Abstract: A marine platform is described incorporating a submerged buoyancy chamber which is anchored to drilled piles in the sea bed by cables. Each anchorage point lies inside a working chamber mounted on the pile to enable workmen inside the chamber to release the coupling between the cable and the pile when the cable needs to be replaced. The buoyancy chamber and the working chamber both have airlocks to enable the transfer of workmen between each chamber and a submersible craft. The buoyancy chamber supports a deck which lies above sea level. The deck is in fluid communication with a wellhead on the sea bed by means of risers. Each riser incorporates two concentric tubes. The inner tube carries oil from the wellhead and the other tube contains a hydraulic fluid through which valves at the wellhead can be controlled from the deck such that the valves close if riser rupture releases hydraulic pressure.