Major Soryu hull life issue? - Maintenance/Emergency Welding in Japan only?

The following discussion of the Soryu hull, short operational life and welding in Japan only issues arises from "S"'s and my discussions on June 3-4, 2015 see Comments section for http://gentleseas.blogspot.com.au/2015/06/problems-with-submarine-purchase-from.html.

"S" explained on June 3, 2015, that one of the key technologies for the Soryu is the hull design and way of building it - which includes welds. "Submarine structure is made of series of cylindrical hulls reinforced by beams. Joints between hulls, connection of seam in hull, and connection between hull and beams are welded."

"In the case of [the] Collins submarine, GMA (Gas Metal Arc) welding was used for these hull buildings.[also called hull sections]"

"But it is not the case for Soryu submarine, GMA welding [is only?] used for hull-beam connection, [while] GTA (Gas Tungsten Arc) welding [is] used for hull-hull joint and seam connection in my opinion. [The] quality of GTA welding is superior to that of GMA welding, this technology is quite difficult to learn and it’s welding speed is very slow. There is no GTA welding technology in Australian submarine building sector. So hull building in Australia is rather risky and I can not recommend it in terms of Aussie submariners’ safety.

I then asked S "is Japan's hull making and welding practices designed for 20 years of submarine operational use, with problems developing if there are 10 more years of use?" [This is important because Japanese subs are typically operational for 20 years while Australian subs are ideally designed for 30 years service.] 

On June 4, 2015 S indicated:

"I think that [the Soryu] submarine may have material degradation issues for very long time use.

1. Fatigue which is structural damage from repeated loading: Submarine is exposed to very high pressure repeatedly. Sometimes tiny crack generates, propagates and finally leads fracture.

2. Corrosion which is damage caused by electrochemical reaction: Especially non-uniform corrosion like pitting is very dangerous. In the case of Soryu type subs, we’d better...pay special attention to the situation of interfaces [with the] anti-vibration rubber/hull.

3. We also must pay attention to stress corrosioncracking (SCC), the growth of crack formation under corrosive (sea water) and stress condition.

Prediction of these damages is difficult. But in the case of Soryu subs, JMSDF [the Japanese Navy] may accumulate important material data including degradation thanks to long operational experiences and consecutive building one of same type of submarine." [There is a life extension program for the Oyashio class (that precedes the Soryu class) but it is not known whether life extensions will be successful.] 

QUESTIONS

I would be most grateful for answers to:

Noting Submarine Matters records January 20, 2015 indicate the strength of steel used in the Soryu is "NS110" (equivalent to HY-156). This is much stronger and harder to weld than most submarines (including those from TKMS) which have a HY-80 strength rating. The safe depth most submarines can dive may be around 400 meters - but Soryus with their stronger steel may be able to dive to 600 meters.

1.  Is the high yield strength (equivalent to HY-156) of Soryu pressure hull steel the reason why GTA (Gas Tungsten Arc) welding is used?

2.  Or has GTA became the standard Japanese submarine welding method for other reasons?

3.  Does the deeper diving ability of the Soryu come at the expense of shorter 20 year old operating life?

4.  Put another why does the use of HY-156 (NS110) which allows deep diving put extra stress on the hull effectively shortening operational life to 20 years?

5.  If "[GTA] technology is quite difficult to learn and it’s welding speed is very slow. [And] There is no GTA welding technology in Australian submarine building sector. So hull building in Australia is rather risky and I can not recommend it in terms of Aussie submariners’ safety." Then

5(a)  How can deep maintenance, involving cutting and rewelding a Soryu hull, be done in Australia?

5(b)  If there is major battle damage to an Australian Soryu requiring cutting and rewelding the hull, how can that be done in Australia?

Pete