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 .