They are generally built on land and floated out to sea on a submersible boat, sometimes as a single module. read more
There are no way to build stationary rig in middle of ocean with decent sea depth. The deepest platform could be built on the shallow waters may be 1000-1500 ft deep only. For other deepwater drilling projects there are drill ships which can be utilized on the very deep sea levels and semi-submersible floating rigs. read more