(Bloomberg) -- Two shuttle tankers used to haul crude from Russia’s Sakhalin-2 project have failed to move cargoes since April, putting the entire burden of shipments on a single vessel.

The project typically ships three or four cargoes a month that have all been delivered to China since the start of 2023. The round trip, including discharging there, has averaged about two weeks. That’s easily doable by three tankers and, possibly in a pinch, two, but not by just one.

The Zaliv Vostok loaded its most recent cargo in early April and was anchored off various locations around Zhoushan Island after discharging at Ningbo. Since the end of April, the ship has been at a yard on Xiaoqian Island, vessel tracking data compiled by Bloomberg show.

The Zaliv Baikal did take on a cargo on May 21 and headed toward Ningbo, as usual. But once it got there, things were anything but normal. The vessel turned back and headed to the small Russian port of Zarubino, about 100 miles (161 kilometers) west of Kozmino, where it appears to have met up with another Russian tanker, the Nikolay Zuyev.

The ships’ owner, Russia’s state-controlled Sovcomflot PJSC, and project operator Sakhalin Energy didn’t provide immediate responses to Bloomberg requests for comments.

The two vessels spent almost three days together before the Zaliv Baikal headed back toward Zhoushan. But by now it was empty, according to the ship’s draft signal, which was updated several hours later. It, too, may be headed for a maintenance yard.

Of the three Sakhalin-2 shuttle tankers, only the Zaliv Aniva appears to be operating at anything resembling normalcy. After discharging a cargo at Zhoushan on June 6, it raced back to Sakhalin Island, arriving June 10.

Swapping in tankers to replace those out of action isn’t straightforward. The vessels, like those used for the Sakhalin 1 project, must be able to take on cargo through a bow loading system, a feature that isn’t common on the Aframax-size ships employed at the project.

The project is due to take delivery of two liquefied natural gas-fueled tankers on a 10-year charter from Sovcomflot, according to a 2021 press report. But both vessels are currently employed elsewhere, with no sign of them heading to Sakhalin.

While the reason for disruption is unclear, it comes at a time when Sovcomflot has publicly said it’s being pressured by Western sanctions. By and large, Russia’s oil exports and revenue have held up despite the measures, but the tanker company has seen 21 of its vessels prevented from loading cargoes following designation by the US Treasury between October and February.

The first of those sanctioned tankers are now being brought back into use.

Unless the Zaliv Vostok is brought back into service soon, the Sakhalin-2 project will struggle to keep delivering at its normal rate. The first June cargo had yet to begin loading by June 10.

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