Thanks to the evolution of hardware and software, the direct computation of dynamic loads is becoming a common practice on the design of offshore floating units for both extreme and fatigue assessments. In the specific case of FPSOs, usually the loads prescribed by Classification Societies for oil tankers are adopted. Nevertheless, the direct computation may be used in order to adapt those prescribed extreme loads to consider the specific operational profile of FPSOs. Also the performance of spectral fatigue analysis is commonly accepted in lieu of the Rules prescriptions. The state-of-the-art is to employ potential theory for the hydrodynamic calculations. In this theory, the fluid is considered ideal and therefore no viscous dissipation is included. In spite of the reasonable accuracy of that theory for the computation of large bodies' behavior, in the case of FPSOs with internal turret it may overpredict the free surface elevation inside the turret around resonance frequencies, which may impact both global and local loads acting on the hull. In reality, however, it is expected that a lot of dissipation is generated by the turret structure and components which would bring the wave elevation to much lower levels. In this paper, the relevance of the phenomenon for the structural assessment of an FPSO is evidenced and the effects of dissipation are discussed based on modified potential theory results and CFD RANS analysis. Nomenclature FPSO Floating Production Storage and Offloading CFDComputational Fluid Dynamics RANS Reynolds-Averaged Navier Stokes
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