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QBlade ForumQBlade: Structural DynamicsSubstructure Modelling

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Substructure Modelling

#1 · 15. May 2025, 01:22

Hello.

I am facing difficulties modelling and simulating a new type of substructure. I used the IEA 15 MW VolturnUS-S substructure definition file as a base and edited it. But it is not behaving as expected. Can you please check the pictures of my structural definition file?

Note that I kept the 6*6 hydrodynamic matrices (SUB_MASS, SUB_HYDROQUADDAMPING, SUB_HYDROSTIFFNESS) as it is for the IEA 15 MW Volturn. I modified the other parts as per my requirement. In my model, the forward strut (M_1) under the tower displaces as much weitght as the RNA, tower and forward strut combine, i.e., they neutralize each other. M-5,6,7 struts and M- 8-13 risers have neutral buoyancy. M-2,3,4 are floats with positive buoyancy and M-14,15,16 are stabilizers with negative buoyancy. The floats and the stabilizers neutralize each other. So, they should float at a constant draft in still water. It is also to be noted that the risers can rotate around all the 3 axes, i.e., like they are connected to the M-5 Strut with U-joints.

The problems—

The floats and stabilizers do not seem to work properly. They exhibit a quiet a good amount of pitch motion rearward in both still water and wavy conditions (JONSWAP_8.1m_12.7s_Unidirectional). This seems so incorrect. And the forward part of the system shows almost no motion as if somehow the z-motion is locked for the forward part.
What are the uses of the 6*6 hydrodynamic matrices (SUB_MASS, SUB_HYDROQUADDAMPING, SUB_HYDROSTIFFNESS)? Are they used only for the potential flow theory or for the Morrison theory too? How can I get the matrices for my structure? Can these be calculated automatically by Qblade?
How to turn off the Potential theory calculation? Can I use only the Morrison Calculation? If I can, then how to do it?
Is it possible to simulate the substructure accurately with Morrison’s theory only?

Hoping for your quick reply.

Sincerely

Labib Mahmud

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#2 · 15. May 2025, 15:12

Quote from David on 15. May 2025, 15:12
Hi Labib,
first of all, the model you’ve built already looks quite impressive!
Regarding your questions: it’s difficult to determine exactly why the substructure behaves in a similar way just by looking at the input files. However, I can certainly address your more general questions:
The 6×6 matrices for mass, damping, etc., are primarily used in combination with potential flow theory-based hydrodynamic modeling. In this approach, hydrodynamic forces are applied at a single point rather than being distributed along the structure. As a result, the floating substructure is often modeled as a rigid body, with mass and damping properties represented in an integrated form, lumped at a single point. For example: the mass matrix represents the total mass and inertia of the entire substructure, acting at the center of gravity. These matrices can be derived analytically or extracted from CAD software that contains the detailed geometry and mass distribution of the floater.
If you want to disable potential flow forces, you can set the parameters USE_RADIATION, USE_EXCITATION, and USE_SUM_FREQS to false, and set DIFF_EVAL_TYPE to 0.
In contrast, Morison theory models hydrodynamic forces as distributed over the substructure. However, selecting appropriate Morison coefficients requires careful calibration and, ideally, validation against experimental or high-fidelity numerical data. While Morison theory can provide accurate results for simple, slender structures and in flow regimes dominated by drag and inertia forces, its applicability is generally limited to cases where the body dimensions are small compared to the wavelength of incident waves (i.e., when the structure is “slender”). This typically corresponds to small Keulegan–Carpenter (KC) number values. Under such conditions, hydrodynamic interactions between different parts of the structure are minimal, and the local flow field can be reasonably approximated as unaffected by the body’s presence.
Happy modeling!
Best regards,
David

Hi Labib,

first of all, the model you’ve built already looks quite impressive!

Regarding your questions: it’s difficult to determine exactly why the substructure behaves in a similar way just by looking at the input files. However, I can certainly address your more general questions:

The 6×6 matrices for mass, damping, etc., are primarily used in combination with potential flow theory-based hydrodynamic modeling. In this approach, hydrodynamic forces are applied at a single point rather than being distributed along the structure. As a result, the floating substructure is often modeled as a rigid body, with mass and damping properties represented in an integrated form, lumped at a single point. For example: the mass matrix represents the total mass and inertia of the entire substructure, acting at the center of gravity. These matrices can be derived analytically or extracted from CAD software that contains the detailed geometry and mass distribution of the floater.

If you want to disable potential flow forces, you can set the parameters USE_RADIATION, USE_EXCITATION, and USE_SUM_FREQS to false, and set DIFF_EVAL_TYPE to 0.

In contrast, Morison theory models hydrodynamic forces as distributed over the substructure. However, selecting appropriate Morison coefficients requires careful calibration and, ideally, validation against experimental or high-fidelity numerical data. While Morison theory can provide accurate results for simple, slender structures and in flow regimes dominated by drag and inertia forces, its applicability is generally limited to cases where the body dimensions are small compared to the wavelength of incident waves (i.e., when the structure is “slender”). This typically corresponds to small Keulegan–Carpenter (KC) number values. Under such conditions, hydrodynamic interactions between different parts of the structure are minimal, and the local flow field can be reasonably approximated as unaffected by the body’s presence.

Happy modeling!

Best regards,

David

Labib Mahmud has reacted to this post.

#3 · 15. May 2025, 22:28

Thanks a lot, Mr. David for your precious support. I really appreciate it.

I have some more question:

Can I model buoyancy explicitly within subelementsrigid and use it into the potential flow calculation without creating the sub_mass matrix?
pot_rad_file, pot_exc_file, pot_diff_file and pot_sum_file— are these the same for all structures or they are different? Do I need to calculate them for different structures? Can these be generated from NEMOH?
Qblade manual states that subdividing cylindrical elements into smaller subelements increases accuracy for morrison calculation. Then what is the purpose of advanced buoyancy?
For only Morrison calculation, shall the staticbuoyancy be set to false? And for morrison calculation in conjuction with potential flow calculation, shall wavekineval_mor and wavekineval_pot be set to 1?
The potential flow theory consists of six matrices and two solvers. Do I need all of them for an accurate simulation? Note that, I am mostly concerned with the turbine motion and loads on the bearing of the blades.
The simulation results seem to me a bit complex for me. The parameters are stated in short form. How roll, pitch, yaw, heave, surge, sway, absolute vertical, absolute horizontal and absolute transverse motion are defined in Qblade? Is there any glossary for the short terms used in Qblade? I have searched but could not manage to find one.

With best regards

Labib Mahmud

#4 · 15. May 2025, 22:58

And another question I would like to add to this—

Is the McCamy-Fuchs Correction applicable to axial Morrison force too? If so, then how to calculate the ratio? I can get the wavelength for normal flow but how can I get wavelength for the axial flow?

#5 · 16. May 2025, 10:44

Quote from David on 16. May 2025, 10:44
Hi Labib,
Buoyancy can be included either via the 6×6 lumped hydrodynamic stiffness matrix or as a distributed force applied to each member. A combination of both approaches is also possible.
These hydrodynamic input files depend on the substructure geometry and can be generated using tools like WAMIT, NEMOH, or similar. This process typically requires meshing the substructure geometry.
The spatial discretization of substructure members is defined in the member table. Wave kinematics are evaluated at each sub-member, so increasing the member resolution resolves changes in wave kinematics along the member length. The advanced buoyancy feature is unrelated to wave kinematics; instead, it enhances the accuracy of buoyancy calculations for partially submerged and potentially tilted members.
When static buoyancy is enabled, buoyancy is evaluated based on the mean sea elevation, rather than the local, time-varying sea surface. This avoids double-counting sea elevation effects already included in potential flow wave excitation forces. If you’re using only Morison elements, this feature can be disabled. The wave kinematics evaluation for Morison elements can be set to “local.”
The modeling approach depends entirely on the designer’s choices and the substructure topology. Multiple valid approaches exist, and each can yield accurate results if applied appropriately.
For an overview of coordinate systems and conventions used in QBlade, refer to:
https://docs.qblade.org/src/user/index_ue.html#coordinate-systems-and-conventions
The McCamy-Fuchs correction applies only to the normal Morison forces acting on a member, not to axial forces. In your current model, no axial Morison coefficients are defined.
Hydrodynamic modeling of substructures is complex and requires informed decisions on many parameters. QBlade provides powerful tools to handle this, and with some experience and careful setup, you can achieve accurate and reliable results. It’s a challenging process and steep learning curve, but very much within reach.
BR,
David

Hi Labib,

Buoyancy can be included either via the 6×6 lumped hydrodynamic stiffness matrix or as a distributed force applied to each member. A combination of both approaches is also possible.
These hydrodynamic input files depend on the substructure geometry and can be generated using tools like WAMIT, NEMOH, or similar. This process typically requires meshing the substructure geometry.
The spatial discretization of substructure members is defined in the member table. Wave kinematics are evaluated at each sub-member, so increasing the member resolution resolves changes in wave kinematics along the member length. The advanced buoyancy feature is unrelated to wave kinematics; instead, it enhances the accuracy of buoyancy calculations for partially submerged and potentially tilted members.
When static buoyancy is enabled, buoyancy is evaluated based on the mean sea elevation, rather than the local, time-varying sea surface. This avoids double-counting sea elevation effects already included in potential flow wave excitation forces. If you’re using only Morison elements, this feature can be disabled. The wave kinematics evaluation for Morison elements can be set to “local.”
The modeling approach depends entirely on the designer’s choices and the substructure topology. Multiple valid approaches exist, and each can yield accurate results if applied appropriately.
For an overview of coordinate systems and conventions used in QBlade, refer to:
https://docs.qblade.org/src/user/index_ue.html#coordinate-systems-and-conventions
The McCamy-Fuchs correction applies only to the normal Morison forces acting on a member, not to axial forces. In your current model, no axial Morison coefficients are defined.

Hydrodynamic modeling of substructures is complex and requires informed decisions on many parameters. QBlade provides powerful tools to handle this, and with some experience and careful setup, you can achieve accurate and reliable results. It’s a challenging process and steep learning curve, but very much within reach.

BR,

David

Labib Mahmud has reacted to this post.