented pipeline was the option left for the pipeline operator to demonstrate the structural integrity of the pipeline for safe operation with the existing dent. Various assessment methods and standards are available for the verification of pipelines with dents and a combination of dents with other defects (Zhao et al. 2022; Tchorzewski and Jaske 2002). For the present study, API 579-1/ASME-FFS-1 Fitness-For-Service (2021) standard is followed, which provides methods to assess whether the operating equipment/pipeline is fit for its intended service or whether a different type of in-service damage threatens its integrity. Since the considered pipeline does not satisfy the conditions for Level-1 and 2 assessment procedures as per ASME-FFS-1, Level-3 integrity assessment is adopted. The Level 3 Assessment rules are intended to evaluate dents in complex geometries subject to general loading conditions. For Level-3 assessment, the simulation of the pipeline is carried out using commercial finite element (FE) software ABAQUS/CAE. To simulate the actual site condition, a 3D soil-structure interaction FE model is prepared, including the native soil, embankment, trench, casing, etc., along with the dented pipeline. The considered pipeline is subjected to overburden pressure due to soil embankment and vehicular loads in addition to internal pressure. Further, the internal pressure and vehicular loads are cyclic in nature. The literature on pipelines subjected to Indian-type traffic loads is limited. Also, the commonly used API RP 1102 (2007) standard for the pipeline stress calculation at highway crossing is limited to standard wheel axle loads and pipelines crossing the highway at 90°. Since the API 579-1 standard allows the usage of arbitrary wheel load configurations, the present work considers different types of vehicular loads on the highway as per IRC:6 (2017). In the first step, the permanent plastic strain developed in the pipeline due to the dent damage is calculated by simulating various stages of construction and operation sequentially. The dented pipeline segment was further assessed for potential failure modes i.e., protection against plastic collapse, local failure, collapse from buckling, and fatigue damage due to vehicular traffic as well as internal pressure cycles. The assessment confirms the integrity of the dented pipeline under all possible loading conditions. The methodology presented in this paper can act as a guide for the integrity assessment of pipelines with dent damage located at highway crossings. 2. Pipeline Details 2.1. Design Data The considered stretch of the pipeline crosses the 10-lane highway at an angle of 57.5 ⁰ with the median of the road. The 10-lane road having a width of 56 m is constructed over an embankment. A typical sketch of the