Method for arranging a crane machine on a wind power station that floats in open water, wherein the wind power station comprises a buoyant platform provided with a wind turbine extending upwards from the buoyant platform, wherein the buoyant platform comprises at least a first and a second support point (3, 4, 13, 14) located horizontally spaced apart in relation to each other and at a substantially similar level above a surface of the open water, wherein the crane machine comprises at least a rotatable crane base structure, a main boom and hoist components for lifting and lowering materials, wherein the crane machine, when arranged on the wind power station with the main boom connected to the rotatable crane base structure and extending in an upwards direction and with the hoist components installed, is configured to lift/lower heavy wind turbine components (7, 8) to/from an upper part of the wind turbine, wherein the method comprises the steps of: - providing the crane machine in an at least partly disassembled state onto one or more vessels and arranging the one or more vessels in association with the buoyant platform, - moving/lifting the at least partly disassembled crane machine from the one or more vessels onto the buoyant platform, wherein the rotatable crane base structure and the main boom are disconnected from each other during moving/lifting, - assembling the crane machine when positioned on the buoyant platform, wherein the moving/lifting-step comprises: - positioning the rotatable crane base structure at the first support point (3, 13); - positioning the main boom substantially horizontally with a first end portion located at the first support point (3, 13), and - positioning the main boom so as to be supported by the second support point (4, 14) as well as by the first support point (3, 13) and/or a further support point arranged between the first and second support points (3, 13, 4, 14), wherein the assembling-step comprises: - connecting the first end portion of the main boom to the rotatable crane base structure when the main boom and the rotatable crane base structure are positioned on the buoyant platform. wherein the wind power station comprises a buoyant platform provided with a wind turbine extending upwards from the buoyant platform, wherein the buoyant platform comprises at least a first and a second support point (3, 4, 13, 14) located horizontally spaced apart in relation to each other and at a substantially similar level above a surface of the open water, wherein the crane machine comprises at least a rotatable crane base structure, a main boom and hoist components for lifting and lowering materials, wherein the crane machine, when arranged on the wind power station with the main boom connected to the rotatable crane base structure and extending in an upwards direction and with the hoist components installed, is configured to lift/lower heavy wind turbine components (7, 8) to/from an upper part of the wind turbine, wherein the method comprises the steps of: - providing the crane machine in an at least partly disassembled state onto one or more vessels and arranging the one or more vessels in association with the buoyant platform, - moving/lifting the at least partly disassembled crane machine from the one or more vessels onto the buoyant platform, wherein the rotatable crane base structure and the main boom are disconnected from each other during moving/lifting, - assembling the crane machine when positioned on the buoyant platform, - providing the crane machine in an at least partly disassembled state onto one or more vessels and arranging the one or more vessels in association with the buoyant platform, - moving/lifting the at least partly disassembled crane machine from the one or more vessels onto the buoyant platform, wherein the rotatable crane base structure and the main boom are disconnected from each other during moving/lifting, - assembling the crane machine when positioned on the buoyant platform, wherein the moving/lifting-step comprises: - positioning the rotatable crane base structure at the first support point (3, 13); - positioning the main boom substantially horizontally with a first end portion located at the first support point (3, 13), and - positioning the main boom so as to be supported by the second support point (4, 14) as well as by the first support point (3, 13) and/or a further support point arranged between the first and second support points (3, 13, 4, 14), - positioning the rotatable crane base structure at the first support point (3, 13); - positioning the main boom substantially horizontally with a first end portion located at the first support point (3, 13), and - positioning the main boom so as to be supported by the second support point (4, 14) as well as by the first support point (3, 13) and/or a further support point arranged between the first and second support points (3, 13, 4, 14), wherein the assembling-step comprises: - connecting the first end portion of the main boom to the rotatable crane base structure when the main boom and the rotatable crane base structure are positioned on the buoyant platform. - connecting the first end portion of the main boom to the rotatable crane base structure when the main boom and the rotatable crane base structure are positioned on the buoyant platform.