Offshore wind farm is a renewable energy that has been commercially operated in Europe over 3.8 GW capacities in 2011 and will be targeted upon 150 GW in 2030. Taiwan also plans to install 3 GW offshore wind farms in 2030. Thus, a number of condition assessments and engineering designs are being studied, such as the site selection, the wind turbines layout, the environmental impact assessment, the maritime engineering, the installation and maintenance fleets. In this paper, we carried out the winds and waves as the critical conditions of sea status for the installation of offshore wind turbines. The winds and waves data were collected by a data buoy which was deployed near the entrance of Taichung Harbor. The wind speeds were measured at 15 m above the sea surface. In the case study, we assumed a jackup vessel will be used for installation of wind turbines. Then, some benchmarking conditions are filled into the assessment. For example, the conditions for operated criteria were suggested as wind speeds below 15 m/s and significant wave heights below 1.5 m in a water depth of 30 m. Moreover, a continuous working duration is considered as 15 hours for installation of a wind turbine from a tower, a nacelle, and blades. Eventually, the yearly available days for the wind turbine installation were assessed about 175 days. Different working modes, such as piling, cable laying, and maintenance, were discussed those yearly available days as well. From the operation and maintenance (O&M) supporting vessels working days (260 days), an offshore wind farm owned 50 wind turbines and each needs maintenance of 260 hours every year that was a benchmark scenario. The supporting vessels require 6 at least that had been estimated.