Abstract: A platform is provided for use in combination with a foundation affixable to a sea bed to provide a fixed/movable marine structure system for use at selected offshore locations. The platform is capable of alternately existing in a fixed mode in which the platform is releasably coupled to the foundation, and in a floating mode in which the platform is uncoupled from the foundation. The platform comprises a working deck, and a supporting structure including buoyancy adjustment means and securing means. The buoyancy adjustment means preferably comprises ballast adjustment means. Also provided is a marine structure system comprising a platform and a foundation, and a method of coupling and uncoupling the platform from the foundation. The system of the present invention is well adapted for use at offshore locations subject to conditions which may require a platform to be both relatively rigid and stable on site at times and to be safely moved off site at other times.