Method and device for mounting of wind turbine blades

1. A method for mounting a wind turbine blade to a wind turbine hub of a wind turbine by a crane boom, comprising:

keeping a blade orientation horizontal when lifting the wind turbine blade off a ground and mounting the wind turbine blade to the wind turbine hub;

bearing a weight of the wind turbine blade by a bearing wire; and

controlling the blade orientation by a control wire and a winch arrangement;

lifting the wind turbine blade with a lifting system comprising a lifting device which is attachable to the wind turbine blade and connects the control wires and the bearing wire;

controlling the blade orientation horizontal when lifting the wind turbine blade off the ground using the control wires; and

fixing the wind turbine blade in a horizontal position to the wind turbine hub,

wherein the control wire connects the wind turbine blade to the winch arrangement via the crane boom and keeps the blade orientation horizontal and controls the blade orientation within a horizontal plane, and

wherein the winch arrangement is located at the lifting device and controls the control wire for controlling a distance of the wind turbine blade from the crane boom and the blade orientation within the horizontal plane,

wherein:

the winch arrangement comprises a first winch and a second winch,

the first winch controls the distance of the wind turbine blade from the crane, and

the second winch controls the blade orientation within the horizontal plane.

2. The method according to claim 1 , wherein

the control wire comprises a free length comprising a first branch and a second branch,

the first branch comprises a first fraction of the control wire,

the second branch comprises a second fraction of the control wire,

the first and the second branches are connected directly or indirectly to the crane boom,

the first winch controls the free length of the control wire, and

the second winch controls a distribution of the free length of the control wire to the first fraction and the second fraction.

3. The method according to claim 1 , further comprising:

lifting the wind turbine hub to a nacelle of the wind turbine with the lifting system prior to lifting the wind turbine blade; and

mounting the wind turbine hub on the nacelle.

4. The method according to claim 1 , further comprising:

lifting the wind turbine hub and a nacelle of the wind turbine together with the lifting system prior to lifting the wind turbine blade; and

mounting the nacelle including the wind turbine hub on a wind turbine tower.

5. The method according to claim 1 , wherein at least two control wires are used which can be controlled independently to each other.

6. The method according to claim 1 , wherein the control wire is pre-tensioned when the wind turbine blade is lifted.