Feedback on Moordyn V1.01.02F - Numerical instability ?

Dear NREL Team,

Thank you for your work.

I just want to report on the forum an issue I experienced, and a possible solution.

I’m using FAST 8.16.00a-bjj, 27-Jul-2016 together with a floating platform and a multi-segmented mooring system in MOORDYN (6lines).

FAST revision : x64
Moordyn parameters dtM = 0.001s
Trying to calculate the RAO at 15s of the platform (regular wave, 1m amplitude, MORISON model) I was facing an instability of platform motions.
After a few investigations, it seems that the instability of motions was starting after a numerical issue in the mooring resolution at time step 512s.

I then tried to reduce moordyn parameters dtM
=> dtM = 0.0005s => numerical issue appears at 256s
=> dtM = 0.0001s => numerical issue appears at 128s

I then tried with :
FAST revision : [b]x64d - same simulations files
Moordyn parameters dtM = 0.001s
The numerical instability disapears.