3D FEA can be used to investigate fatigue characteristics and results in either a stress amplification factor or a relationship between peak stress and applied load. Determining fatigue performance necessitates detailed modelling to capture all possible hot-spots in equipment.
An example would be determining the fatigue performance for a subsea wellhead system. This can be an important tool when looking at extending the life of a well. The ability to feed in accurate fatigue properties into a global riser analysis ensures the overall operation can be assessed to the highest standards.
Determination of fatigue hot-spots is performed with linear elastic material properties with small details that may be removed for structural analysis included in the model to ensure all stress hot spots are found. Due to the more detailed geometry, mesh is generally finer resulting in larger models requiring more computational power. Details such as fillets and blend radii are very important, as perfectly sharp internal corners result in stress singularities in the finite element model.
A fatigue assessment of a tension ring is shown in Figure 1 where maximum principal stress was extracted at hot spot locations on the body as well as welds. As DNVGL-RP-C203 S-N curves for welds account for the notch effect seen in FE models at sharp corners where a singularity exists, the hot spot stress is derived through extrapolation of the surface stress to the weld toe as shown in Figure 2.
Figure 1: Tension Ring Fatigue Assessment
Figure 2: Extrapolation of hot spot stresses at weld toe
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