Introduction  The topside module of an offshore step-up station usually has numerous tube-I section joints with complicated structure, which cannot be checked by general structural design software. Hence, a feasible checking design method and refinement of stiffener schemes are in need.

  Method  A simplified model based on typical tube-I section joints of the topside module of an offshore step-up station was established, and five stiffener schemes were designed for it. Based on the finite element software simulation method, three typical displacement loading conditions, i.e., axial compression displacement, in-plane rotation, and out-of-plane rotation, were applied to analyze the plastic zone development and failure modes of the five stiffener schemes. For the longitudinal stiffener scheme selected, the effective stress section of the stiffener was assumed based on existing codes, and a simplified calculation method for stiffener stress check under complicated loads was proposed. The results were examined using the finite element method.

  Result  The results show that the unstiffened tube-I section joint cannot satisfy the design principle of "strong joint and weak member", and it is necessary to take stiffening measures such as stiffeners. The longitudinal stiffener scheme performs better in terms of bearing capacity and economical efficiency. Longitudinal stiffeners are essential for the shearing capacity of beam webs, while the transverse stiffeners cannot enhance shearing capacity. Finite element analysis indicates that the stress distribution is consistent with the assumption of the simplified calculation method, and the simplified calculation method predicts higher stress levels than the finite element results, which means the proposed simplified calculation method is feasible and safer.

  Conclusion  The longitudinal stiffener scheme is a better stiffening scheme for the joint. The proposed simplified calculation method can be adopted to check the longitudinal stiffener scheme efficiently.