The Three Telltale Signs of an STS Transfer in Open Sources

Ship-to-ship transfers are a normal part of the maritime economy. They are also the single most common physical mechanism for sanctions-evasion oil flows. The trick is telling routine STS — which happens at designated anchorages, with both vessels broadcasting clean AIS, in line with port authority records — from the variant that is not routine at all.

The three signatures below, taken together, are extremely difficult to explain away.

Signature one: paired AIS silence

A standard STS transfer takes between four and sixteen hours, depending on cargo type and sea state. During a clean transfer both vessels broadcast normally — AIS Class A transponders are not affected by mooring alongside another hull, and there is no operational reason to switch them off.

A meaningful STS-evasion signature is when both vessels go dark inside the same 24-hour window, in the same maritime zone, and reappear within minutes of each other. The probability of two random AIS failures coinciding in that way is low. The probability of two intentional ones is high.

Signature two: draft change without port call

AIS messages include a draft field — the depth of the vessel below the waterline, in tenths of a metre. Loaded tankers ride low; empty ones ride high. A 100,000-tonne crude transfer can shift draft by 3–4 metres.

A vessel whose last broadcast draft before the AIS gap was 8.0 metres, and whose first broadcast draft after the gap is 13.5 metres, has either visited a loading terminal — which would be visible in port-call records — or received cargo somewhere else. If port records show no call, the cargo came from another vessel. The arithmetic does the work.

Crews sometimes cheat the draft field manually. They cannot, however, cheat the vessel's actual displacement, which leaves a SAR signature visible on Sentinel-1 imagery: heavier ships sit lower and reflect differently. A SAR pass over the suspected transfer zone can confirm the change independently.

Signature three: geography that should not host an STS

Legitimate STS transfers happen at designated anchorages — Fujairah, Gibraltar, Skaw, Lakonikos, Singapore Eastern OPL. They are listed. They are watched. The variant that matters is the transfer that happens nowhere — in deep water, far from any anchorage, often at the edge of an EEZ or in waters with weak surveillance.

• The Strait of Hormuz outer approaches.
• The Sea of Japan east of South Korea.
• The Gulf of Guinea south of the active piracy zones.
• The eastern Aegean, around the Lakonikos Gulf approaches.
• The eastern Mediterranean, south of Cyprus.

A two-vessel STS in any of those zones, with paired AIS silence and a draft mismatch, is approximately as deniable as a fingerprint on a door handle.

Putting the three together

No single signature is a story. Paired AIS silence inside terrestrial coverage, plus a draft discontinuity unexplained by port records, plus a transfer location outside any designated anchorage, plus — when available — a SAR confirmation: that is a story. It is also the kind of evidence package an editor will publish, because each element cites itself and the chain reproduces.

There is a craftsmanship to this work. The reward, for the analyst willing to learn it, is the rare luxury of writing investigative pieces where the data does the heaviest lifting and the prose is mostly just labelling.