A MATLAB toolbox to generate State-Space Models for Wave Radiation Loads.

by Tiago Duarte
Instituto Superior Tecnico - IST Lisbon, Portugal

SS_Fitting is a MatLab toolbox designed to provide a state-space model based on the WAMIT output files.  These matrices are required to use the new state-space submodule (SS_Radiation) of FAST's HydroDyn hydrodynamics module, in order to compute the wave-radiation forces of a floating wind turbine.  This provides a lower computational time, enables the use of tight coupling schemes, and the complete linearization of the wind turbine problem without discrete-time states.

The toolbox includes four different methods to provide the state-space matrices of an linear, time-independent (LTI) system.  These are presented in detail in the Theory and User Manual. The program outputs the matrices in a *.ss file, which can be read directly by the new SS_Radiation module within HydroDyn. This toolbox was developed in collaboration with WavEC - Offshore Renewables, and can also be downloaded from (

All questions about SS_Fitting should be addressed to Tiago Duarte.

You may download the following files from our server:

  • SS_Fitting Change Log (v1.00.01, 1 KB, 3-October-2013)

    This is a list of changes made to the code. Look at this text file to see if we have made worthwhile changes since you received your previous version of SS_Fitting.

  • SS_Fitting Theory and User Manual (v1.00, 691 KB, 18-September-2013)

    This is the SS_Fitting Theory and User Manual. Please refer to it when trying to understand how the program works.

    If you want to refer to this manual in a report, here is a reference for you:

    Duarte, T. "SS_Fitting Theory and User Manual," NREL/Draft Report. Golden, Colorado: National Renewable Energy Laboratory, 2013.

  • SS_Fitting Archive for Windows (EXE 1.1 MB) (v1.00.01, 3-October-2013)

    This is a self-extracting archive of SS_Fitting.  Simply execute it in an appropriate directory to extract the files.  It contains the change log, the source code, manual, sample input files, test cases, and some archive/web support files.

  • Sample Results Archive for Windows (EXE 144 KB) (25-March-2013)

    This link contains sample SS_Fitting output (*.ss) files for four reference floating turbines: ITI Energy Barge, MIT/NREL TLP, OC3-Hywind Spar and OC4 Semi-submersible.  These can be downloaded and used with HydroDyn's new SS_Radiation module.  Simply place them in the same directory as the corresponding WAMIT output files.

Programming Language:     Technology:     Wind and Water Tools:

You must log in to download this software.

If you want to refer to this website in a report, here is a reference you can use:
NWTC Information Portal (SS_Fitting). Last modified 29-September-2014 ; Accessed 05-April-2020