Abstract: An off-shore oil drilling and pumping platform supported on a downwardly opening leakage control tank assembly which cooperates with the sea bottom to define a substantial enclosure for containing a source of oil leakage near the sea bottom. Collapsible and distensible air bags are attached to the tank assembly above the top wall of the tank for movement between active and inactive positions. Conduits are provided for introducing fluid under pressure into and releasing fluid from the bags to respectively distend or collapse the bags. In inactive position, the bags are collapsed and disposed generally adjacent the top wall of the tank. In active position the bags are distended and extend generally laterally outwardly from the tank assembly to aid in underwater mobility in placing and installing the platform.