This work reports the results of a numerical investigation aiming at assessing the structural safety of an actual offshore topside steel structure exposed to accidental localised fire conditions. Particularly, the paper explores the nonlinear thermomechanical and ultimate strength behaviours obtained by means of two fire approaches: sophisticated CFD (Computational Fluid Dynamics) and “LF-ESF” (Localised Fire with Ellipsoidal Solid Flame), previously developed and validated by the authors. CFD-based approaches, which typically solve nearly compressible flow with input energy given by a combustion model, allow accurate evaluation of a fluid-thermo response for the entire duration of the simulated accident. However, the analysis complexity, massive amount of produced data, excessive computational and time-consuming make CFD approaches unsuitable for the current applied design usage. On the other hand, the proposed LF-ESF approach displays quite accurate estimates in comparison to CFD counterparts. Moreover, the LF-ESF can be directly modelled in FE-based commercial software and used to obtain the steel temperature variation and thermomechanical behaviour. The obtained results indicate that the LF-ESF approach combined with FE-based models can provide reliable fire-safety analyses, ensuring that the main safety (load-bearing) functions of the offshore steel structures are not impaired during accidental fire conditions. •One-way coupling localized fire simulations through CFD-FEM and Localised Fire with Ellipsoidal Solid Flame approaches.•Thermomechanical analyses of localized fires scenarios in offshore topside steel structures through sophisticated models.•The failure mechanisms with and without fire scenarios were investigated.•Unlike conventional methods based on the uniform heating assumption, the LF-ESF method considers the thermal gradient.