An open-source analysis workflow for geometrically imperfect bolted ring-flanges in wind turbine support structures

Bolted ring-flanges connect structures via two circular flanges, secured with pre-loaded bolts. In offshore wind turbine support structures the flanges connect the monopile and transition piece, the transition piece and tower, and adjacent tower sections. Fatigue analysis of the bolted ring-flanges in offshore wind turbine support structures is a critical step during design. The consideration of geometric imperfections of the flanges can be an influential factor in assessed fatigue damage over some given Fatigue Limit State load case. Schemes to analyse the effect of geometric imperfections using Finite Element Method simulations are available in literature. These schemes rely on the use of proprietary tools for the analysis. In this paper we introduce an open-source workflow to assess load transfer through geometrically imperfect bolted ring-flanges. The workflow relies on four open-source toolsets: the FreeCAD 3D parametric modelling package, the Code Aster / Salome-Meca Finite Element Method simulation environment, the ParaView post-processing and visualisation package, and the Python programming language. We present a case study of load transfer assessment in a monopile to transition piece bolted ring-flange typical of modern offshore wind turbine support structures. We decompose the load transfer across the flange under a number of geometric imperfection scenarios to determine the expected stress ranges for fatigue analysis. The results are compared with analytical methods specified in engineering standards. The workflow is configurable as per user needs, and is maintained as a GitHub repository at https://github.com/jhjorg/OpenBoltRF.