Oil and shipping
Ship-to-Ship Oil Transfers: Uses, Rules and Detection
Ship-to-ship transfers explained: legitimate STS uses, MARPOL Annex I rules, the link to sanctions evasion and the dark fleet, and how STS activity is detected.
A ship-to-ship (STS) transfer moves cargo or fuel directly from one vessel to another, either at anchor or while both ships are under way, without using a port terminal. The two ships moor alongside each other, separated by large fenders, and pump oil through hoses. STS operations are routine in the oil trade. They are also a well-documented technique for hiding where sanctioned oil comes from, which is why they attract so much attention from regulators and analysts.
Legitimate uses
STS transfers have several ordinary commercial uses:
- Lightering. A large tanker transfers part of its cargo into smaller ships so it can enter a port with limited depth, or so the cargo can be delivered to ports the large ship cannot reach. The reverse also happens. A 2018 EIA analysis explained that US Gulf Coast ports could not fully load very large crude carriers, which carry about 2 million barrels each, so cargoes were topped up offshore by smaller vessels in designated lightering zones.
- Storage. Tankers used as floating storage receive and discharge cargo by STS, particularly when the forward price curve makes holding oil profitable.
- Blending and consolidation. Traders combine parcels of different grades to meet a buyer's specification or fill a large ship for a long voyage.
- Bunkering. Fuel supply barges deliver marine fuel to ships at anchor, which is a form of STS transfer even though it moves fuel rather than cargo.
STS transfers can also keep trade moving when normal routes are disrupted. In its September 2026 Short-Term Energy Outlook, the EIA noted that Saudi Arabia had reportedly begun using ship-to-ship transfers outside the Persian Gulf while flows through the Strait of Hormuz were constrained, and listed increased STS transfers among the workarounds it expected exporters to use.
How STS transfers are regulated
The international rules for oil cargo transfers sit in Chapter 8 of MARPOL Annex I, regulations 40 to 42, added by IMO resolution MEPC.186(59) in July 2009 and in force from 1 January 2011. The main requirements:
| Requirement | What it says |
|---|---|
| Scope | Oil tankers of 150 gross tonnage and above transferring oil cargo between tankers at sea |
| STS operations plan | Each tanker must carry a plan approved by its flag administration, written in the ship's working language and based on IMO-identified best-practice guidelines |
| Records | STS records must be kept on board for three years and be available for inspection |
| Notification | A tanker planning STS in the territorial sea or exclusive economic zone of a MARPOL party must notify that state at least 48 hours in advance |
Chapter 8 does not apply to bunkering, to transfers involving fixed or floating production and storage platforms, to operations needed for safety or pollution response, or to transfers involving warships and other government ships on non-commercial service.
Coastal states add their own rules. Bunkering in particular falls outside Chapter 8 and is regulated nationally.
Algoa Bay, South Africa
Algoa Bay, off Gqeberha on South Africa's south coast, has hosted offshore STS bunkering since 2016. According to South Africa's Department of Forestry, Fisheries and the Environment, the bay is home to the world's largest breeding colonies of African penguins, and four oil spills since bunkering began have affected 260 of the birds. The maritime regulator SAMSA imposed a moratorium on new bunkering licenses for the bay in 2019, and operations were suspended for several weeks after a fuel spill during a transfer in May 2022.
In August 2025 the government announced the Regulations for the Environmental Management of Offshore Ship-to-Ship Transfer under the Integrated Coastal Management Act. They prohibit STS operations within three nautical miles of the high-water mark and within five nautical miles of marine protected areas and aquaculture zones. In Algoa Bay, operations are confined to designated anchorages with caps on the number of operators and tankers, seasonal restrictions in one anchorage area, and weather limits of winds below 22 knots and waves below two meters. Operators must monitor for penguins and marine mammals. The Southern Africa tanker watch tracks tanker presence in the region.
Why STS is linked to sanctions evasion
An STS transfer breaks the paper trail between a cargo's origin and its buyer. Oil loaded in a sanctioned country can be moved to a second tanker at sea, possibly blended with other oil, and delivered with documents that name a different origin. A May 2020 US advisory from the Treasury, State Department and Coast Guard listed STS transfers among the deceptive shipping practices used to evade sanctions, alongside disabling or manipulating AIS, falsifying cargo documents and frequent flag changes.
In December 2023 the IMO Assembly adopted resolution A.1192(33) on the "dark fleet" or "shadow fleet", defined as ships engaged in illegal operations to circumvent sanctions, evade safety or environmental rules, avoid insurance costs or pursue other illegal activities. The resolution notes the high risk of incidents when such ships carry out STS transfers, urges flag states to consider requiring their ships to notify them of STS operations, especially mid-ocean ones, and calls on coastal states to monitor STS activity in their waters.
Sanctions authorities now target individual vessels. By April 2026 the European Commission reported that 632 vessels in Russia's shadow fleet were listed by the EU, barring them from EU ports and from a broad range of maritime services.
How STS activity is detected
No single data source proves that an STS transfer took place. Analysts combine several:
- AIS behavior. Two tankers broadcasting positions close together at very low speed for several hours, away from a port or terminal, is the classic signature. The guide to AIS explains what those broadcasts contain and where they fail.
- Draught changes. AIS voyage data includes each ship's draught. A rise on one ship and a fall on the other suggests cargo moved. Draught is entered manually by the crew, so it is a hint rather than a measurement.
- AIS gaps. A tanker that stops transmitting near a known transfer area and reappears days later with a different draught is a candidate for a hidden transfer.
- Satellite imagery. Optical images can show two hulls moored side by side, and radar images can do so through cloud and at night. Imagery confirms that ships were alongside each other but cannot identify them on its own, as covered in the guide to Sentinel-2 ship detection.
The limits matter. A lawful lightering operation produces exactly the same pattern as an illicit one, and the IMO resolution itself notes that a ship's AIS going unreceived is not on its own proof of illegal activity. Detection identifies events worth investigating; it does not establish intent. For that reason FXPM Tanker Watch presents its satellite vessel counts at known transfer anchorages as observations, not conclusions.
Key points
- STS transfers move oil between ships at sea and are routine for lightering, storage, blending and bunkering.
- MARPOL Annex I Chapter 8 requires an approved STS operations plan, three years of records and 48 hours' notice to the coastal state for transfers in its waters.
- Bunkering is outside Chapter 8; places such as Algoa Bay regulate it nationally, with South Africa's 2025 regulations tightening controls.
- STS transfers are a known sanctions-evasion technique and a focus of the 2023 IMO resolution on the dark fleet.
- AIS patterns, draught changes and satellite imagery can flag likely transfers but cannot prove intent.