Current and Wave simulation

Tomas.Cortes · June 25, 2019, 12:26pm

Hi everybody,

I was performing OC4 load case 2.4 (current and regular waves) and a similar one but with the wave and current in the opposite direction with the semisubmersible. The idea was to evaluate if there is some kind of symmetrical behavior between the two. The only difference between the hydrodyn files is, thus, CurrSSDir and WaveDir. In one case they are both 0, in the other, they are both 180. The issue is that the results seem to be odd. I’ve attached some of the results below.

What do you think of them? I would say, for instance that that surge in the 180º case would be negative and the sway null… I don’t understand if it is correct and unexpected or if there’s some modelling mistake.

Best regards,

Tomás

Jason.Jonkman · June 25, 2019, 1:41pm

Dear Tomas,

I agree that these results really don’t make sense to me, but I’m not sure what is set incorrectly. From the surge response, it looks like the 180-degree waves and current, causes a reduction in mean surge, but there must still be some force pushing the semi in positive surge. Do you have aerodynamic loads enabled? And I would not expect any sway and roll motion in a simulation with waves and current only.

Best regards,

Tomas.Cortes · June 25, 2019, 10:28pm

Dear Jason,

Thanks for your reply. I do not have any wind in the simulation: CompInFlow=0.
In order to try to understand better what’s going on, I’ve run other two cases splitting the previous one: only current and only waves, both with CurrSSDir and WaveDirr at 180º. The results are below. Somehow it makes more sense (at least as current is concerned), though the wave effect is still not expected. But maybe that’s the way the platform reacts to the waves…

Jason.Jonkman · June 26, 2019, 2:08pm

Dear Tomas,

Are you using a floating platform model NREL has provided as an example (e.g., the OC4-DeepCwind semisubmersible) or a model you made yourself? Do you have second-order difference-frequency effects enabled (mean drift), which will induce a mean load for regular waves? I’d probably need to know more about your model set up to comment more.

Best regards,

Tomas.Cortes · June 26, 2019, 4:18pm

Dear Jason,

Yes, I’m using OC4-DeepCwind semisubmersible and without second-order difference-frequency effects enabled.

Here’s the hydrodyn file used:

------- HydroDyn v2.03.* NREL 5.0 MW offshore False Echo ---------------------- 1025 WtrDens 200 WtrDpth 0 MSL2SWL - ---------------------- 1 WaveMod - 0 WaveStMod - 4600 WaveTMax 0.2 WaveDT 6 WaveHs - 10 WaveTp 1.05 WavePkShp 0.314159 WvLowCOff 1.570796 WvHiCOff 180 WaveDir 0 WaveDirMod - 1 WaveDirSpread - 1 WaveNDir - 0 WaveDirRange - 123456789 WaveSeed(1) 1011121314 WaveSeed(2) FALSE WaveNDAmp “” WvKinFile 1 NWaveElev - 0 WaveElevxi - 0 WaveElevyi - ---------------------- FALSE WvDiffQTF FALSE WvSumQTF 0 WvLowCOffD - Low 1.256637 WvHiCOffD 0.618319 WvLowCOffS 3.141593 WvHiCOffS ---------------------- 0 CurrMod - 0.5 CurrSSV0 30 CurrSSDir 20 CurrNSRef 0 CurrNSV0 - 0 CurrNSDir - 0 CurrDIV - 0 CurrDIDir - ---------------------- 1 PotMod - “…/5MW_Baseline/HydroData 1 WAMITULEN - 13917 PtfmVol0 0 PtfmCOBxt - 0 PtfmCOByt - 1 RdtnMod - 60 RdtnTMax 0.0125 RdtnDT ---------------------- 0 MnDrift - 0 NewmanApp - 12 DiffQTF 12 SumQTF ---------------------- True PtfmSgF True PtfmSwF True PtfmHvF True PtfmRF True PtfmPF True PtfmYF ---------------------- 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 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ---------------------- 2 NAxCoef - AxCoefID AxCd AxCa AxCp
(-) (-) (-) (-)
1 0.00 0.00 1.00
2 9.60 0.00 1.00
---------------------- 44 NJoints JointID Jointxi Jointyi (-) (m) (m) 1 0.00000 0.00000 -20.00000 2 0.00000 0.00000 10.00000 3 14.43376 25.00000 -14.00000 4 14.43376 25.00000 12.00000 5 -28.86751 0.00000 -14.00000 6 -28.86751 0.00000 12.00000 7 14.43376 -25.00000 -14.00000 8 14.43376 -25.00000 12.00000 9 14.43375 25.00000 -20.00000 10 -28.86750 0.00000 -20.00000 11 14.43375 -25.00000 -20.00000 12 9.23760 22.00000 10.00000 13 -23.67130 3.00000 10.00000 14 -23.67130 -3.00000 10.00000 15 9.23760 -22.00000 10.00000 16 14.43375 -19.00000 10.00000 17 14.43375 19.00000 10.00000 18 4.04145 19.00000 -17.00000 19 -18.47520 6.00000 -17.00000 20 -18.47520 -6.00000 -17.00000 21 4.04145 -19.00000 -17.00000 22 14.43375 -13.00000 -17.00000 23 14.43375 13.00000 -17.00000 24 1.62500 2.81500 10.00000 25 11.43376 19.80385 10.00000 26 -3.25000 0.00000 10.00000 27 -22.87000 0.00000 10.00000 28 1.62500 -2.81500 10.00000 29 11.43376 -19.80385 10.00000 30 1.62500 2.81500 -17.00000 31 8.43376 14.60770 -17.00000 32 -3.25000 0.00000 -17.00000 33 -16.87000 0.00000 -17.00000 34 1.62500 -2.81500 -17.00000 35 8.43376 -14.60770 -17.00000 36 1.62500 2.81500 -16.20000 37 11.43376 19.80385 38 -3.25000 0.00000 -16.20000 39 -22.87000 0.00000 40 1.62500 -2.81500 -16.20000 41 11.43376 -19.80385 42 14.43376 25.00000 -19.94000 43 -28.86751 0.00000 -19.94000 44 14.43376 -25.00000 -19.94000 ---------------------- 4 NPropSets - PropSetID PropD PropThck
(-) (m) (m)
1 6.50000 0.03000 2 12.00000 0.06000 3 24.00000 0.06000 4 1.60000 0.01750 ---------------------- SimplCd SimplCdMG (-) (-) (-) 0.00 0.00 0.00 ---------------------- 0 NCoefDpth Dpth DpthCd DpthCdMG (m) (-) (-) ---------------------- 25 NCoefMembers MemberID MemberCd1 (-) (-) (-) 1 0.56 0.56 2 0.61 0.61 3 0.61 0.61 4 0.61 0.61 5 0.68 0.68 6 0.68 0.68 7 0.68 0.68 23 0.68 0.68 24 0.68 0.68 25 0.68 0.68 8 0.63 0.63 9 0.63 0.63 10 0.63 0.63 11 0.63 0.63 12 0.63 0.63 13 0.63 0.63 14 0.63 0.63 15 0.63 0.63 16 0.63 0.63 17 0.63 0.63 18 0.63 0.63 19 0.63 0.63 20 0.63 0.63 21 0.63 0.63 22 0.63 0.63 -------------------- 25 NMembers MemberID MJointID1 (-) (-) (-) 1 1 2 2 3 4 3 5 6 4 7 8 5 42 3 6 43 5 7 44 7 23 9 42 24 10 43 25 11 44 8 12 13 9 14 15 10 16 17 11 18 19 12 20 21 13 22 23 14 24 25 15 26 27 16 28 29 17 30 31 18 32 33 19 34 35 20 36 37 21 38 39 22 40 41 ---------------------- 2 NFillGroups FillNumM FillMList (-) (-) 3 2 3 4 -6.17 3 5 6 7 -14.89 ---------------------- 0 NMGDepths - MGDpth MGThck MGDens
(m) (m) (kg/m^3)
---------------------- 0 NMOutputs - MemberID NOutLoc (-) (-) (-)
---------------------- 0 NJOutputs - 0 JOutLst ---------------------- True HDSum False OutAll 2 OutSwtch - “ES11.4e2” OutFmt “A11” OutSFmt ---------------------- “Wave1Elev” END of output channels Input File --------------------------------------------
baseline floating platform HydroDyn input properties for the OC4 Semi-submersible.
- Echo the input file data (flag)
ENVIRONMENTAL CONDITIONS --------------------------------
- Water density (kg/m^3)
- Water depth (meters)
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 ---------------------------------------------------
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)
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]
- Analysis time for incident wave calculations (sec) [unused when WaveMod=0; determines WaveDOmega=2Pi/WaveTMax in the IFFT]
- Time step for incident wave calculations (sec) [unused when WaveMod=0; 0.1<=WaveDT<=1.0 recommended; determines WaveOmegaMax=Pi/WaveDT in the IFFT]
Significant wave height of incident waves (meters) [used only when WaveMod=1, 2, or 3]
- Peak-spectral period of incident waves (sec) [used only when WaveMod=1 or 2]
- Peak-shape parameter of incident wave spectrum (-) or DEFAULT (string) [used only when WaveMod=2; use 1.0 for Pierson-Moskowitz]
- 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]
- 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]
- Incident wave propagation heading direction (degrees) [unused when WaveMod=0 or 6]
Directional spreading function {0: none, 1: COS2S} (-) [only used when WaveMod=2,3, or 4]
Wave direction spreading coefficient ( > 0 ) (-) [only used when WaveMod=2,3, or 4 and WaveDirMod=1]
Number of wave directions (-) [only used when WaveMod=2,3, or 4 and WaveDirMod=1; odd number only]
Range of wave directions (full range: WaveDir +/- 1/2*WaveDirRange) (degrees) [only used when WaveMod=2,3,or 4 and WaveDirMod=1]
- First random seed of incident waves [-2147483648 to 2147483647] (-) [unused when WaveMod=0, 5, or 6]
- Second random seed of incident waves [-2147483648 to 2147483647] (-) [unused when WaveMod=0, 5, or 6]
- Flag for normally distributed amplitudes (flag) [only used when WaveMod=2, 3, or 4]
- Root name of externally generated wave data file(s) (quoted string) [used only when WaveMod=5 or 6]
Number of points where the incident wave elevations can be computed (-) [maximum of 9 output locations]
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]
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]
- Full difference-frequency 2nd-order wave kinematics (flag)
- Full summation-frequency 2nd-order wave kinematics (flag)
frequency cutoff used in the difference-frequencies (rad/s) [Only used with a difference-frequency method]
- High frequency cutoff used in the difference-frequencies (rad/s) [Only used with a difference-frequency method]
- Low frequency cutoff used in the summation-frequencies (rad/s) [Only used with a summation-frequency method]
- High frequency cutoff used in the summation-frequencies (rad/s) [Only used with a summation-frequency method]
CURRENT ------------------------------------------------- [unused with WaveMod=6]
Current profile model {0: none=no current, 1: standard, 2: user-defined from routine UserCurrent} (switch)
- Sub-surface current velocity at still water level (m/s) [used only when CurrMod=1]
- Sub-surface current heading direction (degrees) or DEFAULT (string) [used only when CurrMod=1]
- Near-surface current reference depth (meters) [used only when CurrMod=1]
Near-surface current velocity at still water level (m/s) [used only when CurrMod=1]
Near-surface current heading direction (degrees) [used only when CurrMod=1]
Depth-independent current velocity (m/s) [used only when CurrMod=1]
Depth-independent current heading direction (degrees) [used only when CurrMod=1]
FLOATING PLATFORM --------------------------------------- [unused with WaveMod=6]
Potential-flow model {0: none=no potential flow, 1: frequency-to-time-domain transforms based on WAMIT output, 2: fluid-impulse theory (FIT)} (switch)
/marin_semi” 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!]
Characteristic body length scale used to redimensionalize WAMIT output (meters) [only used when PotMod=1]
- 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!]
The xt offset of the center of buoyancy (COB) from the platform reference point (meters) [only used when PotMod=1]
The yt offset of the center of buoyancy (COB) from the platform reference point (meters) [only used when PotMod=1]
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]
- 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!]
- 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]
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]
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]
- 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]
- 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]
- Platform horizontal surge translation force (flag) or DEFAULT
- Platform horizontal sway translation force (flag) or DEFAULT
- Platform vertical heave translation force (flag) or DEFAULT
- Platform roll tilt rotation force (flag) or DEFAULT
- Platform pitch tilt rotation force (flag) or DEFAULT
- Platform yaw rotation force (flag) or DEFAULT
PLATFORM ADDITIONAL STIFFNESS AND DAMPING --------------
0 0 0 AddF0 - Additional preload (N, N-m)
0 0 0 AddCLin - Additional linear stiffness (N/m, N/rad, N-m/m, N-m/rad)
0 0 0
0 0 0
0 1451298897 0 0
0 1451298897 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 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
AXIAL COEFFICIENTS --------------------------------------
Number of axial coefficients (-)
MEMBER JOINTS -------------------------------------------
- Number of joints (-) [must be exactly 0 or at least 2]
Jointzi JointAxID JointOvrlp [JointOvrlp= 0: do nothing at joint, 1: eliminate overlaps by calculating super member]
(m) (-) (switch)
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
2 0
2 0
2 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
1 0
9.13000 1 0
1 0
9.13000 1 0
1 0
9.13000 1 0
1 0
1 0
1 0
MEMBER CROSS-SECTION PROPERTIES -------------------------
Number of member property sets (-)
! Main Column
! Upper Columns
! Base Columns
! Pontoons
SIMPLE HYDRODYNAMIC COEFFICIENTS (model 1) --------------
SimplCa SimplCaMG SimplCp SimplCpMG SimplAxCa SimplAxCaMG SimplAxCp SimplAxCpMG
(-) (-) (-) (-) (-) (-) (-)
0.00 1.00 1.00 0.00 0.00 1.00 1.00
DEPTH-BASED HYDRODYNAMIC COEFFICIENTS (model 2) ---------
- Number of depth-dependent coefficients (-)
DpthCa DpthCaMG DpthCp DpthCpMG DpthAxCa DpthAxCaMG DpthAxCp DpthAxCpMG
(-) (-) (-) (-) (-) (-) (-) (-)
MEMBER-BASED HYDRODYNAMIC COEFFICIENTS (model 3) --------
- Number of member-based coefficients (-)
MemberCd2 MemberCdMG1 MemberCdMG2 MemberCa1 MemberCa2 MemberCaMG1 MemberCaMG2 MemberCp1 MemberCp2 MemberCpMG1 MemberCpMG2 MemberAxCa1 MemberAxCa2 MemberAxCaMG1 MemberAxCaMG2 MemberAxCp1 MemberAxCp2 MemberAxCpMG1 MemberAxCpMG2
(-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) ! Main Column
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 ! Upper Column 1
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 ! Upper Column 2
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 ! Upper Column 3
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 ! Base Column 1
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 ! Base Column 2
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 ! Base Column 3
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 ! Base column cap 1
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 ! Base column cap 2
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 ! Base column cap 3
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 ! Delta Pontoon, Upper 1
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 ! Delta Pontoon, Upper 2
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 ! Delta Pontoon, Upper 3
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 ! Delta Pontoon, Lower 1
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 ! Delta Pontoon, Lower 2
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 ! Delta Pontoon, Lower 3
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 ! Y Pontoon, Upper 1
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 ! Y Pontoon, Upper 2
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 ! Y Pontoon, Upper 3
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 ! Y Pontoon, Lower 1
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 ! Y Pontoon, Lower 2
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 ! Y Pontoon, Lower 3
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 ! Cross Brace 1
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 ! Cross Brace 2
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 ! Cross Brace 3
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 -------------------------------------------------
- Number of members (-)
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 1.0000 3 TRUE ! Main Column
2 2 1.0000 3 TRUE ! Upper Column 1
2 2 1.0000 3 TRUE ! Upper Column 2
2 2 1.0000 3 TRUE ! Upper Column 3
3 3 1.0000 3 TRUE ! Base Column 1
3 3 1.0000 3 TRUE ! Base Column 2
3 3 1.0000 3 TRUE ! Base Column 3
3 3 1.0000 3 TRUE ! Base column cap 1
3 3 1.0000 3 TRUE ! Base column cap 2
3 3 1.0000 3 TRUE ! Base column cap 3
4 4 1.0000 3 TRUE ! Delta Pontoon, Upper 1
4 4 1.0000 3 TRUE ! Delta Pontoon, Upper 2
4 4 1.0000 3 TRUE ! Delta Pontoon, Upper 3
4 4 1.0000 3 TRUE ! Delta Pontoon, Lower 1
4 4 1.0000 3 TRUE ! Delta Pontoon, Lower 2
4 4 1.0000 3 TRUE ! Delta Pontoon, Lower 3
4 4 1.0000 3 TRUE ! Y Pontoon, Upper 1
4 4 1.0000 3 TRUE ! Y Pontoon, Upper 2
4 4 1.0000 3 TRUE ! Y Pontoon, Upper 3
4 4 1.0000 3 TRUE ! Y Pontoon, Lower 1
4 4 1.0000 3 TRUE ! Y Pontoon, Lower 2
4 4 1.0000 3 TRUE ! Y Pontoon, Lower 3
4 4 1.0000 3 TRUE ! Cross Brace 1
4 4 1.0000 3 TRUE ! Cross Brace 2
4 4 1.0000 3 TRUE ! Cross Brace 3
FILLED MEMBERS ------------------------------------------
- Number of filled member groups (-) [If FillDens = DEFAULT, then FillDens = WtrDens; FillFSLoc is related to MSL2SWL]
FillFSLoc FillDens
(m) (kg/m^3)
1025
1025
MARINE GROWTH -------------------------------------------
Number of marine-growth depths specified (-)
MEMBER OUTPUT LIST --------------------------------------
Number of member outputs (-) [must be < 10]
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 ---------------------------------------
Number of joint outputs [Must be < 10]
- List of JointIDs which are to be output (-)[unused if NJOutputs=0]
OUTPUT --------------------------------------------------
- Output a summary file [flag]
- Output all user-specified member and joint loads (only at each member end, not interior locations) [flag]
Output requested channels to: [1=Hydrodyn.out, 2=GlueCode.out, 3=both files]
- Output format for numerical results (quoted string) [not checked for validity!]
- Output format for header strings (quoted string) [not checked for validity!]
OUTPUT CHANNELS -----------------------------------------
- Wave elevation at the platform reference point (0, 0)
and end of file. (the word “END” must appear in the first 3 columns of this line)

Best regards,

Tomás

Jason.Jonkman · June 27, 2019, 1:04pm

Dear Tomas,

Actually, I see that you do have the second-order difference-frequency and sum-frequency potential-flow effects enabled (DiffQTF = SumQTF = 12). This likely explains the mean offset in the surge direction.

I’m still not sure about sway and roll, but perhaps this is related to something else in your model, not set in HydroDyn. To start debugging, I would suggest to disable all platform structural degrees of freedom in ElastoDyn and output the total hydrodynamic loads from HydroDyn (HydroFxi - HydroMzi) to verify that the hydrodynamic loads are only in the surge, pitch, and heave directions.

Best regards,

Tomas.Cortes · June 27, 2019, 7:26pm

Dear Jason,

Indeed, I was just looking at the 2ND-ORDER WAVES part, so I missed 2ND-ORDER FLOATING PLATFORM FORCES…

So, I turned off both DiffQTF and SumQTF and the results are now as expected. I just have one doubt regarding this issue. When I have the waves heading at 0º, there’s almost no difference between having DiffQTF and SumQTF turned on or off. On the other hand, if the waves are heading at 180º, the difference is huge. Any hint on why this is happening?

Best regards,

Tomás

Jason.Jonkman · June 27, 2019, 7:57pm

Dear Tomás,

Did you change the second-order WAMIT data for the OC4-DeepCwind semi in any way (marin_semi.12d and marin_semi.12s)? The second-order WAMIT data that NREL has provided is only applicable to the zero-degree wave heading. HydroDyn should abort if you select a wave direction outside of the range of the WAMIT data, but there is a known bug in HydroDyn that has not yet been fixed related to the wave directions for the second-order potential-flow solution–see: github.com/OpenFAST/openfast/issues/9. My guess is you have not changed the second-order WAMIT data and–because of the bug–HydroDyn is not functioning properly when the wave direction is not zero, when it should have aborted in this case.

Best regards,

Dhaneesh.Machavarapu · June 23, 2020, 7:57pm

Sir,

I have simulated wave loading by modeling a sea state with significant wave height 5m and peak spectral period of 12.4 sec using the JONSWAP spectrum for bottom fixed monopile. And I observed that inertia force was more than drag force. But my structure is small compared to the wave.

if so Froude-Krylov theory should be used for calculating wave loads. Am I correct? But in hydrodyn manual relative form of Morison equation is there. I am not getting a clear view of the formulation of wave force calculation on bodies
What are the equations FAST is using for calculating water particle velocities and accelerations and wave forces on a bottom fixed and flexible monopile for a random wave?

Thank you.

Jason.Jonkman · June 23, 2020, 8:11pm

Dear Dhaneesh,

Here are my answers to your questions:

The strip-theory formulation of FAST’s HydroDyn hydrodynamics module uses the relative form of Morison equation, with extensions for end effects, tapered members, buoyancy, etc. The Froude-Krylov force is the integration of the undisturbed pressure field over the surface of the body, which is captured in Morison’s equation, based on the long wavelength approximation (i.e, the “1” in the (1+Ca) term, but HydroDyn uses a user-specified pressure coefficient, Cp, in place of “1”).
The first-order (linear) terms use Airy wave theory. If the second-order wave kinematics are enabled (added to the first-order terms using perturbation theory), these are implemented using analytical expressions developed by Sharma and Dean.

Best regards,

Rahul.Ramachandran · March 10, 2024, 5:47pm

Dear Jason,

Does hydrodyn consider/model wave-current interaction ? Are there any wave frequency limitations while using current and waves together in regular wave simulations?

Best regards,
Rahul.

Jason.Jonkman · March 11, 2024, 5:19pm

Dear @Rahul.Ramachandran,

HydroDyn currently superimposes wave kinematics and current through a simple vector summation. More sophisticated modeling of waves plus current, e.g., to account for the encounter frequency, is not considered. The superimposed wave plus current velocity is used in the strip-theory solution’s calculation of viscous-drag loads.

Best regards,

Rahul.Ramachandran · March 12, 2024, 6:35pm

Dear Jason,

Thank you for your answer.

Rahul.

Marwa.Mohamed · August 15, 2024, 11:45pm

I have a basic question please, why many papers didn’t consider the current in calculation of Design Load Cases according to IEC 61400-3 like (Loads Analysis of a Floating
Offshore Wind Turbine Using Fully Coupled Simulation) or IEA 15 MW, IEA 10 MW OR IEA 22 MW? is it due to that the current effect is fading after 20 m in the NCM when using near surface current model?
I mean in monopile offshore wind turbine, should the current be taken or not?

Regards,
Marwa

Jason.Jonkman · August 16, 2024, 4:10pm

Dear @Marwa.Mohamed,

The importance of current depends on the specific site conditions, as well as the support structure type. In research projects, sometimes current is neglected for simplicity or lack of site-specific current data.

Best regards,

Marwa.Mohamed · August 16, 2024, 6:05pm

Dear Dr. Jason,
Sorry for bothering you with my questions.
in IEC 61400-3

So if I want to include current in OpenFast, in HydroDyn CurrNSV0 - Near-surface current velocity at still water level (m/s) is Uw(0) and can be calculated from converting 10 min wind speed at 10 m above SWL to 1 hour wind speed at 10 m SWL by Durst curve. is this a right approah or not please?

Regards,
Marwa

Jason.Jonkman · August 16, 2024, 8:30pm

Dear @Marwa.Mohamed,

I’m not familiar with the Durst curve, but I would expect that you could assume a simple wind shear profile (such as the power law) to estimate the mean wind speed at 10-m height from the mean wind speed at hub height, which is specified elsewhere in OpenFAST (in InflowWind or TurbSim). HydroDyn does not currently have a built-in way of calculating CurrNSV0 from the wind speed specified in InflowWind.

Best regards,

Marwa.Mohamed · August 16, 2024, 10:06pm

Thanks Dr. Jason for your reply.
Yes that’s what I want to do, but Is the mean wind speed at hub heigt is 10 minute average wind speed or 1 hour mean wind speed?

Regards,
Marwa

Jason.Jonkman · August 16, 2024, 11:01pm

The mean wind speed entered into InflowWind or TurbSim is the mean wind speed over the simulation length, which is typically 10 minutes or 1 hour.

Regardless, except for extreme conditions (such as the extreme 10-minute mean wind speed with a recurrence period of 1- or 50-years), I would say that you could assume the 10 minute and 1 hour means are the same.

Best regards,

Marwa.Mohamed · August 16, 2024, 11:03pm

Thanks Dr. Jason For your responce.

Regards,
Marwa

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