Dear Dainius,
Simply taking the tower-base or tower-top loads from FAST as input to ABAQUS for the jacket design/analysis is a one-way coupled approach that is not common in the offshore wind industry.
The most common approach in use by the offshore wind industry is the sequentially coupled approach whereby the jacket is designed and analyzed in an offshore design tool e.g. ABAQUS, which also exports a superelement for use in an aero-elastic code e.g. FAST for coupled aero-hydro-elastic simulation. The loading at the interface (e.g. tower base) is then passed back from the aero-elastic code to the offshore design tool for stress recovery and code checks.
Another approach that is being applied is the fully coupled approach, whereby the coupled aero-hydro-servo-elastic simulations are performed in one tool e.g. FAST. and only the structural reaction loads are output for import into an FEA tool for stress recovery and code checks. This is discussed e.g. in the following forum topic: Stress Analysis of Blades and a Tower - #13 by Jason.Jonkman. However, if you are changing the design of the jacket in the FEA solution then this would likely require updating and rerunning the coupled aero-hydro-servo-elastic FAST analysis apriori.
I hope that helps.
Best regards,