A damper unit (302, 303, 403, 404) for damping oscillations of a tower structure (101, 301, 401) when secured thereto, the damper unit (302, 303, 403, 404) comprising - a damper unit structure (511, 605, 705) adapted for attachment to the tower structure, - a pendulum structure (501, 512, 513, 601, 701), - a suspension arrangement for suspending the pendulum structure (501, 512, 513, 601, 701) from the damper unit structure (511, 605, 705) such that the pendulum structure (501, 512, 513, 601, 701) is allowed to displace from a neutral position for the pendulum structure (501, 512, 513, 601, 701), the suspension arrangement comprising one or more wires (514-516, 602-603, 702-703) for suspending the pendulum structure (501, 512, 513, 601, 701), - a sensor adapted for measuring movements of the tower structure (101, 301, 401), and - tuning means configured for adjusting the natural frequency of the suspended pendulum structure (501, 512, 513, 601, 701) in response to measured movements of the tower structure (101, 301, 401), characterised in that: the tuning means comprises a suspension length adjuster for adjusting the natural frequency of the suspended pendulum structure (501, 512, 513, 601, 701), said adjuster comprising, - a guide means (606, 706) extending at least partly along the length of said one or more wires (514-516, 602-603, 702-703), - a cart (604, 704) attached to and movable along the guide means (606, 706), - for each of said one or more wires (514-516, 602-603, 702-703), a clamp (608, 712) secured to the cart (604, 704) at one end and to the wire (514-516, 602-603, 702-703) at the other end, and - actuation means for moving the cart (604, 704) along the guide means (606, 706), the actuation means being operably connected to the processor. - a damper unit structure (511, 605, 705) adapted for attachment to the tower structure, - a pendulum structure (501, 512, 513, 601, 701), - a suspension arrangement for suspending the pendulum structure (501, 512, 513, 601, 701) from the damper unit structure (511, 605, 705) such that the pendulum structure (501, 512, 513, 601, 701) is allowed to displace from a neutral position for the pendulum structure (501, 512, 513, 601, 701), the suspension arrangement comprising one or more wires (514-516, 602-603, 702-703) for suspending the pendulum structure (501, 512, 513, 601, 701), - a sensor adapted for measuring movements of the tower structure (101, 301, 401), and - tuning means configured for adjusting the natural frequency of the suspended pendulum structure (501, 512, 513, 601, 701) in response to measured movements of the tower structure (101, 301, 401), characterised in that: the tuning means comprises a suspension length adjuster for adjusting the natural frequency of the suspended pendulum structure (501, 512, 513, 601, 701), said adjuster comprising, - a guide means (606, 706) extending at least partly along the length of said one or more wires (514-516, 602-603, 702-703), - a cart (604, 704) attached to and movable along the guide means (606, 706), - for each of said one or more wires (514-516, 602-603, 702-703), a clamp (608, 712) secured to the cart (604, 704) at one end and to the wire (514-516, 602-603, 702-703) at the other end, and - actuation means for moving the cart (604, 704) along the guide means (606, 706), the actuation means being operably connected to the processor. - a guide means (606, 706) extending at least partly along the length of said one or more wires (514-516, 602-603, 702-703), - a cart (604, 704) attached to and movable along the guide means (606, 706), - for each of said one or more wires (514-516, 602-603, 702-703), a clamp (608, 712) secured to the cart (604, 704) at one end and to the wire (514-516, 602-603, 702-703) at the other end, and - actuation means for moving the cart (604, 704) along the guide means (606, 706), the actuation means being operably connected to the processor.