Ebbie, "Semi-submersible" platforms are like boats, and are powered to their site. Some are left in place, others are moved to a new site.

There are several types of platforms, but scroll down to "semi-submersible" in this Wiki article. The Transocean 'Deepwater Horizon' rig used by BP cost $600 million. I forget where it was built, I think South Korea but not sure.

Near the bottom of the article, a platform off Brazil is shown; it looks like the rig used by BP.

Semi-submersible