If there was an explosion, which has not been proven yet, it would have happened on top of the platform. So a sub would either have to have a really cool torpedo that goes out of the water and hits a target, which someone would have seen, or the sub would have to get close, then a guy would climb all the way up the platform and strap a mine on top without beeing seen.
Lets be rational.
All they needed was a simple delivery boom and some magnets.