The incident involving the 45,879-dwt Sea Hermes (built 2004) took place on 30 March but details have only just emerged.
Details of the attack, which took place off Bonny, feature in the latest piracy report from security consultants AKE Maritime.
In another incident last week on 4 April an unnamed supply vessel was reportedly attacked about 57 nautical miles west southwest of Brass, Nigeria.
“Both attacks highlight the ongoing risk to both large commercial vessels and smaller supply ships off Nigeria, but also underscore the lack of adequate regional piracy reporting and information sharing,” AKE Maritime said.
“All vessels are advised to ensure defensive measures are in place, particularly at anchor, when drifting, or when carrying out ship to ship transfer (STS) operations.”
News of the attacks come as the Movement for the Emancipation of the Niger Delta (MEND) has threatened to resume attacks against the local oil industry.
MEND, one of the largest militant groups in the Niger Delta region, is making the threat in response to the sentencing of the leader of the group by a South African court.
Maritime Asset Security and Training (MAST) is warning organisations operating in the Niger Delta, to heed the increased threat over the coming days and possibly weeks.
“We have reason to believe that this is a serious threat by MEND and have advised our clients operating in Niger Delta to act accordingly,” said MAST chief executive Philip Cable.
“Whilst the Nigerian Joint Task Force has redeployed forces to counter the threat, we would urge organisations to take all possible measures, within national and international law, to protect their facilities and personnel over the coming days.”
Arctic Securities analyst Erik Stavseth says the resurgence of MEND attacks could constrain West African crude exports.
“Although the impact is highly uncertain, MEND’s previous attacks have been known to reduce volumes out of Nigeria and hence impact the crude tanker market in the larger sizes.”
Nigeria derives some 80% of its revenue and almost all of its export earnings from the oil and gas industry.