Abstract
OpenFAST, an open-source wind turbine simulation tool developed by the National Renewable Energy Laboratory, is being adapted to model fixed and floating marine turbines. Since OpenFAST was originally developed for wind turbines, its models were previously limited to simulating rotor operation in air and restricted to certain geometries. Development for marine turbines is focused on capturing additional physics relevant to operation in water and enabling the simulation of more diverse support structure geometries.
The rotor, substructure, and inflow models have been adjusted to allow the support structure and rotor to be defined below the mean sea level. New functionalities include simulating additional loads caused by buoyancy, added mass, and inflow accelerations. Added mass loads resulting from these inflow accelerations, as well as hydroelastic structural deformations, structure motions, and blade pitch accelerations, are included. Functionalities enabling the superposition of wave and current velocities upstream of the rotor, along with the calculation of inflow accelerations from turbulence, have been introduced. Furthermore, an ongoing project is modifying several OpenFAST modules to allow modeling of multiple rotors attached to a single support structure. A check to detect the presence of cavitation on the blades was previously added to the rotor hydrodynamic model. Potential future work includes accounting for flow confinement effects from multiple rotors and from the surface/seabed, addressing more complex coupling between wave and current velocities, expanding the aeroacoustics model for underwater operation, modeling biofouling on the rotor, and simulating additional device topologies.
This work will concentrate on evaluating the effects of added mass on the loads, displacement, and power performance of a reference floating marine turbine. Added mass has been included in OpenFAST’s AeroDyn module via an extended Morison’s equation and user-defined added mass coefficients. These coefficients can be defined for the normal-to-chord, tangential-to-chord, and pitch directions along the span of each blade. Transverse added mass coefficients can also be defined along the length of the support structure. OpenFAST simulations will be run of the Reference Model 1 marine turbine attached to a floating platform, design details of which were presented at the UMERC 2023 conference. Simulations will be run with and without added mass effects. These effects will be evaluated for multiple blade stiffness settings and a range of inflow conditions (i.e., different current and wave combinations). The magnitude of the effects caused by inflow accelerations, hydroelastic structural deformations, structure motions, and blade pitch accelerations will be compared. Comparisons will be made to prior work in this area.
OpenFAST development for marine turbines is supported by the U.S. Department of Energy Water Power Technologies Office and Advanced Research Projects Agency-Energy.