Thanks to the stipulations of the 1936 Montreux Convention.
Due to the stipulations of the 1936 Montreux Convention that gives control of the Bosporus Straits and the Dardanelles to Turkey, Russia is restricted in how it is allowed to move its naval forces—particularly aircraft carriers and submarines—through those waterways. However, Moscow’s loss is Washington and NATO’s gain—but if the Kremlin manages to sway Turkey into its orbit, the tables would turn in Russia’s favor.
The Russians have assigned six newly built Project 636.3 Varshavyanka class diesel-electric submarines—which are referred as Improved Kilos by NATO—to the Black Sea Fleet. Four of those vessels, Novorossiysk, Rostov-na-Donu, Staryy Oskol and Krasnodar arrived at Sevastopol between 2015 and 2017. Two additional boats, Velikiy Novgorod and Kolpino, arrived in the Eastern Mediterranean in September 2017, but have never sailed through the Bosporus to formally join the Russian Black Sea Fleet’s 4th Separate Submarine Brigade.