# Some questions about mooring calculations

Dear everyone，
I’m currently experiencing some problems with my recent study of the three mooring methods MAP++, FEAMooring and MoorDyn, and would appreciate answers.
First of all, I observe that MoorDyn has four coefficients Ca, Cd, Caax, Cdax, indicating that both normal and tangential additional mass and drag are considered, and FEAM’s LineCI, LineCD should only consider normal additional mass and drag, does this mean that MoorDyn is more accurate than FEAM, and also I find that I have used the two respectively At the same time, I found that when I calculate the anchor chain tension with these two modules respectively, the calculation result of FEAM is always slightly larger than that of MoorDyn, is this the reason?
The second point is about the calculation of inertial forces. I observed that none of the three calculation methods seems to take inertial forces into account, only MoorDyn and FEAM take hydrodynamic forces (drag, add mass) into account, is there a problem with my understanding?
The third point, so the accuracy of the calculation of the three methods is MoorDyn > FEAMooring >MAP++,is that right please?
Best regards，

Dear @Yushun.Fu,

I would say that FEAMooring’s FEA formulation is higher fidelity than the lumped mass approach used by MoorDyn, but MoorDyn has more generalized functionality, so, is more powerful as a result. I also find MoorDyn easier to use than FEAMooring. MAP++ provides good mooring stiffness, but includes no dynamics.

Regarding inertial forces, are you referring to fluid inertial forces or the physical inertia of the lines?

Best regards,

I’m glad to receive your reply, I realized that some of my basic theories may be wrong, I will go through them in detail again and come back to continue the discussion with you
Best regards，

1. Regarding the introduction of MAP++, I read: The main disadvantage is that quasi-static models neglect hydrodynamic and inertial forces on the line. does the inertial forces here refer to the additional mass? The inertial forces refer to the additional mass?
2. Here are some of my understandings about MoorDyn and FEAMooring, I don’t know if they are correct or not, and I would like to get your explanation.
(1). MoorDyn and FEAmooring both consider hydrodynamic forces and their own inertial forces, while MAP++ does not.
(2).MoorDyn considers hydrodynamic and inertial forces in the normal and tangential directions through four coefficients Ca, Cd, Cat, and Cdt.
(3).FEAMooring considers axial hydrodynamic and inertial forces by LCI and LCD, the difference between the two is that FEAMooring does not consider tangential direction, I guess it may be due to the fact that tangential direction has very little influence on the calculation results.
3. I try to fix the platform at the position of surge=5m, the condition is no wind and still water.
(1). By plotting the anchor chain tension time domain graph I found that the anchor chain tension of FEAMooring can’t keep fixed in a line. I think the result should be fixed around a value like MoorDyn and MAP++.For this reason I plotted the PSD of the FEAMooring anchor chain tension, I think the result should be 0 (because the anchor chain tension should be a fixed value) but from the result there is a very high influence of higher order frequencies, I am curious where these higher order frequencies come from?

(2). In order to solve this problem, I changed the Total number of elements per line of FEAMooring from 50 to 20, and I can find from the PSD plot that there is a significant decrease in the effect of higher order frequencies, can this not indicate that the fidelity of FEAMooring has a lot to do with the number of segments? Then is the result more accurate for 50 segments or 20 segments?

(3). I used FEAMooring to simulate a lot of conditions, there will be a lot of higher-order frequencies, for which I chose to filter to deal with, generally retaining the frequency below 1HZ, through comparison, found that the filtered results and MoorDyn close to this, which is not to say that in the future I use FEAMooring simulation need to be filtered?

(4). Sometimes when I simulate some working conditions with FEAMooring, the anchor chain tension shows some negative values, which I think is against the rule. How should I solve this?

4. Regarding your statement that the FEA formulation is higher fidelity than MoorDyn, do you mean that the finite element model applied by FEA is more accurate than the centralized mass used by MoorDyn?

Best regards，

Dear @Yushun.Fu,

Here are my responses:

1. I would say that the "fluid-inertia forces are expressed in the hydrodynamic loads, which are neglected by MAP++. MAP++ also neglects the self-inertia of the lines themselves, as well as the internal flexible modes of the lines.
2. I agree.
3. I agree that in still water with a fixed platform that the solution should be and remain in static equilibrium with constant tension. Given that FEAMooring is showing time variability, I’m guessing that the FEAMooring solution is not initialized in a static equilibrium condition; moreover, the nonequilibrium condition is not decaying quickly, implying little internal damping. I would guess the peaks in the PSD are natural frequencies associated with internal vibration modes of the lines. And by reducing the number of segments, the number of high-frequency modes is reducing.
4. I just meant that an FEA formulation is a higher fidelity than a lumped mass formulation for structural analysis. But in practice, I find MoorDyn easier to set up and get reasonable answers from than FEAMooring; plus, MoorDyn has more general functionality. That said, I’m not an expert in FEAMooring and I would suggest reaching out to the developer of FEAMooring (Yoon Bae) for FEAMooring-specific questions.

Best regards,

Hi, Could you tell me how I can run fst file by modifying FEA mooring file?

I’m not sure I understand your question. What mooring-related properties do you want to modify? Which mooring module of OpenFAST are you using (MAP++, MoorDyn, or FEAMooring)? Do you have a question about a specific mooring-related input in the input file?

Best regards,