The semisub was operating for Venezuela's state-run producer PDVSA at the Mariscal Sucre complex development under a five-year contract which came into effect last year.
Venezuela's Oil Minister Rafael Ramirez later told Reuters that the accident was caused by water entering a subsea pontoon, adding that that there are plans to try to retrieve the unit.
In his tweet, Chavez said all 95 crew on the rig were evacuated safely.
