Method of erecting a wind turbine

1. A method of erecting a wind turbine on a wind turbine site, the wind turbine comprising a turbine tower and a nacelle, the method comprising the following steps:

providing a plurality of tower sections being arrangeable upon each other in a vertical orientation in a tower structure to form the turbine tower,

providing at least two damper units configured to dampen oscillations of the turbine tower,

attaching one of the at least two damper units to one of the plurality of the tower sections on an outside thereof, and subsequently arranging said tower section in the tower structure,

attaching a subsequent one of the at least two damper units to another one of the plurality of the tower sections on an outside thereof, and subsequently arranging said tower section in the tower structure, while the previous one of the at least two damper units is still attached to the previous one of the plurality of tower sections.

2. The method according to claim 1 , wherein the nacelle is arranged on top of the tower structure while at least one of the two damper units is attached to a tower section in the tower structure.

3. The method according to claim 1 , further comprising a step of attaching a hoisting system to one of the at least two damper units and releasing said damper unit from the tower section to which it is attached in the tower structure and lowering said damper unit to the site.

4. The method according to claim 3 , wherein the step of arranging one of the tower section in the tower structure comprises a step lifting the hoisting system and said tower section, arranging the tower section in the tower structure, detaching the tower section from the hoisting system, attaching the hoisting system to one of the damper units attached to another tower section in the tower structure, and lowering the hoisting system and said damper unit.

5. The method according to claim 4 , wherein the step of attaching the hoisting system to one of the damper units in the tower structure is carried out before the step of detaching the tower section from the hoisting system.

6. The method according to claim 5 , wherein the hoisting system comprising a tower section lifting sling and a damper unit lifting sling, the damper unit lifting sling have a length exceeding a height of said tower section, and wherein the step of attaching the hoisting system to said damper unit is carried out while the hoisting system is attached to an upper end of said tower section.

7. The method according to claim 1 , wherein the step of attaching one of the at least two damper units to one of the plurality of the tower sections on an outside thereof, and subsequently arranging said tower section in the tower structure is carried out when a predetermined number of tower sections has been arranged in the tower structure.

8. The method according to claim 3 , wherein the step of releasing one of the damper units from a tower section in the tower structure comprises a step of determining tension in the hoisting system, and based on the determined tension, to select a state where said damper unit can be released from said tower section.

9. The method according to claim 3 , wherein the step of lowering a damper unit to the site further comprising a step of rotating said damper unit approximately 90 degrees before attaching said damper unit to a subsequent one of the plurality of tower sections.

10. The method according to claim 3 , wherein the step of lowering a damper unit to the site further comprising a step of parking said damper unit on the site before attaching said damper unit to a subsequent one of the plurality of tower sections.

11. The method according to claim 1 , wherein the step of attaching one of the at least two damper units to one of the plurality of the tower sections on an outside thereof comprises a step of providing at least one through hole in a wall of the tower section and fixing said damper unit to said tower section by a locking structure extending through the through hole.

12. The method according to claim 1 , wherein the step of attaching one of the at least two damper units to one of the plurality of the tower sections on an outside thereof comprises a step of providing at least one shelf support on an outer surface of said tower section and fixing said damper unit to said tower section by engagement between the shelf support and a part of said damper unit.

13. The method according to claim 1 , wherein the step of attaching one of the at least two damper units to one of the plurality of the tower sections on an outside thereof comprises a step of providing at least one pivoting structure between said damper unit and said tower section and aligning said damper by reorienting the pivoting structure relative to said tower section.

14. The method according to claim 1 , wherein a damping frequency of at least one of the damper units is variable, and the method comprises a step of varying the damping frequency in dependency of the natural frequency of the tower structure.

15. The method according to claim 1 , wherein each of the at least two damper units comprises:

a damper unit structure adapted for attachment to the tower structure,

a pendulum structure,

a suspension arrangement for suspending the pendulum structure from the damper unit structure such that the pendulum structure is allowed to displace from a neutral position for the pendulum structure, the suspension arrangement comprising one or more wires for suspending the pendulum structure,

a sensor adapted for measuring movements of the tower structure, and

tuning means configured for adjusting the natural frequency of the suspended pendulum structure in response to measured movements of the tower structure.