A spar platform for use in the offshore drilling or production of fossil fuels includes a hull having a centerwell. An airtight and watertight barrier traverses the centerwell, forming a variable buoyancy compartment in the centerwell. In certain embodiments the centerwell is open at the bottom to the sea, while in certain other embodiments the centerwell is sealed at the bottom. At least one sleeve for accommodating a riser extends through the barrier and the variable buoyancy compartment to the bottom of the centerwell. The sleeve has an open upper end to provide a drain for water accumulating in the centerwell, and it forms an airtight and watertight seal at its juncture with the barrier. In some embodiments, two or more airtight and watertight barriers are provided across the centerwell, defining an airtight and watertight fixed buoyancy chamber between each adjacent pair of barriers.