PSI - Issue 57

Inge Lotsberg et al. / Procedia Structural Integrity 57 (2024) 569–580 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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welded test specimen. The results are comparable with that derived from measurements provided that the recommended methods for finite element modelling from DNV-RP-C203 (2019) are followed. In some situations, with inclusion of correct geometry of the fillet welds it can be difficult to correctly follow the guidelines and several elements are used to represent the doubling plate thickness. Analysts also seem to believe that finer meshes lead to improved results. The guidelines are based on calibration of analysis models with coarser meshes where the calculated stresses at the hot spot stress area have not converged. This means that by refined meshes the results can change which may also affect the calculated hot spot stresses. Especially results from quadratic extrapolation methods are sensitive to how the element mesh is made. In the performed finite element models it is seen that converged results have not been achieved in the fine mesh models. This has led to non-conservative hot spot stress result using Method A in DNV-RP-C203 (2019) and conservative results when using the quadratic extrapolation method. When these results are removed from the derived hot spot stress data, all the other results fall within ± 10 % from the target value. However, it is noted that some of methods provide results slightly on the non-conservative side. The uncertainties and the scatter in results provided by the different analysis methods should be kept in mind when recommendations on safety factors are being made. With many elements over the plate thickness an alternative is to perform a linearization of the calculated stress through the plate thickness at the hot spot area. When one is in doubt of the reliability of a calculated hot spot stress, it may be recommended to check the results also by use of other hot spot stress methods. An alternative is also to perform similar analysis of a well-known detail for a relative calibration of the analysis results using a similar finite element mesh in the two models. Furthermore, the effective notch stress method may be used for analysis. However, in many cases this method may be too time consuming for fatigue assessment of real structures. For fatigue assessment of the weld root the simplest method is normally to use the nominal stress method where the stresses in the weld throat are calculated based on equilibrium of the membrane forces in the plates at the connection. Also, here the effective notch stress method may be used for analysis in special situations. For the doubling plates in test specimen no 1 the test results indicate that the S-N curve for the weld root should be at least 2 curves above the lowest S-N curve recommended for fillet welds in DNV-RP-C203 (2019). References Fricke, W., 2001 . Recommended Hot Spot Analysis Procedure for Structural Details of FPSO’s and Ships Based on Round -Robin FE Analyses. Proc. 11th ISOPE, Stavanger. Also 2002, International Journal of Offshore and Polar Engineering. Vol. 12, No. 1, March 2002. Fricke, W., 2013. IIW guideline for the assessment of weld root fatigue. Welding in the World, Vol. 57, 753-791. Sonsino, C. M., Fricke, W., de Bruyne, F., Hoppe, A., Ahmadi, A. and Zhang, G. 2012. Notch Stress Concepts for the Fatigue Assessment of Welded Joints – Background and Applications. International Journal of Fatigue 34 (2012) 2 – 16. Hobbacker, A., 1996. Fatigue Design of Welded Joints and Components. Recommendations of IIW. Joint Working Group XIII-XV. Abington Publishing, Cambridge, England. Hobbacker, A., 2009. IIW Recommendations for Fatigue Design of Welded Joints and Components. The Welding Research Council, Inc. New York, 2009. ISSN 0043-3226. Niemi, E., 1995.Stress Determination for Fatigue Analysis of Welded Components. IIS/IIW-1221-93. Abington Publishing, Cambridge, England. Niemi, E., Fricke, W. and Maddox, S. J., 2006. Fatigue Analysis of Welded Components. Designer’s Guide to the Structural Hot -spot Stress Approach. Woodhead Publishing Limited, Cambridge, England. Lotsberg, I., Fjeldstad, A., Ro Helsem, M. and Oma, N., 2014. Fatigue Life Improvement of Welded Doubling Plates by Grinding and Ultrasonic Peening, Welding in the World: Volume 58, Issue 6, pp. 819-830. Lotsberg, I., 2016. Fatigue Design of Marine Structures. Cambridge University Press. New York. Simulia User Assistance 2021. [cited 2022-12-14]. Available from: Abaqus - SIMULIA User Assistance 2021 (3ds.com). Poutiainen, I., Tanskanen, P. and Marquis, G., 2004. Finite Element Methods for Structural Hot Spot Stress Determination - a Comparison of Procedures, International Journal of Fatigue 26, 1147 – 1157. Almar-Næss, A. 1985. Fatigue Handbook. Tapor Forlag, NTH, Tronheim, Norway. DNV-RP-C203, 2019. Fatigue Design of Steel Offshore Structures. DNV, Oslo, Norway. DNV-CG-129, 2021. Fatigue Assessment of Ship Structures. DNV, Oslo, Norway.