Session: 09-01-02 Wind Energy: Aero-hydrodynamics 2

Paper Number: 123909

123909 - Recent Floating Wind Modelling Enhancements in Openfast 

OpenFAST is a state-of-the-art physics-based engineering tool for floating offshore wind turbines (FOWTs), supporting coupled aero-hydro-servo elastics. OpenFAST has the ability to (1) run a large number of nonlinear simulations to enable standards-based loads analysis for predicting wind system ultimate and fatigue loads and (2) linearize the underlying nonlinear model about an operating point to understand the wind system response and enable modal analysis, controls design, and stability studies. The objective of this presentation is to summarize a range of new features that have recently been implemented to enhance the modelling of FOWTs within OpenFAST.

To enable the design and optimization of next-generation FOWT technologies, substructure flexibility and member-level load calculations have been implemented in OpenFAST. This involved upgrading the SubDyn substructural dynamics module to solve for the elastic modes in a floating reference frame and to support beam, pretensioned cable, and rigid-link elements as well as cantilevered, pin, universal, and ball joints. The HydroDyn hydrodynamics module was also upgraded to support multiple potential-flow (large-volume) bodies and displacement-dependent buoyancy in the strip-theory formulation for small-volume members.

HydroDyn was also enhanced to capture additional hydrodynamic nonlinearities. Large floater motion is now supported through the introduction of 3D grids of wave kinematics and 2D grids of wave elevation and potential-flow loads, so that proper wave phasing at the displaced position of the floater can be evaluated. Various wave stretching theories—including vertical, extrapolation, and Wheeler stretching—have been introduced to capture free-surface effects. Mover, the hydrostatic solution of strip-theory members is now fully nonlinear and integrated up to the instantaneous free surface. The MacCumy-Fuchs correction has been introduced in the strip-theory formulation to support larger member diameters. Finally, constrained new wave theory has been introduced to support the modelling of extreme individual waves embedded into severe irregular sea states.

Members of substructures for FOWT with rectangular cross sections are becoming more common. As such, the structural dynamics of SubDyn and hydrostatics and hydrodynamics of HydroDyn are now being upgraded to support different properties in the two transverse directions of members.

A new structural control model has also been introduced into the ServoDyn servo-dynamic module to support novel floater-based passive and active controllers, including tuned-mass dampers, tuned liquid-column dampers, and cable and tendon tensioning.

The modelling of stationkeeping systems has been greatly enhanced to keep up with new design needs, through an overhaul of the MoorDyn mooring-dynamics module. Expanded functionality includes support for synthetic rope visco-elasticity, nonlinear spring elements, bending of dynamic power cables, buoyancy cans, seabed bathymetry and friction, and mooring line loss based either on timing or tension exceedance. Furthermore, support for array-level shared-mooring configurations has been introduced into the FAST.Farm wind farm extension of OpenFAST.

Several FOWT concepts make use of stationkeeping systems with turrets or single-point moorings, with passive yaw control through weathervaning. To support analysis with large yaw transients, the structural dynamics and hydrodynamic theories of OpenFAST are now beeing upgraded to support large yaw rotation. This functionality also supports the modelling of mooring line loss events, which can also result in large yaw transients.

As functionality has been added to OpenFAST over time, the computational expense has increased, resulting in slower than desired execution times. To address this, a new tight coupling algorithm has also been introduced, enabling OpenFAST to take larger time steps, resulting in speed ups of 10x to 100x faster (depending on the model) than solutions with the prior loose-coupling algorithm.

Combined, these improvements will facilitate more accurate and still computationally tractable simulations not previously possible with OpenFAST, enabling further advancements in FOWT design and analysis.

Presenting Author: Jason Jonkman NREL

Presenting Author Biography: Jason Jonkman, Ph.D. is a principal engineer at the National Renewable Energy Laboratory (NREL). He is the lead developer of the OpenFAST and FAST.Farm multi-physics engineering tools for designing and analyzing land-based and offshore fixed and floating wind turbines and wind farms. Dr. Jonkman supports several International Energy Agency (IEA) Wind Tasks on wind energy model verification and validation. He is also a member of the International Electrotechnical Commission (IEC) working group to develop design requirements for floating offshore wind turbines. Dr. Jonkman earned a Ph.D. in Aerospace Engineering Sciences from the University of Colorado-Boulder.

Authors:

Jason Jonkman NREL