In a general aspect, a method is presented for manufacturing support structures for offshore wind turbines. In some implementations, the method includes constructing a plurality of modular sections that assemble to define the support structure. One or more of the plurality of modular sections are configured to anchor to an underwater floor. At least one of the plurality of modular sections is constructed by operations that include forming a wall along a perimeter to bound a volume, filling the volume with a castable material, and hardening the castable material. In some instances, forming the wall includes depositing layers of printable material successively on top of each other. The method also includes joining the plurality of modular sections to assemble the support structure.