Abstract: There are disclosed embodiments of hollow-walled structures for marine use, and particularly of form a semi-submersible vessel for use in the off-shore industry. The structure has concrete walls of hollow structure, and void formers are incorporated in at least some of the concrete walls to provide controlled variation in wall density to meet design requirements. In the application of the invention to a semi-submersible vessel, void formers are cast in situ within the concrete walls of upstanding columns of the vessel, and in which the volume and/or distribution of the void formers in the walls varies in such a way that the density of the columns decrease with height above the base.