In a further escalation of tension between the United States and Venezuela, President Donald Trump has ordered a naval blockade to stop sanctioned oil tankers from entering and leaving the South American country.

Venezuela – which has the world’s largest proven oil reserves – is highly dependent on revenues from its oil exports to finance its government spending.

But US sanctions targeting Venezuela’s state-run oil company PDVSA have made exporting oil difficult for the Venezuelan government, leading them to resort to a fleet of “ghost ships”.

So what do we know about these vessels and how they operate?

Trump imposed sanctions on Venezuela’s oil industry as far back as 2019, during his first term as president.

That year, Venezuelan crude exports fell by more than half from roughly 1.1 million barrels per day in January to about 495,000 by the end of 2019, according to the US Energy Information Administration.

Six years later, the sanctions remain in place but Venezuela’s oil exports have grown again to around 920,000 barrels per day as of November, according to the news agency Reuters.

While this falls far short of the country’s peak level of oil exports of 3 million barrels per day in 1998, this partial recovery indicates that the sanctions against Venezuela are not working as the US hoped.

It indicates that the government of Nicolás Maduro has found new ways to sell Venezuelan oil with the “ghost fleet” at their centre.

Ghost fleets are a growing phenomenon, used not just by Venezuela but also by two other oil-producing countries under Western sanctions – Russia and Iran.

Financial intelligence firm S&P Global estimates that one in five oil tankers worldwide are used to smuggle oil from sanctioned countries.

Of these, 10% carry only Venezuelan oil, 20% carry Iranian oil, while 50% are exclusively dedicated to Russian oil. The remaining 20% are not tied to any particular country and can transport oil from more than one of these nations.

Oil sanctions aim to discourage countries or companies from buying or dealing with crude oil from sanctioned nations.

Companies and nations caught buying oil from sanctioned countries like Venezuela risk being sanctioned themselves by the US.

Sanctioned countries offer their oil at steep discounts so that companies or nations are willing to take the risk of buying it whilst applying tricks to disguise its origin.

One of the most common strategies ghost tankers use is to frequently change their name or flag – sometimes several times in a month.

For example, the tanker seized this Wednesday is called The Skipper, according to CBS News, the BBC’s US partner.

The ship has been sanctioned by the US Treasury since 2022 for its alleged role in an oil-smuggling network that helps finance Iran’s Revolutionary Guard and the Lebanese militia Hezbollah, according to CBS.

At that time, the tanker was named Adisa, but it was originally called Toyo. It was one of the vessels linked to Russian oil tycoon Viktor Artemov, who is also under sanctions.

The Skipper is a 20-year-old vessel – another common trait among ghost fleet tankers. Major shipping companies usually dispose of ships after 15 years of service, and after 25 years they are typically scrapped.

Another trick these ships use is to steal the identity of scrapped vessels by using their unique registration numbers assigned by the International Maritime Organization – similar to criminals using the identity of a dead person.

These are known as “zombie ships”.

Last April, a ship called Varada arrived in Malaysian waters after a two-month journey from Venezuela.

It raised suspicions because it was a 32-year-old boat and flew the flag of the Comoros, an island nation off east Africa, which is a popular choice among ships that want to avoid detection.

According to a Bloomberg investigation, it was a zombie ship, as the real Varada had been scrapped in Bangladesh in 2017.

The news agency compared satellite images with historical photos to detect four zombie ships carrying Venezuelan crude oil.

Other common tactics include disguising the origin of crude oil by transferring it in international waters to legally compliant tankers with other flags.

These then deliver the oil to its destination, presenting it as coming from a country that is not sanctioned.

This happened with Venezuelan oil exports to China during Trump’s first term when sanctions were tightened.

Another common trick among these tankers is disabling the Automatic Identification System, which transmits data including the vessel’s name, flag, position, speed or route.

This allows ships to hide their identity and location.

Maritime risk company Vanguard Tech says it believes The Skipper was “spoofing its position for a long time” – that is, transmitting a false signal, making it appear to be in a different location.

According to a report in October by the anti-corruption NGO Transparencia Venezuela, there were 71 foreign tankers at the ports of Venezuelan state oil company PDVSA – of which 15 are under sanctions and nine are linked to ghost fleets.

It found that 24 tankers were operating under stealth, with their mandatory location signals deactivated.

The NGO says it detected six ship-to-ship cargo transfers in waters off western Venezuela.

Most of the ships flew flags of countries considered regulatory havens with lax oversight of sanctions, including Panama, Comoros and Malta.

Many spent more than 20 days without docking at an oil terminal, unlike the Chevron-operated ships that the US has authorised in Venezuela, which load and leave within six days.

“The extended stay in port areas without directly reaching oil terminals raises serious doubts about the type of operations these vessels are conducting,” said Transparencia Venezuela in its report.

Given that the operation to seize the ship on 10 December came from the USS Gerald Ford aircraft carrier – the largest in the world – which is now part of the massive US military deployment in Caribbean waters, Maduro’s ability to rely on the ghost fleet is likely to be significantly curtailed.