Steam trials of the Russian nuclear-powered submarine Podmoskovye have started in the White Sea in the country’s north after a 16-year period of modernization, RIA Novosti reported.

Having undergone extensive modernization in a process that took 16 years at the Zvezdochka shipyard in the northern Russian city of Severodvinsk, the vessel will now be used as carrier for deep-diving special-purpose submarines.

RIA Novosti quoted the Zvezdochka shipyard’s press service as saying that “shipbuilders wrapped up works on repairs and modernization of the Podmoskovye submarine, which took to the sea for steam trials late last week.”

The Russian Navy's BS-64 Podmoskovye is a Delfin-class ballistic missile submarine (NATO reporting name “Delta-IV”). She was originally designated K-64.

The Podmoskovye was originally laid down in 1982 and was in service with the Russian Northern Fleet between 1987 and 1999.

The vessel entered the Zvezdochka shipyard for repairs in late 1999 when the “in-depth modernization” of the submarine was launched. In August 2015, the submarine started moor trials.

During the modernization process, the vessel's missile compartment for Sineva intercontinental missiles was cut out and replaced with compartments for living and scientific work, as well as equipment for docking nuclear deep-diving submarines.

According to the website vesselfinder.com, “one of the submarines for which Podmoskovye most probably will be used as carrier is the Losharik deep diving titanium submarine.” 

The “Losharik” is nicknamed for the peculiar shape of its hull, which resembles a Soviet-era cartoon toy horse made from a string of connected balls; the ship consists of several connected titanium spheres.

The website said that the Podmoskovye will also be capable of carrying the Klavesin-1R, an autonomous unmanned submarine used for surveillance and acoustic studies at depths of up to 6,000 meters.

The craft carries various gears to collect geological data and samples from the seabed. Its first known mission with the Russian Northern Fleet was performed at a depth of 3,000 meters at the Mendeleyev ridge in the Arctic.

Moreover, submarines of this type can be used in various kinds of special operations; hence the extremely limited volume of information about the sub’s features available in public domain.