"What caused the impact is not known."  
Alright all you Sherlock`s out there..what could have been the "impact" that the crew said they felt at 11:20am?  
Some come to mind;
One might first think ice.  Were large chunks still surging in and out with the tide?  Was the tide running at that time?  
Could a structural cross-member beam have finally rusted through and broken loose, or been struck by something damaging the leaking oil pipe attached to it?  It sounds like some critical infrastructure on the bottom of Cook Inlet is reaching it`s designed end of life and beyond.  Do we just keep patching that tenuous infrastructure because nobody wants to get the bill, like musical chairs only musical failing pipelines under Cook Inlet?  That doesn`t sound like a wise policy to this Alaskan. Who assures the integrity of all those lines under the Inlet? Is there an agency who can or does regulate them for engineered structural soundness on behalf of the people of Alaska? Or is it hodge-podge..