Hi Pooria,

I suggest to have a look on DNV-RP-D101 (older ed. 2008), Appendix G. Unfortunately, the newer 2017 Ed. does not include any more the tabulated loading cases.

You'll find there that WIN and "U" Inertial Load (that assimilates Wave effect, of course) are initially summed separately using SRSS method, and then resultant-combined WIN and "U" loads are summed using Scalar method to determine the overall effect.

In general, SRSS summation might be used to avoid the development of those complicated loads cases' series...and the obtained results are on the conservative side...
I've seen similar approaches on some past FPSO projects with reputed engineering companies.

Best regards,