Saptarshi-Sarkar / OC3-Hywind-Spar

A MATLAB based package for dynamic simulation of spar-type floating offshore wind turbine.

1. Introduction

This package provides a MATLAB-based simulation tool to analyse the dynamics of a spar-type floating offshore wind turbine using Kane's method. The model is set up with the 5 MW OC3 Hywind Spar type FOWT. However, the codes are generic and will work for wind turbines with different ratings. For detail on the theory and derivation refer to [1]. The inputs are formatted as in OpenFAST. All structural and aerodynamic input data is read from OpenFAST input files. However, inputs related to the wind field, wave field and control systems are initialized separately as described in section 3.

The mooring cables can be modelled using the MoorDyn or the OpenMoor program. When using OpenMoor the users have the additional capability of modelling wave-current interaction. For details on the theory of wave-current interaction refer to [2].

2. How to use

Three steps to run the code.

Step 1 - Create wind field using TurbSim

Use NREL's TurbSim tool to generate the desired 3D wind field. Please ensure that the rotor-swept area is entirely enclosed. Once generated, place the *.sum and *.wnd files in the Wind_Dataset folder.

Step 2 - Create a wave field using the CreateWaveDataset.m script

Use the MATLAB script CreateWaveDataset.m to create the wave velocity field. The created wave field in placed in the Wave_Dataset folder by the script. Use the script to create regular wave (sinusoidal wave) field or use the PiersonMoskowitz or JONSWAP spectrum. You can also include wave-current interaction in the generated wave field. For details on the wave-current interaction model refer to [2].

The Wave class has been created by Lin Chen. You can download OpenMoor from his page as well.

Step 3 - Run the main simulation using the OffshoreWindTurbine.m script

The MATLAB script OffshoreWindTurbine.m is used to run the simulations. The script is described briefly below. The code starts with some initial housekeeping.

clc; clear -globals;
clear SystemMatrices BaselineControllers;
addpath('./Wind_Dataset','./Wave_Dataset','./src') 
pause(.5);
global GRAVACC FLUIDDENSITY;      
GRAVACC = 9.80655; FLUIDDENSITY = 1025;

MoorDyn of OpenMoor needs to be unloaded if it's loaded.

if libisloaded('MoorDyn') || libisloaded('MoorApiwin64')
    if libisloaded('MoorApiwin64')
    calllib('MoorApiwin64','finish'); unloadlibrary('MoorApiwin64');
    else
    calllib('MoorDyn','LinesClose');      unloadlibrary MoorDyn;  
    end                             % unload library (never forget to do this!)end
end

Read FAST input files

% Read FAST input data files
[Airfoils, Geometry] = ReadWindTurbineAeroDataInterp('rad');
[Blade, Tower]       = ReadWindTurbineStructuralData();
[ElastoDyn]          = ReadElastoDyn();
[Servo]              = ServoDyn();

Create the modal properties required for the dynamic analysis. Also, select the mooring cable model and select if wave loads are to be calculated or not.

% Create tower and blade structures
Twr = CreateTwr(Tower,ElastoDyn); Bld = CreateBld(ElastoDyn,Geometry,Blade); Platform = CreatePlatform();
Platform.Mooring     = 1;    % 1 for Moordyn, 2 for OpenMoor
Platform.WaveLoads   = 1;    % 1 to calculate wave load on spar using Morisson's equation, 0 to not.

If wave loads are included, place the correct file name (name of the .mat file created using the CreateWaveDataset.m script in the below line

    waveopt.wave_file = 'WaveHs3_Tp11_Dir90_NoCur0_Tf50';

String array that contains the list of all available DOFs

% DOFs available
DOFsStr = {'Sg','Sw','Hv','R','P','Y','TFA1','TSS1','TFA2','TSS2','NacYaw','GeAz','DrTr','B1F1','B1E1','B1F2','B2F1','B2E1','B2F2','B3F1','B3E1','B3F2'};

If you want to turn off certain DOFs, include them in the string array. An example showing how to turn off all blade second flapwise modes B1F2, B2F2 and B3F2

TurnedOffDOFsStr = {'B1F2','B2F2','B2F2'};

Next include the correct filename name of the TurbSim generated wind file in the argument of the readBLgrid.m function

% Get the TurbSim-generated wind field and grid
[velocity, Wind.y, Wind.z, Wind.nz, Wind.ny, Wind.dz,...
          Wind.dy, Wind.dt, Wind.zHub, Wind.z1, Wind.SummVars] = readBLgrid('1ETMC');

For steady wind field, uncomment the next line

% [Wind.t_TurbSim, velocity] = SteadyWind(Wind.y, Wind.z, 11.4, 0.2);

Turn on/off Pitt and peters correction for the skewed wake, and Blade pitch control. The default settings are given below. In the default setting the blade pitch control must all be turned on.

WindNom.PittandPeters = true(0);  % Place 0 in the argument for FALSE or 1 for TRUE
WindNom.PitchControl  = true(1);  % Place 0 in the argument for FALSE or 1 for TRUE
WindNom.AeroElastic   = true(1);  % 1 to include aeroelastic effect in inplane direction, 0 to ignore 

Next, select start time t0, final time tf and time step deltat

t0  = 0;
tf  = 1;
deltat = 0.0125;

This concludes the set-up of the OffshoreWindTurbine.m script. The script runs using ODE 4 RK $4^{th}$ order method and plots the results.

Step 4* - Build ./src/NominalSystemMatrix_mex.mex64

*This is an optional step. This package uses MATLAB mex files to accelerate the simulation time. The NominalSystemMatrix.m function is identified as the bottleneck function. Therefore, it has been built into a mex (MATLAB executable) function. The mex function is already provided in the src folder. It should work normally without the need for rebuilding it. However, in case your MATLAB version does not support the pre-build mex file follow the following steps

1. In the OffshoreWindTurbine.m script change the final time tf to 0.1 sec.
2. In the SystemMatrices.m function change the function call in line 109 from

[IM_nom, f_nom, Controls] = NominalSystemMatrix_mex(q_Nom, Controls, ElastoDyn, Airfoils, Twr, Bld, Platform, WindNom, mooring_load, f_Morison);

to

[IM_nom, f_nom, Controls] = NominalSystemMatrix(q_Nom, Controls, ElastoDyn, Airfoils, Twr, Bld, Platform, WindNom, mooring_load, f_Morison);

3. Open the app MATLAB Coder.
4. In the MATLAB Coder app select the NominalSystemMatrix.m function.
5. In the next page, to autodefine the input arguments select the OffshoreWindTurbine.m script. Let the app run the OffshoreWindTurbine.m script and autodefine the inputs.
6. In the next page, generate the code and verify it.
7. Finally, generate a mex file. The coder app with generate a mex file that's ready to be used. The file is called NominalSystemMatrix_mex.
8. Return to the SystemMatrices.m function and change line 109 back to

[IM_nom, f_nom, Controls] = NominalSystemMatrix_mex(q_Nom, Controls, ElastoDyn, Airfoils, Twr, Bld, Platform, WindNom, mooring_load, f_Morison);

9. Note: MATLAB Support for MinGW-w64 C/C++ Compiler must be installed in your MATLAB.

Get in touch

• 👋 Hi, I’m Saptarshi Sarkar, a postdoctoral researcher at the Chalmers University of Technology, Göteborg, Sweden.
• 👀 I’m interested in renewable energy systems and football ⚽.
• 🔨 I’m currently working on the dynamics and control of wind turbines. This includes dynamics of onshore and offshore wind turbines and, structural vibration control.
• 🙌 💪 I’m looking to collaborate on topics related to the dynamics, control and reliability of renewable energy systems, mainly wind turbines and wave energy converters.
• 💻 Check out my Google Scholar profile or my ORCiD or my ResearchGate profile.
• 📫 If my work interests you and you want to collaborate, reach me at [e-mail].

3. Brief description of the functions in src

A brief description of the functions is provided here. The description is brief and often more detail is available as comments in the functions themselves.

AddedMass.m

Create the added mass matrix for the floating platform using the dimensions of the floating platform, later added to the inertia (mass) matrix at the platform DOFs.

BaselineControllers.m

This is the MATLAB representation of the DISCON controller used in FAST. This function uses the same input parameters as the DISCON controller in FAST.

BEMTMex.m

Evaluate the aerodynamic loads on the blades using the Blade Element Momentum theory using Ning's method [5].

BladeModeShapes.m

Function to create the twisted mode shapes of the blades using the structural twist angle as the parameter. For a mathematical definition of the twisted mode shape refer to [1] and [3].

CaseOC3.xml

Input file for the OpenMoor program used to model the mooring cables.

CheckInterpPoints.m

The wind speeds are interpolated on the blades. Therefore, the grid of the wind field created using TurbSim must encompass the point. This function checks whether the point on the blade lies within the grid.

Coordinate_systems.m

Create the inertial coordinate system and all other local coordinate systems required for the simulation.

coprod.m

Multiply the position vector of a point of a turbine with a coordinate system.

CreateBld.m

Create blade structural properties for simulation.

CreatePlatform.m

Initiate hydrodynamic load parameters and the dimensions of the platform.

CreateTwr.m

Create tower structural properties for simulation.

current.dat

The x, y, z coordinates below MSL and corresponding current velocities in the x, y and z coordinates.

FASTTransMat.m

Equation (2-2) in reference [4].

LiftDragCoeffInterp.m

Interpolate the lift and drag coefficient of the airfoils for any give angle of attack using linear interpolation.

InitializeInflowAngle.m

Initialize the inflow angles at all blade nodes. The step is required to help with the convergence at the first evaluation of the BEMT function.

moorapi.h

Headfile for the OpenMoor dll. Can also be downloaded from OpenMoor.

MoorApiwin64.dll

OpenMoor dll is loaded and called by MATLAB to estimate the mooring loads. This dll can also be downloaded from OpenMoor.

MoorDyn.dll

MoorDyn dll that is loaded and called by MATLAB to estimate the mooring loads. This dll can also be downloaded from MoorDyn.

MoorDyn.h

Headfile for the MoorDyn dll. Can also be downloaded from MoorDyn.

Morisons.m

Function to evaluate the wave loads on the floating platform using Morison's equations.

NominalSystemMatrix_mex.mexw64

Nominal System Matrices mex function. Mex function is used to speed up execution.

ode4.m

Runge-Kutta fourth-order method for numerical integration. This is the main function that performs the time integration.

readBLgrid.m

MATLAB function distributed by NREL used to read TurbSim generated 3D wind field in MATLAB.

ReadElastoDyn.m

Function to read the ./5MW_Baseline/NRELOffshrBsline5MW_OC3Hywind_ElastoDyn.dat file.

ReadWindTurbineAeroData.m

Function to read the blade aerodynamic properties from the ./5MW_Baseline/AeroData folder.

ReadWindTurbineStructuralData.m

Read the blade's and tower's structural data from the ./5MW_Baseline/NRELOffshrBsline5MW_Blade.dat and the ./5MW_Baseline/NRELOffshrBsline5MW_OC3Hywind_ElastoDyn_Tower.dat respectively.

Results.m

Function to evaluate the response time histories from the integrated state vector.

RHS.m

Stores the right hand side of the equation $\dot{q} = f(t, q, \dot{q}, u)$

ServoDyn.m

Function to initialize all control parameters. This function is used instead for the ...ServoDyn.dat file used in FAST\OpenFAST.

SteadyWind.m

If you want to simulate the turbine for steady wind, this function creates a steady wind field.

SystemMatrices.m

By adding together the Nominal System Matrix, the loads and the control inputs, this function evaluates the final system matrices i.e., the $\mathbf{M}(\mathbf{q}, t)$ and $\mathbf{f}(\mathbf{\dot{q}}, \mathbf{q}, t)$ matrices in equation 11 of reference [1].

Transform1.m, Transform2.m and Transform2.m

These three functions provide three different options for applying coordinate transformations based on the dimensions of the matrices. The comments provided in the matrices will make it clear.

Like any set of codes, even after thorough verification, bugs may exist. If not bugs, may be possible room for improvement. In case you spot any, please don't hesitate to reach out.

References

[1] Sarkar, S., & Fitzgerald, B. (2021). Use of kane’s method for multi-body dynamic modelling and control of spar-type floating offshore wind turbines. Energies, 14(20), 6635.

[2] Sarkar, S., Chen, L., Fitzgerald, B., & Basu, B. (2020). Multi-resolution wavelet pitch controller for spar-type floating offshore wind turbines including wave-current interactions. Journal of Sound and Vibration, 470, 115170.

[3] Sarkar, S. (2020). Individual blade pitch control strategies for spar-type floating offshore wind turbines. Trinity College Dublin.

[4] Jonkman, J. M. (2007). Dynamics modeling and loads analysis of an offshore floating wind turbine. University of Colorado at Boulder.

[5] Ning, S. A. (2014). A simple solution method for the blade element momentum equations with guaranteed convergence. Wind Energy, 17(9), 1327-1345.