Ship to Ship (STS) Transfer: Maritime Glossary

A ship to ship (STS) transfer is the movement of cargo directly from one vessel to another, at anchor or underway, without going through a port terminal. STS operations are a normal part of tanker and bulk trades, used to lighten large ships for shallow ports or to combine cargoes, and most are entirely legitimate.

The rules for oil STS transfers

Chapter 8 of MARPOL Annex I, in force since 1 January 2011, covers oil transfers between tankers of 150 gross tonnage and above. A tanker planning an STS operation in the territorial sea or exclusive economic zone of a MARPOL party must notify that coastal state at least 48 hours in advance, carry an approved STS operations plan, and record each operation in its oil record book, keeping the records for at least three years.

When an STS transfer becomes a sanctions risk

The May 2020 sanctions advisory from OFAC, the US State Department and the US Coast Guard lists STS transfers used to conceal the origin or destination of cargo as a deceptive practice. The IMO resolution of December 2023 on the dark fleet also urged flag states to act on transfers at sea used to evade rules. The pattern to watch for is a transfer in open water, away from a recognised STS area, often paired with an AIS gap on one or both ships.

What buyers and suppliers should check

Both vessels in the transfer, not only the one you deal with
Where the transfer happened and whether coastal state notice was given
Whether either ship had an AIS gap around the transfer
The ownership, flag and sanctions status of the other vessel

These checks belong in the same workflow as your other counterparty checks. See how to screen a vessel for sanctions.

Normal STS work versus a warning sign

Usually normal	Worth a closer look
Transfer at a recognised anchorage or lightering area	Transfer in open water far from any recognised area
Coastal state notified in advance	No notice given where it was required
Both ships broadcasting AIS throughout	One or both ships go dark around the transfer
Both vessels and their owners screen clear	The other vessel or its owner is on a sanctions list

Tidal screens vessels and counterparties for sanctions, ownership and compliance history, and documents every decision. See maritime sanctions screening software or maritime due diligence.

Sources

Dark fleet AIS gap Bunkering Charter party Bill of lading All glossary terms →

Frequently asked questions

What is a ship to ship transfer?

A ship to ship (STS) transfer moves cargo directly from one vessel to another, at anchor or underway, instead of through a port terminal. It is common in tanker and bulk trades.

Are ship to ship transfers legal?

Yes, most are. Oil STS transfers between tankers of 150 gross tonnage and above follow MARPOL Annex I Chapter 8, which requires an approved operations plan, 48 hours notice to the coastal state where it applies, and records kept for at least three years.

Why are STS transfers linked to sanctions evasion?

US sanctions guidance lists STS transfers used to hide the origin or destination of cargo as a deceptive practice. Transfers in open water, away from recognised areas and paired with AIS gaps, are the main warning sign.

How much notice is needed for an STS transfer?

Under MARPOL Annex I Chapter 8, a tanker planning an oil STS operation in the territorial sea or EEZ of a MARPOL party must notify that coastal state at least 48 hours in advance.

Ship to ship transfer