Two of the Dutch firm's vessels were used during the installation. The Svanen heavy-lift vessel installed 56 of the foundations, while its sister jack-up vessel Aelos was tasked with the remaining 31 monopiles and transition pieces.
The foundations were produced by German firm EEW SPC, Danish group Bladt Industries and their UK joint venture Offshore Structures Britain (OSB).
Van Oord said installation of the 87 foundations took four and a half months.
Dong's Walney Extension is being built in two phases. The western phase will comprise 40 MHI Vestas V164 8.25MW turbines, while the eastern phase will use Siemens' 7MW turbine.
The whole project is due for completion next year.
Van Oord project manager Floren Verweij said it was the first time the Aelous vessel was in floating mode to install the transition pieces as the seabed was unsuitable for it to be jacked-up.