The global piracy watchdog said this month that 44 pirate attacks have been reported in 2012. There were 25 in 2011.
The bureau also said many other attacks have gone unreported in the Atlantic waters off West Africa.
Most of the pirate attacks appear to be the work of Nigerian crime syndicates that have been operating in their own oil-rich country for years.
"Pirates in West Africa is a serious problem," said IMB Director Pottengal Mukundan. "Pirates are getting quite audacious, with increasing violence being used."
While the high-profile piracy by Somali gangs in the Gulf of Aden and the Indian Ocean appears to be declining, largely due to the intervention of international naval task forces, attacks off West Africa are increasing as the region's oil production steadily mounts and tanker activity intensifies.
The countries around the Gulf of Guinea are reportedly losing $2 billion a year to maritime crime.
On Aug. 18, the British tanker Anuket Emerald was seized by 16 heavily armed pirates in the Gulf of Guinea, the marauders' main operational zone, off Lome. Most of its cargo of 3,450 tons of oil was transferred to another tanker, Nigerian authorities reported.
The pirates held the Anuket Emerald for five days before releasing it. Some of the stolen oil was traced to a tank farm near Lagos, Nigeria's main port and commercial center.
Ten days later, the 74,990-ton oil tanker Energy Centurion was seized by pirates as the vessel lay in the Lome Anchorage off the Togo coast. After a brief gun battle with a Togolese navy vessel, the marauders made off with the Greek-owned tanker and its crew of 23 Russians and their Greek skipper.
They stole 3,000 tons of its 50,000-ton cargo of oil products and disappeared. A French warship found the tanker the following day off the coast of Togo, its crew unharmed.
"It's not piracy; it's robbery," an official of the owner-operators, Golden Energy Management in Athens, declared. "The tanker was carrying gasoil and the robbers just wanted the cargo."
In another incident Sept. 5, 20 gunmen in four speedboats struck the Singapore-flagged tanker Abu Dhabi Star 14 nautical miles off Lagos.
"Our radar picked up four unlit boats," the tanker's Indian skipper, Capt. Aron Chandran, told the BBC.
"They were much bigger than the standard boats here. They had twin engines and approached very fast ... on both sides of the ship. Each boat had five well-armed people on board."
The 23-man Indian crew took refuge in a protected room, known as the citadel.
The ship's operator, Pioneer Ship Management Services of Dubai, said the pirates stole several thousand tons of its U.S.-bound cargo of Nigerian crude and fled 10 hours after the hijacking.
The attacks on fuel-laden tankers can be quite brazen. On Aug. 28, pirates seized another Greek-owned tanker off Togo as an anti-piracy conference was being held in the Togolese capital, Lome. The raiders offloaded 3,000 tons of fuel onto barges and disappeared along the swampy coastline.
None of the crewmen aboard the tankers were harmed in these attacks but the pirates frequently use violence and several crewmen have been slain this year.
But unlike the Somali pirates who've been operating on the other side of the continent since 2005, the marauders off West Africa don't generally hold onto ships they seize or detain crews for ransom.
They focus on stealing the tankers' oil, which they load aboard larger tankers operated by criminal syndicates, many of them based in Nigeria, to sell in African, Asian and European markets using forged documents.
Some maritime security specialists suspect many of the attacks are masterminded by a single syndicate with insider access to oil industry and tanker company data on sailings and cargoes.
Nigerian syndicates have been conducting such operations for years in the Niger Delta, the country's main oil-producing zone, where they steal thousands of barrels of oil every day.
Industrial-scale theft of oil in Nigeria, Africa's leading oil producer, has become a national security threat, with the state losing $1 billion a month.