On March 31st, Pakistan announced plans to buy eight new Chinese-made submarines. The submarines are likely to be based of the Type 39B Yuan SSK, of which the export version is designated the S-20. The S-20 displaces about 2,300 tons, but air independent propulsion (AIP) is not standard to the submarine. AIP a closed off propulsion system, like a gas compression Stirling engine or fuel cells, that doesn't require a separate oxygen supply It is a must have for modern SSKs, allowing them to stay underwater for up to four weeks without using noisy snorkels to recharge batteries (often SSK batteries have enough charge to last several days at most). Pakistan's S-20s are likely to have AIP since its Agosta 90B submarines already have the technology; the PLAN's 12 Yuan SSKs all have sophisticated AIP systems.