Hi,
Could you explain why analyzing the platform movements in an still water case, with no wind and no current, the structure is moving upwards and downwards constantly? It shouldn’t be like that, should it?
I can’t understand why this is happening (and you have much more experience). Last week I was abel to reproduce your Hywind platform very well in both versions v7.02 and v 8.0. Now, I’m simulating a semisubergible platform in v 8.0 but it is being strange.
Thank you very much.
Kind regards
Dear Fernando,
I know nothing about the semisubmersible you are modeling in FAST v8, so, I can’t answer your question specifically.
What normally causes motion of a floating structure in the absence of wind, waves, or current would be an initial condition different from the static-equilibrium condition. For static equilibrium in heave in particular, the buoyancy from displaced water should balance the weight of the system (including turbine, tower, platform, and ballast) and mooring system pretension.
Best regards,
Hi Jason,
The platform I’m trying to simulate is similar to the Windfloat platform. I have a doubt. I don’t know how to introduce in FAST’s input files the differences between the Morison members and the rest. I can do it in order to obtain WAMIT files, but I don’t know if I need to introduce any parameters in the Hydrodyn input file or in the code.
Best regards,
Dear Fernando,
I’m sorry, but I don’t understand your question.
Our next release of FAST v8 will include a model of the OC4-DeepCwind semisubmersible that is based on a hybrid combination of WAMIT and Morison members. Hopefully this model will provide some guidance for your model of the WindFloat. We are also working on a HydroDyn user guide and theory manual that should help clarify how the module works.
Best regards,
Hi Jason,
I’ll try to be more specific. I tried two different spar buoys and both are being very well simulated by FAST v8. I’m doing exactly the same (I mean the same methodology to obtain the WAMIT files) for a semisubmergible platform but the behavior of the structure is completely different than the one we simulated with a different model (calibrated). This fact means that introducing directly WAMIT files is not working because added mass is being missed, doesn’t it? Could you tell now if next FAST realease is going to suffer changes acordingly to this fact?
Thank you very much again, your help is always crucial.
Best regards,
Dear Fernando,
How HydroDyn interacts with WAMIT did not change much between FAST v7 and FAST v8 and I’m not aware of any bugs in the FAST v8 implementation of WAMIT-related terms. Is your model of the WindFloat a WAMIT-only model, or a hybrid model of WAMIT and Morison members? Are you treating the WindFloat with the tower installed on one of the columns as opposed to centrally located in the middle of the semi?
Best regards,
Dear Jason,
For a V-shape FOWT like WindFloat whose tower was mounted on one of the column, where can I find some certest input files as reference. I am very confused about how to determine ‘PtfmCMxt’ in ElastoDyn, ‘PtfmRefxt’ and ‘PtfmCOBxt’ in HydroDyn and the origin of the WAMIT model.
Besides, the coordinate of the mooring system is also a problem, I don’t know whether should I input the anchoring point with the vertex of V as the origin of the coordinate systrm or the ‘PtfmRefxt’ defined in the HydroDyn.
I searched through some topics about the similar questions but still didn’t figure it out. The HydroDyn Manual and FAST manual of the FAST v8 was specific but mainly focused on the OC4 DeepC type. Can you give me some more suggestions?
Dear Jason,
Base on my sketch, I have built the FAST v8.16 model with the ‘PtfmCMxt’=-40 in ElastoDyn, ‘PtfmRefxt=0’ and ‘PtfmCOBxt=-40’ in HydroDyn, the origin of the WAMIT model was at Point B(with COG of wamit model (-40,0,0)). The mooring coodinate was in relation to Point B. The model can basically simulate for still water tests but run unsteadliy for wind-wave conditions. I assume the setup of the current model had some problems so I try to turn to OpenFAST v3 to rebuid the model.
Regards,
Dear @TianCheng.Yao,
I agree with your settings of PtfmCMxt and PtfmCOBxt for the situation such as the WindFloat where the floating platform center of mass and center of buoyancy are offset from the tower centerline. NREL does not have such a model shareable in the public domain.
Best regards,
Dear jason
In the latest OpenFAST v3.0, I rebuild the model mentioned above. The main ElastoDyn and HydroDyn parameters are attached below, .hst file which calculated from WAMIT is also included.
In ElastoDyn, PtfmCMxt=-42.97 (More offset than Point A to account for the mass of the RNA and tower, same reason for the PtfmPIner).
In HydroDyn, PtfmRefxt=-40.414(Point A),PtfmVol0 is the same as the WAMIT output, PtfmCOBxt also =-40.414. No strip theory member accounted because the damping are set up according to the model test.
In WAMIT output used here, the WAMIT model was built with the orgin at Point A, and the C44 and C55 in .hst file proved that,these values are same because the WAMIT model was built up with the symmtery geometry, and the Vertical center of mass was zero in WAMIT model.
4.MoorDyn
In moordyn, the mooring parameters are basically set up and the static pull-up test shows good agreement between the FAST and the model test.
Based on the setup above, I can get the static equilibrium position and acceptable Surge and Heave Decay result, however, the Roll and Pitch Decay show different natural periods between these two modes while the model test shows the same(see figure attached). In my assumption, the rotational inertia of the intire system (RNA ,tower, platform)was close between Roll and Pitch mode. The hydrostatic restoring matrix in the hst file also show that C44 and C55 is the same. So the roll and pitch decay test should agree well.
Would you please give my question a look?
Best regards,
ElastoDyn
-42.97 PtfmCMxt - Downwind distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters) //to be decided
0 PtfmCMyt - Lateral distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters) ////to be decided
-7.59 PtfmCMzt - Vertical distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters) //14-6.41
0 PtfmRefzt - Vertical distance from the ground level [onshore] or MSL [offshore] to the platform reference point (meters) //to be decided
---------------------- MASS AND INERTIA ----------------------------------------
0 TipMass(1) - Tip-brake mass, blade 1 (kg)
0 TipMass(2) - Tip-brake mass, blade 2 (kg)
0 TipMass(3) - Tip-brake mass, blade 3 (kg) [unused for 2 blades]
44501 HubMass - Hub mass (kg)
76465 HubIner - Hub inertia about rotor axis [3 blades] or teeter axis [2 blades] (kg m^2)
534.116 GenIner - Generator inertia about HSS (kg m^2) //LY4MW Undefined
140000 NacMass - Nacelle mass (kg)
2.05E+06 NacYIner - Nacelle inertia about yaw axis (kg m^2)
0 YawBrMass - Yaw bearing mass (kg)
8.578E+06 PtfmMass - Platform mass (kg)
8.08E+09 PtfmRIner - Platform inertia for roll tilt rotation about the platform CM (kg m^2)
6.78E+09 PtfmPIner - Platform inertia for pitch tilt rotation about the platform CM (kg m^2)
1.37E+10 PtfmYIner - Platform inertia for yaw rotation about the platform CM (kg m^2)
type or paste code here
HydroDyn
1 PotMod - Potential-flow model {0: none=no potential flow, 1: frequency-to-time-domain transforms based on WAMIT output, 2: fluid-impulse theory (FIT)} (switch)
1 ExctnMod - Wave-excitation model {0: no wave-excitation calculation, 1: DFT, 2: state-space} (switch) [only used when PotMod=1; STATE-SPACE REQUIRES *.ssexctn INPUT FILE]
1 RdtnMod - Radiation memory-effect model {0: no memory-effect calculation, 1: convolution, 2: state-space} (switch) [only used when PotMod=1; STATE-SPACE REQUIRES *.ss INPUT FILE]
60 RdtnTMax - Analysis time for wave radiation kernel calculations (sec) [only used when PotMod=1 and RdtnMod=1; determines RdtnDOmega=Pi/RdtnTMax in the cosine transform; MAKE SURE THIS IS LONG ENOUGH FOR THE RADIATION IMPULSE RESPONSE FUNCTIONS TO DECAY TO NEAR-ZERO FOR THE GIVEN PLATFORM!]
Default RdtnDT - Time step for wave radiation kernel calculations (sec) [only used when PotMod=1 and ExctnMod>1 or RdtnMod>0; DT<=RdtnDT<=0.1 recommended; determines RdtnOmegaMax=Pi/RdtnDT in the cosine transform]
1 NBody - Number of WAMIT bodies to be used (-) [>=1; only used when PotMod=1. If NBodyMod=1, the WAMIT data contains a vector of size 6*NBody x 1 and matrices of size 6*NBody x 6*NBody; if NBodyMod>1, there are NBody sets of WAMIT data each with a vector of size 6 x 1 and matrices of size 6 x 6]
1 NBodyMod - Body coupling model {1: include coupling terms between each body and NBody in HydroDyn equals NBODY in WAMIT, 2: neglect coupling terms between each body and NBODY=1 with XBODY=0 in WAMIT, 3: Neglect coupling terms between each body and NBODY=1 with XBODY=/0 in WAMIT} (switch) [only used when PotMod=1]
"HydroData/Middle" PotFile - Root name of potential-flow model data; WAMIT output files containing the linear, nondimensionalized, hydrostatic restoring matrix (.hst), frequency-dependent hydrodynamic added mass matrix and damping matrix (.1), and frequency- and direction-dependent wave excitation force vector per unit wave amplitude (.3) (quoted string) [MAKE SURE THE FREQUENCIES INHERENT IN THESE WAMIT FILES SPAN THE PHYSICALLY-SIGNIFICANT RANGE OF FREQUENCIES FOR THE GIVEN PLATFORM; THEY MUST CONTAIN THE ZERO- AND INFINITE-FREQUENCY LIMITS!]
1 WAMITULEN - Characteristic body length scale used to redimensionalize WAMIT output (meters) [only used when PotMod=1]
-40.414 PtfmRefxt - The xt offset of the body reference point(s) from (0,0,0) (meters) [1 to NBody] [only used when PotMod=1]
0.0 PtfmRefyt - The yt offset of the body reference point(s) from (0,0,0) (meters) [1 to NBody] [only used when PotMod=1]
0.0 PtfmRefzt - The zt offset of the body reference point(s) from (0,0,0) (meters) [1 to NBody] [only used when PotMod=1. If NBodyMod=2,PtfmRefzt=0.0]
0.0 PtfmRefztRot - The rotation about zt of the body reference frame(s) from xt/yt (degrees) [1 to NBody] [only used when PotMod=1]
9130.78 PtfmVol0 - Displaced volume of water when the platform is in its undisplaced position (m^3) [only used when PotMod=1; USE THE SAME VALUE COMPUTED BY WAMIT AS OUTPUT IN THE .OUT FILE!]
-40.414 PtfmCOBxt - The xt offset of the center of buoyancy (COB) from the platform reference point (meters) [only used when PotMod=1]
0 PtfmCOByt - The yt offset of the center of buoyancy (COB) from the platform reference point (meters) [only used when PotMod=1]
.hst
3 3 3.580E+02
3 4 -8.514E-02
3 5 -1.265E-01
3 6 0.000000E+00
4 1 0.000000E+00
4 2 0.000000E+00
4 3 0.000000E+00
4 4 2.257E+05
4 5 1.377E+00
4 6 8.61E+02
5 1 0.000000E+00
5 2 0.000000E+00
5 3 -1.533109E-02
5 4 0.000000E+00
5 5 2.257E+05
5 6 3.186E+02
6 1 0.000000E+00
6 2 0.000000E+00
6 3 0.000000E+00
6 4 0.000000E+00
6 5 0.000000E+00
6 6 0.000000E+00
Dear @TianCheng.Yao,
In ElastoDyn, the platform inertia is specified about the platform CM location, so, I would expect for a floating substructure that is symmetric (looking like an equilateral triangle from the top) that PtfmRIner = PtfmPIner.
Given the symmetry, why does PtfmCMxt in ElastoDyn not equal PtfmRefxt or PtfmCOBxt in HydroDyn?
Also given the symmetry, why are the (3,4), (3,5), (4,5), (4,6), (5,3), and (5,6) elements in the WAMIT *.hst file nonzero?
Best regards,
Dear Jason,
Thanks for your reply!
1.I don’t know if my understanding of PtfmRIner and PtfmCMxt is correct, in my opinion, these parameters in Elastodyn refer to the characteristics of the platform itself only other than the entire fowt, is that right?
For my model shown in the sketch, the turbine and the tower is mounted on one column, and the whole model is upright. so the inertia of the platform itself is different between PtfmRIner and PtfmPIner and that’s why the PtfmCMxt which parameter refers to the x offset of the CG Platform only(other than the entir system) not equal PtfmRefxt or PtfmCOBxt in HydroDyn.
In my wamit model, the wet surface of the platform under design draft is modeled, the geometry is symmetry with the mass defined equal to the displacement, and mass center is defined at the center of the waterplane. Besides, I checked the .hst file of the OC4 Semi certest file, they are nonzero, either. I am confused about that.
marin semi .hst
3 3 3.800615E+02
3 4 0.000000E+00
3 5 -1.533109E-02
3 6 0.000000E+00
4 1 0.000000E+00
4 2 0.000000E+00
4 3 0.000000E+00
4 4 -3.787550E+04
4 5 0.000000E+00
4 6 0.000000E+00
5 1 0.000000E+00
5 2 0.000000E+00
5 3 -1.533109E-02
5 4 0.000000E+00
5 5 -3.787527E+04
5 6 0.000000E+00
6 1 0.000000E+00
6 2 0.000000E+00
6 3 0.000000E+00
6 4 0.000000E+00
6 5 0.000000E+00
6 6 0.000000E+00
Best regards,
Dear @TianCheng.Yao,
Regarding (1), I’m not sure I understand what you are saying. I agree that the platform mass, center of mass, and inertia are only for the floating substructure (in isolation). The moorings, tower, and RNA are all specified separately in the OpenFAST model. So, unless the offset column upon which the tower is attached is different than the other two offset columns, I would expect the roll and pitch inertias to be equivalent for a symmetric platform.
Regarding (2), there are small nonzero (3,5) and (5,3) values in the WAMIT .hst file for the OC4-DeepCwind semisubmersible. But these are quite small relative to the diagonal elements and likely the result of small numerical round-off errors in WAMIT. In your case, which also uses a symmetric semisubmersible, there are more nonzero off-diagonal elements and some of them are larger than I would expect for numerical round-off errors.
Best regards,
Dear Jason,
Sorry for my unclear expression.
I didn’t set any ballast or filled members in hydrodyn, so for my model ,to obtain a static equilibrium
and upright position, the platform itself has a offset mass center to balance the mass of RNA and tower. Therefore,the roll and pitch inertias of the platform are not equivalent.
Regarding (2), I will cheak my wamit model and thanks for your advice.
Best regards,