• The Navy is planning to instal Air Independent Propulsion (AIP) modules on all six Scorpene submarines to extend their endurance. This will be done when the submarines go for normal refit which is six years after their induction.
• The first Scorpene submarine INS Kalvari is all set to join the Navy by August and would go for its normal refit in 2023. The remaining five submarines are expected to be inducted at nine-month intervals.
• An AIP module is being developed by the Defence Research and Development Organisation (DRDO) and was supposed to be installed on the last two submarines before they roll out of the production line.

About Air Independent Propulsion:

• Air-independent propulsion (AIP) is any marine propulsion technology that allows a non-nuclear submarine to operate without access to atmospheric oxygen (by surfacing or using a snorkel).
• AIP can augment or replace the diesel-electric propulsion system of non-nuclear vessels.

Source:TH & WIKI