Design-oriented predictive models for tubular T-joints based on nonlinear finite element and shakedown analyses

Thin-walled tubular joints are fundamental load-carrying components in offshore and marine structures, where structural integrity under extreme and cyclic loading conditions is a key design concern. This study develops an integrated framework to predict the ultimate limit state and elastic shakedown limit of tubular T-joints subjected to axial compression. A comprehensive parametric finite element analysis was performed, and empirical design-oriented formulas were systematically derived through automated regression methods. The ultimate limit state was evaluated using the Riks method, yielding six predictive models; the best-performing model showed excellent agreement with numerical results (R² = 0.9935, COV = 0.0340) and outperformed existing design standards, reducing mean absolute percentage errors from 23.81 % (API) and 30.68 % (CIDECT) to 15.59 %. The shakedown limit was estimated using the linear matching method, and six additional formulas were proposed. The most accurate model achieved R² = 0.9979 and COV = 0.0190. Validation against incremental cyclic elastoplastic analyses confirmed the predictive capability, with 121 parametric cases correctly classified and the best formula reaching 97.66 % accuracy. The proposed formulas provide a practical design tool for thin-walled tubular joints, enabling safer and more efficient offshore structural applications.