Cargo had been very volatile Attaka Crude with a 7.9 psia RVP.
Ship not inerted.
All P/V valves and ullages covers open.
This was ships SOP to avoid over-pressuring tank during ballasting.
Ballasting pushed vapors on deck.
5W whose expansion trunks were under the midships house
had been converted to cargo to take advantage of additional deadweight
associated with 1966 regulations.
Wind was light from the stern,
allowing vapors to collect in protected area between houses.
Vent trunk to machinery spaces in mid-ship house were
on aft side of house above this area.
Ship had a draft forward/aft of 12/26 ft, over 7 m of trim.
Cargo tank vent piping had numerous holes on underside.
USCG thinks some of the vapor was pulled into the midship
machinery space, ignited, deck between houses went up
and then the tanks themselves.
Given that everything was open doubt if the holes in the piping
made much difference.
As a result of this explosion which cause a lot of damage
and injuries on-shore, the USCG, 8 years after Mactra et al.
belatedly decided to require inerting on all tankers above 20,000 tons
trading to US ports.
Not sure when this came into effect.
The USCG had already required inerting on all tankers above 100,000 dwt
(all combo carriers above 50,000 dwt) with keel-laying after Dec 31, 1974.
As slow as this was, USCG was well ahead of IMO.
Requirements with respect to closed loading were much slower in coming.