Sudden change in the bending moment of the Floater member


Currently, I am doing the flexible floater modeling in OpenFAST. The floating type is semi-sub model. I used the SubDyn to output the center column internal loads. I found that the bending moment at center column suddenly change at the node at the water level. I wonder if this is caused by the Hydrodynamic loads. I used the hybrid hydrodynamic model (Potential flow+Strip theory member). From what I understand, hydrodynamic-applied point forces and moments at strip theory analysis nodes distributed along the substructure, while the radiation and diffraction effect are applied at the reference point of the floating body. In other word, the loads pre-calculated by Potential flow theory can only be applied at reference point of the floating body. My reference point for the Potential flow solution is at the water surface. Therefore, the sudden (step) change in the bending moment was observed at the water surface. Do I understand this correctly?

Best Regards,

Dear @Danupon.Subanapongm,

Sounds like a reasonable explanation, but you haven’t provided enough detail about your model set up to answer with certainty.

I agree with your statements, but please note that we typically match potential-flow bodies in HydroDyn with rigid bodies and SuDyn and strip-theory members in HydroDyn with flexible beam members in SubDyn. Please also note that HydroDyn supports multiple potential-flow bodies, so, you can have multiple potential-flow reference points.

Best regards,

Dear Jason,

Thank you very much for your kind explanation.

Does this mean that in order to include the radiation and diffraction effects at each members (as specified in SubDyn and HydroDyn for Strip-Theory part) I can on use the panel method software (e.g., WAMIT and AQWA) to analyze the multiple potential-flow bodies?

Best Regard,

Dear @Danupon.Subanapong,

The potential-flow and strip-theory solutions of HydroDyn are independent calculations.

The potential-flow solution does support multiple potential-flow bodies, which can be precalculated from WAMIT or AQWA, although WAMIT output file format is expected.

Best regards,

Nice to meet you.

I am facing a similar problem.
What I want to know is how to accurately calculate the sectional force of a floating body.

I am modeling a floating body using SubDyn to obtain the sectional forces of a semi-submersible floating body with Nmodes=21 and modeling the body as an elastic body.

I set PotMod in HydroDyn to 1 and PropPot for all members to True.

With these settings, the bending moment calculated by SubDyn changed abruptly after sea level (0,0,0), the location where the hydrodynamic forces are defined. The bending moment above sea level is similar to the response at the base of the tower, and is considered to be a reasonable value. The bending moment below sea level is similar to that obtained when SubDyn is set to Nmodes=0 (rigid body) and HydoroDyn is set to Morrison’s equation only (no potential theory is used). In other words, we believe that we have not obtained an accurate elastic response.

Is it possible to use potential theory to determine the motion of the floating body and SubDyn to determine the cross-sectional forces assuming an elastic body at the same time? If so, I would like to know how to do it.

If the above methods are not possible, the best I can think of right now is the following: 1) Find the RAO of the floating body using potential theory and a model that takes into account the drag coefficient (Cd) of the HydroDyn. 2) Model the floating body in SubDyn and reproduce the RAO in 1) using a model that takes into account the added mass coefficient (Ca), drag coefficient (Cd), and pressure coefficient (Cp) in HydroDyn. 3) Using the model in 2), determine the cross-sectional force of the floating body.

Let us know what you think.

Dear @Yosuke.Matsumoto,

OpenFAST supports the modeling of hydro-elastics and member-level loads, but to get proper member-level loads, you must properly distribute the hydrodynamic loads across the member. This is possible through the strip-theory of solution of HydroDyn, but not through the potential-flow solution. In the potential-flow solution of HydroDyn, the potential-flow body is assumed to rigid whereby hydrodynamic loads are lumped at a point.

This is why in my forum post above I suggested that one typically matches potential-flow bodies in HydroDyn with rigid bodies (called rigid links) in SubDyn and strip-theory members in HydroDyn with flexible beam members in SubDyn. That said, HydroDyn, does support multiple potential-flow bodies, so, you can lump the loads at multiple points, which allows you to obtain proper loads on flexible bodies that interconnect the rigid bodies.

It looks like you are modeling a semisubmersible. For a semisubmersible, I would normally expect that you’d model the large-volume bodies (offset columns) as potential-flow bodies in HydroDyn and rigid bodies in SubDyn, and the thin members (pontoons, braces) as strip-theory bodies in HydroDyn and flexible beams in SubDyn. In this case, OpenFAST can calculate the member-level loads in the pontoons and braces, but not in the offset columns.

Best regards,

Thank you for your answer.

I understood it as shown in the attached figure. If my understanding is wrong, please let me know.

I have an additional question.

  1. I am modeling a pontoon of the same width as the center and side columns. I want to find the cross-sectional forces for the center column, side columns, pontoons, and all members. Am I correct in my understanding that in such a case, I must use strip theory and flexible beam elements to obtain the cross-sectional forces? I am not sure if I can apply the Morrison formula to such a large cross section.

  2. Can the position of action of the concentrated load in potential flow theory be defined at depths below the water surface? For example, I was wondering if it is possible to define the potential theory load action location at the bottom of the center column, then it would be possible to model above that with flexible beam elements to obtain the cross-sectional forces.

Dear @Yosuke.Matsumoto,

I agree with your figure.

Regarding (1), if you want the cross-sectional loads throughout the structure, I agree that using strip theory in HydroDyn and flexible beam elements in SubDyn for all members is appropriate. You haven’t stated what the member diameters of your semisubmersible are, but I’ve been surprised how well strip theory can do (with the proper hydrodynamic coefficients), except perhaps for mild seastates where the wavelengths are small.

Regarding (2), yes, HydroDyn allows you to define the reference point x/y/z locations of each potential-flow body, where z need not be zero (assuming the potential-flow solution is defined relative this reference point). But if the entire center column is modeled with a single potential-flow body, I would not expect choice of the z-coordinate to make any difference in the results. To use the potential-flow solution on the center column and still get cross-sectional loads, you’d have to partition the center column into multiple potential-flow bodies, which would allow you to access the cross-sectional loads at the intersections between each body.

Best regards,

Thank you for your answer.

(1) I understand now.

(2) I understand that if I create a WAMIT calculation model according to the element division intervals of HydroDyn and SubDyn, I can obtain the fluid force corresponding to the position of each element. However, as discussed so far, I am aware of the following problems in obtaining the cross-sectional forces of a floating body. Is my understanding correct?

  1. The element to which the potential theory is applied will be a rigid body, so the obtained SubDyn cross-sectional forces will be different from those of a flexible element. The sectional forces obtained by this method are not reliable.
    (2) If the WAMIT model is divided into smaller parts, the accuracy of the obtained fluid forces is lower than if the WAMIT model is not divided.

Dear @Yosuke.Matsumoto,

I don’t agree with your comments to (2).

Regarding (2.1), it is not possible to extract loads from rigid links in SubDyn. It is only possible to extract loads from flexible members.

Regarding (2.2), the accuracy does not decrease if you subdivide a potential-flow body into multiple potential-flow bodies (assuming your model is set up properly). However, the model is more complicated to make and will have a higher computational cost.

Best regards,

Thank you very much.
I agree with your comments.