US seizure of oil tankers: The shadow fleet that transports sanctioned oil, and how it operates

While seizure of sanctioned tankers using military force is an extreme step and has evoked strong responses from countries like Russia and China, the Donald Trump administration in Washington has said that it will continue to seize sanctioned oil tankers.

Shadow fleet vessels have been the backbone of the transportation of oil and petroleum products from countries like Iran, Venezuela, and Russia, which are under various degrees of sanctions imposed by the Western powers, primarily the US. Shadow fleet or opaque fleet are terms used to describe vessels that use various deceptive maritime practices and opaque business and ownership structures in order to conceal their true ownership and actual origin of the commodity being transported. The ultimate aim is to evade sanctions or restrictions by operating in a manner that makes tracking, detection, and enforcement difficult and complicated. According to various estimates, the shadow fleet comprises over 3,000 vessels.

‘Dark’ and ‘grey’ shadows

Shadow fleet is a broad term that is used to refer to vessels exhibiting deceptive behaviour in order to avoid sanctions enforcement, even as they transport sanctioned or restricted commodities. Within the larger shadow fleet ecosystem are two broad categories of vessels — dark and grey fleet — as per maritime intelligence provider Windward, even as these terms are often used interchangeably.

“The dark fleet represents the highest-risk, non-cooperative core. These vessels deliberately hide activity through AIS (Automatic Identification System)-off behaviour, GNSS (global navigation satellite system) manipulation, false or fraudulent flags, identity laundering, and covert ship-to-ship transfers. They are central to opaque and sanctions-linked commodity flows,” Windward said.

AIS is a transponder system that transmits a vessel’s identity, position, path, and other important data so that other vessels and authorities can track movements on a real-time basis.

Windward added that the grey fleet is a newer, behaviour-based category that emerged after the Russia-Ukraine war. Grey fleet vessels are not necessarily sanctioned and are not automatically off-limits for business, but they exhibit risk indicators like irregular trading routes, rapid ownership changes, high-risk port sequences, or structural ambiguity designed to appear compliant.

A large number of shadow fleet tankers have complex and obscure ownership structures and are registered in geographies with lax regulatory oversight, like Gabon, the Marshall Islands, the Cook Islands, Liberia, Panama and even landlocked Mongolia. Often, such vessels, which are usually quite old with an age of over 15 or 20 years, operate with substandard insurance and certifications.

It is worth noting that a mysterious Mumbai-based shadow fleet shipper—Gatik Ship Management—had emerged out of nowhere to become one of the largest transporters of Russian crude between 2022 and 2023. At its peak, Gatik was the commercial manager of around 60-odd crude oil and petroleum product tankers, valued at over $1.5 billion. By August 2023, however, it had transferred all the tankers it managed to a web of related companies in the wake of the global attention and scrutiny it attracted for moving Russian oil.

Tell-tale signs of shadow fleet operations

Shadow fleet tankers often display a combination of deceptive behaviour aimed at making the identification of their location and cargo difficult. Agencies tasked to identify sanctions evasion and enforce sanctions look for these signs to ascertain whether a tanker was involved in a sanctioned transaction. If so, the vessels and those associated with them are usually sanctioned. But outright seizure of tankers using military force was previously almost unheard of.

Shadow fleet tankers frequently disable their AIS transponders during high-risk activities like entering sanctioned ports or conducting transfers, making it difficult to pinpoint their location while they are participating in a sanctioned or restricted commodity transfer. At times, they are also involved in GNSS spoofing—broadcasting false location data—that makes them appear hundreds of miles away from their actual location.

These vessels are often involved in ship-to-ship (STS) transfers of oil. An STS transfer by itself doesn’t mean that the ships involved are suspicious, as there can be various logistical reasons for such transfers as well. But in the case of shadow fleet tankers, STS transfers are often done in high seas far away from ports’ oversight, and frequently involve mixing sanctioned crude with non-sanctioned oil, or doing a number of STS transfers between multiple vessels with the objective of obscuring the true origin of the oil.

These vessels also frequently change their country of registration, often between countries with minimal oversight. Some even operate under fraudulent flags. Frequent changes in the vessels’ names and companies managing the vessels have also been observed. In the maritime industry, vessel ownership structures can be extremely opaque, which often renders it impossible to trace the actual owner of a ship and those operating the tankers.

There are various ownership-related categories in the maritime industry, including commercial managers, registered owners, and beneficial owners. Put simply, the commercial manager is the effective manager of the fleet and is responsible for the commercial decisions pertaining to the vessels. The registered owner of the vessel is the company in whose name the vessel is registered in an international shipping registry. Often, registered owners are located in offshore safe havens for tax-related purposes and to maintain secrecy. The beneficial owner is the ultimate owner of a vessel, but its name is often buried deep under layers of interconnected offshore shell companies.

According to commodity market analytics firm Kpler, between January and October of 2025, 904 vessels were sanctioned by the US, European Union, or other Western powers. Of these, 234 were sanctioned after dark STS transfers, 261 after location

spoofing, and 168 after dark port calls. Dark STS transfers refer to cargo transfers between ships while their AIS transponders are off. Similarly, dark port calls mean a vessel’s approach, docking, or leaving a port with the AIS transponder off.

Kpler’s analysis showed that as of October 2025, out of 2,974 vessels that were not sanctioned but still exhibited at least one suspicious behaviour, 302 stood out as facing a high risk of facing sanctions within the next 12 months.

“The 302 high-risk vessels are all tankers with an average age of just over 20 years. Panama (101), Liberia (29), and Palau (24) dominate (as vessel flags)…nearly half, 137 vessels, operate without clear P&I (protection and indemnity insurance) coverage,” Kpler said in a report from November.