Ship-to-Ship Transfers: How Sanctioned Oil Reaches Global Markets

April 7, 2026 · 7 min read · Maritime Security

Ship-to-ship (STS) transfers are the primary mechanism through which sanctioned crude oil and refined products from Russia, Iran, and Venezuela enter legitimate global supply chains. An STS transfer involves two vessels mooring alongside each other at sea — typically at anchor or while drifting — to transfer cargo via hoses and manifolds without either vessel calling at a port. It is a lawful and routine practice in the oil industry, but it has become the critical enabler of large-scale sanctions evasion.

As of 2026, an estimated 1.5 to 2 million barrels per day of sanctioned crude oil move through STS transfer networks, according to analyses by Kpler, TankerTrackers.com, and the Centre for Research on Energy and Clean Air (CREA). Understanding how these networks operate is essential for port operators, terminal security teams, and compliance officers who must avoid facilitating sanctioned cargo.

How Do Ship-to-Ship Transfers Work?

In a standard STS operation, a laden tanker (the discharging vessel) moors alongside an empty or partially loaded tanker (the receiving vessel) using pneumatic fenders and mooring lines. Cargo hoses connect the vessels' manifolds, and oil is pumped from one to the other. The operation typically takes 24 to 48 hours depending on cargo volume and pumping rates.

STS transfers are governed by guidelines from OCIMF (Oil Companies International Marine Forum) and are standard practice for lightening operations, cargo blending, and logistics optimization. The problem is not the practice itself — it is how it is being exploited.

How Is STS Used for Sanctions Evasion?

The sanctions evasion model follows a consistent pattern that maritime intelligence analysts have documented extensively:

The Originating Vessel

A tanker loads crude oil at a sanctioned origin — Kozmino (Russia), Kharg Island (Iran), or Jose (Venezuela). This vessel is typically part of the shadow fleet: elderly, opaquely owned through shell companies, and insured by non-Western P&I clubs.

The Dark Transit

During the voyage to the STS zone, the originating vessel engages in AIS spoofing or goes dark by switching off its transponder. This obscures the origin of the cargo and makes tracking difficult.

The Transfer Zone

STS operations concentrate in specific locations chosen for their distance from naval patrols and their proximity to major shipping lanes. Key hotspots in 2026 include:

Laconian Gulf, Greece — The largest STS hub for Russian crude in Europe, handling an estimated 400,000 barrels per day.
Ceuta and Gibraltar anchorages — Transfer points for Russian diesel and fuel oil entering the Mediterranean market.
South and Southeast Asia — Waters off Malaysia, Singapore, and Indonesia serve as the primary STS zone for Iranian crude destined for Chinese refineries.
West Africa — STS operations off Togo and Ghana facilitate the blending and re-origination of Venezuelan crude.

The Clean Vessel

After the transfer, the receiving vessel — which may be a mainstream tanker with Western insurance and a clean trading history — delivers the cargo to its final destination. The bill of lading shows only the STS location as the load port, and the crude is often blended or regraded to further obscure its origin.

What Are the Numbers?

The scale of STS-enabled sanctions evasion is staggering:

Russia exported approximately 3.3 million barrels per day of crude by sea in Q1 2026, with an estimated 40% transshipped via STS at some point in the supply chain (Kpler data).
Iran's seaborne crude exports reached 1.6 million barrels per day, nearly all of which involved at least one STS transfer (TankerTrackers.com).
The shadow fleet conducting these operations comprises over 800 tankers, representing approximately 15% of the global VLCC and Suezmax fleet by deadweight tonnage (Lloyd's List Intelligence).
Insurance losses related to shadow fleet incidents have exceeded $200 million since 2023, with environmental damage from aging vessels adding further costs.

Why Should Port Operators Care?

Port and terminal operators face direct regulatory exposure from STS-enabled sanctions evasion.

Receiving sanctioned cargo. If a tanker that received sanctioned crude via STS calls at your terminal, your facility has handled sanctioned cargo — regardless of what the shipping documents say. OFAC, EU sanctions authorities, and HMRC have all issued guidance making clear that due diligence obligations extend beyond paperwork.

Anchorage security. Several STS hotspots overlap with port anchorage areas. If unregulated STS operations occur in your port's anchorage, your facility is implicated in the supply chain.

Environmental liability. Shadow fleet vessels conducting STS operations are disproportionately old, poorly maintained, and under-insured. An oil spill during STS near your port creates environmental and reputational damage even if your terminal was not directly involved.

How Can Technology Help Detect Illicit STS Transfers?

Detecting illicit STS transfers requires fusing multiple data sources:

Satellite SAR imagery can identify vessels moored alongside each other at sea, even when AIS transponders are switched off.
AIS behavioral analysis can flag vessels that drift or anchor in known STS zones, show unexplained draft changes (indicating cargo was loaded or discharged), or exhibit other suspicious patterns.
Dark vessel detection using satellite imagery and RF signals can identify tankers operating without AIS in STS hotspots.
Trade document analysis can flag cargo with inconsistent origin documentation or pricing that suggests sanctioned crude.

The combination of these techniques enables AI-driven monitoring systems to identify probable STS events and alert compliance teams before suspect cargo reaches port.

Key Takeaways

Ship-to-ship transfers are the primary mechanism for moving an estimated 1.5–2 million barrels per day of sanctioned crude into global markets.
STS operations concentrate in specific geographic hotspots including the Laconian Gulf, Ceuta, Southeast Asia, and West Africa.
Port operators face sanctions compliance risk if STS-laundered cargo is received at their terminals.
Multi-source intelligence combining satellite imagery, AIS behavioral analysis, and dark vessel detection is essential for identifying illicit STS activity.
Relying solely on shipping documents and AIS data is insufficient — operators must invest in technology that can verify the actual origin of cargo.