NEW DELHI: “Sea denial” is the name of the game for conventional diesel-electric submarines. Their primary role is to interdict an enemy’s trade and energy routes, block its ports, sink its warships, and sometimes attack land targets with long-range cruise missiles.Nuclear-powered submarines armed with nuclear-tipped ballistic missiles, called SSBNs in naval parlance, of course, are all about strategic deterrence. While India took a major step forward to complete its nuclear weapons triad by commissioning its first small SSBN called INS Arihant last year, to add to the existing Agni land missiles and fighter-bombers, its old and depleting conventional underwater combat arm has been a huge worry for the last several years.So, when PM Narendra Modi commissions India’s first new diesel-electric submarine in over 17 years in Mumbai on Thursday, it will be a red-letter day for the beleaguered Navy . The force, after all, is grappling with just 13 ageing conventional submarines, only half of them operational at any given time.The new 1,565-tonne submarine, named INS Kalvari (tiger shark, a deadly deep-sea predator) after the first-ever submarine inducted by India from Russia in December 1967, is to be followed by five of her Scorpene sisters under the Rs 23,652 crore “Project-75” underway at Mazagon Docks in collaboration with France.The Scorpene project, of course, has faced huge time and cost overruns after the contract with French shipbuilder DCNS (the Naval Group) was inked way back in October 2005.INS Kalvari, for instance, was to be ready by 2012, with the other five coming by 2017. Now, the second one INS Khanderi will be commissioned by mid-2018, with the third INS Karanj following by early-2019. All the six will be inducted by 2020-2021 now.“It’s better late than never. INS Kalvari is the most potent platform to have been constructed in India, capable as it is of undertaking offensive operations spanning the entire spectrum of maritime warfare,” said an officer.The submarine, which has a speed of 20 knots, is equipped with sea-skimming SM-39 Exocet missiles and heavy-weight wire-guided surface and underwater target torpedoes. “The submarine has superior stealth and the ability to launch a crippling attack on the enemy using precision-guided weapons,” said another officer.Even as it pops the bubbly for INS Kalvari, the Navy is also keen to kick-start the long-delayed “Project-75-India” for construction of six new stealth submarines, with both land-attack missile capabilities and air-independent propulsion for greater underwater endurance. “We hopeful of beginning the project by end-2018,” said Navy chief Admiral Sunil Lanba.Four global ship-builders — Naval Group-DCNS (France), ThyssenKrupp Marine Systems (Germany), Rosoboronexport Rubin Design Bureau (Russia) and Saab Kockums (Sweden) — have responded to the initial RFI (request for information) issued by the Navy for this project estimated to be over Rs 60,000 crore.Under P-75I, which was accorded “acceptance of necessity” in November 2007, a foreign collaborator will be selected to tie-up with a domestic shipyard under the “strategic partnership” policy. It will take seven to eight years after the contract is inked for the first submarine to roll out.India needs at least 18 diesel-electric and six nuclear attack submarines, backed by four SSBNs armed with long-range ballistic missiles, to guard against the two-front threat scenario from China and Pakistan as well as achieve credible nuclear deterrence.