PSI - Issue 71

V. Thondamon et al. / Procedia Structural Integrity 71 (2025) 226–232

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Wu et al. (2021) performed re-rounding studies on API 5L X80 grade steel pipes. Indentation was made without internal pressure using rigid indenters and then various magnitudes of internal pressure was applied to study the effect of internal pressure on re-rounding of dents. Naghipour et al. (2018) studied re-rounding behaviour on pipe of 50 mm outer diameter and thickness of 5 mm with internal pressure and without internal pressure. It was inferred that if the dent depths are higher, the magnitude of re-rounding is lesser and internal pressure helps in re-rounding of dents. Sha et al. (2021) carried out re-rounding studies on API 5L X65 grade steel pipes and studied the strain variation around the dent. It was observed that all strains are tensile in nature when measured in radial direction of the dent. Zhang et al. (2020) performed numerical studies on API 5L X60 grade steel pipes of 508 mm outer diameter. Re rounding of dent depths were studied for various wall thicknesses and internal pressure values. The results showed that the strain values are insignificant when measured at 1000 mm away from the center of the indenter position. Range of indent influence was shown by Zhao et al. (2020) to be within 1.5 times the diameter of the indenter. Zhao et al. (2020) observed that the strains along longitudinal and circumferential direction of the pipe shows much prominent variation and strains measured at 45 degrees to the pipe axis shows lesser variation. Numerical analysis was performed by Yu et al. (2019) on API 5L X65 grade steel pipes of 660 mm outer diameter. Maximum plastic strain was observed between the edge of indenter (flank) and edge of dent (rim). Strain variation in radial direction up to a distance of approximately 1.5 times the indenter diameter from the edge of the dent (rim) is studied. Pipe of various thickness to outer diameter ratio between 3% to 10% was modelled by Rezaee et al. (2018) using 8-node linear brick elements and indenter was meshed by 4-node bilinear rigid quadrilateral shell element. General contact (Explicit) was assigned and the mechanical contact property of tangential behaviour with friction coefficient of 0.15 and normal behaviour of hard contact was used. It was observed that higher internal pressure resulted in lower plastic deformations. API 5L X46 grade steel is one of the most commonly used grades of steel in oil and gas industry. Numerical studies were carried out on API 5L X46 grade steel pipes to understand the re-rounding response. Dents of spherical shape were imparted. Two different dent sizes were considered in proportion to the outer diameter of the pipe. Diameter of indenters were 0.25 times and 0.4 times the outer diameter of the pipe. Dent depths at indentation forces of 90 kN and 105 kN were studied. Dent depths proportional to the outer diameter of the pipe (10% and 12.5% of the outer diameter) were considered for re-rounding studies. Under commissioning conditions, dents were imparted without internal pressure. Under in-service conditions, internal pressure was applied and dents were imparted. 2. Material properties Specimens were fabricated from in-service pipes as per ASTM E8M. Tension tests were performed to evaluate the mechanical properties of API 5L X46 grade steel pipe. From the tension test results, the average yield strength, ultimate tensile strength, modulus of elasticity and percentage of elongation of the material were evaluated as 436.79 MPa, 526.91 MPa, 220.63 GPa and 17.9, respectively. 3. Numerical studies In the present study, the stress-strain curve obtained from the tension test was used as material input for the numerical analysis. A 2000 mm long pipe was modelled in finite element software, “ABAQUS”. The outer diameter and thickness of the pipe were considered as 356 mm and 7.8 mm, respectively. Non-linear static analysis was performed. Mechanical contact property was assigned with tangential and normal behaviour parameters. Under tangential behaviour, the penalty friction formulation was used with a friction coefficient of 0.15. Under normal behaviour, hard-contact pressure-overclosure was used and separation was allowed after contact. Standard linear 8 noded brick element C3D8R was used for modelling the pipe. Dents were imparted by modelling a discrete rigid indenter of diameter 89 mm and 142 mm, which correspond to 0.25 times and 0.4 times the outer diameter of the pipe respectively. The indenter is placed such that it is atleast 1000 mm away from the end of the pipe. At the indenter location, the pipe was partitioned to enable finer meshing at dent location. Surface supports were assigned to the bottom three-fourth circumference of the pipe opposite to the indenter, in the partitioned region at the indentation location. Simple support was assigned to the surface. This surface support was assigned to restrain the pipe during indentation and to avoid ovalization of the cross-section during indentation. Typical indenter position and support of pipe in FE model are shown in Fig. 1. Table 1 shows the summary of cases considered in dent formation study. For dent formation studies, indentation