There are tons of pirates in Indonesia, raiding freighters and oil tankers in the Straits of Malacca, between Indonesia, Malaysia, and Singapore. To a less extent, there's also piracy in other parts of Southeast Asia, the Horn of Africa (as in this story), and pretty much anywhere that it's easy to get fast engines for small boats, commercials ships are confined to a small area, AK-47's are easy to get and cheap, and the government is corrupt and/or incompetent.
The typical raid is a small boat or two with a dozen locals with AK-47's. Since the crew of an oil tanker is maybe 10 guys at most, and there's not typically a lot of weapons on a tanker (though captains usually do have guns), it's fair easy for them to overpower the crew. They then steal the personal possessions of the crew, the payroll (still often in cash), and anything that's not nailed down. Often, they will also kill the crew and steal the whole ship and cargo, bringing it port under a false name.
The Simpsons had a episode about all this, in which Homer was partying on Mr Burn's yacht when Asian pirates attacked. They stole the yacht, tossed Homer overboard, and headed for "Secret Pirate Island, AKA Hong Kong".