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

The spectral fatigue methodology is a widely accepted methodology to compute the fatigue life of an offshore platform. The ever-increasing demand for life extension of the existing floating platforms worldwide continues to grow. 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. 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 SCF’s (stress concentration factors) for these locations. The fatigue lives thus obtained using SCF approach with 2-D elements 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. The paper also illustrates the applicability of Engineering Criticality Assessment (ECA) using stress-histogram based Fracture Mechanics Evaluation (FME) approach. 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, FEA is performed for both the models to calculate fatigue lives using the S-N curve-based approach. In addition, FME is also 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. Also, presented is a comparison of fatigue lives based on the solid and shell element FME.