 The "Breton Rig 20" was 160 feet by 85 feet, and could work in 20 feet water depth.By 1958, the number of submersible drilling rigs had increased to around 30.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.