A semi-submersible structure is adapted to carry consumable solid or liquid material such as mud, and removable equipment, for example drilling tubulars. Both consumables and removable equipment are removed from the structure during use, thereby reducing the structure's total mass. The structure is provided with pontoon means (4, 8) containing a first set of compartments (66, 68, 70, 72, 84, 86, 88) for consumable material and ballast control means for introducing ballast into the first set of compartments to compensate for the removal of consumable material. The structure also includes dedicated ballast tanks (40, 42, 56, 106, 108), but the arrangement is such that, if the first set of compartments is empty, and no consumable material or removable equipment is being carried by the structure, the filling of the ballast tanks with ballast will not cause the structure to be submerged to an extent sufficient to achieve a semi-submerged condition (the structure therefore still being over-buoyant), whereas said condition would be achieved if the first set of compartments (66, 68, 70, 72, 84, 86, 88) were also at least partially filled. The structure thus includes "dual purpose" compartments which can accept either consumable materials or ballast, thereby increasing the effective deck load of the structure.
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