The shadow fleet, a group of tanker ships used by the likes of Iran and Russia to sell oil on the global market despite Western sanctions, is growing.

These ships often operate in a legal gray area. And they now comprise a whopping 17% of all international tankers worldwide, according to a report from Allianz.

In the early 2010s, the West slapped big sanctions on Iran’s energy exports. So to keep its oil revenue flowing, Iran turned to the so-called shadow fleet, said Elisabeth Braw, a senior fellow at the Atlantic Council.

“It really began as a way for Iran to export and import because it was under so many sanctions,” she said.

Braw said Russia now also uses shadow vessels to sell its oil — the country was sanctioned for invading Ukraine in 2022. The size of the international shadow fleet has more than doubled since then.

“They’re essentially like a tumor on the global shipping system,” said Braw.

These shadow vessels commonly use deceptive tactics. They spoof GPS, for example, to claim they’re loading oil at an Iraqi port when they’re actually in Iran. They carry fake insurance documents. And they transfer their cargo from one ship to another, to another, just to confuse anyone watching by satellite.

And Braw said these vessels aren’t exactly in ship-shape.

“They’re often old,” she said. “They should really be scrapped. An old vessel that is poorly maintained poses significant risks of accidents and incidents, just like an old car.”

There have already been some minor collisions and oil spills caused by shadow fleet ships. Tomer Raanan, a maritime risk analyst at Lloyd’s List Intelligence, fears that a major environmental disaster could be next.

“It’s a massive accident waiting to happen,” Raanan said.

And because these ships are underinsured at best, clean-up costs aren’t borne by the operators of the shadow vessels, said Joshua Tallis with the Center for Naval Analyses.

“All of those costs are gonna fall on the international community and most likely, quite heavily off the coast of whatever country that accident actually happens on,” Tallis said.

The continued growth of these risks question whether western sanctions have had their desired effect, said Ian Ralby, CEO of the maritime consultancy I.R. Consilium.

“No amount of sanctions have put Russia, Iran, North Korea, or anyone out of business,” Ralby said. 

Ralby added that intercepting these shadow ships isn’t a great option. That would go against the principle of freedom of navigation on the high seas.

“Most Navy's coast guards and marine police forces around the world are not gonna have an impetus to try to take on this kind of situation,” Ralby said, “particularly given the extreme political blowback that would accompany any kind of interdiction.”

For now, the shadow fleet continues to sail.