So I am making an oil rig and carved out a piece of styrofoam in the main shape and was going to cover it in white fondant and then add the decorative pieces to that, however, when i roll out the fondant and put it on the base it rips at the top level and pulls down. So how can i put fondant on the styrofoam without this happening?

Make sure you grease the syrofoam with shortening before you put the fondant on it. Anytime you are using fodant on strofoam, cakeboards and the like you need shortneing not water to make it stick.

I can get it to stick...however, it just rips right off the top of the styrofoam?

When I cover styrofoam, I cover it first with melted white chocolate (candy melts - whichever color best matches the color of your fondant.) I put on a couple of thin layers until I can no longer feel the sharp (cutting edges & points!) of the styrofoam. Then I smooth it with a hot knife, let it cool, and then cover it with fondant. I usually brush on a very tiny bit of apricot glaze to ensure good stick. I've had really good luck with this. Good luck to you!

that seems much easier and less fondant also...what else can i use instead of apricot glaze i dont have any of that?

Try softening the sharp edges by filing with an emery board or sand paper.
The fondant should stick just by using a bit of water or shortening. It is less expensive and neater than using piping gel or anything else sugary.

Thank you all for the help it worked out great lastnight! However, i usually just use wiltons fondant but this time i needed black so i bought the duff brand real fast and it takes FOREVEr to harden, so i will be doing the other half tonight!

Duff brand fondant is actually Fondarific and doesn't get hard like Satin Ice or Fondx does.

Another tip for covering cakes in styrfoam, use piping gel. It holds really well.