Use of built-up H sections as secondary members in topside modules

There have been many attempts to widely utilize built-up H sections to secure flexibility in structural design. Much research into the structural strengths and limit states of built-up H sections, therefore, has been carried out. However, a practical redesign methodology taking advantage of built-up H beams has yet to be introduced into the offshore industry. In this study, a comprehensive investigation into built-up H sections is carried out, based on which, a new redesign procedure for weight reduction herein is suggested. First of all, on the basis of the ANSI/AISC 360-10, the differences between the rolled H and built-up H sections are investigated in terms of their various strengths. Then, a secondary-member redesign procedure is established as a means of reducing structural weight by replacing rolled H sections with built-up H sections. In that procedure, the built-up H section cross-section is modified according to the failure modes of reference rolled H sections. The redesign procedure is verified by a nonlinear finite element analysis and four-point bending test. Through the nonlinear finite element analysis and experiment on the reference rolled H section and built-up H section obtained by the redesign procedure, it is observed that the weight of the built-up H section is reduced by about 15% while a flexural strength similar to that of the reference rolled H section is maintained. The suggested redesign procedure is then applied to three floating production storage offloading (FPSO) topside modules for demonstration purposes. In the results, the total structural weights of the reference topside modules are reduced by approximately 3%–5% by employing built-up H sections in secondary members in lieu of rolled H sections. The results indicate that, in many cases, built-up H sections can be used as secondary members to reduce the structural weight of topside modules.