Hi Jason,
I am trying to measure the heave value from the center of mass instead of mean sea level (MSL).
For this, I have to change the value of PtfmRefzt from 0 to =-8.6588 which is similar to PtfmCMzt=-8.6588. However, my heave value remains unchanged and it still seems to be measured from MSL. Do I also need to make changes somewhere else?
I am using 5MW_OC4Semi_WSt_WavesWN, Open FAST v3.0.0.
Dear Kamran,
With such a small moment arm, I would guess the heave displacement would only be minimally effected by pitch and roll motions when comparing the motion relative to the MSL versus relative the CM.
Alternatively, you could keep the platform reference point at PtfmRefzt = 0 and shift the outputs to the CM with a bit of post-processing, as discussed, e.g., in the following forum topic: Time domain analysis by AQWA - #2 by Jason.Jonkman.
Best regards,
Dear @Jason.Jonkman,
I am using the StC module and i wanna put a TMD in the spar platform.
In the StC input file, there is part called StC LOCATION. One should put the location of the TMD relative to the reference origin of component attached to. I wanna make sure that the reference point of the spar is at SWL and not at the tower base. Let’s say i wanna put a TMD in the spar at -20 m from the SWL, then i should put -20 m right ? or -30 m (with respect to the coordinate system at the tower base) ?
Also, i was a bit lost about the meaning of DEL. According to MLife, DEL is a kind of stress. I read a lot of papers where they mention DEL tower base fore aft moment and it has a unit of kN.m
What do you think ?
Best Regards,
Riad
Dear @Riad.Elhamoud,
For the substructure-based StC, the reference origin is the platform reference point as defined within ElastoDyn (typically PtfmRefzt = 0, corresponding to MSL).
DELs can be computed for forces, moments or stresses. OpenFAST calculates sectional loads (forces, moments), not stresses, so if you compute a DEL directly from the OpenFAST output, it would have units of force or moment.
Best regards,
Dear @Jason.Jonkman,
Thank you for your reply.
If i understand your answer considering the Damage Equivalent Moment, one should obtain the TwrBsMyt (for example) from OpenFAST. Then, apply rainflow counting cycle to extract moment range and apply the DEL formula.
However, i was worried about the m coefficient which is the slope of the SN curve. In fact, steel has a bilinear SN curve. There are two slopes depending on the number of cycles as defined in DNV.
I was wondering whether you or anyone in the community have worked with this case (bilinear SN curve) and how you have dealt with it. In the litterature, i did not find any solution for the bilinear SN curve case.
Best Regards,
Riad
Dear @Riad.Elhamoud,
I agree that bilinear SN curves should be considered at some point, but this functionality has not been introduced in NREL post-processors (MCrunch, MLife, pCrunch) yet.
Best regards,