Operational air-independent propulsion for Kalvari submarines: A critical technology for underwater stealth

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New Delhi | Updated: November 14, 2019 12:32:22 PM

The Made in India AIP system is therefore already planned to be retrofit onboard first submarine which enters a major refit. The indigenous AIP system is still at a developmental stage whilst pending four Submarines under the project are scheduled for commissioning in the next two years.

INS Kalvari IMAGEKalvari class submarines are powered by conventional diesel-electric propulsion systems, as per the original plans, the last two submarines were to be equipped with an indigenously developed AIP technology

The recent announcement by Defence Research and Development Organisation (DRDO) about the Land-Based testing of the indigenous Air-independent Propulsion (AIP) is going to take time before it gets operationally ready to be fitted on the submarines of the Indian Navy.

Before a Defence Quality Assurance (QA) approved ‘productionised’ version is available for operational exploitation onboard Kalvari-class submarine , the Ministry of Defence/Indian Navy have options open to procure AIP system from an international supplier. “While the preference will be given to the indigenously made AIP, keeping the submarine performance issues in mind in the face of the hostile situation in the waters, options are always open to buy urgently from vendors who meet all parameters,” said a top source.

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Six Kalvari class (Scorpene) diesel-electric attack submarines (SSKs) under Project 75 are being built by Mazagon Dock Limited (MDL) with the support and technology transfer (ToT) from French company Naval Group (former DCNS). As has been reported earlier, two out of six submarines have been commissioned.

What is the function of AIP?

“An Air-independent Propulsion (AIP) system onboard a submarine allows conventionally powered submarine to operate without access to the outside air. A Kalvari Class submarine when fitted with an AIP system onboard shall be able to run its electric propulsion motor and electrical network while bypassing the conventional batteries. This shall allow it to remain submerged for a longer duration by reducing the need to surface often to take in the air for running diesel engines for charging conventional batteries” explains Milind Kulshreshtha, a C4I expert.

Though the Kalvari class submarines are powered by conventional diesel-electric propulsion systems, as per the original plans, the last two submarines were to be equipped with an indigenously developed AIP technology. This state of the art Fuel-cell based AIP design was taken up by the DRDO for indigenous development since such technology from abroad was extremely expensive. The fuel cell-based AIP uses hydrogen and oxygen to generate electricity has almost no moving parts (making them quieter) and produces minimal waste.

The indigenous AIP system is still at a developmental stage whilst pending four Submarines under the project are scheduled for commissioning in the next two years. The Made in India AIP system is therefore already planned to be retrofit onboard first submarine which enters a major refit.

Tests & Trials of Indigenous AIP System

It has to undergo a well-defined stringent Testing & Trial stage prior to being qualified even for installation onboard (an operational ship or submarine). According to Kulshreshtha, “A system like AIP shall initially be undergoing multiple Land Based Test Site (LBTS) trials to prove its efficacy ashore. This ashore system version shall be re-designed for fitment onboard a Kalvari class submarine and subsequently installed for experimental trials. The risk here with an R&D version system fitted on an operational vessel is that it may keep going for sea trials as part of multiple iterative improvement cycles, leading to a very undesirable operational situation.”

Indian Navy shall be always eager to enhance the submarine’s operational capabilities (especially in hostile waters) for an active role like intelligence gathering etc. but stuck with an experimental system. Further, it is all too well known that getting an operational vessel tied down ashore for experimental work (as a priority over and above its Operational commitments) is not easy.

Once the R&D version fitted onboard achieves all the desired parameters as per laid down Naval requirements (like noise signatures, efficiency etc.), a ‘production’ version design of indigenous AIP will evolve.

“Then the design shall be handed over to a manufacturing partner (like a Defence PSU) under a ToT for further supply to the Indian Navy. The local Industrial agency shall setup an AIP production division and the first of the system manufactured shall undergo a rigorous Director General Quality Assurance (DGQA) Test Schedule. This procedure includes specialised tests like Type Testing and Environmental Tests. A Type Test procedure establishes the suitability of the manufactured system for Defence application,” the expert adds.

Environmental Checks for Indigenous AIP

As per DGQA procedure, the first AIP system shall undergo Environmental checks like Vibration Test, High Temperature, Damp test, Drip Proof and Tropical test as laid down in the Joint Services Standard (JSS) 55555. While being verified against the laid down Environmental specs, the system usually gets ‘Yellow Banded’ i.e. earmarked as ‘not fit’ for use onboard and kept in the workshop as a reference set.

Installation & Commissioning of AIP

Installation on board a submarine is an involved activity and follows some tough guidelines. The supplier has to provide System Specialists to assist in supervising the work and inspection of work throughout onboard fitment including Connectorisation, Setting to Work, Harbour and Sea Trials. T

Explaining the process, Kulshreshtha adds, “Board of officers from Naval Headquarters, submarine, and Dockyard with assistance from OEM, Shipyard or other suitable agencies shall be constituted for the promulgation of Feasibility of fitment document. This will be further approved by the HQtrs so as to ensure a smooth retro fitment of AIP system. Final acceptance of the system shall be the responsibility of a designated Naval Trial & Testing team for handing over of a completely proven system to the Submarine staff for exploitation and maintenance onboard.”

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