An oil tanker runs aground off eastern China, leaking fuel into the water. Another is caught in a collision near Cuba. A third is seized in Spain for drifting out of control.

These vessels were part of a "shadow" fleet of tankers carrying oil last year from countries hit by Western sanctions, according to a Reuters analysis of ship tracking and accident data and interviews with more than a dozen industry specialists.

Hundreds of extra ships have joined this opaque parallel trade over the past few years as a result of rising Iranian oil exports as well as restrictions imposed on Russian energy sales over the war in Ukraine, said the industry players, who include commodity traders, shipping companies, insurers and regulators.

"The risk of having an accident is definitely going up," said Eric Hanell, CEO of tanker operator Stena Bulk. "We might be affected being at a port... because someone is running into us or loses control, which is a much bigger risk on those kinds of ships because they are older and not as well-maintained."

Many leading certification providers and engine makers that approve seaworthiness and safety have withdrawn their services from ships carrying oil from sanctioned Iran, Russia and Venezuela, as have a host of insurers, meaning there's less oversight of vessels carrying the flammable cargoes.

Some industry figures fear this parallel trade carrying tens of millions of barrels of oil around the world could undermine decadeslong industry efforts to increase shipping safety following disasters including the 1989 Exxon Valdez spill in Alaska, which caused devastating environmental damage.

Last year there were at least eight groundings, collisions or near misses involving tankers carrying sanctioned crude or oil products, including the events off China, Cuba and Spain, according to a Reuters analysis based on ship-tracking information and Lloyd's List Intelligence data on vessel incidents.

That's the same number as the previous three years combined, although still a fraction of the overall 61 incidents recorded across the whole shipping industry in 2022, the analysis found.

None of the eight incidents caused any injuries or significant pollution. Some executives are worried, though.

"You have the dark fleet which has not been vetted so much and that is a concern," said Jan Dieleman, president of commodities group Cargill's ocean transportation division. "We do not have visibility on maintenance and safety as no one is really boarding the ships and doing checks - that is missing."

Government officials from Iran, Venezuela and Russia, which do not recognize Western sanctions, didn't immediately respond to requests for comment for this article.

Several of the shipping players interviewed said oil producers hit by sanctions had little choice but to use less tightly vetted vessels to keep their exports flowing and shore up their stumbling economies.

Invisible feet?

Estimates of the size of the shadow fleet vary, with industry participants putting the number at anything from more than 400 to north of 600, or roughly a fifth of the overall global crude oil tanker fleet.

"Our data shows that it has now reached around 650 units," said Andrea Olivi, head of wet freight at commodity trader Trafigura, which estimates that two-thirds of that number are crude tankers.

The number of tankers transporting Iranian crude and products – excluding the state's own fleet – has risen to above 300 this month from 70 in November 2020, said Claire Jungman, chief of staff at U.S. advocacy group United Against Nuclear Iran (UANI), which tracks Iranian-related tanker traffic via satellite data.

Iran's oil minister said this month that the country's oil exports had reached their highest level since the reimposition of U.S. sanctions in 2018, with 83 million more barrels exported in the past year versus the year before.

Meanwhile, economic penalties imposed on Moscow by Washington and other Western capitals over the Ukraine conflict have led to dozens more ships plying the shadow trade, the industry participants said.

Some cautioned that the size of the shadow fleet was becoming more difficult to gauge given the complex layers of compliance around sanctions on Russian oil, which is banned from many Western ports and subject to a price cap by G-7 countries.

Reuters was unable to independently verify the numbers regarding the size and growth of the shadow fleet.

The U.S. Treasury didn't immediately respond to a request for comment on ships carrying sanctioned oil. A State Department spokesperson said the U.S. strove to identify sanctions evasion in the shipping sector in an effort to bolster navigation safety and minimize the risk of environmental hazards.

‘Endangered waters’

Among the eight incidents identified last year, the Linda I tanker carrying Russian oil was detained at the southern Spanish port of Algeciras in November, according to the Reuters analysis.

Spain's Merchant Fleet authority confirmed the incident and the cargo, telling Reuters the vessel had been authorized to pick up spare parts outside port limits but was found drifting toward anchored ships due to navigation system faults.

"The vessel was detained for having endangered the vessels anchored in its vicinity and for a series of deficiencies," said the agency, part of the transport ministry.

The Linda I was also in contravention of U.N. pollution regulations by not having an exhaust gas cleaning system, or scrubber, while using high-sulfur marine fuel, said the Merchant Fleet, which fined the ship 80,000 euros ($85,800) and detained it from Nov. 2 to Dec. 27 while its faults were fixed.

The Linda I's owner, Spastic Oceanway, is listed in the Equasis public shipping database as care of Chanocean Management. There was no reference to either company at Chanocean's corporate office listed in downtown Hong Kong when a Reuters reporter visited the building.

In eastern China, the Arzoyi tanker – which UANI analysis showed was carrying Iranian oil – ran aground while unloading at the Qingdao Haiye Mercuria Terminal on March 23 last year, causing a small oil spill in port waters, according to data from Lloyd's List Intelligence.

Three days later, the Petion carrying Venezuelan crude from the country's Jose port was involved in a collision with another tanker off the Cuban port of Cienfuegos, although the cause wasn't clear, according to the Reuters analysis.

Most of Venezuela's oil exports are subject to U.S. sanctions.

The Arzoyi's owner, listed as Panama-based owner Vitava Shipping, couldn't be reached for comment, while there are no contact details listed for the Petion.

Chinese and Cuban maritime authorities didn't immediately respond to requests for comment.

The potential perils posed by the shadow fleet were shown in 2021 when Israel said a tanker transporting Iranian oil spilled its cargo in the eastern Mediterranean, causing ecological damage to a swathe of coastline.

Ship-to-ship transfers

Around 774 tankers out of 2,296 in the overall global crude oil fleet are 15 years old or more, according to data provider VesselsValue.

Although it is not known how many of those older vessels are part of the shadow fleet, the strict vetting policies of oil majors and commodity traders mean they typically use tankers aged under 15 years.

Some industry participants said ship-to-ship (STS) transfers of oil and other fuel cargoes involving shadow tankers at various locations at sea, outside the oversight of port authorities, posed significant safety and environmental risks.

In 2019, two tankers caught fire in the Black Sea region while transferring fuel at sea, leaving at least 10 crew dead, after one vessel was barred from using a port due to U.S. sanctions.

"We are seeing older vessels with unknown technical management companies performing STS in the middle of the Atlantic," Trafigura's Olivi said.

"The risk of a major pollution incident is very high."