STS • Rendezvous • Pair-Trawling vs Suspicious • Taiwan Waters
A ship-to-ship (STS) transfer, or "rendezvous," is two vessels lying alongside at sea to move cargo without entering port. It is often perfectly legal — pair trawling, resupply, bunkering — but it is also a key tactic for sanctions evasion and smuggling to hide where cargo comes from. This site detects STS events via spatio-temporal AIS matching and separates lawful operations from suspicious transfers. An STS event is a lead to investigate, not a legal finding.
A Ship-to-Ship Transfer (STS), often called a "rendezvous" or "alongside" operation, is when two vessels lie close together at sea to move cargo, fuel, or catch from one ship to another — without ever entering a port.
This is a long-established, perfectly normal practice: large tankers decant crude into smaller ships offshore, motherships receive catch from fishing boats on the grounds, and bunker vessels refuel fleets working far from land. The problem is that the very same maneuver can be used to hide the true origin and destination of cargo.
An STS is hard to see from a single ship's track — it is fundamentally a relationship between two vessels. This site finds it through spatio-temporal matching of AIS positions:
When a set of positions meets all of these, the system records an "STS event," logging the two vessels involved, the time, location, and duration.
Detecting an STS is only the first step. The vast majority of alongside operations are legitimate; the key is to tell routine from anomalous.
This site's STS detection labels events as "pair-trawling" or "suspicious"; only the latter feeds further threat assessment.
STS becomes a gray-zone focus because it can move cargo without leaving a port record — exactly the capability anyone evading oversight needs most.
This site does not treat every rendezvous as a threat. The goal is to filter out the huge volume of routine operations so the genuinely anomalous few surface:
Live STS results are on the STS detection page; the full scoring logic is in our methodology.
This page is for research and education, aggregating public data under the MIT License. When citing, please attribute the source and note the limitations above.
Public shipping investigations and sanctions-monitoring reports repeatedly describe one recurring playbook. Crude oil from a sanctioned source is first loaded onto a "shadow fleet" tanker — typically an older hull, owned through layers of shell companies, with opaque insurance.
That tanker then sails to a loosely regulated patch of sea — for example off Southeast Asia or in particular Mediterranean gulfs — and lies alongside a second tanker. During the transfer both vessels often switch off or spoof their AIS, making it hard for outsiders to connect "the ship that carried the source cargo" to "the ship that eventually enters port."
When the second ship arrives carrying "blended" or origin-obscured oil, the documented cargo origin has been severed. This is exactly why STS sits at the heart of sanctions evasion: on the high seas, unobserved and deniable, it resets the traceability of the cargo.
This site's STS detection relies on AIS positions, so several inherent limits must be stated honestly:
For these reasons, STS detection is positioned to "generate leads to investigate," not to "establish illegality." SAR satellites can catch some AIS-off targets, but their coverage has spatial and temporal gaps — it is not all-weather, all-ocean. Only by cross-referencing multiple sources (AIS, SAR, sanctions lists, identity records) does the genuinely suspicious minority surface.
The concepts on this page can be verified against these public, authoritative sources:
This page is for research and education; detection signals are leads to investigate, not legal or factual findings. See the methodology.