When is peacetime damage to a submarine major?

HMCS Corner Brook after the accident. Sonar smashed (Photo at Canada's CBC News)

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When is serious, serious? The submarine didn't catastrophically rupture as the article argues, but...

HMCS Corner Brook suffered a grounding accident on June 4, 2011. The sub surfaced and returned to port under her own power. 

CANADIAN ARTICLE

Tim Dunne, wrote “Journalists sub-par on sub debate” on March 3, 2012 in the Halifax, Nova Scotia, Canada, Chronicle Herald. Here is part:

...The boat struck bottom in 45 metres of water near Nootka Sound on western Vancouver Island, June 4 of last year, cutting a four-by-five-metre hole in the boat’s front.

...Missing from the debate was that the damage was to the front of the submarine’s fibreglass casing. Four metres inside the damaged casing is the pressure hull, made of 3.8-centimetre (38mm) HY 80 steel, and this is the main compartment where the crew and controls are located...While the casing was damaged, the pressure hull, able to withstand incredible stresses, was untouched.

...HMCS Corner Brook’s grounding should not be trivialized. It was a serious incident and had the potential to be a tragedy. However, commentators should not overstate the accident and ignore the important contributions which Canada’s submarines make to training, sovereignty and prevention of drug trafficking...” see WHOLE CANADIAN ARTICLE

ANOTHER ACCOUNT

Wikipedia advises "On 4 June 2011, Corner Brook while diving off the coast of British Columbia slammed into the seafloor at 5.9 knots (11 km/h) at a depth of 45 metres (148 ft). Two sailors were injured in the collision and the submarine suffered significant damage, with a 2-metre (6 ft 7 in) hole in the bow. Two torpedo tube doors were torn off in the collision.<sup>[25]

ADVICE FROM "S" ON DAMAGE TO THE PRESSURE HULL</sup>

Drawing on-translating S’s Comments (below) of 14/3/17.

Based on a cross section [this diagram] and pictures of the damaged HMCS Corner Books, relevant issues may include:

-  the fibreglass casing, outer (non-pressure) hull, acted as a shock absorber

-  torpedo tubes penetrate the pressure hull, so distortion of torpedo tubes can distort the presure hull.
   This is especially if the collided-with tubes act as levers.

-  the pressure hull appears to have been damaged. Damage can be visually obvious ("yielding")
   or invisible (electronic sensors can detect "microcracks" especially around the hull-tube 
   joints) 

After a range of inspections of damage to the pressure hull the front part of pressure hull was probably repaired by weld-cutting it. A replacement part was then welded on. The hull cutting-welding activity may have put new strains on undamaged parts.

These repair activities may have caused a slight inconsistency between the new part and the undamaged parts of the pressure hull leading to a reduction in maximum submerge depth capability.

It is fortunate that the grounding only occurred 45 metres down. If the grounding was 250 metres down this might have caused catastrophic failures to the torpedo tube seals and to the pressure hull - sinking the sub. 

By current Japanese standards high yield HY-80 steel is not that strong. [Pete comment - this is noting the much more modern Soryu has HY-156 equivalent steel.] A pressure hull 38mm or 39mm thick is rather thin. A hull thickness of 45mm-65mm would be more effective for safer, deeper, diving. [Pete comment - Pressure hull tolerances are, of cause, linked to likely threats. Canadian subs monitoring drug-organised crime is not the same as monitoring Chinese facilities (with ASW forces that may kill a sub)].

On assessing damage, or what could have happened, who’s right or wrong?

Pete