The present invention provides a controllable float module for a modular offshore support structure assembly. The inventive float module comprises (i) a first ballast chamber provided within a predetermined first portion of said float module, having at least one first controllable fluid connection, adapted to provide controlled fluid flow between an interior of said first ballast chamber and a first fluid reservoir containing a first fluid, and at least one second fluid connection, adapted to provide controlled fluid flow between said interior of said first ballast chamber and a second fluid reservoir containing a second fluid; (ii) at least one second ballast chamber provided within a predetermined second portion of said float module, having at least one first controllable fluid connection, adapted to provide controlled fluid flow between an interior of said second ballast chamber and said first fluid reservoir containing said first fluid, and at least one second fluid connection, adapted to provide controlled fluid flow between said interior of said second ballast chamber and said second fluid reservoir containing said second fluid, and (iii) a control system, adapted to control each of said at least one first and second controllable fluid connection and selectively vary the proportional quantity of said first fluid and said second fluid within any one of said first and at least one second ballast chamber, so as to selectively vary the buoyancy of any one of said at least one predetermined first and second portion of said float module when in use.