Undersea Watch

MAREEM AIP has been developed for navy’s Scorpene SSKs

Prasun K. Sengupta

A shore-based Air Independent Propulsion (AIP) system developed by the Defence Research and Development Organisation (DRDO) and built by an industrial consortium led by Larsen & Toubro Ltd (L&T) successfully demonstrated a 14-day endurance trial-run to the Indian Navy early last December. The Land-Based Prototype (LBP) is located at the DRDO’s Ambernath-based Naval Materials Research Laboratory (NMRL). Primary performance parametres as per the navy-mandated trial directives were met satisfactorily. The LBP project was sanctioned in August 2010 at a cost of Rs 191.60 crore but has so far consumed about Rs 218 crore. The project is slated for closure this June. The next phase of R&D calls for developing a Marinised Engineered AIP Energy Module (MAREEM), for which Rs 182 crore has been sanctioned since June. MAREEM’s user-trials are expected to concluded by August 2020.

INS Khanderi CM-2000 Scorpene SSK-3

The MAREEM project aims to package the AIP module’s sub-systems, reactant tanks and waste tanks inside the simulated hull section of a CM-2000/Project 75 Scorpene diesel-electric submarine (SSK). The AIP module has in-situ provisioning of hydrogen through hydrolysis of borohydride and oxygen through liquid oxygen. MAREEM’s pre-production floor model (PPFM) has already been tested, while the hull simulated model is presently undergoing testing. Hydrogen generator, phosphoric acid fuel cell (PAFC) stacks have also been fabricated.

The NMRL, along with the DRDO’s Kochi-based Naval Physical and Oceanographic Laboratory (NPOL), have since 2002 been trying to develop an AIP system that will enable an SSK to stay submerged continuously for about 25 days. The methanol-based steam reforming system suitable for a SSK comprises a storage vessel for methanol, a storage vessel for oxygen, a steam reformer assembly, a gas purification stage, and a carbon dioxide handling system. Methanol is a liquid at room temperature and can be stored in tanks. The methanol will be consumed as it is used by the NMRL-developed Borohydride Hydrolysis/Phosphoric Acid-based fuel cell, and a hard conformal tank requires compensation to accommodate the changing volume to prevent it from collapsing.

Direct water contact with methanol is unacceptable because the two are miscible. External storage of methanol in soft conformal bags is now being tried out. The bags are fabricated from methanol-resistant material and, during operation, the seawater naturally displaces the consumed methanol without coming into contact with it. Methanol is a toxic, flammable liquid that burns without a flame, but is easily contained and therefore, if the system is correctly designed, it should not pose a safety hazard.

There is also considerable interest in methanol reformer systems for use in automobiles and buses. Alcohols and hydrocarbons can, in theory, act as fuel for a fuel cell and be directly oxidised like hydrogen. One of the commonest fuels of this type is methanol, which is used in the Direct Methanol Fuel Cell (DMFC) of the type now being developed by Germany’s HDW. Apart from the NMRL and NPOL, other DRDO laboratories and industrial entities that are involved with this R&D venture are Larsen & Toubro, THERMAX, IOCL, TEXOL, Indian Institute of Petroleum, AKSA, CEEFES, C-DAC, DIGITRONICS, NSTL, RCI, ROLTA and MDL.

Without snorting, a diesel-electric SSK can only expect to stay continuously submerged for a maximum of about 100 hours if cruising continuously at 4 Knots. The SSK must snorkel on a regular basis to preserve the charge in its main battery. The ratio of time spent snorkelling to not snorkelling is referred to as the Indiscretion Ratio and will normally be kept as low as possible. Indiscretion ratios vary from 30 per cent during transits to 5 per cent for a SSK in an operational area. By and large, snorkelling is the Achilles Heel of the SSK, exposing it to counter-detection.

Firstly, snorkelling requires a periscope/optronics mast, an ESM mast and a snorkel induction mast to be raised, all of which expose the SSK to enemy visual sensors and radar. It is possible for an enemy to detect masts and their plumes and wakes visually, particularly during the day. Visual counter-detection opportunities increase with the number of masts exposed, the speed of the SSK and the calmness of the sea. Designers try to minimise mast visual profiles by minimising mast sizes, using camouflage to blend masts into the background environment and streamlining masts to reduce plumes and wakes.

Operators try to minimise plumes and wakes by minimising snorkelling speeds; a simple rule-of-thumb being Knots = sea state + one. Radar counter-detection is also a function of the number of masts exposed and the speed of the SSK, although good radar performance is not limited to daylight. Techniques used to minimise visual counter-detection generally work equally well in also minimising radar counter-detection.

Additionally, radar absorbent material and shape optimisation are used. ESM masts and systems are employed to determine the presence of dangerous radar signals and masts are lowered when rackets approach dangerous levels. ‘Gulping’ can be used to reduce visual/radar counter-detection opportunities, particularly in scenarios where there is a heavy airborne ASW presence, but a pressing need to snorkel. ‘Gulping’ involves raising the snorkelling mast just above the surface of the water. Wave action results in the mast washing over from time to time — which causes discomfort to the crew as vacuums are pulled inside the SSK and then released on an alternate basis.

Despite all the methods employed by submariners to minimise counter-detection while snorkelling, modern optronic systems and periscope detection radars, particularly airborne, still present challenges to submariners. Another significant snorkelling counter-detection source stems from running diesels and associated equipment noises. Snorkelling can increase a SSK’s acoustics radiate noise source-level between 20 and 30 decibels. Assuming propagation losses of six decibels per doubling of range, and all other things being equal, the acoustic counter-detection ranges of a snorkelling SSK can increase eight- to 16- fold!

Of course, SSK Commanding Officers will take advantage of any increases in ambient noise such as that caused by evening or fish choruses and heavy rain. They will also top up the battery packs with short snorkellings whenever tactically possible. Nonetheless, snorkelling presents significant challenges to SSK commanders. AIP-equipped SSKs don’t have the same Achilles Heel as diesel-electric SSKs. Whilst conventional AIP systems don’t assist SSKs in transits or in high-speed runs, they do allow them to operate at low-speed for up to three weeks (or 504 hours) without the need to snorkel.