PSI - Issue 60

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

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general-purpose finite element software ABAQUS 2019. The governing equation for the nonlinear static finite element analysis (Reddy 2020) is given below [ ({ ∆ } )] { ∆ } +1 ={ } where [ ] is the element stiffness matrix, { } is the element force vector, and {∆} denotes the solution at the n th iteration. Note that the element stiffness matrix is nonlinear and asymmetric. Hence the assembled nonlinear equations are solved after imposing boundary conditions using iterative methods. ABAQUS generally uses Newton's method as a numerical technique for solving nonlinear equilibrium equations. The steps followed for the integrity assessment of the dented pipeline are listed below: 1) Caliper data of the dent profile is filtered for removal of repeated data. Further, the noise in the data is minimized using FFT-based low-pass filtering. 2) FE model of the native soil, embankment (Old and expanded stretch of road), casing, and pipeline are modeled and assembled at respective locations. 3) First, the Geostatic state of stress in the native soil medium is achieved by carrying out the Geostatic analysis step in ABAQUS. In this Step, only the native soil, embankment of the old 4-lane road, and casing are activated. However, the soil in the trench, the embankment corresponding to the new additional lanes of the highway, and the pipeline itself are not activated. 4) Next, the pipeline (without dent) is activated and a static analysis is carried out with the gravity load on the pipeline. 5) From the filtered dent profile, an artificial indenter of the shape of the dented profile is generated in the form of a deformation profile, and the same is applied to the respective section of the pipeline. A nonlinear material model is applied for this stretch of pipeline. In this step, the pipeline is also applied with an internal pressure of 14.0 kg/cm 2 , which is the pressure in the pipeline during the ILI. 6) After the entire indentation is applied, the deformation indenter is withdrawn. By this procedure, the dented profile in the pipeline model can be generated with residual strains and locked-in stresses. 7) Now, the trench of the pipeline is filled by activating soil in the model. To simulate a gradual backfilling of the pipeline trench, the trench soil is activated in five layers. 8) After this, the embankment corresponding to the new lanes is activated, and static analysis is carried out. To realistically simulate the embankment construction, the activation of the soil of the new embankment is done in four layers. 9) With this, the stresses in the dented pipeline with only gravity loads are generated. This analysis is further continued for various other loading conditions (like vehicular loads, internal pressure, etc.) as applicable for assessment based on API 579-1/ASME FFS-1 Level 3 Assessment, for protection against following failure modes. a. Plastic Collapse b. Local Failure c. Collapse From Buckling d. Failure from Cyclic Loading 3.1. Caliper data filtering The raw data obtained from ILI contains several repeated/redundant entries. Thus, the raw data is initially filtered for repeated and redundant readings. Further, the data is smoothened using an FFT Low Pass filter to minimize noise and false readings from surface irregularities. However, the profile of the dent generated with twelve circumferential points is not enough to generate a dent profile suitable for FE analysis. Hence as a widely used approach, spline interpolation is performed between available data points to generate the 3D profile of the dent. The generated 3D profile of the dent is shown in Fig.3.