Large Pitch/Roll in Multi-Body Model

[Haoxuan.Shen]

Dear Community,

I modeled a multi-body floating wind turbine platform (Nbody=5) for the first time. I calculated the potential flow files (PotFile) using AQWA, setting the center of gravity (COG) of each body to Z=0 in AQWA. The center of buoyancy(COB) was automatically calculated by AQWA. To avoid double counting, I followed the suggestion in the following forum post and modified the platform center of gravity position: hydrodynamic model for semisubmersible FOWT - #7 by Jason.Jonkman
Below is a schematic diagram of the model and the calculation of AQWA. The blue cross marks indicate the center of buoyancy positions, and the black squares indicate the center of gravity positions:

I currently have the following questions for advice:

First, when I ran the simulation with the above settings, the platform experienced large pitch and roll displacements, as shown below, what could be causing these large displacements?

Second, for a multi-body model, does ElastoDyn also require separate center of gravity values for each floating body (as in HydroDyn), or is it sufficient to define only one overall platform center of gravity?

My relevant HydroDyn settings are as follows:

---------------------- FLOATING PLATFORM ---------------------------------------
1 PotMod - Potential-flow model
1 ExctnMod - Wave-excitation model
1 ExctnDisp - Method of computing Wave Excitation
10 ExctnCutOff - Cutoff frequency (Hz)
0 PtfmYMod - Model for large platform yaw offset
0 PtfmRefY - Platform reference yaw offset (deg)
10 PtfmYCutOff - Cutoff frequency (Hz)
36 NExctnHdg - Number of platform yaw angles
1 RdtnMod - Radiation memory-effect model
80 RdtnTMax - Analysis time for radiation kernels (sec)
“DEFAULT” RdtnDT - Time step for radiation kernels (sec)
5 NBody - Number of WAMIT bodies
1 NBodyMod - Body coupling model
“../../HydroData/study” PotFile - Root name of potential-flow data
1 WAMITULEN - Characteristic body length scale (m)
-23.75 -23.75 0 47.54 0 PtfmRefxt - xt offset of body reference points (m)
-41.14 41.14 0 0 0 PtfmRefyt - yt offset of body reference points (m)
-14 -14 -20 -14 -10 PtfmRefzt - zt offset of body reference points (m)
0 0 -15 0 0 PtfmRefztRot - Rotation about zt (deg)
4277 4277 13975 4277 839 PtfmVol0 - Displaced volume (m^3)
0 0 0 0 0 PtfmCOBxt - xt offset of COB (m)
0 0 0 0 0 PtfmCOByt - yt offset of COB (m)

My relevant Elastodyn settings are as follows:

---------------------- TURBINE CONFIGURATION -----------------------------------
3 NumBl - Number of blades
149.693 TipRad - Blade tip radius (m)
2.443 HubRad - Blade root radius (m)
-3 PreCone(1) - Blade 1 cone angle (deg)
-3 PreCone(2) - Blade 2 cone angle (deg)
-3 PreCone(3) - Blade 3 cone angle (deg)
0 HubCM - Hub mass offset (m)
0 UndSling - Undersling length (m)
0 Delta3 - Delta-3 angle (deg)
0 AzimB1Up - Azimuth for blade 1 up (deg)
-8.08262 OverHang - Overhang length (m)
0.0 ShftGagL - Shaft strain gage location (m)
-8 ShftTilt - Rotor shaft tilt (deg)
2.361 NacCMxn - Nacelle CM x-offset (m)
0.014 NacCMyn - Nacelle CM y-offset (m)
2.113 NacCMzn - Nacelle CM z-offset (m)
2.361 NcIMUxn - Nacelle IMU x-offset (m)
0.014 NcIMUyn - Nacelle IMU y-offset (m)
2.113 NcIMUzn - Nacelle IMU z-offset (m)
2.675117 Twr2Shft - Tower-top to shaft vertical distance (m)
168.9 TowerHt - Tower height (m)
18.2 TowerBsHt - Tower base height (m)
0.0686 PtfmCMxt - Platform CM x-offset (m)
0 PtfmCMyt - Platform CM y-offset (m)
-11.774 PtfmCMzt - Platform CM z-offset (m)
0 PtfmRefzt - Platform reference point z-offset (m)

Best regards,

[Jason.Jonkman]

Dear @Haoxuan.Shen,

It sounds like you are using five potential-flow bodies with full hydrodynamic coupling in HydroDyn but are modeling the substructure rigidly in ElastoDyn. The multi-body capability of HydroDyn is meant to be used in conjunction with the structural dynamics modeled in SubDyn to account for hydro-elasticity. If the substructure is modeled rigidly in ElastoDyn, all hydrodynamic loads will get lumped to the single rigid body anyway, so, there is no real benefit of using the five potential-flow bodies with full hydrodynamic coupling, and a single potential-flow body would suffice.

Regarding your first question, I’m not sure but if you represent the entire floater as a single potential-flow body, do you get the response you expect? If so, this would suggest an issue with how you’ve defined the multiple bodies or their coupling.

Regarding your second question, ElastoDyn is treating the substructure as a single rigid body, so, only one lumped center of gravity is needed. Additional structural details would be needed if you decide to represent the substructure structural dynamics in SubDyn.

Best regards,

[Haoxuan.Shen]

Dear @Jason.Jonkman,

Thank you for your response. You are absolutely correct—I am currently still modeling the substructure as rigid in ElastoDyn. As this is my first attempt at multi-body hydrodynamics modeling, my plan is to first debug HydroDyn before enabling SubDyn for further adjustment. My ultimate goal is to calculate the stress at specified cross-sections.

To follow up on your question regarding my first issue**:** When using a single potential-flow body, I obtained the expected response. However, after switching to multiple bodies in HydroDyn (only the HydroDyn file was modified, with all other files kept unchanged. The specific configurations in hydrodyn were provided in my original post), the simulation runs initially, and the results for the first few tens of seconds remain consistent with the single-body model. Yet after a period of time, the displacement increases abnormally and the hydrodynamic response begins to diverge.

Therefore, I would like to ask for further guidance: How should HydroDyn be configured correctly in this context, especially in the FLOATING PLATFORM section? Could the positions of the reference point and the center of buoyancy (COB) play a critical role in the motion response?

Additionally, I have a few related questions:

1. Could the fact that SubDyn is not enabled (with the structure remaining rigid) be the direct cause of this diverging displacement issue?

2. Where can I find an example model of a semi-submersible platform that uses multi-body HydroDyn coupled with SubDyn? I haven’t been able to locate a relevant case study so far. Additionally, would such a modeling approach enable me to achieve my ultimate goal of calculating stress at cross-sections?

I will provide the relevant figures separately for your reference. Thank you again for your time and support.

Best regards,