Submarine ARA San Juan only had a Weak Hull (eg. 100 Meter Operating Depth)

From November 18, 2017 to December 1, 2018 Submarine Matters reported many aspects of the loss of the Argentine submarine ARA San Juan. San Juan remains lost without a trace, even though several countries continue to look for it.

On January 30, 2017 International Business Times reported  :

" The ARA San Juan was “’limited’ in its operational depth at 100 meters...because at greater depth [it] could not ‘guarantee its watertightness’, according to the document" [the document may be from the Comprehensive-Nuclear-Test-Ban-Treaty-Organization (CTBTO) or US Naval Intelligence (?)]

The new internal document may be able to shed light on what happened to the ARA San Juan. Officials still had not determined the cause of the submarine’s disappearance, though a report released in January by SaveTheRoyalNavy(dot)org said it was likely that all crew on board died from an explosion on the vessel.

An acoustic signal was detected the day the submarine disappeared that authorities said was consistent with the sound of an implosion of pressure inside the ship at a depth of 388 meters... The report stated the explosion was equivalent to 5,669 kilograms of TNT which would have destroyed the hull in 40 milliseconds." [this is according to a CTBTO and/or US Naval Intelligence report]

COMMENT

By setting a safe operating depth of 100 meters the Argentine naval authorities must have decided that San Juan had a severely weakened pressure hull. Only a 100 meter safe operating depth is surprisingly shallow. Even German Type XXI submarines built in 1943 had "Test" (ie. operating) depths of 240 meters (see sidebar). A weak pressure hull may be due to one or several factors including:

-  age (San Juan was completed 35 years ago in 1983). A 25 to 30 year operating life for a well
   maintained submarine is considered standard.
-  age in conjunction with too many deep immersion cycles (repeated deep diving, then surfacing
   gradually causes metal fatigue in the pressure hull over a number of years).
-  the Argentine Navy is notorious for poor maintenance
-  extensive and/or improper hull cutting for engine maintenance (cutting or welding pressure hulls
   weakens them) and/or
-  too much rust and saltwater corrosion of the pressure hull shell and of strengtheners.

In general the newer the submarine the stronger its steel alloy pressure hull and the deeper it can safely dive (see here and here). Titanium alloy permits much deeper diving (eg. Russian Alfa class submarines) but titanium is more expensive to process, weld, use in construction and repair.

Pete

China's January 11, 2018, Type 093 Submarine Incursion A SIGINT Probe?

Comments here by "PUNTER" and others, has prompted me to revisit the January 11. 2018 incursion of a (probably) Chinese Type 093 SSN into the Japanese claimed “Senkaku” islands-sea area. The incursion appears to be a:

-  Chinese politico-strategic demonstration that China also claims those islands-waters, 

-  a reconnaissance-intelligence exercise to test the efficiency of Japanese and probably America

   ASW fixed and mobile sensors, OR

-  a miscalculation by China's Navy (PLA-N) that its 093 could carry out a mission undetected by
   Japanese (and possibly US) sensors.

The Chinese exercise would allow China to “trace” or “track” predictable responses by Japan’s intelligence agencies and politico-military chain of command the exercise could yield some decoded SIGINT benefits and undecoded traffic analysis analytical benefits. 

The incursion by the 093 and a Chinese Type 054/054A Frigate was beneficial to Chinese electronic interception as it attracted Japanese MPAs and surface craft. If a Japanese submarine was also sent then Chinese tracking of it would be an even greater intelligence prize. 

BACKGROUND

Two Type 093s, were built in the 2000s, and at least 2 or 3 more improved (could be called Type 093As) were commissioned in 2016. The Type 093A reputedly have vertical launch YJ-18 supersonic anti-ship cruise missiles and may be as stealthy as unimproved US Los Angeles class SSN-688s) and  much quieter than China’s first and noisy Type 091 SSNs.

Side view of a Chinese Type 093. Note old fashioned (maybe 1990-2009 design) cruciform rudder, instead of modern x-plane rudder. See much larger.

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The January 11, 2018, Type 093 forced to surface or it surfaced to show the Chinese flag. Note that the cruciform rudder may indicate that the Type 093 might have only have been "old style" "unimproved?"  (Mark. 1(?)) build in the 2000s (Photo via Kyodo News).

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For security and deception China has variously called the Type 093 (or allowed it to be called) "unimproved [2000s build]" "improved [2016 build], plain 093, 093A, 093B and 093G. "Shang" is a NATO reporting name. What the 5 Type 093s are called seems to rely more on what the non-Chinese observers decide to call it rather than usually secret Chinese designations.

FURTHER THEORIES ON THE MISSION

The Type 093 perhaps entered the Miyako-jima (island) part of the Senkakus:

-  without an expectation of detection, or

-  to see how close it could get to Japan’s seafloor SOSUS array or Japan's Integrated Undersea

   Surveillance System (IUSS) (here's the US IUSS) which is SOSUS + many additional sensor 
   platforms). The map below may be the US-Japanese SOSUS-IUSS.

This method of gauging the sensitivity of Japanese and US sensors is very useful to China. Also ease or difficulty of detection would assist China in indicating how many additional stealth improvements or efforts with existing stealth features are required.

If, it is an old Type 093 then China may wish to deceive Japan-US into conclusions that the submarine “could be too easy to detect”

I would conclude “there is not enough information to back up [the easy to detect] theory”.

Better to be a bit pessimistic in intelligence assessments than too optimistic.

“Map 4. The US ‘Fish Hook’ Undersea Defense Line” in Desmond Ball and Richard Tanter, The Tools of Owatatsumi Japan’s Ocean Surveillance and Coastal Defence Capabilities (2015, ANU Press) page 54. The map may depict past or current locations of the eastern Asia - inner western Pacific SOSUS-IUSS seafloor array. 

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Pete

French Mini-Sub Likely to be used for Australia's Future Submarine

French Special Forces-External Intelligence Directorate (DGSE) "Propulseur Sous-Marins 3rd  Generation (PSM3G) for fitting on top of French Barracuda-Suffren Class SSNs and Australian Future Subs (SSKs). (Diagram and further background courtesy H. I. Sutton's Covert Shores).

---

Noting much equipment on Australia's Future Submarine will be specifically compatible with France's Barracuda-Suffren class SSN DefenseNews reports January 25, 2017, in part:

"PARIS ― The head of France’s special forces has presented a wish list to lawmakers that includes a modified NH90 helicopter and a mini-submarine to complement the Barracuda nuclear attack submarine."

"Mini-submarine

Naval special forces are “waiting impatiently for entry into service of the PSM3G in 2018,” he noted. That is a third-generation, propelled mini-submarine that would work in conjunction with the Barracuda submarine.

That mini-sub would work with the Mistral projection and command ship as an interim measure, he said. “The PSM3G is a rare and immediately available capability effective against the famous ‘area-denial’ threat — the special forces have to be able to get close to an enemy coast to call in a strike,” he said.

The mini-submarine is a relatively large unit, capable of being launched from the Barracuda while submerged, he explained. “They are undetected from departure to return from the mission; that is, to say, an intelligence-gathering mission without being seen, without a signature, without particular risk and with an impressive efficiency,” he said.

The particularly high-performing mini-submarine, according to Isnard, would be fully autonomous, run on battery power and have a navigation system.

...[also significant] The Suffren, the first-of-class Barracuda boat, [laid down 2007!] is due for delivery in 2019 - a year late."

Penske MTU 4000 competition to Kawasaki Diesel for Submarine - PART FOUR

Against my preference for Kawasaki diesels (see PART THREE) sourced through Naval Group’s European partner, MAN, is MTU/Penske/ASC’s promotion of the MTU 4000 for Australia’s future submarine.

ASC’s Media Release of October 6, 2017 reported:

“Australia’s sovereign submarine company, ASC, and Penske Power Systems have announced their intention to enter into an agreement to put MTU engines through their paces at ASC’s engine test facilities in Adelaide as part of an ongoing research and development activity.”

On September 9, 2017 Penske was more specific, reporting:

“In a world first, Penske Power Systems will unveil the much anticipated MTU 12V4000U83 submarine engine at Pacific 2017 on [October 3, 2017].

This will be the global reveal of MTU’s next generation of submarine charging unit engines, suitable for projects such as the upgrade of the current Collins class and inclusion within the Australian future submarine.

With a well proven pedigree, the 12V4000U83 boasts greater power ([only] up to 1,500 kW) than its 16V396SE84 predecessor while being IMO Tier II compliant and compact.”

COMMENT

With only an output of 1.5MW 12V4000U83 diesels fall short of the 2MW required in a 8MW total, 4 diesel arrangement for the Australian Future Submarine. The option of further developed more powerful MTU diesels may deserve consideration - though this is risky.

An unhelpfully blurry MTU diesel chart, with the very narrow power range MTU 4000 for submarine 3rd from bottom. See chart a little larger and clearer here http://www.mtu-online.com/mtu/applications/marine-defense/?L=eyeusejkvzrfvkp .

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In a January 6, 2018 comment MHalblaub suggests MTU 4000 diesels for submarine more powerful with more cylinders than the current with MTU 4000 12V (2185mm Height x 1850mm Width x 2870mm Length (without gearbox).

Those larger maybe 2MW, more cylinder, MTU 4000s are:

-  16 V: 2185 x 1850 x 3510 or

-  20 V: 2240 x 1470 x 4040

But there may be risks of developing specially Australia Only (therefore potentially orphan) MTU diesels for submarine. The precedent of selecting untried Garden Island-Hedemora V18B14SUBs (with turbocharger) for the Collins submarines should be remembered. The most unique, orphan part of the Garden Island- Hedemora V18B14SUB was the turbocharger, the precursor to over reliance on Australian technical solutions and exorbitant maintenance rather than relying on a large industrial development base like Kawasaki, MAN or MTU 12V4000s.

Additional issues are unstealthy vibration from diesels causing excessive noise as n the Hedemora V18B14SUB [source?]. 

High power Kawasaki diesels already used on Japanese submarines may be a less risky and more efficient choice.

Pete 

Translation of Japanese J-Ships Article which uses Submarine Matters Articles

Tanaka (in a comment here) has kindly translated (from Japanese to English) an article by Isaku Okabe (a popular  Japanese military critic) in the J-Ships February 2018 issue. J-Ships is a popular Japanese maritime magazine.

Isaku Okabe's article concerns the Soryu Mk.2's improved, more stealthy, snorting/snorkeling performance. The information on the Soryu Mk.2 performance is based on Submarine Matters' articles written by Anonymous and Pete ( here, here and here and especially here, here and here ) .

Tanaka's Translation [with some further Pete translation] is:

"THE INCREASING CAPABILITY OF STEALTH CRUISING THANKS TO THE LONGER UNDERWATER DURATION".

"There are some estimations of performance comparison between [the current] Soryu class [Mk. 1] and [submarine number] 27SS [see TABLE below] in a blog [Submarine Matters] written by a foreigner [ :) ] who is familiar with submarine matters.

In the blog, 27SS is called "Soryu Mk.2" and its maximum period of continuous [fully submerged] cruising at a speed of 4 knots is estimated to be 7~9 days compared to that of Soryu's 3~3.5days.

The blog says the general period of continuous [Soryu Mk.2] fully submerged cruising will increase to 6~8 days though the current Soryu class can cruise only 1~1.5 days.

The so called "Soryu Mk.2" will be able to conduct secret patrols without snorting for a much longer time. In regard to cruising at a speed of 18 knots, "Soryu Mk.2" will be able to do it continuously for 3~4 hours which is 3~4 times longer than current Soryu's.

The time required for charging batteries for the current Soryu Mk. 1 is estimated to be 5~10 hours, whereas the "Soryu Mk.2" needs just 1~2 hours, which makes the latter much more stealthy.

Supposing the mission period is 60 days, the "Soryu Mk.2" [needs to snort] 8~10 times compared to 40~60 times for the Soryu Mk.1. Snorting [with loud diesels running] reduces stealthiness - so a submarine needs to move out of its operational area to be safe.

[Lithium-ion Batteries (LIBs)] can reduce the number and length of snorting times which increases the stealthy operational time. This results in much more efficient use of submarines.

It goes without saying that these are estimations by a foreigner [ but kindly provided by Anonymous ] and no one knows if they are correct or not. But at least they can be good indicators to infer the progress of submarines using LIBs [rather than shorter operating Lead-acid Batteries (LABs)] .

Converting from [the Soryu Mk.1's] AIP + LABs to [the Soryu Mk.2's] LIBs is not as simple as just changing the type of batteries. Many parts of the submarine need to be modified such as whole power system and safety management system for the realistic and safe use of LIBs.

The 11th [27SS] and 12th [28SS] Soryu class submarines, which correspond to what the foreigner calls "Soryu Mk.2", will undergo major battery-electrical system conversions but not other major changes. Soryu Mk.2's hulls, weapons, sensors and control systems remain almost the same as current Soryu class." [The author, Okabe, did not mention the major change to the more powerful, storm tolerant, snorkel system. Maybe that will be introduced on the Soryu Mk.3?].

TABLE - SORYU & Oyashio Program as at January 25, 2018 

SS No.	Build No Name	Pennant No.	MoF approved amount ¥ Billions FY	LABs, LIBs, AIP	Laid Down	Laun -ched	Commi ssioned	Built By
5SS Oyashio	8105 Oyashio	SS-590/ TS3608	¥52.2B FY1993	LABs only	Jan 1994	Oct 1996	Mar 1998	KHI
6SS-15SS Oyashios 10 subs	8106 -8115 various	SS-591-600	¥52.2B per sub FY1994-FY2003	LABs only	15SS Feb 2004	15SS Nov 2006	15SS Mar 2008	MHI & KHI
16SS Soryu Mk 1	8116 Sōryū	SS-501	¥60B FY2004	LABs + AIP	Mar 2005	Dec 2007	Mar 2009	MHI
17SS	8117 Unryū	SS-502	¥58.7B FY2005	LABs + AIP	Mar 2006	Oct 2008	Mar 2010	KHI
18SS	8118 Hakuryū	SS-503	¥56.2 FY2006	LABs + AIP	Feb 2007	Oct 2009	Mar 2011	MHI
19SS	8119 Kenryū	SS-504	¥53B FY2007	LABs + AIP	Mar 2008	Nov 2010	Mar 2012	KHI
20SS	8120 Zuiryū	SS-505	¥51B FY2008	LABs + AIP	Mar 2009	Oct 2011	Mar 2013	MHI
No 21SS			No 21SS built
22SS	8121 Kokuryū	SS-506	¥52.8B FY2010	LABs + AIP	Jan 2011	Oct 2013	Mar 2015	KHI
23SS	8122 Jinryu	SS-507	¥54.6B FY2011	LABs + AIP	Feb 2012	Oct 2014	7 Mar 2016	MHI
24SS	8123 Sekiryū	SS-508	¥54.7B FY2012	LABs + AIP	15 Mar 2013	2 Nov 2015	13 Mar 2017	KHI
25SS	8124 Seiryū	SS-509	¥53.1B FY2013	LABs + AIP	22 Oct 2013	12 Oct   2016	Mar? 2018	MHI
26SS	8125 Shoryu	SS-510	¥51.7B FY2014	LABs + AIP	2014	6 Nov 2017	Mar 2019?	KHI
27SS First Soryu Mk 2	8126	SS-511	¥64.4B FY2015	LIBs only	2015	2018?	Mar 2020	MHI
28SS  Second Soryu Mark 2	8127	SS-512	¥63.6B FY2016	LIBs only	2016?	2018?	Mar 2021?	KHI
29SS First Soryu Mk 3 (1) (3)	8128	?	¥76B FY2017	LIBs only	?	?	2023?	MHI?
30SS Second Soryu Mk 3 (2)	8029?	?	¥71.5B FY2018	LIBs only	?	?	2024?	KHI?

Table from information exclusively provided to Submarine Matters. LABs = lead-acid batteries, AIP = air independent propulsion, LIBs = lithium-ion batteries. ¥***B = Billion Yen. MHI = Mitsubishi Heavy Industries, KHI = Kawasaki Shipbuilding Corporation of Kawasaki Heavy Industries.

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Tanaka, Anonymous and Pete

Australian Future Sub's Diesel Genset Requirements - PART THREE

The critical issues of providing sufficiently powerful and reliable diesels for Australian future submarines are complex. Unreliable diesels and generators seem to have been the main weakness of the Collins.

Australia enthusiasm for selecting a Japanese submarine in the years up to 2016 relied heavily on an Australian assessment that the KAWASAKI 12V/25/25SB was the best high power submarine diesel available worldwide. The 12V25/25SB has been successfully used on Japanese subs since 2005 (see sidebar). 

So far I think Naval Group should work through the MAN Diesel & Turbo SE (which owns SEMT Pielstick diesel builder) grouping to obtain the 12V/25/25SB diesels under licence. Or perhaps obtaining Kawasaki's newly developed 12V25/31S (to be installed in 29SS, to be laid down in 2018/19 - see TABLE) as this will gradually replace the 12V/25/25SB. This is noting MAN and Kawasaki are already closely associated on marine diesels.

As outlined in PART TWO diesels (around 1.4MW or less) for smaller European designed submarines are inadequate in power to meet Australian submarine needs. Four more powerful diesels (around 2MW each) are required. This would avoid the need for 6 “euro” diesels (an arrangement which would inefficiently stretch the Australian submarine's length and/or diameter.

On the need for four diesels submarine expert Hans J. Ohff, with much knowledge and many Adelaide and Canberra contacts, commented on Selection of Naval Group Day (April 26, 2016) that: 

"The [Australian future submarine] will be equipped with four diesel alternators to generate electricity, a  >7 megawatt permanent magnet motor and ample battery storage." 

A Kawasaki-MAN effort to introduce Kawasaki diesels for Australian use should preferably avoid  major alterations, like an Australia Only turbocharger-supercharger system, due to technical/reliability risk. 

Australian submarine diesels will not function in isolation. Other power train components matter. In that vein it appears that Naval Group has already selected Jeumont Electric as the official supplier of the generator pack which includes manufacturing permanent-magnet synchronous motors and the associated power electronics cabinets. Hence the diesel supplier will need to work closely with Jeumont Electric on a business and technical level.

Pete

Tension Between Western Delegations on Evacuation of Foreign Nationals from S Korea

This is an interesting matter between Western military, diplomatic and security agencies. It is within the context of sensitivities around the 2018 Winter Olympics (also see). 

These Olympics will be held in PyeongChang, South Korea from 9 to 25 February 2018. PyeongChang is a mountainous area of South Korea just 100 km south of the tense Demilitarized Zone (DMZ) with North Korea.

 If any of North Korea's frequent nuclear/missile threats occurred in February this could wreck the usual good feelings of the Winter Olympics. Mass evacuations of foreign nationals is a possibility, however slight.

This deals with the sensitivies of:

1.   promising North Korea that no military exercises will be held in South Korea prior to the February 2018 Winter Olympics in South Korea. Summary of a January 2018 Washington Post report: Trump and South Korean President Moon Jae-in have agreed to postpone the large annual Foal Eagle Exercise until after the Winter Olympics to reduce tensions with North Korea. 

2.  North Korea might decide to feel cheated that "secret" plans were reported in the Japanese media (below) in January 2018 concerning international preparations made in South Korea (SK) for evacuating foreign nationals in case of high tension or conflict with North Korea (NK). 

[PETE COMMENT

From the following it appears there is a high degree of ill-feeling on security issues, between and within Western delegations (including Japan and SK) in dealings with NK. The Chinese and Russians must be perplexed.]

On January 21, 2018 Anonymous translated [right-click mouse] page 2 of a Japanese language Gendai article of January 20, 2018. Pete did further translation for context:

"In a period of emergency in the Korean Peninsula, when President Trump has taken military action against NK, concrete measures to rescue US, UK, Canadian, Australian, French and Japanese and Americans nationals from SK are being planned

See Osan Airforce Base where the international evacuation meeting was held.

In November 2017, the first meeting of Japan and five members of UN Command in SK (US, UK, Canada, Australia and France) was held secretly at Osan US Air Force (USAF) Base 64 km south of Seoul to discuss rescue/evacuation planning for their foreign nationals. 

[Osan hosts US 7th Air Force Headquarters]. 

"It was a military official consultation and SK was excluded because of possible information leakages to NK through the SK "Blue House"." 

[the "Blue House" is a large complex that is the Executive Office and Official Residence of SK President Moon Jae-in]

Japanese attendance included Counsellor Level officials from the National Security Bureau of the Cabinet Secretariat and uniformed staff from the Japanese Ministry of Defense. They were transferred by USAF aircraft travelling from USAF Base Yokota (Tokyo) to Osan USAF Base.

See Pyeongchang Olympic site and Busan+Inchon evacuation ports (Map courtesy worldatlas)

Numbers of the foreign nationals in each region of South Korea, identified by the US, were displayed on a huge screen and projector in the November 2017 meeting room at Osan Base. Measures for transporting the nationals to Busan Port and the Port of Incheon via air, land and sea were explained.

The US State Department informed SK diplomatic and security authorities who had been excluded from the November 2017 meeting. SK reacted by complaining strongly and could [did?] attend the second meeting held in December 2017.

Until [November 2017?] these kinds of meetings have only been held secretly between the US National Security Council, US Department of Defense, Japanese NSC and Japanese Ministry of Foreign Affairs. The US State Department and Japanese Ministry of Defense have been excluded because the security of their information management is considered suspect.

"Under multilateral talks discussing the situation in North Korea, it can be understood that leakage of confidential information is a deadly and serious problem."

Translated by Anonymous and by Pete.

Chinese undersea sound surveillance sensors (SOSUS) near Guam

"The Chinese Academy of Sciences recently disclosed that underwater acoustic sensors have been monitoring sea activity near Guam since 2016."

Source: Russia's SputnikNews relaying a Hong Kong, South China Morning Post report of  

January 22, 2018. 

COMMENT

The report indicates "The sensors collect water temperature and salinity data that impact how sound moves through water". These make the underwater acoustic (SOSUS) sensors more accurate AND also provides a scientific research "cover" for basically ASW sensors. 

Once China perfects node sensors near Guam, and in the South, China Sea, China will lay 500+ km  undersea SOSUS cable arrays, stringing many sensors together

-  thereby providing electrical and communications channels to the sensors.

The main (near Guam) Chinese sensor targets would be the US Submarine Squadron 15 (SUBRON 15), at Naval Base Guam island. 

Anderson Air Base (at right) is also on Guam island, as is Guam (Apra) Naval Base, where

SUBRON 15 has long been based. South China Sea on left. All "targets" for China's developing SOSUS systems.

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SUBRON 15 mainly consists of:

-  the large submarine tender USS Frank Cable (AS-40)

-  four Los Angeles class SSNs, namely USS Key West (SSN 722), USS Oklahoma City (SSN 723), 

   USS Topeka (SSN 754) and USS Asheville (SSN 758). 

-  also an Ohio class SSGN occasionally visits.

FOR FURTHER BACKGROUND

See Submarine Matter's June 17, 2015 article, Guam Nuclear Submarine and Bomber Air Base.

Pete

Suspicious Russian Submarine Fire, Vladivostok, Diesel Vapour? Battery Hydrogen?

KQN has drawn Submarine Matter's attention to a January 21, 2018 Russian claim that the Russian Navy intentionally set a Russian conventional, diesel-electric, Kilo class submarine on fire, on January 21, as part of a “damage control exercise”. This fire was/is at Vladivostok Main Naval Base, Russian Pacific Fleet.

The Kilo is most likely part of the Russian Pacific Fleet's, Vladivostok, 19th Submarine Brigade and may well be an unimproved Project 877 Kilo (likely Pennant Number "B-260", "B-445", "B-394", "B-464", "B-494", "B-187", "B-190" or "B-345"). Intentionally starting a fire is highly unlikely given the cost of a Kilo and close proximity of several other Kilo subs near the fire (see photo below).

Russian Kilo submarine on fire, January 21, 2018, at Vladivostok. (Photo courtesy Anonymous Russian via a scoop by Tyler Rogoway, The Drive January 21, 2018, website).

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REASONS FOR FIRE?

It will be interesting if the Russians admit the real reason, eg:

-  residual diesel fuel vapour catching alight 

-  angle grinder or welding equipment started a fire of insulation or diesel fuel vapour between the 

   outer hull and pressure hull,

-  in what supports diesel burning KQN suggested January 22, 2018 "From the photo [above] it
   appears smoke is coming out from the rear escape hatch? Diesel generator fire? The diesel engines
   and motor is in the back. There seems to be diesel fire on water as well? [also noticed by
   Anonymous 23/1/18 11:51 AM]. There appears to be light ice on sea surface, I am sure it is cold
   there so do they need to run the generators or main engine to maintain the inside at a controlled
   temperature for the sake of electronics?"

-  Pete suggests another theory - that extreme cold surface temperature or water temperature
   caused diesel fuel tank/fuel in the diesel engines/or pipes to freeze-expand-and-split the
   tanks/engines/pipes with some diesel leaking into the water. A short term rise in temperature
   (Sun during the day) and ignition (bored sentry/crew smoking?) all caused the fire. 

I doubt that one would put up a real fire as an exercise on a $300M submarine. Strange things have happened with the Russian armed forces like the time some soldier cooked inside an IFV and then the IFV ended up being barbecued...

-  battery maintenance causing release of flammable Hydrogen gas from the batteries, leading to fire. 

OR

-  fire of highly volatile torpedo propellant, then subsequent "cooking-off" of torpedo warheads

Precents include:

-   the welding caused fire between outer and pressure hulls on the Russian Oscar class SSGN "Orel" 
    on April 8, 2015

-  the fire and explosion of Indian Kilo submarine INS Sindhurakshak, on August 14,  2013.

Pete