I am on a calculation in a FPSO and I am a first timer. I am a bit perplexed on how to combine the structure deflections due to the ship motion with the motion accelerations. could anyone confirm the following load cases ( suppose there is only one P/T and U and D represent acceleration and deflection respectively)

L1 W+P+T
L2 W+P+T+U+D
L3 W+P
L4 L2-L1
L5 L3+L4 (scalar) this case would have to be checked for fatigue as well.

if the above load cases are sound, I would like to know why I had to combine displacement with acceleration and check them as an occasional case. as far as I know displacement is always a secondary load, right?