CFD code comparison, verification and validation for decay tests of a FOWT semi-submersible floater

Abstract

With the advancement of high-performance computation capabilities in recent years, high-fidelity modelling tools such ascomputational fluid dynamics are becoming increasingly popular in the offshore renewable sector. To justify the credibilityof the numerical simulations, thorough verification and validation is essential. In this work, preparatory heave decay tests fora freely floating single cylinder are modelled. Subsequently, the surge and sway decays of a linearly moored floating offshorewind turbine model of the OC4 (Offshore Code Comparison Collaboration Continuation) phase II semi-submersible platformare simulated. Two different viscous-flow CFD codes are used: OpenFOAM (open-source), and ReFRESCO (communitybasedopen-usage). Their results are compared against each other and with water tank experiments. For the single-cylinderdecay simulations, it is found that the natural period is accurately modelled compared to the experimental results. Regardingthe damping, both CFD codes are overly dissipative. Differences and their potential explanations become apparent in theanalysis of the flow field data. Meanwhile, large numerical uncertainties especially in later oscillations make a distinctconclusion difficult. For the OC4 semi-submersible decay simulations, a better agreement in damping can be achieved,however discrepancies in results are observed when restricting the degrees of freedom of the platform. Flow field data againreveals differences between the CFD codes. Meanwhile, through the effort to use similar numerical settings and quantifythe numerical uncertainties of the CFD simulations, this work represents a stepping stone towards fairer and more accuratecomparison between CFD and experimental results.

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