During exploitation, offshore floating structures are subjected to random loads with repeating cycles that may occur the fatigue in the mooring lines. That can be one of the important causes of incidents for offshore floating structures while exploiting oil and gas as well as exploiting renewable energy. Random fatigue analysis for mooring systems of floating offshore units in general and mining semisubmersible platforms in particular is a complex problem and has an important significance in ensuring the sustainability of these structures. The fatigue damages assessment for mooring lines of offshore floating structures is a nonlinear random problem, which cannot use the deterministic analysis method with conventional S-N fatigue curves as for fixed offshore structures. Therefore, the paper focuses on the methodology of the fatigue damages assessment problem for mooring system using nonlinear random dynamic tension simulations of mooring lines in time domain under the effect of annual statistical sea states. From there, the T-N fatigue curves for mooring lines are studied and the “Rainflow” method is applied to count the cycles of tension expressions in the mooring lines. Finally, the Palmgren-Miner rule is applied to calculate the accumulated fatigue damages and the fatigue life for mooring systems. In the numerical simulation section, the paper conducts the fatigue analysis to assess the sustainability for the mooring system of FPU DH01 semisubmersible platform at Dai Hung field, in the South of Vietnam Sea.