Topsides Deck Structure Stiffness Modeling and its Effects on Hull and Deck Design

Offshore floating production platform truss type topsides structure often consists of truss, deck plates and stiffeners. Since these deck plates are usually thin, they are traditionally viewed as shear plates in topsides structure design, or ignored in the modeling to be conservative. While it seems to be conservative for hull structure evaluation to ignore the effects of deck plates completely, it is also possible to underestimate the loads transmitted to deck structure with an integrated deck and hull structure model. On the other hand, while taking into account the deck stiffened plates with normal stress and shear stress capacity, the hull structure design could benefit from less distributed loads on hull for an integrated deck and hull structure model, provided that the deck plate buckling capacity can be justified. In this study, a semisubmersible global FE model of hull and topsides deck is analyzed with three approaches: considering deck stiffened plates with full capacity, considering deck stiffeners and plates as shear plates, and ignoring deck stiffened plates. The differences in hull critical structure internal loads among these three approaches are quantified. For the integrated model, ignoring deck stiffened plate approach induces the least deck structure loads, and results in the highest hull structure loads especially in diagonal seas. The full capacity deck plate model induces the highest deck structure loads, and results in the lowest hull structure loads. The model in which the deck plate is modeled as a shear plate functions similarly to the model in which the deck plate is modeled as a regular plate for the internal load distribution. It is thus recommended that in all design stages, appropriate deck stiffened plates stiffness and its effects on the design should be taken into account.