The problem isn´t the derating factor, I will use D.F.=3-4. (This give a static/dynamic safety ratio of 24-32, to avoid fatigue, impact, and other unusuall condition)
This operation seems very unsafe for the wheight vessel operators.
If the change of the vessel height is more than 0,50 meters.
You need a telescopic structure fixed to the platform and sliding on the ship and into the platformstructure(5m sliding) to guide the weight to avoid swinging the weight.