Analysis and Design of Stiffened Columns in Offshore Floating Platforms Subjected to Supply Vessel Impacts

Abstract

In this work the resistance of a stiffened column exposed to stern corner impacts is studied. The effect of ring and vertical stiffeners are explored, as well as the effect of decks and bulkheads. It is found that the ring stiffener is an important part of the load carrying structure for radial loads. These are therefore isolated and studied in greater detail. The ring stiffeners are analysed as a semicircular arch using several analytical models, as well as with the finite element method. It is shown that the collapse load for a ring stiffener with a stocky cross section can be well estimated by both a static and a kinematic model. For a more slender cross section local effects such as bucking of the web and top flange is important for the response. A folding line model for the local collapse of the cross section is proposed, and included in the kinematic model. This increases the accuracy of the model significantly, especially for slender cross sections at large displacements. The accuracy of the statical model was improved by introducing interaction equations between the moment capacity and the axial load. This resulted in a iterative formula. Finite element analyses of stern corner impacts have been performed for several different designs of stiffened columns to determine how a strength design can be obtained. It is found that the collapse of the ring stiffeners give a reduction in capacity and subsequently large deformations. This collapse load is therefore proposed as a limit for strength design. The vertical stiffening is shown to influence the collapse load by restraining the lateral displacement of the ring stiffener web, in addition to providing the necessary strength to carry the impact load to the ring stiffeners. The effect of decks was found to be minor with respect to the initial collapse, but important for the energy dissipation at large deformations. Bulkheads were found to give little to no extra capacity apart from the case where they are directly inline with the impact direction.