Dear Dr. Jason,
I met some problems when modeling Telwind using OpenFastv3.0. The Telwind floating platform consists of two floaters and six tension cables connecting them.
1.If two floaters are assumed rigid, do I only need to model the six cables in SubDyn? And the twelve joints of cables are all set as interface joints?
2.In the properties of cables, does the positive T0 value mean stretch?
3.In ElastoDyn, do I need to definite the mass and inertia of the two floaters separately?
I would appreciate it if you could answer these questions.
Are you trying to model this floater as a single six degree of freedom rigid body? If so, you can model the entire floater as the platform in ElastoDyn.
Or are you trying to model the floater as a two six degree of freedom rigid bodies connected by elastic cables? If so, youâll need to model the floater in the SubDyn module. SubDyn models the substructure as an interconnection of beam, taut cable, and/or rigid members. Presumably youâd want to model the six cables with the taut cable elements, but Iâm not fully sure how youâd want to model the two hulls.
For cable elements in SubDyn, the pretension force must be positive, indicating stretching of the line (to keep the cable taut).
Yes, I want to model the floater as two 6-Dof rigid bodies connected by elastic cables in SubDyn, in that case, how to define the PtfmRefzt and PtfmMass properities in ElastoDyn, should I follow the rules applied in fixed offshore wind turbine and set those parameters as the TP(Transiton piece) 's position and mass, which is PtfmCMzt=PtfmRefzt=TP center height, PtfmMass=TP mass?
It probably makes sense to model the entire floating substructure (upper and lower rigid bodies and cables) in the same module (SubDyn). In that case, the 6 platform DOFs should be enabled in ElastoDyn, but the mass, center of mass, and inertia of the upper rigid body needs to be modeled in SubDyn. So, you can set PtfmRefzt = PtfmCMzt = PtfmMass = 0 (etc. for all platform mass, center of mass, and inertia properties in ElastoDyn, except PtfmYIner, which you should set equal to the rotational inertia of the undeflected tower about its centerline, at least when YawDOF = True, as discussed in other forum topics).
To model the upper rigid body in SubDyn, ideally youâd place rigid links between the interface node and the nodes connecting the cables, but there is a bug in SubDyn that was recently found regarding this functionality, as reported recently on the OpenFAST issues page: github.com/OpenFAST/openfast/issues/854. As such, Iâd recommend using quite stiff beam elements. You can add lumped (concentrated) masses/inertias at the node endpoints of the beams to ensure that the overall mass, center of mass, and inertia of the upper rigid body are as you want.
To model the lower rigid body in SubDyn, you can use rigid links between nodes of the cables, again, adding lumped masses/inertias as needed to get the overall mass, center of mass, and inertia of the lower rigid body are as you want.
SirďźIf I want to built up new offshore floating foundationďźhow can I need to do in FAST to built up my floating foundationďźhow I need to have data about foundationďźThanksďź
Iâm not sure I really understand your question. Youâll obviously need the properties of the floating substructure, including structural and hydrodynamic properties of the floater, the properties of the mooring system and tower, and any modifications necessary in the wind turbine controller.
Thank for your help! So where should I modify the floating foundation ďźIs HydroDyn or SubDnyďźAnother question is how can I obtain data about floating substructure ďźis other softwareďźbecause I want to built up new floating foundation by FAST to analyse.
Be regardsďź
The hydrodynamic properties are defined in HydroDyn, the floating substructure properties are defined in SubDyn (if the substructure is modeled as flexible) or ElastoDyn (if the substructure is modeled as a lumped mass/inertia), the tower properties are defined in ElastoDyn, and the mooring properties are defined in one of the mooring modules (e.g., MAP++, MoorDyn, or FEAMooring).
The data necessary to build a floating substructure model depends on the floater. If the floater is of large volume, presumably youâd want to model the hydrodynamics via potential flow theory, which requires the use of WAMIT or similar software to generate the hydrodynamic coefficients as a preprocessing step.
I am trying to get the internal forces of the OC4 semi-submersible substructure by using OpenFAST. Following your previous guidance, I modeled the platform in SubDyn, the tower and the turbine was built in ElastoDyn, but I felt confused when I going into HydroDyn.
Knowing how the hydrodynamic loads distribute along the substructure members is essential for the internal force calculation. Instead of the potential flow theory which is mainly used for global hydrodynamic analysis, it seems that only the strip-theory-based (Morisonâs equation) modeling approach in HydroDyn can provide such information to SubDyn. However, the OC4 semi-submersible is a large volume floater, we know it is better to use potential flow theory. It is also difficult to accurately determine the member-based C_A(added-mass coefficient) if the Morisonâs equation is applied.
Could you give me some advise on the hydrodynamic modeling for my case? Thanks a lot!
When the ability to model structural flexibility and member-level loads in floating platforms was introduced in OpenFAST v2.6 and newer, we introduced the functionality in HydroDyn to model multiple potential flow bodies, with optional hydrodynamic interaction. So, for the OC4-DeepCwind semi, you could model each offset column (and perhaps the center column) as distinct potential-flow bodies with the pontoons and braces represented via the strip-theory formulation. Then in SubDyn, you could model the offset columns (and perhaps the center column) as rigid links with the pontoons and braces represented as beam element. This set-up would allow you to capture the large-volume effects from potential flow, while still capturing the structural flexibility and member-level loads within the pontoons and braces.
I read a paper (by Luan et al. from NTNU) you cited in the NREL technical report. To get the internal forces within columns/pontoons, the authors divided the columns/pontoons into several segments, as the following figure shows:
I would like to do the same thing, what should I do in OpenFAST? I know we can get the multibody hydrodynamic coefficients via potential flow solvers like WAMIT (I would like to use the NBodyMod=1 mode, because I donât want to generate too many .1/.3/.hst files), but I am quite confused on how OpenFAST maps those member-level potential coefficients to SubDyn. Is it done by using PtfmRefxt/PtfmRefyt/PtfmRefzt/PtfmRefztRot?
Similar to Shengtao Zhou, I aim to model the OC4 semi-sub in OpenFAST with SubDyn to compute internal loads in the substructure, but I am just starting to learn OpenFAST. I am doing this in the context of my master thesis, which deals with design, analysis and optimization of a semi-submersible substructure for floating offshore wind, using OpenFAST (in a higher-fidelity model approach). I realize that for optimization practices, the floater is often modeled as a rigid body, but in my case I would like to verify structural integrity of the optimized floater design. Initially, I am trying to model the OC4 semi-sub for verification of my programming framework, but ultimately I will end up with a different semi-submersible platform geometry. As for the hydrodynamics, I am using Nemoh to calculate the hydrodynamic coefficients.
I am aware that there is no public model available for modeling flexible floating platforms, therefore I followed Jason Jonkmanâs guidelines from this github discussion to set-up the simulation with SubDyn: (FeaCoupling of an External floater (platform+mooring) with Openfast #791 ¡ OpenFAST/openfast ¡ Discussion #801 ¡ GitHub). However, I am not sure how to set the FEA and Craig-Bampton parameters in the SubDyn input file. Could anyone please provide guidance in how to choose the settings in this section for a flexible floating platform?
OpenFAST uses its internal spatial mesh-to-mesh mapping functionality to connect the hydrodynamic nodes from HydroDyn to the structural nodes from SubDyn: 4. User Documentation â OpenFAST v3.4.1 documentation. Because HydroDyn and SubDyn use point-element meshes, effectively a nearest-neighbor mapping is used. The HydroDyn potential-flow mesh is based on HydroDyn inputs PtfmRefxt/PtfmRefyt/PtfmRefzt.
Please note, however, that the structure you show seems to have members that are quite small in volume, so, perhaps a strip-theory solution is satisfactory? (We typically recommend that large-volume members be represented in the potential-flow solution and small-volume members be represented in the strip-theory solution.) Large-volume members requiring potential flow are likely quite stiff and may need not be split out into multiple potential-flow bodies in HydroDyn and beam elements in SubDyn. Rather, it may be satisfactory to treat each large-volume body as a separate potential-flow body in HydroDyn with rigid-link elements in SubDyn.
Here are my general recommendations for setting the FEA and Craig-Bampton parameters in SubDyn for floating wind applications:
FEAMod = 3
NDiv = 1, unless the members arenât manually discretized by adding joints, in which case NDiv > 1. That said, NDiv only supports a uniform division of all members, so, unless all members are roughly equal length, it may be better to manually discretize by adding joints.
CBMod = True
Nmodes = large enough to capture the lowest modes of the substructure, say up to 5-10 Hz, as calculated by SubDyn. This is typically less than 10-20 for most floating substructures. Because you likely wont know the frequencies until you run SubDyn, this will likely take some trial and error (by running SubDyn with different Nmodes). You can use the SubDyn mode visualizer (Mode shape visualization) to visualize each mode.
JDampings = 1 (if you know nothing) or tuned to match known full-system damping ratios.
I aims to transfer the hydrodynamic load distribution of the large-volume members (e.g. columns) to SubDyn and obtain their internal forces, so I split the large-volume members into 50 potential-flow bodies in HydroDyn, but OpenFAST (v3.2.1) cannot run properly with such number of bodies (NBodyMod=3 is used). The following errors occur:
I guess it is due to the memory overflow because when I reduced the number of potential-flow bodies to 5, OpenFAST can run normally. I switched to a higher-performance PC (128GB RAM), the 50-body case still doesnât work. Is there any way to fix this problem? If not, I think that representing the large-volume members in strip-theory solution is the only option for my case.
Dear Dr.Jason
Thanks for reading my email. Iâm currently using the subdyn of openfast to simulate the flexiblity of the floating offshore wind turbine. Some errors happened during the simulation.
I want to discuss the effects of the flexiblity of the floating platform on the dynamic response. The windfloat wind turbine is used and the extreme sea condition(Turbulent wind speed:37.4 s, Hs=12.58m, Tp=17.28s) is considered
When I considered the flexiblity of the floating in Subdyn module, the simulation ran very well.
However, when I set the platform to be rigid according to your previous comments(As shown below).
I didnât change other parameters except for the Nmodes=0, CBmode=true, and SttcSolve=false.
And I used the Morison equation only to solve the hydrodynamics of the Windfloat platforms.
Besides, the simulation under the normal operation condition ran well for both the flexible model and the rigid model. Below is the comparison between these two models under the normal operation condition.
Do you have any suggestions for solving this problem?
Thanks very much!
I would normally expect hitting a memory limit would trigger an error such as âError allocating space forâŚâ, not an access violation error. But after discussing internally, we recalled that that HydroDyn has some issues involving error handling, and so, the more graceful simulation abort may not always happen. In such cases, hitting a memory limit could result in an access violation error.
Thanks very much for your comments. Yes, the DT_UJac was set to be 2 which is around 1/10 of the pitch natural period. The simulation is aborted. However, when I set DT_UJac=0.5, the simulation runs normally. Is there any possible reason causing this? Iâm very curious about the set of DT_UJac. In the reference manual, the recommended value of DT_UJac is also 1/10 of the natural period of roll, pitch or yaw. Can you give me some suggestions or is there any detailed reference manual about the parameter âDT_UJacâ?
I generally agree with the guidance for DT_UJac that you referenced. Perhaps the yaw mode has a lower natural period than pitch and is driving the need for a smaller value of DT_UJac than you anticipated? Or perhaps the pitch motion is larger at lower periods than at the natural period?
In the SubDyn case, the surge, heave and pitch behaviour in the plot reveal that the modelling has not been successful. For example, there is a negative offset in heave. My initial guess is that by modelling the platform in SubDyn the total mass of the platform has increased, which could explain the difference in mean heave, and also the increased surge period. As a matter of fact, the way that I modeled the floater now (see visualisation below), I did not correct for overlapping members (yet, although I doubt it would have this much of an influence).
However, when I look at the SD.sum output file (see below), the mass of the platform is actually lower than the mass specified in the OC4 semisub definition report, which is 3.8522E+6 kg
I looked around on the forum and Iâve found some comments about DT_Ujac but this value does not seem to have an impact on the observed behaviour. As I am a new user of OpenFAST, I am afraid that I may have overlooked a certain setting in one of the input files, or that Iâve over-determined the floater geometry. I did not change anything in the HydroDyn file. I enabled the 6 platform DOFs in ElastoDyn, but did not touch the âmass and inertiaâ section w.r.t. the reference case
Clearly I am missing something. Please, could you help me towards a solution?
