Potential Flow - Strip Theory hybrid model

Dear all,

I am trying to make a hybrid Potential Flow and Strip Theory model for a semisub platform in OpenFAST. To this end, I am adding Strip Theory elements to the original Potential Flow model (WAMIT output files) in Hydrodyn, to model viscous effects on the platform. All Strip Theory element coefficients, except the drag coefficients, are set to 0.

I have substracted the submerged volume of the Strip Theory members to the submerged volume of the Potential Flow model (PtfmVol0). This way, the equilibrium position of the platform remains the same. However, I don’t know if I also have to correct the hydrostatic stiffness coefficients of the Potential Flow model in the hybrid version.

I have made some FreeDecays to compare the hybrid model and the Potential Flow model (setting the drag coefficients of the Strip Theory elements in the hybrid model to 0), and the results suggest that I don’t have to correct any hydrostatic coefficients in the hybrid model. However, logic tells me that this is what should be done.

Many thanks in advance.

Best regards,

NicolĂĄs Deza

Dear NicolĂĄs Deza,

HydroDyn will calculate buoyancy, etc. for any strip-theory member marked with PropPot = False. You should update the additional linear stiffness matrix with the hydrostatic restoring associated with these strip-theory members. More information is provided in the draft HydroDyn User’s Guide and Theory Manual.

Best regards,

Dear all,

I have a query regarding the different theories in the HydroDyn module. According to the HydroDyn manual when applying potential-flow theory-only there should not be any joints or members. I have attempted to remove the members and joints in the OC4 Semi file but I haven’t managed to make it run without any members.

So I was wondering, if applying potential-flow theory only can be achieved also by just setting the drag coefficients to zero and setting PropPot=TRUE to all members?
In this case however, the results of the free decay tests do not agree with hybrid and only strip solution.

Thanks in advance.

Best,
Dimitra Karystinou

Dear @Dimitra.Karystinou,

Yes, to model with potential-flow only, you should be able to run HydroDyn without any joints or members in the strip-theory solution. Can you clarify what issue you run into when you eliminate all joints and members?

I agree that the alternative is to set all drag coefficients to zero and set PropPot = True for all members; this should be equivalent to not having a strip-theory solution, albeit with more computational expense than the recommended approach.

I would not expect a potential flow-only solution to provide identical results to a solution with viscous drag, unless the viscous drag effects are negligible for the specific floater you are simulating.

Best regards,

Dear Jason,
Thank you for the quick response. When removing the members and joints I get the following error:


I use the 5MW_OC4Semi_WSt_WavesWN files, with openFAST 3.3.0 with MAP (but I get the same with MoorDyn as well). In the HydroDyn I removed all from the Member Joints, Member Cross-section properties, Member-based coeff, members and filled members. I don’t if I should change something else or if I am ignoring something that I should have considered.

Kind regards,
Dimitra

Dear @Dimitra.Karystinou,

Thanks for clarifying. Now I understand what is happening. The strip-theory solution is used both for external hydrodynamic loads (buoyancy, fluid inertia, viscous, added mass) due to water external to the member, as well as internal hydrodynamic loads (negative buoyancy, inertial) due to ballasted/flooded/filled members. When you eliminate the strip-theory joints and members, you are also eliminating the water ballast, which makes the OC4-DeepCwind semisubmersible hydrostatically unstable. If you want to eliminate the strip-theory solution, you should ensure that the mass/inertia of the water ballast is accounted for in the structural model.

Best regards,

1 Like

Dear @Jason.Jonkman ,

Following your advice I managed to eliminate the strip theory by removing the members and adjusting the mass and inertia of the platform to include the ballast based on the OC4 definition.

The results that I am getting when comparing the three different theories (for surge free decay) are the following and I was wondering if they are what I should expect. Without the strip theory and therefore the viscous effects the damping ratio is very low (five times lower than that of hybrid) also there is a small difference between the natural frequencies (105s and 99s respectively).

Thanks in advance.
Kind regards,
Dimitra

Dear @Dimitra.Karystinou,

Yes, I would expect less damping in the potential-flow only solution. You could address through by approximating the viscous contribution through an additional linear (AddBLin) or quadratic drag (AddBQuad) matrix.

I’m not sure I fully understand the change in natural frequency, but perhaps the added mass from the strip theory solution did not fully match that captured by the potential-flow solution in the surge direction.

Best regards,

1 Like

Dear jason,

I currently have a similar problem, I am using the Openfast to explore the motion of the OC3 system in irregular waves. When calculating hydrodynamic loads, I only want to use potential-flow theory, so I set “PotMod=1” and eliminate all joints and members. However, when I looked at Openfast’s calculation results, I found that even if I eliminated all joints and members, the effects of strip-theory still remained, which made the calculation result of total integrated hydrodynamic load (HydroFxi,HydroFyi,HydroFzi,HydroMxi,HydroMyi,HydroMzi) different from the First-order wave excitation force from WAMIT(B1WvsFxi,B1WvsFyi,B1WvsFzi,B1WvsMxi,B1WvsMyi,B1WvsMzi), the difference is as follows:


------- HydroDyn v2.03.* Input File --------------------------------------------
NREL 5.0 MW offshore baseline floating platform HydroDyn input properties for the OC3 Hywind.
False            Echo           - Echo the input file data (flag)
---------------------- ENVIRONMENTAL CONDITIONS --------------------------------
     "default"   WtrDens        - Water density (kg/m^3)
     "default"   WtrDpth        - Water depth (meters)
     "default"   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 ---------------------------------------------------
             3   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]
          100   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]
             2   WaveHs         - Significant wave height of incident waves (meters) [used only when WaveMod=1, 2, or 3]
            10   WaveTp         - Peak-spectral period of incident waves       (sec) [used only when WaveMod=1 or 2]
"DEFAULT"        WavePkShp      - Peak-shape parameter of incident wave spectrum (-) or DEFAULT (string) [used only when WaveMod=2; use 1.0 for Pierson-Moskowitz]
             0   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   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]
             0   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] 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]
           3.5   WvHiCOffD      - High frequency cutoff used in the difference-frequencies (rad/s) [Only used with a difference-frequency method]
           0.1   WvLowCOffS     - Low  frequency cutoff used in the summation-frequencies  (rad/s) [Only used with a summation-frequency  method]
           3.5   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)
             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>0; 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!]
        0.0125   RdtnDT         - Time step for wave radiation kernel calculations (sec) [only used when PotMod=1 and ExctnMod>0 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]
"../5MW_Baseline/HydroData/Spar"    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]
       8029.21   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 6*NBody 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)
             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      98340000
        100000             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 6*NBody x 6*NBody matrix; if NBodyMod>1, NBody size 6 x 6 matrices]
             0        100000             0             0             0             0
             0             0        130000             0             0             0
             0             0             0             0             0             0
             0             0             0             0             0             0
             0             0             0             0             0      13000000
             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 6*NBody 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 --------------------------------------
             0   NAxCoef        - Number of axial coefficients (-)
AxCoefID  AxCd     AxCa     AxCp
   (-)    (-)      (-)      (-)
---------------------- MEMBER JOINTS -------------------------------------------
             0   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)
---------------------- MEMBER CROSS-SECTION PROPERTIES -------------------------
             0   NPropSets      - Number of member property sets (-)
PropSetID    PropD         PropThck
   (-)        (m)            (m)
---------------------- SIMPLE HYDRODYNAMIC COEFFICIENTS (model 1) --------------
     SimplCd    SimplCdMG    SimplCa    SimplCaMG    SimplCp    SimplCpMG   SimplAxCd  SimplAxCdMG  SimplAxCa  SimplAxCaMG  SimplAxCp   SimplAxCpMG
       (-)         (-)         (-)         (-)         (-)         (-)         (-)         (-)         (-)         (-)         (-)         (-)
       0.0        0.00        0.00        0.00        0.00        0.00        0.00        0.00        0.00        0.00        0.00        0.00 
---------------------- 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) --------
             0   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
   (-)         (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)
-------------------- MEMBERS -------------------------------------------------
             0   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)
---------------------- 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]
"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!]
---------------------- OUTPUT CHANNELS -----------------------------------------
"Wave1Elev"               - Wave elevation at the platform reference point (0,  0)
"B1Surge"
"B1Sway"
"B1Heave"
"B1Roll"
"B1Pitch"
"B1Yaw"
"HydroFxi"
"HydroFyi"
"HydroFzi"
"HydroMxi"
"HydroMyi"
"HydroMzi"
"B1WvsFxi"
"B1WvsFyi"
"B1WvsFzi"
"B1WvsMxi"
"B1WvsMyi"
"B1WvsMzi"
"B1WvsF1xi"
"B1WvsF1yi"
"B1WvsF1zi"
"B1WvsM1xi"
"B1WvsM1yi"
"B1WvsM1zi"
END of output channels and end of file. (the word "END" must appear in the first 3 columns of this line)

My hydrodyn file is also uploaded, I want the hydrodynamic load to contain only the wave excitation force, so that the results in the figure above are exactly the same. I wonder why the effects of the strip-theory still persist after all joints and members have been eliminated? How to set up the hydrodyn file to completely eliminate the influence of the strip-theory? I would appreciate it if you could give me some advice.

Best regards
LinYang

Dear @Lin.Yang2,

You’ve disabled the strip-theory solution by eliminating all joints and members. But your model still has hydrodynamic loads other than wave excitation (ExctnMod = 1) in the potential-flow solution. I also see that you’ve enabled wave-radiation (RdtnMod = 1) in the potential-flow solution, as well as addition platform stiffness and linear damping (AddCLin, AddBLin). HydroDyn output B1WvsFxi (etc.) will only contain the loads from ExctnMod = 1 whereas HydroDyn output HydroFxi (etc.) will contain the full contribution of all loads from HydroDyn.

Best regards,

Dear jason,

Thanks for your quick reply, through your explanation, I understand the reason for the difference in the above figure. Even if I set RdtnMod, AddCLin and AddBLin to 0, the above figure will still have a difference, which is due to the existence of radiation force. Thank you again for your kind help!

Best regards
LinYang

Hi Jason, I am also trying to use a hybrid approach (PF + Morison).
I am using strip theory to include the viscous effects. I have defined the MemberCd values in the hydrodyn input. Does this help include viscous effects in all 6 DOFs of motion? Or in heave, roll and pitch atleast?

Second order wave effects are important for me, so if I used the strip theory solution only, will hydrodyn calculate heave, roll and pitch accurately?

Best regards,
Rahul.

Dear @Rahul.Ramachandran,

The strip-theory viscous forces are computed normal to the member (for transverse drag) and axially along the member (for tapered members and end/joint effects). The direction of these forces in terms of six DOF motion will depend on the orientation of the members.

The strip-theory solution assumes long wavelengths relative to the size of the structure, and so, works better for thin members and low frequency (long wavelength) waves. The strip-theory solution will not account for diffraction, scattering, etc. (if important for your substructure).

Best regards,

Hi Jason,

The platform has vertical columns only.

I have set up a hybrid approach. ie. PotMod = 1, ExctnMod and RdtnMod set to 1, use full sum & difference qtfs, PropPot=True for all columns. Will this result in a ‘double calculation’ ? or the Wamit based solver with take care of diffraction, radiation etc. and strip theory will take care of the viscous effects?

I hope my question is clear.

Thank you,
Rahul.

Dear @Rahul.Ramachandran,

With PropPot = TRUE in HydroDyn, there is no double counting of terms. The potential-flow (WAMIT) solution will cover diffraction, radiation, and hydrostatic effects and the strip-theory (Morison) solution will cover viscous effects.

Best regards,

Dear Jason,

Thanks for the help.

Best regards,
Rahul