4.2.8. HydroDyn User Guide and Theory Manual

HydroDyn is a time-domain hydrodynamics module that has been coupled into the OpenFAST wind turbine multi-physics engineering tool to enable aero-hydro-servo-elastic simulation of offshore wind turbines. HydroDyn is applicable to both fixed-bottom and floating offshore substructures. The current release of HydroDyn integrates with OpenFAST through the FAST modularization framework. HydroDyn can also be driven as a standalone code to compute hydrodynamic loading uncoupled from OpenFAST.

In addition to this documentation, the following materials including presentation slides, development plans, and publications are made available for reference. Note that some of these may be outdated and pertain to older versions of HydroDyn.

HydroDyn allows for multiple approaches for calculating the hydrodynamic loads on a structure:

  • Potential-flow theory solution

  • Strip-theory solution

  • Hybrid combination of the tower

Waves generated internally within HydroDyn can be regular (periodic) or irregular (stochastic) and long-crested (unidirectional) or short-crested (wave energy spread across a range of directions). Wave elevations or full wave kinematics can also be generated externally and used within HydroDyn. Internally, HydroDyn generates waves analytically for finite depth using first-order (linear Airy) or first plus second-order wave theory [SD81] with the option to include directional spreading, but wave kinematics are only computed in the domain between the flat seabed and still-water level (SWL) and no wave stretching or higher order wave theories are included. The second-order hydrodynamic implementations include time-domain calculations of difference- (mean- and slow-drift-) and sum-frequency terms. To minimize computational expense, Fast Fourier Transforms (FFTs) are applied in the summation of all wave frequency components.

The potential-flow solution is applicable to substructures or members of substructures that are large relative to a typical wavelength. The potential-flow solution involves either frequency-to-time-domain transforms or fluid-impulse theory (FIT). In the former, potential-flow hydrodynamic loads include linear hydrostatic restoring, the added mass and damping contributions from linear wave radiation (including free-surface memory effects), and the incident-wave excitation from first- and second-order diffraction (Froude-Kriloff and scattering). The hydrodynamic coefficients (first and second order) required for the potential-flow solution are frequency dependent and must be supplied by a separate frequency-domain panel code (e.g., WAMIT) from a pre-computation step. The radiation memory effect can be calculated either through direct time-domain convolution or through a linear state-space approach, with a state-space model derived through the SS_Fitting preprocessor. The second-order terms can be derived from the full difference- and sum-frequency quadratic transfer functions (QTFs) or the difference-frequency terms can be estimated via Standing et al.’s [RGSW87] extension to Newman’s approximation, based only on first-order coefficients. The use of FIT is not yet documented in this manual.

The strip-theory solution may be preferable for substructures or members of substructures that are small in diameter relative to a typical wavelength. Strip-theory hydrodynamic loads can be applied across multiple interconnected members, each with possible incline and taper, and are derived directly from the undisturbed wave and current kinematics at the undisplaced position of the substructure. The strip-theory loads include the relative form of Morison’s equation for the distributed fluid-inertia, added-mass, and viscous-drag components. Additional distributed load components include axial loads from tapered members and static buoyancy loads. Hydrodynamic loads are also applied as lumped loads on member endpoints (called joints). It is also possible to include flooding or ballasting of members, and the effects of marine growth. The hydrodynamic coefficients required for this solution come through user-specified dynamic-pressure, added-mass, and viscous-drag coefficients.

For some substructures and sea conditions, the hydrodynamic loads from a potential-flow theory should be augmented with the loads brought about by flow separation.  For this, the viscous-drag component of the strip-theory solution may be included with the potential-flow theory solution.  Another option available is to supply a global damping matrix (linear or quadratic) to the system to represent this effect.

When HydroDyn is coupled to OpenFAST, HydroDyn receives the position, orientation, velocities, and accelerations of the (rigid or flexible) substructure at each coupling time step and then computes the hydrodynamic loads and returns them back to OpenFAST. At this time, OpenFAST’s ElastoDyn structural-dynamics module assumes for a floating platform that the substructure (floating platform) is a six degree-of-freedom (DOF) rigid body. For fixed-bottom offshore wind turbines, OpenFAST’s SubDyn module allows for structural flexibility of multi-member substructures and the coupling to HydroDyn includes hydro-elastic effects.

The primary HydroDyn input file defines the substructure geometry, hydrodynamic coefficients, incident wave kinematics and current, potential-flow solution options, flooding/ballasting and marine growth, and auxiliary parameters. The geometry of strip-theory members is defined by joint coordinates of the undisplaced substructure in the global reference system, with the origin at the intersection of the undeflected tower centerline with mean sea level (MSL). A member connects two joints; multiple members can use a common joint. The hydrodynamic loads are computed at nodes, which are the resultant of member refinement into multiple (MDivSize input) elements (nodes are located at the ends of each element), and they are calculated by the module. Member properties include outer diameter, thickness, and dynamic-pressure, added-mass and viscous-drag coefficients. Member properties are specified at the joints; if properties change from one joint to the other, they will be linearly interpolated for the inner nodes.

See Installation and Getting Started for details on how to download or compile the HydroDyn and OpenFAST software executables, as well as instructions for running HydroDyn standalone and coupled to OpenFAST.