Nonlinear Finite Element Assessment of an Offshore Wind Turbine Ring-Flange Joint under Adjacent Missing-Bolt Configurations

Bolted ring-flange connections are critical to the structural integrity of wind-turbine towers, while the absence of several adjacent bolts can significantly alter joint clamping and load-transfer mechanisms. This study presents a nonlinear finite element assessment of a full ring-flange connection under three predefined configurations: an intact joint, a joint with three adjacent missing bolts, and a joint with five adjacent missing bolts. Grade 10.9 bolts were pretensioned before applying a combined quasi-static load vector consisting of axial force, bending moment, torsional moment, and shear force through a remote-point configuration. Frictional contact was modeled using the Augmented Lagrange formulation with a coefficient of friction of 0.20. Numerical robustness was evaluated through mesh-sensitivity analysis, with global displacement changes below 0.20% after further mesh refinement. The maximum bolt von Mises stress increased from 1150.5 MPa in the intact configuration to 1361 MPa and 1683 MPa in the three- and five-missing-bolt configurations, respectively. The corresponding bolt force increased from 1857 kN to 2043 kN and 2323.4 kN. Maximum flange opening increased from 2.13 mm to 5.02 mm, while maximum contact pressure increased from 358.50 MPa to 440.52 MPa. These results demonstrate increasing load concentration, local flange separation, and reduced clamping restraint near the missing-bolt sector. The analysis assumes independently defined static configurations and does not simulate sequential bolt fracture, dynamic effects, or fatigue-life accumulation; experimental validation also remains necessary. The findings can support inspection prioritization and maintenance planning for ring-flange joints containing clusters of adjacent missing or failed bolts.