This is a delicate situation if yu really, really want to get in. There's really no right or wrong answer to this. Several suggestions made are good ones, but no gaurantees they will get in the door.
In my former company, because of my position and contacts there, I had access to everything and took a lot of 'industrial' images - with permission. Naturally, all I had to do was 'inform' really and never really had to ask permission. Many of my images were used within the company and for external publication too.
Now, some photographer - 'better' than me, more experienced, bettr equipment, etc... - if they asked 'the powers' - say the VP I reported for permission - inevitably they would be denied. Regardless of the fact he/she would have performed the same activities with the same caution and professionalism as I - even far 'better' images. Why? Liability? Sort of. It really boild down to time. When I did it, because I was an employee I could juts do it on my own time and I knew when not to bother anyone.
An outsider has no luxury to know production schedules, etc... for one thing and secondly, you can't just let a stranger loose running around taking pictures. That person will need supervision. No time for that. That's the real reason. So companies citing liability, safety, etc... sure those reasons make sense, but the real reason is time. They have no time baby sit an outsider. Period.
So, with that being said, none of that helpsyou get in. Here's some methods:
1. Someone said find the decision maker. That's a good start. Better yet, find the guy that has the influence to get you in and walk you around a bit.
2. Propose something. Free images, whatever. I always had my camera when something 'big' was going on. One of projects required the side of the building to be removed so a large machine could be brought in by helicopter and crane. Had my camera there. If some photographer wanted to take images for free that day, I would have said sure. As long as I got images to use.
3. Find a contractor that doing a job there. Combine that with #2 above. During the same event, one of the contractors asked me if his sone could take images. I said sure. As long as I got free images to use.
4. Take images from the public sidewalk. I did that here in New Haven of the oil tanks at the harbour. Sure I was stopped a couple times by the cops, by security. Eventually, they got to know me and now I take images regularly. In fact I've gotten to know the site manager, showed him my portfolio and he's promised to let me in one day!
5. Find a beautiful model, bring her with you, and say you'd like to use the place as a backdrop for a 'fashion editorial' shoot. A beautiful young female model does wonders when it comes to men.
Regards, Art.