The Abu Dhabi Star is the latest ship to have been attacked in the Gulf of Guinea in recent weeks, highlighting a growing threat in the region.
A total of 40 attacks have been reported this year – many others are hushed up.
News that the vessel was carrying petrol from Nigeria to the US has also raised eyebrows. Nigeria is a major exporter of crude oil but generally imports its refined fuel products.
Some of the 23 crew described the terrifying moment when, in the darkness, they realised the pirates were approaching.