Long time player / lurker -
I watch SubBrief religiously on youtube, Aaron is a former submariner who has great knowledge about Navy matters - seems to think it was a barge with explosives. Nothing definitive here of course, and he probably has an ingrained bias towards a Naval explanation - but something more to throw into the pot:
(from comments: "it has been a long time but here is my take, the explosion was low yield , think ammonium nitrate-fuel oil (AN-FO). It is designed to push up rather than fragment the bridge, Lifting the section off of its supports. this explains the lack of visible damage, the burn on the top was the truck laden with ordinance.or something with bang potential and was a secondary , this explains the guard rails, large sparklers ect."
Also a Naval perspective on the Gas line sabotage: