Dear Yingyi Liu,
I ran your four simulations and confirm the behavior your are seeing. I then ran a fifth case where in MooDyn I added two additional connect nodes i.e. nodes 5 and 6 are identical to node 4, a vessel node at 0, 0, -14. In this case, I change the lines so that line 2 connects to node 5 and line 3 connects to node 6. See my updated MoorDyn_SPM file attached.
Running this fifth simulation gives the results that you’d expect, with fairlead tensions that are consistent with MAP++. My guess is what is going on in case 4 is that the fairlead tension calculated by MoorDyn refers to the vector sum of the tensions from the three lines because all three lines connect to the same vessel node. In case 5, the vessel nodes are now independent (even though they are co-located), resulting in the tension you expect.
Perhaps this explains why the mooring tensions where different between MoorDyn and MAP++ in our related forum discussion in another topic: Platform motion appears strange for an onshore wind turbine, where again, the SPM models are set up with multiple lines connected to the same vessel node.
I hope that helps.
Best regards,
NRELOffshrBsline5MW_OC4DeepCwindSemi_MoorDyn_SPM_JJ.dat.txt (2.49 KB)