Introduction
Structural Integrity of equipment in Oil and Gas Industry is a major issue which invariably encountered during all stages of operation. Pipeline, for instance, might be found defects due to fabrication or operating services which may deteriorate its remaining useful life. The importance of assessing the integrity of these equipment needs to consider underlying physic of fracture mechanics since one could determine pertinent parameters contributed to failure phenomena such as crack propagation. Failure of equipment due to crack propagation can be induced by several factors such as internal pressure, corrosion, or pre-existing dents. Furthermore, this phenomenon could emerge in surfaces of equipment or in weld joints, in particular circumferential weld joints which are considered as weakest links in pipeline [2]. In which, crack occurs when a critical parameter in fracture mechanics such as Stress Intensity Factor (SIF) or J-Integral reaches its critical value. For example, figure 1 exhibits interplay between SIF and Crack Growth Rates (m/s), whereby high SIF proportionally leads to higher crack growth rates
Figure 1. SIF and Crack Growth Rate [3]
Roles of Advance Computational Models
Intricate loading conditions in oil and gas industry prompts robust analysis methods to provide more accurate results. Conventional Finite Element Method (FEM) presumably has shortcomings in generating desirable results in cumbersome cases like crack propagation due to its computational cost and outcomes of analysis could be mesh-dependent leading to discrepancies towards the results when size of mesh is modified [5]. Additionally, crack propagation considers a wide range of parameters to fully represent its physic phenomenon. Therefore, several advance computational models in predicting crack propagation are proposed, including Extended Finite Element Method (XFEM), Gurson Tvergaard-Needleman (GTN), Fracture Locus Curve (FLC), Cohesive Zone Model(CZM) etc. Nonn et al, attempted to compare 3 models in analyzing ductile crack propagation with GTN, FLC, and CZM as can be seen in Figure 2, where proper results could be produced once parameters of models were set appropriately [7]
Figure 2. Crack Simulation between 3 Different Models [7]
Cohesive Zone Model (CZM)
Among other models, CZM is arguably a common model used to evaluate crack propagation in fracture mechanics. In CZM, damage of materials is initiated when the cohesive traction is zero [5]. Herein, cohesive forces in a cohesive zone would play critical roles in resisting crack to grow larger. A main objective of harnessing this model is to encounter a parameter namely Crack-Tip Opening Angles (CTOA) where this parameter has acquired broad acceptance to be used as a fracture-resistance parameter by ASTM and ISO [4]. Indication of crack propagation can be obtained when there is a change of CTOA. CTOA signifies a driving force required to make crack propagation, where CTOA also indicates the toughness of materials. Figure 3 shows a relationship between CTOA and crack length, where a material TH4 possessed larger fracture energy by 15.188 (MPA.mm) compared to a material TH3 (14.580 MPa.mm) ,TH2 (14.175 MPa.mm) ,and TH1 (12.150 MPa.mm), as can be concluded higher CTOA could be found in the material with bigger fracture energy [4]
Figure 3. CTOA and Crack Length [4]
References
[1]Xiaohua Zhu, Zilong Deng, Weiji Liu, Dynamic fracture analysis of buried steel gas pipeline using cohesive model, Soil Dynamics and Earthquake Engineering, Volume 128, 2020,105881,ISSN 0267-7261,https://doi.org/10.1016/j.soildyn.2019.105881.
[2] Polasik, S. J., Jaske, C. E., & Bubenik, T. A. (2016). Review of Engineering Fracture Mechanics Model for Pipeline Applications. Volume 1: Pipelines and Facilities Integrity. doi:10.1115/ipc2016-64605
[3] Matvienko, Y. G. (2011). A Damage Evolution Approach in Fracture Mechanics of Pipelines. NATO Science for Peace and Security Series C: Environmental Security, 227–244. doi:10.1007/978-94-007-0588-3_15
[4] Dunbar, A., Wang, X., Tyson, W. R., & Xu, S. (2014). Simulation of ductile crack propagation and determination of CTOAs in pipeline steels using cohesive zone modelling. Fatigue & Fracture of Engineering Materials & Structures, 37(6), 592–602. doi:10.1111/ffe.12143
[5] Shahzamanian, M. M., lin, M., Kainat, M., Yoosef-Ghodsi, N., & Adeeb, S. (2021). Systematic literature review of the application of extended finite element method in failure prediction of pipelines. Journal of Pipeline Science and Engineering, 1(2), 241–251. doi:10.1016/j.jpse.2021.02.003
[6]Nie, H., Ma, W., Sha, S., Ren, J., Wang, K., Cao, J., & Dang, W. (2020). Dynamic Impact Damage of Oil and Gas Pipelines. Journal of Physics: Conference Series, 1637, 012094. doi:10.1088/1742-6596/1637/1/012094
[7] Nonn, A., & Kalwa, C. (2012). Simulation of Ductile Crack Propagation in High-Strength Pipeline Steel Using Damage Models. Volume 3: Materials and Joining. doi:10.1115/ipc2012-90653
Muhamad Alim 
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