The influence of flexible topside supports on the fatigue damage on main deck of a floating production storage and offloading unit

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School of Engineering | Master's thesis
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64 s. + liitt.
A spectral fatigue analysis has been performed on a structural detail on a barge type FSPO in North Atlantic environment. Topside modules situated 3-4 meters above FPSO main deck need to be isolated from the global deformations of the vessel to prevent damage in the topside production facilities. The target of the thesis is to evaluate the effect of connecting topside to hull with elastomeric bearings compared to the case when the topside is welded to the hull. Although over 150 FPSO:s are already operating worldwide, the research in this area is usually project specific and few articles on the subject have been published. The analysis is based on linear response theory and carried out in frequency domain. Apart from the linearity assumption other assumptions are made, mainly: assumed rigidity of topside module and modelling of hull girder according to beam theory. The considered global load components are vertical hull wave bending and translational accelerations. The supporting arrangement consists of four supporting stools. The topside support design is chosen such that it would work as a "hinge" i.e. to perform as a flexible link when welded. The analysis has been performed on a turret moored and a spread moored hull. It has been carried out in accordance with DNV load component stochastic method with some simplifications. Two parameter Weibull probability theory and two slope S-N curves by DNV have been applied for extreme value and fatigue damage evaluation. Results indicate decreases in forces and hot spot stresses when supports with elastomeric bearings are used. The most probable extreme forces and the fatigue effective stresses are about 20% higher for the welded supports. Fatigue life is 60-70% higher at the most critical location along vessel length when elastomeric bearings are used. The analysis predicts about two times higher fatigue life of the considered detail on a spread moored vessel. Additionally it was found that the wave bending moment for the barge type FPSO is clearly higher in the bow than the DNV design wave bending moment.
Romanoff, Jani
Thesis advisor
Mikkola, Timo
Remes, Heikki
FPSO, FPSO, topside supports, topside, fatigue, trötthet, spectral, spektral, barge type, pråmformad, elastomeric
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