Here's a paper that fills in the holes left in ship transport logs when they turn off their transponders. That happens a lot--more than 40% of global seaborn oil seems to be in the black market avoiding sanctions.
The (Un)Intended Consequences of Oil Sanctions Through the Dark Shipping of Sanctioned Oil
Xiwen Bai, Jesús Fernández-Villaverde, Yiliang Li, Le Xu & Francesco Zanetti, NBER Working Paper 33486 DOI 10.3386/w33486 February 2025
Abstract: We examine the rise of dark shipping—oil tankers disabling AIS transceivers to evade detection—amid Western sanctions on Iran, Syria, North Korea, Venezuela, and Russia. Using a machine learning-based ship clustering model, we track dark-shipped crude oil trade flows worldwide and detect unauthorized ship-to-ship transfers. From 2017 to 2023, dark ships transported an estimated 7.8 million metric tons of crude oil monthly—43% of global seaborne crude exports—with China absorbing 15%. These sanctioned flows offset recorded declines in global oil exports but create distinct economic shifts. The U.S., a net oil exporter, faces lower oil prices but benefits from cheaper Chinese imports, driving deflationary growth. The EU, a net importer, contends with rising energy costs yet gains from Chinese demand, fueling inflationary expansion. China, leveraging discounted oil, boosts industrial output, propagating global economic shocks. Our findings expose dark shipping's central role in reshaping oil markets and macroeconomic dynamics."
"from 2017 to 2023, an average of 7.8 million metric tons of crude oil per month were exported via dark ships from Iran, Syria, Venezuela, and Russia, accounting for approximately 43% of total world seaborne crude oil exports recorded in UN Comtrade data.
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"China is the largest importer of crude oil transported by dark ships from sanctioned countries. Between 2017 and 2023, China’s imports exceeded 97 million metric tons, accounting for 15% of total sanctioned oil exports. Other notable importers include South Korea, India, Egypt, Turkey, and Russia itself."