Assessment of mesh sensitivity of hotspot methods applied to offshore structures

Resumo

To execute fatigue analysis of offshore structures, it is important to adequately determine the stresses field, especially on welds.  Welded regions tend to have significant stress concentration.  To deal with this problem, some international standards suggest the hot spot stress (HSS) method to estimate the stresses near welds, including guidelines for finite element analysis. Two strategies of HSS calculations are considered here: surface-extrapolated stress and through-thickness linearized stress. Recommendations exist for elements sizes of mesh depending on element type, geometry and method for HSS calculation. In this work, the calculations are applied initially to a simple flat model with different meshes, with varying element sizes and proportions between the three dimensions. Next, the strategies are applied to more complex structure, representing a bell mouth used in offshore oil platforms. Being mostly a cylindrical geometry, meshing is controlled mainly by the number of elements in thickness as well as element size in circumferential and axial direction. Different loads were applied on each case to verify mesh sensitivity relation with load type. Results indicate different levels of effect on HSS results for each direction of mesh refinement. The relation of sensitivity of the mesh with the load applied is shown and discussed.