The W-type is a motion-compensated offshore crane capable of hoisting up to 2 mt of cargo, thereby supporting the maintenance processes for the rapidly growing offshore wind farms. However, the rated cargo mass is limited by the dynamic amplification factor (DAF), which is a governing parameter in the structural design of offshore cranes. During the design phase, the industry currently makes use of a one-dimensional spring-mass model to estimate the DAF. In this study, a simplified flexible multibody dynamic model was developed, and validated with experimental data, to simulate the DAF instead. It was found that the previous method underpredicts the DAF by up to 56%, because it neglects the effects of crane inertia and structural damping. Besides, the active controller of the luffing cylinders was modelled for the first time, which reduced the DAF up to 13%. Therefore, the simple spring-mass model is not deemed accurate for this application.