Submersible drilling rig

Once on the location, the pontoon structure is slowly flooded until it rests securely on its anchors, of which there are usually two per corner.

The operating deck is elevated 100 feet above the pontoons on large steel columns to provide clearance above the waves.

After the well is drilled, the water is pumped out of the buoyancy tanks and the vessel is re-floated and towed to the next location.

[2] In 1961, Shell Oil successfully converted an existing submersible rig Blue Water Rig No.1 into the first semi submersible drilling unit for operation in the Gulf of Mexico when it was found to have good stability and motions whilst being towed at a partial draught.

[3] Alden J. Laborde designed and constructed the first purpose-built V-shaped semi-submersible drilling rig, Ocean Driller, delivered in 1963.