OKITIPUPA, NIGERIA (AP) — Until early last year, a rusting oil ship named the Trinity Spirit floated off the coast of Nigeria, pulling crude oil from the ocean floor. Then, last February, it exploded, collapsing into the ocean along with 40,000 barrels of oil.

Five workers were killed and two others presumed dead, their bodies never found. Oil slicks were visible in satellite imagery for days.

The Associated Press drew on ship databases, court documents, and the accounts of three survivors to offer an inside look at the yearslong decline of the aging ship, the numerous warning signs, and the explosion’s messy aftermath — as the survivors, who complained of dangerous working conditions and withheld wages, were accused of setting the ship ablaze.

The Trinity Spirit also fits a wider pattern of old tankers put to work storing and extracting oil even while on the brink of mechanical breakdowns. Here are takeaways from AP’s report.

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This story was supported by funding from the Walton Family Foundation. The AP is solely responsible for all content.

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WHAT KIND OF SHIP WAS THE TRINITY SPIRIT?

The Trinity Spirit was part of a class of vessels that extracts oil offshore and stores it at sea. They are known as floating production storage and offloading units — FPSOs — or as FSOs, floating storage and offloading units, when used only for storage. Since the 1970s, they’ve become increasingly popular for developing oil in deep waters and in places where no pipelines exist. According to the environmental group SkyTruth, there are some 240 in operation today.

FPSOs are unlike most ships for one key reason: They stay in place. Once attached to the ocean floor, they can linger at the same oil field for years or even decades. Many spent the first half of their lives as oil tankers, and were later repurposed into stationary ships.

WHAT CAUSED THE OIL SHIP TO EXPLODE?

Nigerian authorities haven’t published a conclusive reason as to why the Trinity Spirit exploded, but photos of the ship shortly before it exploded and the accounts of three survivors point to it being in a state of near-total disrepair. The engine room flooded twice, one crew member told AP, and the main generator plant was damaged and never repaired. The surviving crew members said maintenance had all but stopped on the 46-year-old ship.

Though a company that had operated the ship accused two surviving crew members of illegally storing oil on the ship and setting it on fire, the two men told AP they were sleeping when the explosion happened. The criminal charges against them were later dropped.

HAVE THERE BEEN BREAKDOWNS ON OTHER OIL SHIPS?

Yes. The AP found at least eight oil ships that have been shut down after a fire, a major safety hazard, or the death of a worker in the last decade. That figure includes an FPSO called the Bunga Kertas, floating off the coast of Malaysia, which paused operations in the same month that the Trinity Spirit caught fire because “integrity issues” were discovered in the ship’s hull. Soon after, a diver involved in the repair process was killed.

According to press coverage, the Bunga Kertas was at one point intended for use only through 2014. Yet the safety issues were discovered in 2022.

Until this fall, another oil ship, the FSO Safer, had for years risked a catastrophic spill in the Red Sea. “It could break up at any time – or explode,” the United Nations said in a statement this spring. The Safer was built in the same year as the Trinity Spirit and fell into disrepair while it was still carrying more than a million barrels of oil.

WHAT ABOUT OIL SHIPS THAT ARE STILL OPERATING?

More than 30 ships are older than the Trinity Spirit and still operating around the world, according to AP’s review. Among them is the Al-Zaafarana, floating off the coast of Egypt, which at 54 years is one of the oldest FPSOs still in service. Close behind it are oil ships in Malaysia and Brazil, each at least half a century old. In Nigeria, the FPSO Mystras is still in service at 47 years old, despite industry reports that the ship was originally built to operate only through 2014.

As a fleet, the ships are getting older. The average hull age of FPSOs has increased from 22 to nearly 28 years since 2010, according to Rystad Energy, and in 2021, the American Bureau of Shipping said several dozen ships were nearing the end of their intended lives.

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Contact AP’s global investigative team at [email protected] or https://www.ap.org/tips/