The spectral fatigue methodology is a widely accepted methodology to compute the fatigue lives of an offshore platform. The ever-increasing demand for life extension of the existing floating platforms worldwide continues to grow. The offshore industry currently relies on more efficient and reliable engineering solutions to sustain the challenging oil market. The spectral method is a very efficient method using the frequency domain S-N curve-based approach. ABS Guide for Fatigue Assessment of Offshore Structures and DNVGL-RP-C203 have established guidelines for employing finite element analysis (FEA) to calculate fatigue lives using the spectral fatigue method.In the past, the spectral fatigue methodology has been applied mainly to the finite element (FE) models with 2-D elements (shell or beam) due to computational efficiency. But, with the technological advancements and the availability of high-performance computing systems, the spectral fatigue method can successfully be applied to the FE models with 3-D solid elements without computational efficiency loss. For complex structural details, the FE models with 2-D elements may not be able to capture the actual geometric details accurately. Hence, detailed FE models with solid (3-D) elements are utilized to capture geometric (stress concentration factors) SCF’s for these locations. These SCF’s are then applied to obtain fatigue damages from the 2-D element FE model using spectral analysis. The fatigue lives thus obtained can be highly conservative or inaccurate. To overcome unreliable fatigue results for such complex locations that need using 3-D elements for a better definition of the local structure, this paper presents an extension to the defined guidelines by employing spectral fatigue methodology to 3-D solid elements. An option to generate a stress range histogram that can be utilized for Fracture Mechanics Evaluation (FME) is also provided.A comparative study is performed for a critical weld location on an offshore platform using solid 3-D and shell 2-D FE models. First, an FEA is performed for both the models to calculate fatigue lives using the S-N curve-based approach. An FME is then performed for the same critical weld location in order to provide a more accurate and reliable solution that will enable Clients to plan their in-service inspections and maintenance programs. The FME requires fatigue stress range histograms as one of the input parameters for the FME implementation. Also presented is a comparison of fatigue lives based on the solid and shell element FME.

Application of Fracture Mechanics to Structural Fatigue Assessment Based on Spectral Method

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/OMAE2021/volume/85116

OMAE 2021

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