Dark Transits are a Stopgap not a Solution

Washington has accomplished an important feat by stealthily moving oil through the Persian Gulf despite Iran’s proclaimed closure. But the means it has relied on can’t become normal maritime behavior. Treasury Secretary Scott Bessent said on August 27 that the United States had facilitated the movement of 130 million barrels of oil through the Strait of Hormuz in the preceding two weeks. That amounts to over 9 million barrels per day (bpd), roughly half of pre-war exports through the chokepoint. Most of these vessels make “dark transits,” when ships sail with their automatic identification system (AIS) turned off. A ship’s AIS transponder broadcasts its location, course, speed, and other data via dedicated maritime radio frequencies to nearby ships, shore-based AIS receivers, and low-earth satellites, which convey that data globally. Dark transits are typically used by sanctioned vessels or ships moving illicit cargo because standard maritime trade, under normal conditions, doesn’t derive benefit from sailing without AIS. But licit maritime trade has increasingly been forced to move through the Middle East waterways on dark trips to avoid attack. Kpler data identified more than 1,500 dark transits through Hormuz between March 1 and late August. Saudi Arabia’s tankers are also transiting the Red Sea without AIS after the Iran-backed Houthis announced a maritime blockade of the kingdom on July 20. Riyadh rerouted most of its oil exports from the Persian Gulf to the Red Sea using the country’s east-west pipeline after Iran closed Hormuz. Washington needs to move Gulf oil through both waterways to market to reduce Tehran’s economic leverage. Dark transits are an important tool to accomplish this, but they are not a long-term solution in the Middle East. The International Convention for the Safety of Life at Sea (SOLAS), the most important maritime treaty on vessel safety, requires ships, with very limited exceptions, to use AIS for safety reasons. Most importantly, the technology is used to prevent collisions, but it also improves a ship’s navigation capacity and knowledge of its environment. Given the potential safety implications, the shipping industry traditionally eschews dark transits. In many instances, sailing without AIS will void a ship’s insurance policy as it is a violation of international standards. While there can be legitimate reasons for a ship to disable its AIS — including for its own safety — this does not necessarily compel the insurance company to continue coverage. Furthermore, transiting without AIS does not guarantee a ship won’t be attacked, as Iran’s recent attacks on Emirati vessels demonstrates. United Arab Emirates’ state-owned Abu Dhabi National Oil Company (ADNOC) has been shuttling oil through the Persian Gulf — sending vessels through Hormuz on dark transits to offload their product onto larger ships waiting outside the strait that then travel onto buyers. While a successful mission — ADNOC is expanding this service to Iraq and Saudi Arabia appears to be replicating the practice — it is not without danger. Abu Dhabi said that Tehran attacked seven Emirati ships in roughly the first half of August alone. Ships can be tracked by radar or satellite even when their AIS is disabled. The confined space of the Strait of Hormuz enables drone and fast attack crafts to more easily surveil and attack vessels. While the United States has targeted Iranian coastal radars, small craft, and drone capabilities, these can all be replenished. Iranian partners would likely support this effort. China considered giving Iran advanced radar technology early in the conflict, according to U.S. intelligence agencies. Russia has allegedly been supporting Iran’s ability to build ram jets to propel hypersonic anti-ship missiles. Both China and Russia have also provided the regime with targeting support via access to satellites or their data intelligence. Iran possesses anti-ship missiles that are launched based on satellite or radar data but lock onto a ship’s heat or electronic signature for terminal guidance, enabling them to target a dark vessel. So long as Iran retains both the desire and residual capability to strike—bolstered by partners willing to assist—even dark transits through the strait carry some risk. Similar threats exist in the Red Sea, where the Houthis utilize Iranian anti-ship missiles, but they also attack ships using small arms and RPGs from small vessels. Dark transits through the Red Sea grew substantially following Houthi attacks on commercial shipping over the war in Gaza in late 2023. This has given the group experience in monitoring and menacing vessels moving through the waterway without AIS. These ongoing maritime threats are, in part, the cost of not decisively defeating the Houthis when they spent two years menacing commercial shipping in the Red Sea. Gulf countries and Washington should not repeat that mistake in the Persian Gulf. The lasting solution is the removal of Iranian threats to maritime shipping and the restoration of freedom of navigation in both critical waterways of the Middle East. Bridget Toomey is a research analyst at the Foundation for Defense of Democracies. Rear Admiral (Ret.) Mark Montgomery serves as senior director of FDD’s Center on Cyber and Technology Innovation (CCTI) and as an FDD senior fellow. 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