Session: Hydrodynamic and Coupled Analyses II

Paper Number: 98094

98094 - Recent Hydrodynamic Modeling Enhancements in Openfast 

OpenFAST is a widely used open-source engineering tool for coupled nonlinear time-domain aero-hydro-servo-elastic simulations of both onshore and offshore wind turbines developed by the National Renewable Energy Laboratory through support from the U.S. Department of Energy. To better support the offshore wind industry and research communities, a restructuring of the HydroDyn hydrodynamics module of OpenFAST was recently undertaken and a slew of new features has recently been implemented to enhance the capabilities of OpenFAST in modeling nonlinear hydrodynamics.

The restructuring of HydroDyn entails the separation of wave-field generation from HydroDyn into a new OpenFAST module called SeaState. Unlike previous versions of HydroDyn, which precomputed the wave kinematics at the reference (undisplaced) positions of the hydrodynamic nodes in a preprocessing step, the new SeaState module computes the complete time history of the wave field, including free-surface elevation, fluid velocity, fluid acceleration, and dynamic pressure, at the vertices of a user-defined three-dimensional wave grid. During the simulation, the wave kinematics at the displaced hydrodynamic nodes can be efficiently evaluated with interpolation in space and time, allowing the evaluation of wave-excitation and viscous-drag loads in the strip-theory solution to be based on the instantaneous position and orientation of the structure, including large displacements. Along with this change, a phase correction has also been implemented for the potential-flow wave-exciting loads for large structure displacement by precomputing the loads with the structure at different positions defined by a horizontal grid and interpolating the instantaneous wave-exciting loads during the simulation using the actual position of the structure within the grid. Included here is an optional low-pass filter so that only low-frequency motions are used in the interpolation so as not to double count second-order effects when the quadratic interaction of first-order quantities is accounted for in the second-order potential-flow solution.

The wave grid constructed by SeaState also enables wave-stretching, which provides wave kinematics in wave crests above the still water level to allow wave loads to be computed up to the instantaneous free surface. Several wave-stretching formulations have been implemented in the strip-theory solution including vertical stretching, Wheeler stretching, and extrapolation stretching. Of interest is the development of a new wave-load redistribution technique to be used in conjunction with wave stretching and a discrete hydrodynamic mesh. The load redistribution modifies the hydrodynamic loads on nodes just below the free surface to ensure the local loads vary continuously and smoothly with time even as the free surface moves past nodes, while maintaining the total hydrodynamic force and moment. This load redistribution is critical when modeling hydro-elastic effects using the SubDyn substructure dynamics module of OpenFAST.  Without it, the hydrodynamic loads as nodes enter and exit the water would be discontinuous and could excite unphysical high-frequency structural vibrations. Along with wave stretching, the nonlinear hydrostatic loads in the strip-theory solution have now been updated to be based on the exact wetted surface assuming circular member cross-sections.

Other new features implemented in HydroDyn include the MacCamy-Fuchs diffraction model in the strip-theory solution and the capability to insert a constrained NewWave of user-specified crest elevation or crest height into extreme stochastic sea states with directional spreading.

The present article documents the formulations of the new improvements to HydroDyn with example applications and numerical results. The restructuring and improvements to HydroDyn greatly enhance the hydrodynamic modeling capabilities of OpenFAST, which are expected to lead to more accurate predictions and provide better support for standards-based load analysis of fixed and floating offshore wind turbines.

Presenting Author: Lu Wang National Renewable Energy Laboratory