The design of offshore structures for extreme/abnormal waves assumes that there is sufficient air gap such that waves will not hit the platform deck. Due to inaccuracies in the predictions of extreme wave crests in addition to settlement or sea-level increases, the required air gap between the crest of the extreme wave and the deck is often inadequate in existing platforms and therefore wave-in-deck loads need to be considered when assessing the integrity of such platforms. The problem of wave-in-deck loading involves very complex physics and demands intensive study. In the Computational Fluid Mechanics (CFD) approach, two critical issues must be addressed, namely the efficient, realistic numerical wave maker and the accurate free surface capturing methodology. Most reported CFD research on wave-in-deck loads consider regular waves only, for instance the Stokes fifth-order waves. They are, however, recognized by designers as approximate approaches since “real world” sea states consist of random irregular waves. In our work, we report a recently developed focused extreme wave maker based on the NewWave theory. This model can better approximate the “real world” conditions, and is more efficient than conventional random wave makers. It is able to efficiently generate targeted waves at a prescribed time and location. The work is implemented and integrated with OpenFOAM, an open source platform that receives more and more attention in a wide range of industrial applications. We will describe the developed numerical method of predicting highly non-linear wave-in-deck loads in the time domain. The model’s capability is firstly demonstrated against 3D model testing experiments on a fixed block with various deck orientations under random waves. A detailed loading analysis is conducted and compared with available numerical and measurement data. It is then applied to an extreme wave loading test on a selected bridge with multiple under-deck girders. The waves are focused extreme irregular waves derived from NewWave theory and JONSWAP spectra.

The increasing demand for offshore wind energy is leading the industry to look for expansions into deep oceans. This development has compelled the industry to venture in to floating offshore installations for wind turbines. The floating installations lead to complex rotor motions in 6 degrees of freedom. The current work focusses on various aerodynamic effects on the turbine rotor due to coupled wind and wave forces. In this research work, the dynamic effects of the platform pitching motion on the rotor for OC3 phase IV case 5.1 are studied with rigid body assumption using high fidelity CFD (Computational Fluid Dynamics) commercial tool and semi empirical tool called FAST which is developed by NREL, USA. The hydrodynamic effects leading to the pitching motion of the turbine platform are extracted from the NREL FAST software. These pitching motions are coupled with the rotating blades to study transient flow behaviors using CFD tools. The results are compared with the standard BEM based methods to identify the discrepancies if any. Moreover, fine variation of rotor power/load due to dynamic pitching of the floating platform is also compared. The induction factors are extracted from the CFD results for every time step and are compared with those obtained with BEM based methods. Moreover, the application of CFD induction factor methods are introduced for cyclic load variations due to the rotor plane motions to check the validity of the BEM for FOWT application.

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Modelling the aerodynamic forces on floating offshore wind turbines (FOWTs) is a challenging task due to the motion of the floating platform, which result in flow transients and associated aerodynamic effects. Each of these needs to be modelled and implemented, before a complete aerodynamic model of the FOWT can be presented. Of special interest is the dynamic wake effect, a result of the time lag between rotor forces and the air flow deceleration within the wake. The dynamic wake effect may present itself significantly in several scenarios including gust loads, changing wind speeds and direction, and the oscillatory motion of the rotor due to platform motion from wave forces. The complexity of the problem is substantiated by the added mass effect, which must be accounted for when considering rotor motions through the air. Computational fluid dynamics (CFD) simulations and blade element momentum (BEM) computations have been performed for the NREL 5MW virtual wind turbine in axial flow and surge motions at different frequencies and amplitudes, to quantify and model the dynamic wake effect, including the added mass effect for rotor motion. This dynamic wake model is then implemented in an unsteady blade element momentum (uBEM) code, for a more complete model of wind turbine aerodynamics.