3.1. Three-legged semi-submersible floating foundation
(1) ‘Drijfwind’ project
In 2002, a project called ‘Drijfwind’ [36] was sponsored by ECN, MARIN, DUT, TNO, MSC (now part of GustoMSC). With the purpose of improving vertical motion response and reducing the overall construction volume, a three-legged floating foundation, named as
Dutch Tri-floater, was proposed for the first time.
More specifically, the Dutch tri-floater, which is a fully-welded steel structure, used 3 hollow columns to provide necessary buoyance. Each column, whose diameter is 8 m, is composed by two layers of shells. In order to eliminate the possibility of local bucking occurring at external shells, a series of vertical plate stiffeners are attached to the external shell with variety of plating thickness (8–10 mm) according to the height of the columns. In addition, horizontal ring webs are added to increase the integrity and total stiffness of the foundation (see Fig. 4). Each column is