Could the old version of WAMIT hydrodynamic files still be used in the new version of openfast?

[tclblby]

I am reproducing a surge free-decay test case based on a published paper. When I used OpenFAST v2.0.0, my simulation results matched those reported in the article very well. However, when I performed the same case with a newer version of OpenFAST, I observed a noticeable discrepancy in the decay amplitude, even though the natural period remains consistent.

To rule out possible input errors, I have carefully updated all input files to the format required by the new version. Therefore, I suspect that the discrepancy may be related to the hydrodynamic data files generated by WAMIT, which were originally prepared for the older OpenFAST version. Whether legacy WAMIT hydrodynamic files are fully compatible with the newer releases of OpenFAST?

If the issue is not caused by the WAMIT files,is there other possible factors that could lead to such differences in decay amplitude between the two versions?

Thanks for help.

[jjonkman]

Dear @tclblby,

I'm not aware of changes in WAMIT version compatibility that you are referencing. Can you clarify what specific changes you are referring to?

Can you clarify more about your simulation set up? What sources of damping are included in your model? (potential flow?, strip-theory drag?, mooring drag?, aerodynamics?) If there are multiple sources of damping, can you isolate which one is causing the change between versions?

Best regards,

[tclblby]

dear @jjonkman ,
Thanks for your reply. The potmod i picked is frequency-to-time-domain transforms based on WAMIT output, and the RdtnMod was convolution. I only used the ElastoDyn, HydroDyn and the MoorDyn, so I think the main source of damping is the Rdt force. I used the same WAMIT output file. What could lead to the difference between the two results?

Here are the Hydrodyn input files i used.

This is from openfast v2.0.0.

------- HydroDyn v2.05.* Input File --------------------------------------------
IEA 15 MW Activefloat Gran Canaria COREWIND, Estyco (idi@esteyco.com,esteyco@esteyco.com), COBRA (central@grupocobra.com, ideasinnovacion@grupocobra.com), M.Youssef Mahfouz (mahfouz@IFB.Uni-Stuttgart.de)
False Echo - Echo the input file data (flag)
---------------------- ENVIRONMENTAL CONDITIONS --------------------------------
1025 WtrDens - Water density (kg/m^3)
200 WtrDpth - Water depth (meters)
0 MSL2SWL - Offset between still-water level and mean sea level (meters) [positive upward; unused when WaveMod = 6; must be zero if PotMod=1 or 2]
---------------------- WAVES ---------------------------------------------------
0 WaveMod - Incident wave kinematics model {0: none=still water, 1: regular (periodic), 1P#: regular with user-specified phase, 2: JONSWAP/Pierson-Moskowitz spectrum (irregular), 3: White noise spectrum (irregular), 4: user-defined spectrum from routine UserWaveSpctrm (irregular), 5: Externally generated wave-elevation time series, 6: Externally generated full wave-kinematics time series [option 6 is invalid for PotMod/=0]} (switch)
0 WaveStMod - Model for stretching incident wave kinematics to instantaneous free surface {0: none=no stretching, 1: vertical stretching, 2: extrapolation stretching, 3: Wheeler stretching} (switch) [unused when WaveMod=0 or when PotMod/=0]
5400.0 WaveTMax - Analysis time for incident wave calculations (sec) [unused when WaveMod=0; determines WaveDOmega=2Pi/WaveTMax in the IFFT]
0.25 WaveDT - Time step for incident wave calculations (sec) [unused when WaveMod=0; 0.1<=WaveDT<=1.0 recommended; determines WaveOmegaMax=Pi/WaveDT in the IFFT]
1.38 WaveHs - Significant wave height of incident waves (meters) [used only when WaveMod=1, 2, or 3]
7.0 WaveTp - Peak-spectral period of incident waves (sec) [used only when WaveMod=1 or 2]
1.0 WavePkShp - Peak-shape parameter of incident wave spectrum (-) or DEFAULT (string) [used only when WaveMod=2; use 1.0 for Pierson-Moskowitz]
0.0001 WvLowCOff - Low cut-off frequency or lower frequency limit of the wave spectrum beyond which the wave spectrum is zeroed (rad/s) [unused when WaveMod=0, 1, or 6]
3.1416 WvHiCOff - High cut-off frequency or upper frequency limit of the wave spectrum beyond which the wave spectrum is zeroed (rad/s) [unused when WaveMod=0, 1, or 6]
0 WaveDir - Incident wave propagation heading direction (degrees) [unused when WaveMod=0 or 6]
0 WaveDirMod - Directional spreading function {0: none, 1: COS2S} (-) [only used when WaveMod=2,3, or 4]
1 WaveDirSpread - Wave direction spreading coefficient ( > 0 ) (-) [only used when WaveMod=2,3, or 4 and WaveDirMod=1]
1 WaveNDir - Number of wave directions (-) [only used when WaveMod=2,3, or 4 and WaveDirMod=1; odd number only]
0 WaveDirRange - Range of wave directions (full range: WaveDir +/- 1/2*WaveDirRange) (degrees) [only used when WaveMod=2,3,or 4 and WaveDirMod=1]
123456789 WaveSeed(1) - First random seed of incident waves [-2147483648 to 2147483647] (-) [unused when WaveMod=0, 5, or 6]
1011121314 WaveSeed(2) - Second random seed of incident waves [-2147483648 to 2147483647] (-) [unused when WaveMod=0, 5, or 6]
FALSE WaveNDAmp - Flag for normally distributed amplitudes (flag) [only used when WaveMod=2, 3, or 4]
"" WvKinFile - Root name of externally generated wave data file(s) (quoted string) [used only when WaveMod=5 or 6]
1 NWaveElev - Number of points where the incident wave elevations can be computed (-) [maximum of 9 output locations]
0 WaveElevxi - List of xi-coordinates for points where the incident wave elevations can be output (meters) [NWaveElev points, separated by commas or white space; usused if NWaveElev = 0]
0 WaveElevyi - List of yi-coordinates for points where the incident wave elevations can be output (meters) [NWaveElev points, separated by commas or white space; usused if NWaveElev = 0]
---------------------- 2ND-ORDER WAVES ----------------------------------------- [unused with WaveMod=0 or 6]
FALSE WvDiffQTF - Full difference-frequency 2nd-order wave kinematics (flag)
FALSE WvSumQTF - Full summation-frequency 2nd-order wave kinematics (flag)
0 WvLowCOffD - Low frequency cutoff used in the difference-frequencies (rad/s) [Only used with a difference-frequency method]
1.256637 WvHiCOffD - High frequency cutoff used in the difference-frequencies (rad/s) [Only used with a difference-frequency method]
0.618319 WvLowCOffS - Low frequency cutoff used in the summation-frequencies (rad/s) [Only used with a summation-frequency method]
3.141593 WvHiCOffS - High frequency cutoff used in the summation-frequencies (rad/s) [Only used with a summation-frequency method]
---------------------- CURRENT ------------------------------------------------- [unused with WaveMod=6]
0 CurrMod - Current profile model {0: none=no current, 1: standard, 2: user-defined from routine UserCurrent} (switch)
0 CurrSSV0 - Sub-surface current velocity at still water level (m/s) [used only when CurrMod=1]
"DEFAULT" CurrSSDir - Sub-surface current heading direction (degrees) or DEFAULT (string) [used only when CurrMod=1]
20 CurrNSRef - Near-surface current reference depth (meters) [used only when CurrMod=1]
0 CurrNSV0 - Near-surface current velocity at still water level (m/s) [used only when CurrMod=1]
0 CurrNSDir - Near-surface current heading direction (degrees) [used only when CurrMod=1]
0 CurrDIV - Depth-independent current velocity (m/s) [used only when CurrMod=1]
0 CurrDIDir - Depth-independent current heading direction (degrees) [used only when CurrMod=1]
---------------------- FLOATING PLATFORM --------------------------------------- [unused with WaveMod=6]
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)
"HydroData/Activefloat" 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]
36431.45 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!]
0 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]
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]
100 RdtnTMax - Analysis time for wave radiation kernel calculations (sec) [only used when PotMod=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; DT<=RdtnDT<=0.1 recommended; determines RdtnOmegaMax=Pi/RdtnDT in the cosine transform]
---------------------- 2ND-ORDER FLOATING PLATFORM FORCES ---------------------- [unused with WaveMod=0 or 6, or PotMod=0 or 2]
0 MnDrift - Mean-drift 2nd-order forces computed {0: None; [7, 8, 9, 10, 11, or 12]: WAMIT file to use} [Only one of MnDrift, NewmanApp, or DiffQTF can be non-zero]
0 NewmanApp - Mean- and slow-drift 2nd-order forces computed with Newman's approximation {0: None; [7, 8, 9, 10, 11, or 12]: WAMIT file to use} [Only one of MnDrift, NewmanApp, or DiffQTF can be non-zero. Used only when WaveDirMod=0]
0 DiffQTF - Full difference-frequency 2nd-order forces computed with full QTF {0: None; [10, 11, or 12]: WAMIT file to use} [Only one of MnDrift, NewmanApp, or DiffQTF can be non-zero]
0 SumQTF - Full summation -frequency 2nd-order forces computed with full QTF {0: None; [10, 11, or 12]: WAMIT file to use}
---------------------- FLOATING PLATFORM FORCE FLAGS -------------------------- [unused with WaveMod=6]
True PtfmSgF - Platform horizontal surge translation force (flag) or DEFAULT
True PtfmSwF - Platform horizontal sway translation force (flag) or DEFAULT
True PtfmHvF - Platform vertical heave translation force (flag) or DEFAULT
True PtfmRF - Platform roll tilt rotation force (flag) or DEFAULT
True PtfmPF - Platform pitch tilt rotation force (flag) or DEFAULT
True PtfmYF - Platform yaw rotation force (flag) or DEFAULT
---------------------- PLATFORM ADDITIONAL STIFFNESS AND DAMPING --------------
0 0 0 0 0 0 AddF0 - Additional preload (N, N-m)
0 0 0 0 0 0 AddCLin - Additional linear stiffness (N/m, N/rad, N-m/m, N-m/rad)
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0 AddBLin - Additional linear damping(N/(m/s), N/(rad/s), N-m/(m/s), N-m/(rad/s))
0 0 0 0 0 0
0 0 6924540 0 0 0
0 0 0 855358790 0 0
0 0 0 0 855358790 0
0 0 0 0 0 0
0 0 0 0 0 0 AddBQuad - Additional quadratic drag(N/(m/s)^2, N/(rad/s)^2, N-m(m/s)^2, N-m/(rad/s)^2)
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
---------------------- AXIAL COEFFICIENTS --------------------------------------
3 NAxCoef - Number of axial coefficients (-)
AxCoefID AxCd AxCa AxCp
(-) (-) (-) (-)
1 0.00 0.00 0.00
2 5.70 0.00 0.00
3 40.09 0.00 0.00
---------------------- MEMBER JOINTS -------------------------------------------
14 NJoints - Number of joints (-) [must be exactly 0 or at least 2]
JointID Jointxi Jointyi Jointzi JointAxID JointOvrlp [JointOvrlp= 0: do nothing at joint, 1: eliminate overlaps by calculating super member]
(-) (m) (m) (m) (-) (switch)
1 0 0 9.0 1 0 !center column top
2 0 0 -15.0 2 0 !centercolumn end of taper
3 -34 0 -26.5 3 0 !1st side column bottom
4 -34 0 9.0 1 0 !1st side column top
5 17 29.445 -26.5 3 0 !2nd side column bottom
6 17 29.445 9.0 1 0 !2nd side column top
7 17 -29.445 -26.5 3 0 !3rd side column bottom
8 17 -29.445 9.0 1 0 !3rd side column top
9 -4.91 0 -20.75 1 0 !1st pontoon
10 -25.50 0 -20.75 1 0 !1st pontoon
11 2.455 4.2522 -20.75 1 0 !2nd pontoon
12 12.75 22.0836 -20.75 1 0 !2nd pontoon
13 2.455 -4.2522 -20.75 1 0 !3rd pontoon
14 12.75 -22.0836 -20.75 1 0 !3rd pontoon
---------------------- MEMBER CROSS-SECTION PROPERTIES -------------------------
4 NPropSets - Number of member property sets (-)
PropSetID PropD PropThck
(-) (m) (m)
1 11.00 0.0
2 19.62 0.0
3 17.00 0.0
4 11.50 0.0
---------------------- SIMPLE HYDRODYNAMIC COEFFICIENTS (model 1) --------------
SimplCd SimplCdMG SimplCa SimplCaMG SimplCp SimplCpMG SimplAxCa SimplAxCaMG SimplAxCp SimplAxCpMG
(-) (-) (-) (-) (-) (-) (-) (-) (-) (-)
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.00 1.00
---------------------- DEPTH-BASED HYDRODYNAMIC COEFFICIENTS (model 2) ---------
0 NCoefDpth - Number of depth-dependent coefficients (-)
Dpth DpthCd DpthCdMG DpthCa DpthCaMG DpthCp DpthCpMG DpthAxCa DpthAxCaMG DpthAxCp DpthAxCpMG
(m) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-)
---------------------- MEMBER-BASED HYDRODYNAMIC COEFFICIENTS (model 3) --------
7 NCoefMembers - Number of member-based coefficients (-)
MemberID MemberCd1 MemberCd2 MemberCdMG1 MemberCdMG2 MemberCa1 MemberCa2 MemberCaMG1 MemberCaMG2 MemberCp1 MemberCp2 MemberCpMG1 MemberCpMG2 MemberAxCa1 MemberAxCa2 MemberAxCaMG1 MemberAxCaMG2 MemberAxCp1 MemberAxCp2 MemberAxCpMG1 MemberAxCpMG2
(-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-)
1 0.737 0.687 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
2 0.699 0.699 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
3 0.699 0.699 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
4 0.699 0.699 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
5 2.050 2.050 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
6 2.050 2.050 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
7 2.050 2.050 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
-------------------- MEMBERS -------------------------------------------------
7 NMembers - Number of members (-)
MemberID MJointID1 MJointID2 MPropSetID1 MPropSetID2 MDivSize MCoefMod PropPot [MCoefMod=1: use simple coeff table, 2: use depth-based coeff table, 3: use member-based coeff table] [ PropPot/=0 if member is modeled with potential-flow theory]
(-) (-) (-) (-) (-) (m) (switch) (flag)
1 1 2 1 2 1.000 3 TRUE
2 3 4 3 3 1.000 3 TRUE
3 5 6 3 3 1.000 3 TRUE
4 7 8 3 3 1.000 3 TRUE
5 9 10 4 4 1.000 3 TRUE
6 11 12 4 4 1.000 3 TRUE
7 13 14 4 4 1.000 3 TRUE
---------------------- FILLED MEMBERS ------------------------------------------
0 NFillGroups - Number of filled member groups (-) [If FillDens = DEFAULT, then FillDens = WtrDens; FillFSLoc is related to MSL2SWL]
FillNumM FillMList FillFSLoc FillDens
(-) (-) (m) (kg/m^3)
---------------------- MARINE GROWTH -------------------------------------------
0 NMGDepths - Number of marine-growth depths specified (-)
MGDpth MGThck MGDens
(m) (m) (kg/m^3)
---------------------- MEMBER OUTPUT LIST --------------------------------------
0 NMOutputs - Number of member outputs (-) [must be < 10]
MemberID NOutLoc NodeLocs [NOutLoc < 10; node locations are normalized distance from the start of the member, and must be >=0 and <= 1] [unused if NMOutputs=0]
(-) (-) (-)
---------------------- JOINT OUTPUT LIST ---------------------------------------
0 NJOutputs - Number of joint outputs [Must be < 10]
0 JOutLst - List of JointIDs which are to be output (-)[unused if NJOutputs=0]
---------------------- OUTPUT --------------------------------------------------
True HDSum - Output a summary file [flag]
False OutAll - Output all user-specified member and joint loads (only at each member end, not interior locations) [flag]
2 OutSwtch - Output requested channels to: [1=Hydrodyn.out, 2=GlueCode.out, 3=both files]
"ES11.4e2" OutFmt - Output format for numerical results (quoted string) [not checked for validity!]
"A11" OutSFmt - Output format for header strings (quoted string) [not checked for validity!]

this is from openfast v3.5.2:

------- HydroDyn Input File ----------------------------------------------------
NREL 5.0 MW offshore baseline floating platform HydroDyn input properties for the OC4 Semi-submersible.
False Echo - Echo the input file data (flag)
---------------------- ENVIRONMENTAL CONDITIONS --------------------------------
1025 WtrDens - Water density (kg/m^3)
200 WtrDpth - Water depth (meters)
0 MSL2SWL - Offset between still-water level and mean sea level (meters) [positive upward; unused when WaveMod = 6; must be zero if PotMod=1 or 2]
---------------------- WAVES ---------------------------------------------------
0 WaveMod - Incident wave kinematics model {0: none=still water, 1: regular (periodic), 1P#: regular with user-specified phase, 2: JONSWAP/Pierson-Moskowitz spectrum (irregular), 3: White noise spectrum (irregular), 4: user-defined spectrum from routine UserWaveSpctrm (irregular), 5: Externally generated wave-elevation time series, 6: Externally generated full wave-kinematics time series [option 6 is invalid for PotMod/=0]} (switch)
0 WaveStMod - Model for stretching incident wave kinematics to instantaneous free surface {0: none=no stretching, 1: vertical stretching, 2: extrapolation stretching, 3: Wheeler stretching} (switch) [unused when WaveMod=0 or when PotMod/=0]
5400.0 WaveTMax - Analysis time for incident wave calculations (sec) [unused when WaveMod=0; determines WaveDOmega=2Pi/WaveTMax in the IFFT]
0.25 WaveDT - Time step for incident wave calculations (sec) [unused when WaveMod=0; 0.1<=WaveDT<=1.0 recommended; determines WaveOmegaMax=Pi/WaveDT in the IFFT]
1.38 WaveHs - Significant wave height of incident waves (meters) [used only when WaveMod=1, 2, or 3]
7.0 WaveTp - Peak-spectral period of incident waves (sec) [used only when WaveMod=1 or 2]
1.0 WavePkShp - Peak-shape parameter of incident wave spectrum (-) or DEFAULT (string) [used only when WaveMod=2; use 1.0 for Pierson-Moskowitz]
0.0001 WvLowCOff - Low cut-off frequency or lower frequency limit of the wave spectrum beyond which the wave spectrum is zeroed (rad/s) [unused when WaveMod=0, 1, or 6]
3.1416 WvHiCOff - High cut-off frequency or upper frequency limit of the wave spectrum beyond which the wave spectrum is zeroed (rad/s) [unused when WaveMod=0, 1, or 6]
0 WaveDir - Incident wave propagation heading direction (degrees) [unused when WaveMod=0 or 6]
0 WaveDirMod - Directional spreading function {0: none, 1: COS2S} (-) [only used when WaveMod=2,3, or 4]
1 WaveDirSpread - Wave direction spreading coefficient ( > 0 ) (-) [only used when WaveMod=2,3, or 4 and WaveDirMod=1]
1 WaveNDir - Number of wave directions (-) [only used when WaveMod=2,3, or 4 and WaveDirMod=1; odd number only]
0 WaveDirRange - Range of wave directions (full range: WaveDir +/- 1/2WaveDirRange) (degrees) [only used when WaveMod=2,3,or 4 and WaveDirMod=1]
123456789 WaveSeed(1) - First random seed of incident waves [-2147483648 to 2147483647] (-) [unused when WaveMod=0, 5, or 6]
1011121314 WaveSeed(2) - Second random seed of incident waves [-2147483648 to 2147483647] for intrinsic pRNG, or an alternative pRNG: "RanLux" (-) [unused when WaveMod=0, 5, or 6]
FALSE WaveNDAmp - Flag for normally distributed amplitudes (flag) [only used when WaveMod=2, 3, or 4]
"" WvKinFile - Root name of externally generated wave data file(s) (quoted string) [used only when WaveMod=5 or 6]
1 NWaveElev - Number of points where the incident wave elevations can be computed (-) [maximum of 9 output locations]
0 WaveElevxi - List of xi-coordinates for points where the incident wave elevations can be output (meters) [NWaveElev points, separated by commas or white space; usused if NWaveElev = 0]
0 WaveElevyi - List of yi-coordinates for points where the incident wave elevations can be output (meters) [NWaveElev points, separated by commas or white space; usused if NWaveElev = 0]
---------------------- 2ND-ORDER WAVES ----------------------------------------- [unused with WaveMod=0 or 6]
FALSE WvDiffQTF - Full difference-frequency 2nd-order wave kinematics (flag)
FALSE WvSumQTF - Full summation-frequency 2nd-order wave kinematics (flag)
0 WvLowCOffD - Low frequency cutoff used in the difference-frequencies (rad/s) [Only used with a difference-frequency method]
1.256637 WvHiCOffD - High frequency cutoff used in the difference-frequencies (rad/s) [Only used with a difference-frequency method]
0.618319 WvLowCOffS - Low frequency cutoff used in the summation-frequencies (rad/s) [Only used with a summation-frequency method]
3.141593 WvHiCOffS - High frequency cutoff used in the summation-frequencies (rad/s) [Only used with a summation-frequency method]
---------------------- CURRENT ------------------------------------------------- [unused with WaveMod=6]
0 CurrMod - Current profile model {0: none=no current, 1: standard, 2: user-defined from routine UserCurrent} (switch)
0 CurrSSV0 - Sub-surface current velocity at still water level (m/s) [used only when CurrMod=1]
"DEFAULT" CurrSSDir - Sub-surface current heading direction (degrees) or DEFAULT (string) [used only when CurrMod=1]
20 CurrNSRef - Near-surface current reference depth (meters) [used only when CurrMod=1]
0 CurrNSV0 - Near-surface current velocity at still water level (m/s) [used only when CurrMod=1]
0 CurrNSDir - Near-surface current heading direction (degrees) [used only when CurrMod=1]
0 CurrDIV - Depth-independent current velocity (m/s) [used only when CurrMod=1]
0 CurrDIDir - Depth-independent current heading direction (degrees) [used only when CurrMod=1]
---------------------- FLOATING PLATFORM --------------------------------------- [unused with WaveMod=6]
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)
0 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]
100 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 6NBody x 1 and matrices of size 6NBody x 6NBody; 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]
2 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/Activefloat" 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) [1 to NBody if NBodyMod>1] [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) [1 to NBody if NBodyMod>1] [only used when PotMod=1]
0.0 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]
36431.45 PtfmVol0 - Displaced volume of water when the body is in its undisplaced position (m^3) [1 to NBody] [only used when PotMod=1; USE THE SAME VALUE COMPUTED BY WAMIT AS OUTPUT IN THE .OUT FILE!]
0.0 PtfmCOBxt - The xt offset of the center of buoyancy (COB) from (0,0) (meters) [1 to NBody] [only used when PotMod=1]
0.0 PtfmCOByt - The yt offset of the center of buoyancy (COB) from (0,0) (meters) [1 to NBody] [only used when PotMod=1]
---------------------- 2ND-ORDER FLOATING PLATFORM FORCES ---------------------- [unused with WaveMod=0 or 6, or PotMod=0 or 2]
0 MnDrift - Mean-drift 2nd-order forces computed {0: None; [7, 8, 9, 10, 11, or 12]: WAMIT file to use} [Only one of MnDrift, NewmanApp, or DiffQTF can be non-zero. If NBody>1, MnDrift /=8]
0 NewmanApp - Mean- and slow-drift 2nd-order forces computed with Newman's approximation {0: None; [7, 8, 9, 10, 11, or 12]: WAMIT file to use} [Only one of MnDrift, NewmanApp, or DiffQTF can be non-zero. If NBody>1, NewmanApp/=8. Used only when WaveDirMod=0]
0 DiffQTF - Full difference-frequency 2nd-order forces computed with full QTF {0: None; [10, 11, or 12]: WAMIT file to use} [Only one of MnDrift, NewmanApp, or DiffQTF can be non-zero]
0 SumQTF - Full summation -frequency 2nd-order forces computed with full QTF {0: None; [10, 11, or 12]: WAMIT file to use}
---------------------- PLATFORM ADDITIONAL STIFFNESS AND DAMPING -------------- [unused with PotMod=0 or 2]
0 AddF0 - Additional preload (N, N-m) [If NBodyMod=1, one size 6NBody x 1 vector; if NBodyMod>1, NBody size 6 x 1 vectors]
0
0
0
0
0
0 0 0 0 0 0 AddCLin - Additional linear stiffness (N/m, N/rad, N-m/m, N-m/rad) [If NBodyMod=1, one size 6NBody x 6NBody matrix; if NBodyMod>1, NBody size 6 x 6 matrices]
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0 AddBLin - Additional linear damping(N/(m/s), N/(rad/s), N-m/(m/s), N-m/(rad/s)) [If NBodyMod=1, one size 6NBody x 6NBody matrix; if NBodyMod>1, NBody size 6 x 6 matrices]
0 0 0 0 0 0
0 0 6924540 0 0 0
0 0 0 855358790 0 0
0 0 0 0 855358790 0
0 0 0 0 0 0
0 0 0 0 0 0 AddBQuad - Additional quadratic drag(N/(m/s)^2, N/(rad/s)^2, N-m(m/s)^2, N-m/(rad/s)^2) [If NBodyMod=1, one size 6NBody x 6*NBody matrix; if NBodyMod>1, NBody size 6 x 6 matrices]
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
---------------------- AXIAL COEFFICIENTS --------------------------------------
3 NAxCoef - Number of axial coefficients (-)
AxCoefID AxCd AxCa AxCp
(-) (-) (-) (-)
1 0.00 0.00 0.00
2 5.70 0.00 0.00
3 40.09 0.00 0.00
---------------------- MEMBER JOINTS -------------------------------------------
14 NJoints - Number of joints (-) [must be exactly 0 or at least 2]
JointID Jointxi Jointyi Jointzi JointAxID JointOvrlp [JointOvrlp= 0: do nothing at joint, 1: eliminate overlaps by calculating super member]
(-) (m) (m) (m) (-) (switch)
1 0 0 9.0 1 0 !center column top
2 0 0 -15.0 2 0 !centercolumn end of taper
3 -34 0 -26.5 3 0 !1st side column bottom
4 -34 0 9.0 1 0 !1st side column top
5 17 29.445 -26.5 3 0 !2nd side column bottom
6 17 29.445 9.0 1 0 !2nd side column top
7 17 -29.445 -26.5 3 0 !3rd side column bottom
8 17 -29.445 9.0 1 0 !3rd side column top
9 -4.91 0 -20.75 1 0 !1st pontoon
10 -25.50 0 -20.75 1 0 !1st pontoon
11 2.455 4.2522 -20.75 1 0 !2nd pontoon
12 12.75 22.0836 -20.75 1 0 !2nd pontoon
13 2.455 -4.2522 -20.75 1 0 !3rd pontoon
14 12.75 -22.0836 -20.75 1 0 !3rd pontoon
---------------------- MEMBER CROSS-SECTION PROPERTIES -------------------------
4 NPropSets - Number of member property sets (-)
PropSetID PropD PropThck
(-) (m) (m)
1 11.00 0.0
2 19.62 0.0
3 17.00 0.0
4 11.50 0.0
---------------------- SIMPLE HYDRODYNAMIC COEFFICIENTS (model 1) --------------
SimplCd SimplCdMG SimplCa SimplCaMG SimplCp SimplCpMG SimplAxCd SimplAxCdMG SimplAxCa SimplAxCaMG SimplAxCp SimplAxCpMG
(-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-)
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.00 1.00
---------------------- DEPTH-BASED HYDRODYNAMIC COEFFICIENTS (model 2) ---------
0 NCoefDpth - Number of depth-dependent coefficients (-)
Dpth DpthCd DpthCdMG DpthCa DpthCaMG DpthCp DpthCpMG DpthAxCd DpthAxCdMG DpthAxCa DpthAxCaMG DpthAxCp DpthAxCpMG
(m) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-)
---------------------- MEMBER-BASED HYDRODYNAMIC COEFFICIENTS (model 3) --------
7 NCoefMembers - Number of member-based coefficients (-)
MemberID MemberCd1 MemberCd2 MemberCdMG1 MemberCdMG2 MemberCa1 MemberCa2 MemberCaMG1 MemberCaMG2 MemberCp1 MemberCp2 MemberCpMG1 MemberCpMG2 MemberAxCd1 MemberAxCd2 MemberAxCdMG1 MemberAxCdMG2 MemberAxCa1 MemberAxCa2 MemberAxCaMG1 MemberAxCaMG2 MemberAxCp1 MemberAxCp2 MemberAxCpMG1 MemberAxCpMG2
(-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-)
1 0.737 0.687 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
2 0.699 0.699 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
3 0.699 0.699 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
4 0.699 0.699 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
5 2.050 2.050 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
6 2.050 2.050 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
7 2.050 2.050 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
-------------------- MEMBERS -------------------------------------------------
7 NMembers - Number of members (-)
MemberID MJointID1 MJointID2 MPropSetID1 MPropSetID2 MDivSize MCoefMod PropPot [MCoefMod=1: use simple coeff table, 2: use depth-based coeff table, 3: use member-based coeff table] [ PropPot/=0 if member is modeled with potential-flow theory]
(-) (-) (-) (-) (-) (m) (switch) (flag)
1 1 2 1 2 1.000 3 TRUE
2 3 4 3 3 1.000 3 TRUE
3 5 6 3 3 1.000 3 TRUE
4 7 8 3 3 1.000 3 TRUE
5 9 10 4 4 1.000 3 TRUE
6 11 12 4 4 1.000 3 TRUE
7 13 14 4 4 1.000 3 TRUE
---------------------- FILLED MEMBERS ------------------------------------------
0 NFillGroups - Number of filled member groups (-) [If FillDens = DEFAULT, then FillDens = WtrDens; FillFSLoc is related to MSL2SWL]
FillNumM FillMList FillFSLoc FillDens
(-) (-) (m) (kg/m^3)
---------------------- MARINE GROWTH -------------------------------------------
0 NMGDepths - Number of marine-growth depths specified (-)
MGDpth MGThck MGDens
(m) (m) (kg/m^3)
---------------------- MEMBER OUTPUT LIST --------------------------------------
0 NMOutputs - Number of member outputs (-) [must be <= 99]
MemberID NOutLoc NodeLocs [NOutLoc < 10; node locations are normalized distance from the start of the member, and must be >=0 and <= 1] [unused if NMOutputs=0]
(-) (-) (-)
---------------------- JOINT OUTPUT LIST ---------------------------------------
0 NJOutputs - Number of joint outputs [Must be < 10]
0 JOutLst - List of JointIDs which are to be output (-)[unused if NJOutputs=0]
---------------------- OUTPUT --------------------------------------------------
True HDSum - Output a summary file [flag]
False OutAll - Output all user-specified member and joint loads (only at each member end, not interior locations) [flag]
2 OutSwtch - Output requested channels to: [1=Hydrodyn.out, 2=GlueCode.out, 3=both files]
"E15.7e2" OutFmt - Output format for numerical results (quoted string) [not checked for validity!]
"A11" OutSFmt - Output format for header strings (quoted string) [not checked for validity!]

Best regards,

[jjonkman]

Dear @tclblby,

Thanks for clarifying. Looking at your HydroDyn model, it looks like you have three sources of damping: wave-radiation from the potential-flow solution (PotMod = 1 with RdtnMod = 1), additional linear damping (AddBLin), and viscous drag from the strip-theory solution (joints, members). Given that you are using MoorDyn, you could also have damping from the stationkeeping system. I would suggest trying to isolate each effect to figure out the relative damping contribution from each term and to identify which one changes between OpenFAST v2.0.0 and v3.5.2.

Best regards,

[tclblby]

Dear @jjonkman,
Sorry to reply so late. It took me a few days to do some tests on them. Now I have got some interesting results.
Firstly, I test the Moordyn V1(used in OpenFast v2.0.0) and Moordyn V2(used in OpenFast v3.5.2). They can get the same force under the same body motion. To be on the safe side, I set the hydrodynamic coefficients in MoorDyn to 0 in subsequent tests.
Then I closed the strip-theory solution (joints, members). It's ok in OpenFast v3.5.2 but there is a bug in OpenFast v2.0.0 preventing it from functioning properly when a model has no strip-theory nodes (NJoints = NMembers = 0). So I set up my HydroDyn module with two joints and one member, but with all member properties and coefficients set to zero，as discussed in the previous issues. I think there is no viscous drag from the strip-theory solution (joints, members).
Inadditionally, I set additional linear damping all zero. So the only damping source is the wave-radiation from the potential-flow solution (PotMod = 1 with RdtnMod = 1) now.
However, the results from the two visions still keep different. As shown in figure below, there nearly no damping the openfast v3.5.2 can get from the WAMIT files. The reson i suppose is the old WAMIT output file using by the new version of openfast may introduce some errors.
Best regards,

[jjonkman]

Dear @tclblby,

If I understand correctly, you'd expect the red and green curves (all damping sources) to match, as well as the blue and black curves (only potential-flow damping) to match. I'm surprised how large of difference you are seeing between the blue and black curves, which are more different than the original model with all damping sources.

It is probably worth doing a similar comparison isolating the other sources of damping (mooring only, viscous only, additional linear damping only) to ensure that it is indeed the potential flow solution, and only the potential flow solution that is causing the differences.

Best regards,

[tclblby]

Dear @jjonkman ,
Thanks for your advice. Im trying to test these cases you mentioned but there are some problems:
if I set PotMod = 0 (0: none=no potential flow): FAST_InitializeAll:HydroDyn_Init:HydroDynInput_ProcessInitData:A member cannot have PropPot set to TRUE if PotMod = 0 in the FLOATING PLATFORM section.
But I must keep two joints and one member in openfast v2.0.0.
If I set PotMod = 1 and RdtnMod = 0, it will influence the mooring's results, as shown in the first figure below. I test them in Openfast v3.5.2.
Could you give me some suggestions on the settings?
By the way, the second picture is the results under damping from mooring only but PotMod = 1 and RdtnMod = 0. It looks strange too, they started the same but later became different.

Best regards,

[jjonkman]

Dear @tclblby,

I would not set PotMod = 0; otherwise, you'll have to treat all hydrodynamic terms via the strip-theory solution, which is very different than you have been doing, which is undesirable. With PotMod = 1 with RdtnMod = 0, you will also eliminate radiation added mass, which will change the natural frequency and is also undesirable. To disable only the wave-radiation damping, I would keep PotMod = 1 with RdtnMod = 1, but set RdtnTMax = 0 s, which will eliminate the damping through the radiation memory effect.

Best regards,

[tclblby]

Dear @jjonkman,
Thank you for your continuous attention and answers. There are some new results i got.
Firstly, I test with damping from mooring only. There is a surprising difference here, as shown in fig.1. It seems that a relatively large damping was suddenly introduced during the calculation in Openfast v2.0.0. I suppose it may be caused by the version of MoorDyn. Openfast v2.0.0 uses MoorDyn v1, but Openfast v3.5.2 uses MoorDyn v2.
Therefore, in the second test I changed the calculation method of mooring to map++ while keeping other conditions unchanged. As shown in fig.2, they show good agreement.
Based on the second test, I added the viscous drag from the strip-theory solution (joints, members). They still show good agreement in fig.3.
Finally, I added the radiation memory effect to the test. I believed they would also exhibit good agreement, but I was wrong. The result is shown in fig.4. The differences in decay amplitude appear again.
In summary, there are two problems that still confuse me here.
Why do the calculation results of Moordyn v1 and Moordyn v2 differ as shown in Fig.1? The settings of Moordyn are as depicted in Fig.5. Although some parameters have been renamed, their meanings and values remain the same according to the Moordyn user manual.
From all the results, it appears that openfast v2.0.0 and openfast v3.5.2 have calculated the same added mass based on the same WAMIT file, but there are some differences in damping. Is this possible? Is there any way to correct this result? By the way, the decay periods under different calculation conditions are shown in Fig.6.

     Best regards,

[jjonkman]

Dear @tclblby,

Thanks for running these additional tests, which helps isolate the issue.

Regarding Figure 2, I still see damping, but MAP++ is a quasi-static mooring module that does not add damping. Perhaps this case still has the additional linear damping (AddBLin)?

I'm not sure why MoorDyn v1 and v2 produce different damping levels in Figure 1. Hopefully a MoorDyn expert can comment.

Regarding differences in the wave-radiation calculation, perhaps the following improvement made in HydroDyn v3.5.1 is the source of the change: #1806? If so, my guess is those differences would go away if you compared v2.0.0 with v3.5.0 (but v3.5.2 would be more accurate).

Best regards,

[tclblby]

Dear @jjonkman,
Thank you for your guidance. You are absolutely right — in the test using MAP++ alone, there should theoretically be no damping. The damping observed in the previous test actually originated from the influence of added damping in other degrees of freedom (heave, roll, and pitch) on the surge motion. Previously, since the motion amplitudes in those directions were very small in the surge decay case, I assumed their effect on surge would be negligible and therefore overlooked the presence of added damping in those directions. This was my mistake. As shown in the new test results in Fig. 1, added damping in other directions can indeed have a significant impact.

Additionally, I downloaded OpenFAST v3.5.0 and compared its simulation results with those from OpenFAST v2.0.0. The two show good agreement, as illustrated in Fig. 2. This confirms that the differences previously observed in the wave-radiation calculation indeed stem from the change of HydroDyn v3.5.1#1806.

The cause of the result discrepancies due to different MoorDyn versions is still under investigation.

Once again, thank you very much for your recent guidance and assistance.

Best regards,

[jjonkman]

Dear @tclblby,

I'm glad to hear that the changes in wave-radiation damping are now understood!

I've forwarded your MoorDyn question along to the MoorDyn experts.

Best regards,

[RyanDavies19]

Hi @tclblby,

I don't remember the exact change that would cause these damping differences (@mattEhall might). In general, MoorDyn v2 is significantly improved from v1 and should be taken as the correct result when comparing the two.

If you need to track down the specific PR that caused this damping change, the line tensions can be informative.