Moscow: The Russian Navy will refit, modernise and recommission two Sierra class (Project 945) titanium-hull nuclear-powered attack submarines by 2017.
The Zvezdochka shipyard said the Sierra class has a light and strong titanium pressure hull, enabling the vessels to dive to depths of up to 550 metres and enhancing their survivability, as well as having a low magnetic signature.
A total of four Project 945 and 945A submarines were built in Russia.
In addition to the Karp and the Kostroma, the Pskov and the Nizhny Novgorod were built in the early 1990s and are currently in service with the Northern Fleet.
The K-239 Karp (Carp) was the first Project 945 (Barrakuda) submarine, which entered service with the Soviet Navy in 1984.
The K-276 Kostroma was commissioned in 1986 and had to be repaired after a collision in February 1992 with the American submarine USS Baton Rouge (SSN-689) while on patrol off Kildin Island near Severomorsk.
The US Navy said the collision occurred more than 22 km from shore, in international waters.
The shipyard signed a modernisation contract for the Karp and the Kostroma with the defence ministry in December 2012, a Zvezdochka official said.
Karp has been sitting at the shipyard since 1994, he said, adding the submarine's titanium hull "is in a very good condition" but a significant part of its equipment is missing.
The submarines' sonar and navigation systems will be replaced and their reactors refuelled, he said.