Renewables: K02 10MW Fixed-bottom OWT - Orcina
www.orcina.comK02 10MW fixed-bottom OWTThis example models the 10MW reference wind turbine (RWT), developed as part of theInternational Energy Agency’s (IEA) Wind Task 37. The system design basis is documented by theSystems Engineering Wind Energy - WP2.1 Reference Wind Turbines technical report, which canbe accessed from the open-source IEA-10.0-198-RWT GitHub repository.The model of the offshore wind turbine (OWT) is shown below, where the turbine takes the formof a three-bladed rotor, with variable-speed and collective blade-pitch control capabilities.The turbine hub is connected to the nacelle, which houses a direct-drive synchronous generator.The nacelle is supported by the tower which is mounted upon a fixed-bottom monopile foundationembedded to a depth of 45m below the seabed.Note: the model properties considered in this example are based on our interpretation of theinformation and data available at the time of writing. If you use, or refer to, this model as part ofyour own analysis requirements then you must carry out the appropriate checks to confirm theimplemented data are correct.Rotor:Blade length 97.3mHub radius 2.3mHub height 118.4mTowerMean sea levelWater depth 30mMud lineMonopileLength 85mMonopile embedment depth 45mPython installation requirementsAs this example uses a Python script for external function purposes, Python 3 is required to viewand run this example. It is also possible to view this model using the demo version of OrcaFlexwhich is available for download from our website: https://www.orcina.com/orcaflex/demo/.For more information about Python installation requirements, and dealing with installationproblems which may occur when interfacing to OrcaFlex, please refer to our dedicated PythonInterface: Installation help page on the subject.Renewables: K02 10MW fixed-bottom OWTPage 1 of 12
www.orcina.comController modellingControl system modelling for the turbine is supported through a pair of Python external functionswhich are used as a wrapper for the NREL Reference Open Source Controller (ROSCO) DLL.For more details about implementing ROSCO in OrcaFlex, please refer to our K03 15MW semi-subFOWT and to our guide to using Python external functions to model turbine controllers.Before opening and/or running this example, it is important to note the following: The DLL (libdiscon.dll), included with this example, represents the latest 64-bit ROSCOversion (v2.4.1, released 11th November 2021), available at the time of writing. The latest32-bit and 64-bit versions of that controller may be downloaded from the ROSCO GitHubrepository.The control DLL calls a controller input file (IEA-10.0-198-RWT DISCON.IN) – downloadedfrom the IEA-10.0-198-RWT GitHub repository – which lists the necessary controllerparameters. The contents of the input file can be viewed and edited using a text file editor,such as Notepad .The wind speed estimator part of the ROSCO code has a known bug which is triggeredwhen the turbine rotor starts from a stationary position, as is the case with the turbine inthis example. Consequently, we have set the wind estimator mode (WE Mode) – on line 15of the parameter input file – to a value of 0.Included amongst the example files is a rotor performance file (IEA-10.0-198RWT Cp Ct Cq.txt) containing tables of coefficients for the rotor power (C P), thrust (CT) andtorque (CQ). The rotor performance file is called by the controller if WE Mode 0; meaningit is not used by this example simulation. Regardless, we have included the performancefile, and included the required settings to call it, with this example for completeness.As we are running a single simulation, we have set the SetCWDToModelDir tag to ‘True’ onthe tags page of the turbine data form. This sets the current working directory to the modeldirectory. This is useful if the DLL is operating in a mode where it needs to read anotherfile, such as the rotor performance file (the path to which is specified on line 79 of thecontroller input file) and a relative path has been given (the DLL uses a path relative to thecurrent working directory).Note: If running multiple simulations, over multiple threads, it is not safe to do this.Instead, the SetCWDToModelDir tag must be omitted, or set to ‘False’, and a full absolutepath must be specified for the rotor performance file. The image below shows where thepath to the performance file must be set if WE Mode 0, where the word ‘PATH’ should bereplaced by the path to the rotor performance file of interest.Renewables: K02 10MW fixed-bottom OWTPage 2 of 12
www.orcina.comBuilding the modelTurbine objectWe have gathered the necessary input data for the turbine object from the available 10MW RWTdocumentation and supporting online resources.This example document does not provide any further details about modelling the turbine rotoritself. For a more detailed discussion on this topic, please refer to our K01 5MW spar FOWTexample.Blade pre-bendOne important turbine feature, not discussed in the above example, is blade pre-bend. This allowsfor representation of blades which are not straight in their unstressed state.The blade pre-bend feature is very similar to pre-bend for OrcaFlex line objects, where it is possibleto specify the local curvature components in radians per unit length.On the blade structure page of the turbine data form, we have specified the y-curvature valuesnecessary to model the blade pre-bend for the 10MW RWT. Here, negative curvature values arespecified to pre-bend the blades out of the rotor plane and away from the tower. Note thatOrcaFlex applies pre-bend irrespective of any structural twist in the blades.Further details about this feature may be found on the Modelling, data and results Turbines Turbine data Blade profile page of the OrcaFlex help.Tower and monopile modellingThe turbine support structure comprises a Tower and Monopile, modelled using line objects. Thecorresponding line types utilise the homogeneous pipe category, which allows for modelling of thevariable outer and inner diameter profiles of these items as well as their physical properties.Although it is possible to model the support structure using a single line, with multiple sections,we have intentionally chosen to model these items as separate line objects, connected togetherusing line-to-line connections. This provides the option to check the interface loads at the lineconnection using the line end load results available in OrcaFlex.Renewables: K02 10MW fixed-bottom OWTPage 3 of 12
www.orcina.comAs the Tower and Monopile lines are very stiff, and already close to their static equilibrium positionsin the reset state, we have set the line statics step 1 & step 2 policies to ‘None’ on the statics pageof the general data form. This bypasses the line statics stage of the statics analysis process,meaning OrcaFlex will move straight to solving whole system statics. This helps to improve theefficiency of the statics solve.Another consequence of the support structure being very stiff is that it can give rise to resultsprecision problems in dynamics. It is possible that single precision logging, which is the defaultsetting specified on the dynamics page of the general data form, will not log the node positionresults with enough significant figures to allow certain results to be accurately derived from them.For this reason, we have decided to log the dynamic results using double precision. Further detailsabout this can be found on the Modelling, data and results General data Logging help page.We have also implemented the Bak tower influence model, which is set on the tower page of theturbine data form (pictured below). This makes use of Bak’s model which incorporates a correctionto the classic potential theory model.When using this model, the tower connections must be specified along with the tower profiles. Theprofiles are defined at several discrete unstretched arc lengths, starting at End A of the tower line.The outer diameter for each tower profile is also specified here. The data are assumed to varylinearly between the arc lengths at which they are given.Further details about the available tower influence models can be found on the Modelling, dataand results Turbines Turbine data Tower help page.Renewables: K02 10MW fixed-bottom OWTPage 4 of 12
www.orcina.comSoil modellingThe soil model, required to capture the interaction between the seabed and the monopilefoundation, represents the monopile-to-soil interaction using the ‘elastic halfspace model’. Here,the interaction is represented using linear springs which resist the translational and rotationalmovement of the embedded monopile in 6 degrees of freedom (DOFs).The P-y models, available in OrcaFlex, cannot account for these spring constants in 6 DOFs so wehave chained four calculated DOFs constraints together to capture the translational and rotationalspring stiffnesses at each embedded Monopile line node: (1) horizontal (x, y), (2) vertical (z), (3)rocking (Rx, Ry), (4) twisting (Rz).Each line node is then connected to its respective ‘chain’ of constraints, to create the followingconnection sequence:Node torsional constraint (Rz) rocking constraint (Ry & Rx) vertical constraint (z) horizontalconstraint (x & y) Global (anchored)The stiffnesses calculated for each constraint account for the embedded depth (h) of eachmonopile line node and the constraint chains work to resist the loads applied to the embeddedmonopile in all 6 DOFs. Note that the sequence terminates at the horizontal constraint (x & y)constraint, which is connected (anchored) to the global axis system.For each constraint, the corresponding DOFs are set to free, on the degrees of freedom page of theconstraint data form. The necessary translational/rotational stiffnesses are calculated andassigned to each constraint, through the stiffness & damping page.The stiffnesses required for the horizontal (x, y) and vertical (z) constraints are input using thetranslational stiffness data item. The stiffnesses required for the rocking (Rx, Ry) and twisting (Rz)constraints are input through the rotational stiffness data item.As an example, the right-hand images (below) show the constraint settings required to capture thehorizontal spring stiffness (x & y) at the line node which sits level with the mud line.Mud lineEmbedded monopile:Each line nodeconnected to a chainof 4 constraintsRenewables: K02 10MW fixed-bottom OWTPage 5 of 12
www.orcina.comTo ensure each of the Monopile lines nodes are connected to their respective constraint chains,End B of the line is connected to its constraint chain through the monopile’s end connection. Theother embedded nodes are connected using mid-line connections, as shown by the image below.As these constraints effectively take over from the seabed model in OrcaFlex, the seabed normaland shear stiffness – specified on the seabed page of the environment data form – are both set to 0.Note: this method of ‘chaining’ constraints is strictly only valid for small angular displacements. Inthis case, the soil stiffnesses are very high and so the displacement of each embedded monopilenode is expected to be small. However, chaining constraints in this way may not be appropriate incases where the constraint out-frame rotates significantly relative to the in-frame.Renewables: K02 10MW fixed-bottom OWTPage 6 of 12
www.orcina.comEnvironmentThe simulation considers a build-up (stage 0) duration of 100s followed by a stage 1 duration of100s. Irregular (JONSWAP) waves, with a significant wave height (HS) of 5m and spectral peak waveperiod (TP) of 8s are also considered.Turbulent wind field:Mean wind speed 15m/sWind Direction 0 Irregular (JONSWAP) waves:HS 5mTP 8sDirection 0 On the wave calculation page of the environment data form, we have specified a wave kinematicscutoff depth of 30m. This is measured downwards from the mean sea surface and, below thisdepth, the wave kinematic fluid velocity and acceleration are taken to be zero. The default valuehere is infinity, which is equivalent to not having a cutoff depth at all.As OrcaFlex does not automatically exclude wave kinematics below the seabed (mud line),specifying a nominal depth ensures the embedded monopile does not experience any waveloading. Note, the wave kinematics cutoff depth has no effect on fluid velocity due to current.Renewables: K02 10MW fixed-bottom OWTPage 7 of 12
www.orcina.comIn this example, we have chosen to apply a turbulent wind flow in the simulation environment.This is applied using the full field wind model, which captures the variation of wind velocity, in bothspace and time, with data specified in an external TurbSim file (wind.bts). The file contains a timeseries of wind velocity, in 3-dimensions, at points on an evenly spaced 2-dimensional grid (in thevertical yz-plane).On the wind page of the environment data form, the name of the .bts file is specified along with thewind direction and origin; which determines how the .bts file’s coordinate system is mapped on tothe OrcaFlex coordinate system. Clicking file header provides some useful information about theimported wind data, including the size, position, and resolution of the 2D grid.OverviewFile: wind.btsID: 7Periodic: NoMean wind speed: 15.000m/sHeight of reference point: 145.000mHeight of bottom of the grid: 20.000mTimeNumber of time steps: 6334Time step: 0.050sDuration: 316.650sGridNumber of vertical grid points: 31Number of horizontal grid points: 31Vertical grid spacing: 8.33333mHorizontal grid spacing: 8.33333mVertical grid size: 250.000mHorizontal grid size: 250.000mTowerNumber of tower points: 3Tower spacing: 8.33333mTower size: 16.6667mDuring the simulation, the grid moves forward at a mean speed of 15m/s. In doing so, the gridsweeps through a volume in the model space within which the turbine is positioned; thus,imparting aerodynamic loading on the system. The images below provide a visual representationof this.Wind field progression through simulation2D Grid:Mean wind speed 15m/s250 x 250mGrid spacing 8.333m20mMean sea levelTower pointsThe file header also reports some information about the tower points included as part of the .btsfile. As the bottom of the 2D grid is positioned 20m above the sea surface, the tower points helpto ensure wind loading is captured on the length of tower between the grid and the sea surface.Based on the reported file header information, we have specified a Z wind origin value of , whichmeans that the vertical origin is at the mean water level.Renewables: K02 10MW fixed-bottom OWTPage 8 of 12
www.orcina.comA wind direction of 0 is also considered, which is the direction in which the 2D grid will propagatethrough the model space.It is also possible to set the wind to ramp during build-up by ticking the corresponding checkbox.We have not ticked that option, for this example, which means the wind velocity at the simulationstart time is applied during statics. When the wind is ramped, no wind is applied during statics andthe wind velocity increases from zero to the full value during stage 0.For further details related to wind modelling in OrcaFlex, please see the Modelling, data andresults Environment Wind data help page.Renewables: K02 10MW fixed-bottom OWTPage 9 of 12
www.orcina.comResultsWindOpening the workspace file named K02 Full field wind.wrk provides some visibility on the appliedwind conditions.The turbulent nature of the wind field is made evident by the speed and direction profiles shownover stage 1 at the rotor hub position (0, 0, 118.5 m). Note that the wind speed and direction bothremain constant during the build-up phase (stage 0). This is governed by the starting speed anddirection from the .bts file: 13.98m/s and -3.86 respectively.Rotor and generator responseThe workspace file, named K02 Rotor and generator response.wrk, opens some time history resultsfor the generator power (94.36%) and blade pitch response over stage 1.The top right-hand graph shows that the applied wind is, in the most part, above the ‘rated’ windspeed for the turbine (11m/s), meaning the pitch controller is active. This regulates power outputfrom the turbine near its rated power (10MW).Note, the generator power results account for the efficiency of the generator (94.36%). To achievethis, we have implemented a user defined result, specified on the corresponding page of thegeneral data form. Here we have selected the embedded source option (pictured below), meaningthe user defined results script is embedded directly in the data form, rather than in an external file.The script itself makes use of an existing results variable (generator power) to which a factor of0.9436 is applied to calculate a new result named generator power (94.36%).For further details and examples related to user defined results, please see the Modelling, dataand results General data User defined results help page.Renewables: K02 10MW fixed-bottom OWTPage 10 of 12
www.orcina.comBlade tip responseOpening the workspace file named K02 Blade tip response.wrk produces time history results, overstage 1, showing the out of plane deflection and in plane deflection at the tip of blade 1 (End B).The blade deflection results, available at blade nodes, report a node's position relative to itsunstressed position. The deflection component in the turbine's z-axis direction represents the outof plane deflection and its component normal to both the turbine's z-axis direction & the blade'spitch axis, is reported as the in plane deflection (pictured below).BladeunstressedpositionOut of planedeflection at bladetip (End B)Deflectioncomponent inturbine’s z-axisdirectionIn plane deflectionat blade tip (End B)BladeunstressedpositionDeflectioncomponent normalto the turbine’s z-axisdirection and theblade’s pitch axisTurbinez-axis directionThe bottom left-hand graph shows a time history of the line clearance results for blade 1. Withreference to the left-hand image below, the results selection form lets you choose how to reportclearances: (i) from the blade to all other lines or (ii) from the blade to a specified line. In this case,we have chosen to report the blade tip clearance from all lines, so the line clearance graph showsthe shortest distance between the node at End B of the blade and the outer edges of the linesegments in the model.Note: the line clearance result does not account for the cross-sectional extent of the blade.For further details relating to the turbine results, available in OrcaFlex, please refer to theModelling, data and results Turbines Turbine results help page.Line clearanceShortest distance betweenblade node (End B) andouter edge of line segmentsRenewables: K02 10MW fixed-bottom OWTPage 11 of 12
www.orcina.comMonopile responseOpening the workspace file named K02 Monopile response.wrk displays some instantaneous valuerange graphs showing the monopile response mid-way through stage 1.The upper right-hand graph shows the x Morison force experienced by the monopile line. Thisrepresents the x-component of the total force defined by Morison's equation, which is the sum ofthe x drag force and the x fluid inertia force (also shown by the bottom two graphs).Note that the embedded monopile – arc length range of 40 to 85m – does not experience anywave loading because the wave kinematics cutoff depth is set to 30m.Soil responseFinally, the workspace file named K02 Soil response.wrk, displays some results showing how theconstraint chain, at the mud line (h 0m), works to represent the interaction between the monopileand the soil.For the constraint capturing the horizontal soil stiffness (Soil-h 0m: y-x), the lower left-hand graphshows a time history of the out-frame X; which is the global X position of the out-frame. For theconstraint capturing the rotational spring stiffness of the soil (Soil-h 0m: Rz), the lower right-handgraph shows a time history of the out-frame dynamic Rz. This represents the orientation of the outframe relative to its static orientation.In this case, the soil stiffnesses assigned to each degree of freedom are very high. As a result, thecalculated out-frame displacements are generally very small.The out-frame rotations are also very small. This is important when considering the small-angleapproximation which accompanies this method of chaining constraints together.Lastly, the upper right-hand graph shows the in-frame connection force results for the horizontalconstraint (x & y). With this constraint being connected to the global axis system (anchored), thechain effectively terminates here and so the reported force is the resultant load the line nodeimparts on the constraint chain.Further details about the available results for constraints can be found on the Modelling, data andresults Constraints Results help page.Renewables: K02 10MW fixed-bottom OWTPage 12 of 12
to specify the local curvature components in radians per unit length. On the blade structure page of the turbine data form, we have specified the y-curvature values necessary to model the blade pre-bend for the 10MW RWT. Here, negative curvature values are specified to pre-bend the blades out of the rotor plane and away from the tower. Note that
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