A ship-to-ship transfer moves cargo from one vessel to another without either entering a port. It is a routine commercial operation: it is used for lightering, where a large vessel discharges part of its cargo so it can enter a shallow port; for consolidating parcels; and for offshore loading at terminals with no jetty.
Done normally it is announced in advance, insured, conducted in daylight, logged by both masters, and attended by a surveyor who records what moved. Almost every element of that description generates an independent record held by somebody who is not a party to the scheme, which is the practical reason the irregular version dispenses with them.
How it works
The transfer itself is unremarkable seamanship. What makes it a technique is where the documentary chain sits.
A cargo loaded at a restricted origin and discharged at a port arrives with a customs entry, a consignee, an origin certificate and an inspection. A cargo transferred at sea arrives at its second vessel with whatever documents the parties choose to create. There is no port state, no customs authority and no independent surveyor unless someone arranges one.
That makes the transfer the point at which a cargo’s documented history can be replaced. It is also the point at which cargoes of different origin can be combined, which is why blending and ship-to-ship transfer appear together throughout the enforcement literature.
What the advisories actually say
The 2020 global advisory issued by the Departments of State and the Treasury and the US Coast Guard treats ship-to-ship transfers as a category of deceptive practice1 , and it is careful about the qualifier: such transfers are frequently used to evade sanctions especially at night or in areas determined to be high risk for sanctions evasion. It does not say that transfers in poor weather, or outside designated transfer areas, are themselves indicators. An earlier version of this page said it did; that was wrong, and the correction is recorded here rather than made silently.
The 2019 Syria shipping advisory states the underlying point plainly: ship-to-ship transfers can conceal the origin or destination of cargo2 . The 2023 Price Cap Coalition advisory repeats it and asks industry to conduct enhanced due diligence around such transfers, especially in areas at higher risk for illicit trading activity or AIS manipulation3 .
Each element has an innocent explanation on its own. Weather windows close, anchorages get congested. What the documents describe is a combination — an at-sea transfer1 , in an area assessed as high risk, by vessels whose behaviour is irregular in other respects — and it is the combination rather than any single feature that carries weight.
The voyage-level version of the same point is in the 2020 advisory, not the 2019 Syria advisory as this page once stated. The 2020 document has a section on voyage irregularities4 covering indirect routing, unscheduled detours and transshipment of cargo through third countries5 , and it separately flags suspicious deviations in routes — changes with no legitimate reason to go off-route, such as unsafe ports, extreme weather or emergencies6 .
Why AIS behaviour and transfers travel together
A transfer at sea requires two vessels to be in the same place at the same time. If both are transmitting position, the meeting is visible in the record and can be reconstructed afterwards. That is why irregular transfers and transponder gaps appear together so consistently in the published material: the second technique exists to conceal the first.
The consequence for detection is that the gap itself becomes the evidence. A vessel that stops transmitting, resumes some hours later at a different draught, and cannot account for the interval has produced a documentary problem that the absence of data does not solve.
How it is caught
Track reconstruction. Two vessels converging, holding station together and separating is a recognisable pattern in position data, and the resulting draught change indicates whether cargo moved. Where transmission stops, the entry and exit points of the gap bound the possible transfer location.
Physical observation. Satellite imagery and maritime patrol resolve directly what position data infers. Where AIS is out or suspected of being manipulated, the 2023 advisory recommends that stakeholders with access to long-range identification and tracking use it to determine a vessel’s true location7 .
Draught and volume reconciliation. A vessel’s reported cargo, its draught readings and the volumes recorded at discharge should reconcile. Where a vessel discharges more than it loaded, something was taken on in between.
Documentary contradiction. The cargo documents assert a voyage. Where that voyage cannot have happened as described, the transfer is the missing step.
Identifying which specific vessels were involved, their ownership and their registry history is vessel-level work and is not carried on this site.
What a compliant transfer looks like
Setting out the legitimate version is the clearest way to explain the illegitimate one, because the indicators are all absences from this list.
A conventional ship-to-ship operation is planned in advance and notified to the coastal state where one has jurisdiction. It is conducted in a designated transfer area, in daylight, within stated weather and sea-state limits. Both vessels have a joint plan of operation and compatible equipment. A superintendent or surveyor attends and records the quantities moved. Both vessels’ insurers know it is happening, because an unnotified transfer can void cover. Both masters log it. The cargo documentation is amended to reflect the transfer, and the receiving vessel’s papers show where the cargo came from.
Almost every one of those steps produces an independent record held by somebody who is not a party to the scheme, which is why the irregular version dispenses with all of them.
Why the transfer is the documentary hinge
The reason this operation matters more than its physical simplicity suggests is that it is the only point in a voyage where cargo changes vessels without passing through a jurisdiction.
Cargo loaded at a terminal has a loading port, a shore tank, a terminal record, a customs entry and a quantity certificate. Cargo discharged at a terminal has the same on the other end. Between those two points, at sea, there is no authority, no inspector and no register — and a cargo that arrives at its second vessel there arrives with whatever documents the parties have chosen to create for it.
That is why transfer, blending and origin fraud travel together. The 2020 advisory names falsifying cargo and vessel documents12 as its own category of deceptive practice, and the 2023 advisory records that transfers can be used to conceal the origin or destination of cargo3 . They are not three techniques that happen to co-occur; they are one operation described at three levels.
The insurance and flag correlation
A vessel conducting irregular transfers has a problem beyond the transfer itself: mainstream insurers underwrite by reference to trade, and a vessel whose declared trade does not match its actual voyages is either misrepresenting to its insurer or has left the mainstream market.
The indicators do cluster, though across two documents rather than one, and it is worth being exact about which says what. The 2020 advisory covers transfers, AIS manipulation8 and flag hopping9 together. Unverifiable insurance is not in the 2020 advisory: it is the 2023 Price Cap Coalition advisory that describes ships relying on unknown, untested, sporadic or fraudulent insurance10 .
Read together the two describe a recognisable sequence: the trade changes, mainstream cover becomes unavailable, the vessel reflags to a registry with lighter requirements and takes cover that cannot be confirmed, and the transponder behaviour changes to match.
Any one of those has an innocent explanation. The sequence does not.
Where the specificity comes from
Financial regulators publish indicators; UN Panels of Experts publish evidence, and the reports of the 1718 Committee’s panel are the fullest public record of at-sea transfers in a sanctions context. The Panel’s 2019 midterm report states the finding directly: the conducting of illicit ship-to-ship transfers is a primary means of importing refined petroleum13 .
The reports run to hundreds of pages a year, so this site cites them by report symbol and paragraph rather than as “the Panel”. The 2018 report describes the tactics as a combination — indirect routes, detours, loitering, false documentation, transshipment through third countries and manipulation of AIS signals14 — used, in the Panel’s words, to obfuscate actual routes, conceal port calls and give the impression that cargo was loaded elsewhere15 .
That is also where the loitering claim on this page comes from. It is a Panel finding, not an OFAC one, and the earlier version of this page attributed it to the wrong document.
The advisories’ maritime indicators are unusually concrete — night transfers in high-risk areas, painted-over identifiers11 , specific AIS behaviours — and the Panel material is where that concreteness comes from. The Panel’s 2019 report records evasion techniques at a level no regulator publishes, including the use of class B Automatic Identification Systems by feeder vessels and multiple transfers using smaller vessels16 .
The limits of the detection picture
Two honest caveats belong on this page.
The first is that transponder data is not surveillance-grade. Terrestrial receivers have limited range, satellite collection has revisit gaps, and message collisions in congested waters cause genuine data loss. A gap in a commercial dataset is not proof that a vessel stopped transmitting, and analysis that treats every gap as deliberate will generate false positives at scale.
The second is that the transfer itself, observed, does not establish what moved. Imagery shows two vessels alongside. Establishing that a particular cargo of a particular origin passed between them requires the draught readings, the discharge records and the documentation, and those are held by parties who are not obliged to produce them.
This is why the strongest published findings in this area combine several independent sources, and why the Panels of Experts — which can request information from member states — produce more conclusive material than commercial analysis alone.