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Development of Novel Surrogate Models for Stress Concentration Factors in Composite Reinforced Tubular KT-Joints

Mohsin Iqbal, Saravanan Karuppanan, Veeradasan Perumal, Mark Ovinis, Muhammad Iqbal

Abstract


Circular hollow section (CHS) joints are among the most critical components in offshore jackets, often requiring rehabilitation to maintain structural integrity. The structural stress approach, based on stress concentration factors (SCFs) for hot-spot stress (HSS) calculations, is commonly used to estimate the fatigue life of critical structural elements such as CHS joints. Various empirical models exist for the rapid estimation of SCF in composite-reinforced CHS joints; however, most studies focus on SCF at the crown and saddle positions under uniplanar loading. This limitation reduces their applicability to multi-planar loading conditions, potentially leading to the underestimation of HSS. This study investigates the use of fiber-reinforced polymer (FRP) composites to strengthen CHS KT-joints under complex loading, focusing on reducing SCF and improving fatigue life. A total of 5,429 finite element simulations were conducted to examine the effects of geometric and reinforcement parameters on SCF. The simulation data were used to train artificial neural networks (ANNs), which were incorporated into a computational tool for the rapid approximation of hot-spot stress in FRP-reinforced KT-joints. The application of composites to CHS joints significantly reduces SCF, particularly with an increased number of reinforcement layers, a higher elastic modulus, and an orthogonal fiber orientation to the weld toe. This study presents a novel methodology for developing efficient models to estimate SCF in composite-reinforced CHS joints under complex loading, addressing a key gap in fatigue design for such joints. The developed computational tool enables the rapid calculation of hot-spot stress in CHS joints.

 

Doi: 10.28991/CEJ-2025-011-04-012

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Keywords


Circular Hollow Section Joints; Composite Reinforcement; Stress Concentration Factors (SCF); Artificial Neural Networks (ANN); Structural Rehabilitation; Surrogate Modelling.

References


Nassiraei, H. (2025). Identification of the most suitable probability models for local joint flexibility in T/Y-connections stiffened with collar or doubler plates. Results in Engineering, 25, 104387. doi:10.1016/j.rineng.2025.104387.

Yang, Y., Min, S., Peng, Y., Wang, H., & Chen, C. (2025). Experimental and numerical investigation on stress concentration factor of large-scale welded tubular T-joints. Ocean Engineering, 320, 120337. doi:10.1016/j.oceaneng.2025.120337.

Zavvar, E., Rosa-Santos, P., Ghafoori, E., & Taveira-Pinto, F. (2025). Analysis of tubular joints in marine structures: A comprehensive review. Marine Structures, 99, 103702. doi:10.1016/j.marstruc.2024.103702.

Iqbal, M., Karuppanan, S., Perumal, V., Ovinis, M., & Rasul, A. (2023). Rehabilitation Techniques for Offshore Tubular Joints. Journal of Marine Science and Engineering, 11(2), 461. doi:10.3390/jmse11020461.

Iqbal, M., Karuppanan, S., Perumal, V., Ovinis, M., & Iqbal, M. (2024). A systematic review of stress concentration factors (SCFs) in composite reinforced circular hollow section (CHS) joints. Composites Part C: Open Access, 15, 100515. doi:10.1016/j.jcomc.2024.100515.

ASME-PCC-2018. (2018). Repair of Pressure Equipment and Piping. The American Society of Mechanical Engineers, New York, United States.

ISO 24817:2015. (2015). Petroleum, petrochemical and natural gas industries - Composite repairs for pipework - Qualification and design, installation, testing and inspection. International Organization for Standardization (ISO), Geneva, Switzerland.

Zuo, G., Tong, L., Zhao, Z., Wang, H., Shi, W., & Yan, Y. (2025). SCF formulae of corrugated-web steel girders: Experiments and numerical analysis. Journal of Constructional Steel Research, 228, 109445. doi:10.1016/j.jcsr.2025.109445.

Sadat Hosseini, A., Bahaari, M. R., & Lesani, M. (2019). Parametric Study of FRP Strengthening on Stress Concentration Factors in an Offshore Tubular T-Joint Subjected to In-Plane and Out-of-Plane Bending Moments. International Journal of Steel Structures, 19(6), 1755–1766. doi:10.1007/s13296-019-00244-0.

Sadat Hosseini, A., Bahaari, M. R., & Lesani, M. (2019). Stress concentration factors in FRP-strengthened offshore steel tubular T-joints under various brace loadings. Structures, 20, 779–793. doi:10.1016/j.istruc.2019.07.004.

Tong, L., Xu, G., Zhao, X. L., Zhou, H., & Xu, F. (2019). Experimental and theoretical studies on reducing hot spot stress on CHS gap K-joints with CFRP strengthening. Engineering Structures, 201, 296–313. doi:10.1016/j.engstruct.2019.109827.

Xu, G., Tong, L., Zhao, X. L., Zhou, H., & Xu, F. (2020). Numerical analysis and formulae for SCF reduction coefficients of CFRP-strengthened CHS gap K-joints. Engineering Structures, 210, 369–86. doi:10.1016/j.engstruct.2020.110369.

Sadat Hosseini, A., Bahaari, M. R., & Lesani, M. (2020). Experimental and parametric studies of SCFs in FRP strengthened tubular T-joints under axially loaded brace. Engineering Structures, 213, 110548. doi:10.1016/j.engstruct.2020.110548.

Nassiraei, H., & Rezadoost, P. (2020). Stress concentration factors in tubular T/Y-joints strengthened with FRP subjected to compressive load in offshore structures. International Journal of Fatigue, 140, 105719. doi:10.1016/j.ijfatigue.2020.105719.

Hosseini, A. S., Bahaari, M. R., & Lesani, M. (2020). SCF distribution in FRP-strengthened tubular T-joints under brace axial loading. Scientia Iranica, 27(3 A), 1113–1129. doi:10.24200/SCI.2018.5471.1293.

Nassiraei, H., & Rezadoost, P. (2021). Stress concentration factors in tubular T/Y-connections reinforced with FRP under in-plane bending load. Marine Structures, 76, 102871. doi:10.1016/j.marstruc.2020.102871.

Nassiraei, H., & Rezadoost, P. (2021). Parametric study and formula for SCFs of FRP-strengthened CHS T/Y-joints under out-of-plane bending load. Ocean Engineering, 221, 108313. doi:10.1016/j.oceaneng.2020.108313.

Sadat Hosseini, A., Zavvar, E., & Ahmadi, H. (2021). Stress concentration factors in FRP-strengthened steel tubular KT-joints. Applied Ocean Research, 108, 1187–221. doi:10.1016/j.apor.2021.102525.

Nassiraei, H., & Rezadoost, P. (2021). SCFs in tubular X-joints retrofitted with FRP under out-of-plane bending moment. Marine Structures, 79, 103010. doi:10.1016/j.marstruc.2021.103010.

Nassiraei, H., & Rezadoost, P. (2021). SCFs in tubular X-connections retrofitted with FRP under in-plane bending load. Composite Structures, 274, 114314. doi:10.1016/j.compstruct.2021.114314.

Zavvar, E., Sadat Hosseini, A., & Lotfollahi-Yaghin, M. A. (2021). Stress concentration factors in steel tubular KT-connections with FRP-Wrapping under bending moments. Structures, 33, 4743–4765. doi:10.1016/j.istruc.2021.06.100.

Nassiraei, H., & Rezadoost, P. (2022). Development of a probability distribution model for the SCFs in tubular X-connections retrofitted with FRP. Structures, 36, 233–247. doi:10.1016/j.istruc.2021.10.033.

Xu, X., Shao, Y., Gao, X., & Mohamed, H. S. (2022). Stress concentration factor (SCF) of CHS gap TT-joints reinforced with CFRP. Ocean Engineering, 247, 110722. doi:10.1016/j.oceaneng.2022.110722.

Mohamed, H. S., Zhang, L., Shao, Y. B., Yang, X. S., Shaheen, M. A., & Suleiman, M. F. (2022). Stress concentration factors of CFRP-reinforced tubular K-joints via Zero Point Structural Stress Approach. Marine Structures, 84, 103239. doi:10.1016/j.marstruc.2022.103239.

Mohamed, H. S., Yang, X. S., Shao, Y. B., Shaheen, M. A., Suleiman, M. F., Zhang, L., & Hossian, A. (2022). Stress concentration factors (SCF) of CFRP-reinforced T/Y-joints via ZPSS approach. Ocean Engineering, 261, 112092. doi:10.1016/j.oceaneng.2022.112092.

Sadat Hosseini, A., Bahaari, M. R., & Lesani, M. (2022). Formulas for Stress Concentration Factors in T&Y Steel Tubular Joints Stiffened with FRP under Bending Moments. International Journal of Steel Structures, 22(5), 1408–1432. doi:10.1007/s13296-022-00651-w.

Zavvar, E., Henneberg, J., & Guedes Soares, C. (2023). Stress concentration factors in FRP-reinforced tubular DKT joints under axial loads. Marine Structures, 90, 429–52. doi:10.1016/j.marstruc.2023.103429.

Mohamed, H. S., Yang, X. S., Liao, F. Y., & Fu, T. (2024). SCF determination of the CFRP- fortified T/Y-joints exposed to IPB or OPB moment via ZPSS approach. Structures, 61. doi:10.1016/j.istruc.2024.105997.

Rashnooie, R., Zeinoddini, M., Ghafoori, E., & Sharafi, M. (2024). Experimental and numerical study on the in-plane bending behaviour of FRP-strengthened steel tubular welded T-joints. Thin-Walled Structures, 201, 112000. doi:10.1016/j.tws.2024.112000.

Zavvar, E., Sousa, F., Giannini, G., Taveira-Pinto, F., & Santos, P. R. (2024). Probability of maximum values of stress concentration factors in tubular DKT-joints reinforced with FRP under axial loads. Structures, 66, 106809. doi:10.1016/j.istruc.2024.106809.

Zavvar, E., Sousa, F., Taveira-Pinto, F., & Rosa Santos, P. (2024). Multivariate Data Analysis of Maximum Stress Concentration Factors in FRP-Retrofitted Two-Planar KT-Joints under Axial Loads for Offshore Renewables. Journal of Marine Science and Engineering, 12(8), 1451. doi:10.3390/jmse12081451.

Rezadoost, P., & Nassiraei, H. (2024). Best Probability Distribution for SCFs in FRP-Strengthened T/Y-Shaped Connections under Out-of-Plane Bending Loading. 21-22 May, 2024, University of Zanjan, Zanjan, Iran.

Iqbal, M., Karuppanan, S., Perumal, V., Ovinis, M., & Nouman, H. (2023). Empirical modeling of stress concentration factors using finite element analysis and artificial neural networks for the fatigue design of tubular KT-joints under combined loading. Fatigue and Fracture of Engineering Materials and Structures, 46(11), 4333–4349. doi:10.1111/ffe.14122.

Ahmadi, H., Lotfollahi-Yaghin, M. A., & Aminfar, M. H. (2011). Effect of stress concentration factors on the structural integrity assessment of multi-planar offshore tubular DKT-joints based on the fracture mechanics fatigue reliability approach. Ocean Engineering, 38(17–18), 1883–1893. doi:10.1016/j.oceaneng.2011.08.004.

Sadat Hosseini, A., Bahaari, M. R., Lesani, M., & Hajikarimi, P. (2021). Static load-bearing capacity formulation for steel tubular T/Y-joints strengthened with GFRP and CFRP. Composite Structures, 268, 263–81. doi:10.1016/j.compstruct.2021.113950.

Moffat, D. G., Kruzelecki, J., & Blachut, J. (1996). The Effects of Chord Length and Boundary Conditions on the Static Strength of a Tubular T-Joint under Brace Compression Loading. Marine Structures, 9(10), 935–947. doi:10.1016/0951-8339(96)00007-X.

Smedley, P., & Fisher, P. (1991). Stress concentration factors for simple tubular joints. Health and Safety Executive - Offshore Technology Report.

ARSEM. (1987). Design Guides for Offshore Structures, Volume 1 - Welded Tubular Joints. Association de Recherche sur les Structures Métalliques Marines (ARSEM), Paris, France.

API (2014). Recommended Practice for Planning, D. and C.F.O.P.-W.S.D. (2014). API RP 2A WSD 22nd Edition, vol. 2014. American Petroleum Institute (API), Washington, United States.

Ahmadi, H., & Nejad, A. Z. (2016). Stress concentration factors in uniplanar tubular KT-joints of jacket structures subjected to in-plane bending loads. International Journal of Maritime Technology, 5, 27-39.

Ahmadi, H. (2019). Probabilistic analysis of the DoB in axially-loaded tubular KT-joints of offshore structures. Applied Ocean Research, 87, 64–80. doi:10.1016/j.apor.2019.03.018.

Ahmadi, H., & Lotfollahi-Yaghin, M. A. (2013). Experimental and numerical investigation of geometric SCFs in internally ring-stiffened tubular KT-joints of offshore structures. Journal of the Persian Gulf, 4(12), 1-12.

Ahmadi, H., & Zavvar, E. (2015). Stress concentration factors induced by out-of-plane bending loads in ring-stiffened tubular KT-joints of jacket structures. Thin-Walled Structures, 91, 82–95. doi:10.1016/j.tws.2015.02.011.

Zhang, Y., Zhang, K., Zhao, H., Xin, J., & Duan, M. (2018). Stress analysis of adhesive in a cracked steel plate repaired with CFRP. Journal of Constructional Steel Research, 145, 210–217. doi:10.1016/j.jcsr.2018.02.029.

Ullah, H., Harland, A. R., & Silberschmidt, V. V. (2012). Experimental and numerical analysis of damage in woven GFRP composites under large-deflection bending. Applied Composite Materials, 19(5), 769–783. doi:10.1007/s10443-011-9242-7.

Ganesh, V. K., & Naik, N. K. (1993). Some strength studies on FRP laminates. Composite Structures, 24(1), 51–58. doi:10.1016/0263-8223(93)90054-T.

Iqbal, M., Karuppanan, S., Perumal, V., Ovinis, M., & Khan, A. (2025). Stress Concentration Factors in CFRP-Reinforced KT-Joints under Multiplanar Bending Loads: Experimental and Numerical Investigation. Results in Engineering, 25(103745), 103745. doi:10.1016/j.rineng.2024.103745.

Ahmadi, H., & Zavvar, E. (2020). Degree of bending (DoB) in offshore tubular KT-joints under the axial, in-plane bending (IPB), and out-of-plane bending (OPB) loads. Applied Ocean Research, 95, 1187–206. doi:10.1016/j.apor.2019.102015.

Ahmadi, H., Yeganeh, A., Mohammadi, A. H., & Zavvar, E. (2016). Probabilistic analysis of stress concentration factors in tubular KT-joints reinforced with internal ring stiffeners under in-plane bending loads. Thin-Walled Structures, 99, 58–75. doi:10.1016/j.tws.2015.11.010.

Hobbacher, A. (1996). Fatigue Design of Welded Joints and Components. Recommendations of IIW Joint Working Group XIII-1539–96/XV-845–96, The International Institute of Welding, Genoa, Italy.

Gurney, T. R. (1991). The Fatigue Strength of Transverse Fillet Welded Joints. The Fatigue Strength of Transverse Fillet Welded Joints, Abington Publishing, Parks Blvd Nashville, United States. doi:10.1533/9780857093257.

Poutiainen, I., Tanskanen, P., & Marquis, G. (2004). Finite element methods for structural hot spot stress determination - A comparison of procedures. International Journal of Fatigue, 26(11), 1147–1157. doi:10.1016/j.ijfatigue.2004.04.003.

Radaj, D., Sonsino, C., & Fricke, W. (2006). Fatigue assessment of welded joints by local approaches, Second Edition. CRC Press, Boca Raton, United States. doi:10.1201/9781439832806.

Maddox, S. J. (2002). Fatigue Strength of Welded Structures. In Fatigue Strength of Welded Structures. Woodhead Publishing, Sawston, United Kingdom. doi:10.1016/c2013-0-17455-7.

Manson, S. S. (1965). Fatigue: A Complex Subject-Some Simple Approximations some approximations useful in design are outlined and their application illustrated. Experimental Mechanics, 5(4), 193–226.

Niemi, E., Fricke, W., & Maddox, S. J. (2018). The Structural Hot-Spot Stress Approach to Fatigue Analysis. In: Structural Hot-Spot Stress Approach to Fatigue Analysis of Welded Components. IIW Collection, Springer, Singapore. doi:10.1007/978-981-10-5568-3_2.

Hobbacher, A. F. (2017). Erratum to: Recommendations for Fatigue Design of Welded Joints and Components. Recommendations for Fatigue Design of Welded Joints and Components, E1–E1, Springer, Cham, Switzerland. doi:10.1007/978-3-319-23757-2_7.

Lesani, M., Bahaari, M. R., & Shokrieh, M. M. (2015). FRP wrapping for the rehabilitation of Circular Hollow Section (CHS) tubular steel connections. Thin-Walled Structures, 90, 216–234. doi:10.1016/j.tws.2014.12.013.

Hawileh, R. A., Musto, H. A., Abdalla, J. A., & Naser, M. Z. (2019). Finite element modeling of reinforced concrete beams externally strengthened in flexure with side-bonded FRP laminates. Composites Part B: Engineering, 173, 106952. doi:10.1016/j.compositesb.2019.106952.

Fu, Y., Tong, L., He, L., & Zhao, X. L. (2016). Experimental and numerical investigation on behavior of CFRP-strengthened circular hollow section gap K-joints. Thin-Walled Structures, 102, 80–97. doi:10.1016/j.tws.2016.01.020.

Efthymiou, M. (1988). Development of SCF formulae and generalized influence functions for use in fatigue analysis. OTJ 88. Recent Developments in Tubular Joints Technology, 4-5 October, 1988, Surrey, United Kingdom.

N’Diaye, A., Hariri, S., Pluvinage, G., & Azari, Z. (2007). Stress concentration factor analysis for notched welded tubular T-joints. International Journal of Fatigue, 29(8), 1554–1570. doi:10.1016/j.ijfatigue.2006.10.030.

Ahmadi, H., Lotfollahi-Yaghin, M. A., & Yong-Bo, S. (2013). Chord-side SCF distribution of central brace in internally ring-stiffened tubular KT-joints: A geometrically parametric study. Thin-Walled Structures, 70, 93–105. doi:10.1016/j.tws.2013.04.011.

Mohammed, A., Dasari, S. R., Khandelwal, S. K., & Desai, Y. M. (2024). Advancements in stress concentration factor computation for tubular X joints through Bayesian-optimized neural networks. Structures, 67, 106962. doi:10.1016/j.istruc.2024.106962.

Iqbal, M., Karuppanan, S., Perumal, V., Ovinis, M., Iqbal, M., & Rasul, A. (2024). Optimization of fibre orientation for composite reinforcement of circular hollow section KT-joints. International Journal of Structural Integrity, 15(4), 717–730. doi:10.1108/IJSI-04-2024-0054.


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DOI: 10.28991/CEJ-2025-011-04-012

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Copyright (c) 2025 Mohsin Iqbal, Saravanan Karuppanan, Veeradasan Perumal, Mark Ovinis2, Muhammad Iqbal

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