The Dutch Offshore Wind Energy Converter project (DOWEC) aims to design large wind turbines for application in large-scale offshore farms. A baseline windfarm has been considered 43 km off the Dutch coast, with 80 wind turbines, each of 6 MW capacity. In this paper, access methods for O and M crew have been investigated. The methods include: (i) inflatable boats (zodiacs), (ii) special offshore access systems and (iii) helicopters. For the methods considered, operational limits have been established, expressed as 'maximum-mean' wind speed and as significant wave height. On the basis of the NEXT database the statistics of the corresponding weather-windows have been determined. These statistics, together with the failure statistics and the regular maintenance demand of the wind turbines and the other offshore windfarm components determine its overall-availability at the specific site. Estimates of this overall-availability of the offshore windfarm have been obtained with a sophisticated Monte Carlo simulation model of the operations within the windfarm. The simulations show that use should be made of access methods with which an accessibility of at least 82% of time can be achieved. This rules out standard options, e.g. direct inflatable boat access, in order to maintain a windfarm availability greater than 90%.