We claim:
1. A method of operating (1) a wind turbine (200) comprising:
determining (10) a reference accumulated fatigue of at least one wind turbine component;
determining (20) an operational time of at least one wind turbine component comprising load obtaining periods;
obtaining (30) a partial load indicator from the at least one wind turbine component for each of the load obtaining periods;
determining (40) a real accumulated fatigue in the at least one wind turbine component comprising obtaining an accumulated load indicator by adding the partial load indicators of each of the load obtaining periods;
comparing (50) the real accumulated fatigue and the reference accumulated fatigue; and
controlling (60) the operation of the wind turbine (200) based at least on a result of comparing the real accumulated fatigue and the reference accumulated fatigue.
2. The method of operating (1) a wind turbine (200) as claimed in claim 1, wherein determining (40) a real accumulated fatigue comprises determining an equivalent fatigue load based at least on the obtained partial load indicator from the at least one wind turbine component.
3. The method of operating (1) a wind turbine (200) as claimed inclaim 1, wherein obtaining (30) a partial load indicator from the at least one wind turbine component comprises:
obtaining (31) wind speed;
obtaining (32) turbulence intensity; and
calculating loads on the at least one wind turbine component during a load obtaining period based at least on the wind speed and the turbulence intensity.
4. The method of operating (1) a wind turbine (200) as claimed inclaim 3, wherein
obtaining (30) a partial load indicator from the at least one wind turbine
component comprises:

obtaining (33) wind shear data; and
calculating loads of the at least one wind turbine component during a load obtaining period is further based on the obtained wind shear data.
5. The method of operating (1) a wind turbine (200) as claimed inclaim 3, wherein
obtaining (30) a partial load indicator from the at least one wind turbine
component comprises:
obtaining (34) length scale turbulence ; and
calculating loads of the at least one wind turbine component during a load obtaining period is further based on the obtained length scale turbulence.
6. The method of operating (1) a wind turbine (200) as claimed inclaim 3, wherein
obtaining (30) a partial load indicator from the at least one wind turbine
component comprises:
obtaining (37) a water parameter comprising at least one of wave direction, JONSWAP spectrum, wave height, wave frequency and tide;
calculating loads of the at least one wind turbine component during a load obtaining period is further based on the obtained water parameter.
7. The method of operating (1) a wind turbine (200) as claimed inclaim 3, wherein calculating loads on the at least one wind turbine component comprises determining (36) a time during which the wind turbine (200) is subjected to each combination of wind speed and turbulence intensity during a load obtaining period, and specifically to each combination of wind speed, turbulence intensity and at least one of wind shear data, length scale turbulence and a water parameter during a load obtaining period.
8. The method of operating (1) a wind turbine (200) as claimed inof claim 1, wherein obtaining (30) a partial load indicator comprises determining (28) a nacelle orientation.
9. The method of operating (1) a wind turbine (200) as claimed inclaim 1 , wherein determining a reference accumulated fatigue in the at least one wind turbine component comprises determining a reference accumulated fatigue at design lifetime.

10. The method of operating (1) a wind turbine (200) as claimed inclaim 9, wherein determining a reference accumulated fatigue in the at least one wind turbine component comprises determining a reference accumulated fatigue trend.
11. The method of operating (1) a wind turbine (200) as claimed inclaim 9 , wherein controlling (60) the operation of the wind turbine comprises:
performing a maintenance operation on the at least one wind turbine component when the real accumulated fatigue exceeds the reference accumulated fatigue at design lifetime.
12. The method of operating (1) a wind turbine (200) as claimed in claim9 ,
wherein controlling (60) the operation of the wind turbine comprises:
extending the expected lifetime of the at least one wind turbine component when the reference accumulated fatigue at design lifetime is greater than the real accumulated fatigue.
13. The method of operating (1) a wind turbine (200) according to any of claims
10 - 12, wherein controlling (60) the operation of the wind turbine comprises:
de-rating the wind turbine (200) for reducing loads on the at least one wind turbine component when the real accumulated fatigue exceeds the reference accumulated fatigue trend.
14. The method of operating (1) a wind turbine (200) as claimed inclaim 10 ,
wherein controlling (60) the operation of a wind turbine comprises:
uprating the wind turbine (200) when real accumulated fatigue is lower than the reference accumulated fatigue trend.
15. A wind turbine controller (240) for controlling the operation of a wind turbine
(200), wherein the wind turbine controller (240) is configured to:
determine an operational time of at least one wind turbine component comprising load obtaining periods;
obtain a partial load indicator from the at least one wind turbine component for each of the load obtaining periods;
determine a real accumulated fatigue in the at least one wind turbine

component by obtaining an accumulated load indicator by adding the partial load indicators of each of the load obtaining periods;
obtain a reference accumulated fatigue;
compare the real accumulated fatigue and the reference accumulated fatigue; and
control the operation of the wind turbine (240) based at least on a result of comparing the accumulated fatigue and the reference accumulated fatigue.