The company has now installed 54 of the 108 turbines on the Scottish Power and Dong-owned project, 14 kilometres off England's north west coast.
A2Sea's Sea Installer vessel hit the milestone after installing six of the Siemens 3.6MW turbines in the last five days alone.
The first electricity was generated from the project in January, exporting power to the National Grid via an onshore substation at Heysham, Lancashire.
Inter-array and export cabling work has been completed at the project, and the substations are operational. As such, commissioning is slated for the end of the year.
West of Duddon Sands will mark Iberdrola's first operating offshore wind project.