A method for use with maintenance of an offshore wind farm comprising a plurality of offshore wind turbines installed in open sea, in which the method comprises: - providing a shuttle vessel (8, 9) for transfer of service engineers and/or goods to and from the wind turbines; - sailing the shuttle vessel (8, 9) to one of the wind turbines; - landing the shuttle vessel (8, 9) against the wind turbine for allowing transfer of the service engineers and/or goods between the shuttle vessel (8, 9) and the wind turbine; - sailing the shuttle vessel (8, 9) to another wind turbine for allowing transfer of service engineers and/or goods between the shuttle vessel (8, 9) and that other wind turbine; and - sailing the shuttle vessel (8, 9) to a harbour facility and mooring it there for allowing exchange of the service engineers and/or the goods between the shuttle vessel (8, 9) and the harbour facility before sailing the shuttle vessel (8, 9) out to another wind turbine; in which the method makes use of an assembly of a plurality of the abovementioned shuttle vessels (8, 9) and a seaworthy harbour ship which serves as the abovementioned harbour facility and for this comprises an internal harbour basin within outlines of the ship and a plurality of mooring positions for mooring the plurality of shuttle vessels (8, 9) to the ship when lying inside the basin, the basin being provided with an entrance opening for the shuttle vessels (8, 9) to sail into and out of the basin, wherein the method further comprises: - sailing the assembly of the plurality of shuttle vessels (8, 9) and the harbour ship towards a halt position near or in the wind farm and substantially keeping the harbour ship in its halt position; - sailing the plurality of shuttle vessels (8, 9) back and forth between the mooring positions of the halted harbour ship and distinctive ones of the wind turbines for performing the maintenance thereto; and - mooring the plurality of shuttle vessels (8, 9) to the mooring positions when lying inside the basin, in which both during the sailing of the assembly towards the halt position as well as during the sailing of the plurality of shuttle vessels (8, 9) back and forth between the halted ship and the wind turbines, the internal basin is kept filled with sea water, characterized in that, both during the sailing of the assembly towards the halt position as well as during the sailing of the plurality of shuttle vessels (8, 9) back and forth between the halted ship and the wind turbines, the internal basin is kept in open connection with the surrounding sea water by means of its entrance opening being open for the shuttles (8, 9) to sail into and out of the basin at all times, and/or by means of the basin having its bottom at least partially open for the surrounding sea water to flow into and out of the basin. - providing a shuttle vessel (8, 9) for transfer of service engineers and/or goods to and from the wind turbines; - sailing the shuttle vessel (8, 9) to one of the wind turbines; - landing the shuttle vessel (8, 9) against the wind turbine for allowing transfer of the service engineers and/or goods between the shuttle vessel (8, 9) and the wind turbine; - sailing the shuttle vessel (8, 9) to another wind turbine for allowing transfer of service engineers and/or goods between the shuttle vessel (8, 9) and that other wind turbine; and - sailing the shuttle vessel (8, 9) to a harbour facility and mooring it there for allowing exchange of the service engineers and/or the goods between the shuttle vessel (8, 9) and the harbour facility before sailing the shuttle vessel (8, 9) out to another wind turbine; in which the method makes use of an assembly of a plurality of the abovementioned shuttle vessels (8, 9) and a seaworthy harbour ship which serves as the abovementioned harbour facility and for this comprises an internal harbour basin within outlines of the ship and a plurality of mooring positions for mooring the plurality of shuttle vessels (8, 9) to the ship when lying inside the basin, the basin being provided with an entrance opening for the shuttle vessels (8, 9) to sail into and out of the basin, wherein the method further comprises: - sailing the assembly of the plurality of shuttle vessels (8, 9) and the harbour ship towards a halt position near or in the wind farm and substantially keeping the harbour ship in its halt position; - sailing the plurality of shuttle vessels (8, 9) back and forth between the mooring positions of the halted harbour ship and distinctive ones of the wind turbines for performing the maintenance thereto; and - mooring the plurality of shuttle vessels (8, 9) to the mooring positions when lying inside the basin, in which both during the sailing of the assembly towards the halt position as well as during the sailing of the plurality of shuttle vessels (8, 9) back and forth between the halted ship and the wind turbines, the internal basin is kept filled with sea water, characterized in that, both during the sailing of the assembly towards the halt position as well as during the sailing of the plurality of shuttle vessels (8, 9) back and forth between the halted ship and the wind turbines, the internal basin is kept in open connection with the surrounding sea water by means of its entrance opening being open for the shuttles (8, 9) to sail into and out of the basin at all times, and/or by means of the basin having its bottom at least partially open for the surrounding sea water to flow into and out of the basin.