VI Energy Politics & Supply Security
Ship-to-ship transfer (STS)
The transfer of an oil cargo from one tanker to another, pumped across while the two ships lie alongside at sea, without entering port.
How it works
A ship-to-ship transfer, or STS, moves cargo from one tanker to another on the open sea or at an anchorage. The method is used to unload a large ship without bringing it into a shallow port, to combine small parcels, or to route a cargo onto a different voyage.
The international framework is Chapter 8 of Annex I to the MARPOL Convention. It requires oil tankers above 150 gross tonnes to carry a transfer plan approved by the flag state and to notify the coastal state in advance. Regulations issued by the United Kingdom in 2020 apply both requirements in its own waters and confine transfers to permitted areas.
The measurement trap is that a transfer can be hidden in ship-tracking data. Once a cargo changes hands at sea, its origin and buyer can change on paper. When the method is used to lose the trail of sanctioned oil, the transfer is usually detected only afterwards, through satellite and AIS data.
Why it matters here
When a strait is not fully closed, oil often leaves it through transfers. In October 2026 a 2-million-barrel cargo loaded inside Hormuz was transferred to a second ship outside, so a single cargo tied up two vessels. As the transfer pattern grows, the fleet constraint deepens even if supply returns. We use this mechanism to read the contradiction between freight rates and transit counts.