Large-diameter (4 to 6m) monopiles are often used as foundations for offshore wind turbines. The monopiles are subjected to large horizontal forces and overturning moments and they are traditionally designed based on the p-y curve method (Winkler type approach). The p-y curves recommended in offshore design regulations were developed for piles with diameters up to approximately 2.0m and are based on a very limited number of tests. Hence, the method has not been validated for piles with diameters of 4 to 6 m. This paper aims to assess different calculation approaches for monopiles subjected to quasi-static loading. The methods are: 1) a traditional Winkler model as recommended in the offshore standards; 2) a modified Winkler model in which the initial stiffness of the p-y curves depends on the pile diameter, depth below mudline as well as the internal angle of friction; 3) a three-dimensional continuum model established by means of the commercial program FLAC3D. The approaches are compared based on a monopile used as foundation for a wind turbine at Horns Rev which is located in the North Sea west of Denmark.