not shown the hoisting devices 100 are attached directly to the compartment 5 so said hoisting devices are arranged to hoist at the same time the compartment 5 itself the docking unit 4 and the auxiliary gantry crane 3 up to the nacelle 1 thus the compartment 5 carries the auxiliary gantry crane 3 in order to also lift this structure to the nacelle 1 therefore an external third party crane is not needed the auxiliary gantry crane 3 further comprises at least a crab 9 located on the top beam 21 displaceable in a direction parallel to said top beam 21 enabling the positioning of the crane hoisting point 25 at any position inside and outside of the nacelle 1 the mobility of the crab 9 enables the movement of the load with three transitional degrees of freedom such that the hoisting point 25 can get access to any point in the nacelle 1 the independent mobile crane system can have an intermediate hoisting frame structure 7 which is provided with at least one anchor point 8 for linking to an internal crane 35 schematically shown in fig4 by means of a wire 33 and at least three anchor points 27 for connection to the independent mobile crane.