The Tanzanian navy foiled a pirate attack on an offshore oil drilling ship and captured all seven bandits, the national transport authority and the vessel's owner said Tuesday.

The attack on the 228-metre long Ocean Rig Posedion, owned by Ocean Rig of Norway, took place as it was carrying out exploration drilling for Brazilian firm Petrobras.

"The attack occured at around 8:15pm on Monday, 23 nautical miles south-east of Mafia (island), about 82 miles south-east of Dar es Salaam," the Surface and Marine Transport Regulatory Authority (SUMATRA) said in a statement.

"The naval troops guarding the ship responded, overpowered the attackers and captured them," SUMATRA said in a statement.

"The security plan worked as expected," a spokesman for Ocean Rig told AFP by telephone.

In September 2010 pirates attacked a drill ship exploring for British Ophir Energy off the southern port town of Mtwara. The navy repulsed the attack and captured one pirate.