Numerical models for SPAR platform dynamics

The deep draft Spar platform concept is considered a competitive alternative for deep-water field developments. The complete system of Spar floater, mooring lines and risers interacts as an integrated dynamic system subject to the combined excitation of wind, waves and current. This paper will deal only with wave effects. Wave induced loads on the floater are essentially a potential flow effect, while loads on mooring and risers have a major viscous fluid contribution due to combined current and wave loads. However, damping of wave induced floater response in certain modes is strongly affected by viscosity, and also other effects like floater vortex shedding may come into play. It is industry practise to take account of viscous effects on the wave induced response by adding empirically based viscous damping to the potential flow damping. However, to account properly for viscous damping effects it may be advantageous to utilise viscous flow solvers together with traditional wave diffraction flow solvers. The present paper focuses on this aspect, the co-processing of potential flow simulations and Navier-Stokes flow simulations. A new method is presented for combined co-processing of wave diffraction and viscous flow calculations in time domain. A potential flow Rankine source method is combined with a state-of-the-art CFD code using the Message Passing Interface (MPI) technique to handle both synchronisation and data transfer between the two simulation codes. The new method is validated using model test results for a classical Spar geometry.