You may test following procedure:
1. extract stress histogram from dynamic analysis runned for reasonable time T
2. calculate stress range (i.e. rainflow counting method or similar method - I am not familiar with offpipe so i do not know if offpipe could be of any help in this regard)
3. divide...
Are you using regular wave in your analysis? if so, results will be conservative. Aslo, fatigue damage in DNV is for one full cycle (Max-Min-Max) not half cycle (Max-Min).
