They are fabricated on land. Typically they mobilized (moved) to location by barges. A jackup rig may be assembled in pieces, with the jacket or the rig structure being installed first, and then the topsides (all housing and equipment that rests on the structure) is installed second, by large cranes. See related link.
A semi-submersible, like the Deepwater Horizon, may be towed by tugboats to its location. See second related link.
Oil rig oil Derick
There is not one, but there numerous oil rigs in the Gulf of Mexico. Their purpose is to pump the oil from the oil deposits located deep within the sea bed in that region.
Oil deposits were found beneath the ocean, too far from the shore to drill from the land. Eventually someone had the bright idea to go and produce these rigs that drill at sea and the first oil rig was born. So, oil rigs were invented to pump the oil located beneath the sea bed far from shore.
toukiai
Most oil rig platforms today do not float. The platform is usually positioned atop several pillars that extend to the bottom of the sea bed on which they are located. Newer technology however, has developed barges that contain oil drilling equipment. These newer barges do float but must rely on precise electronics to keep them positioned within a couple of feet about the drill site on the sea bed.
Pipes under sea
Piper Alpha
yes oil is available under the sea , theres lots of plates of oil under the sea bed
Yes and No The "derrick" is the drilling tower which is part of the complete oil platform (which could be moored at sea and have crew quarters etc) which is the "oil rig"
They are built on land and towed to sea to the drilling site where the oil or gas is.
Piper Alpha
If the rig is relatively close to the sea shore and is located on shallow waters, oil pipes running through the ocean floor are enough to transport the unrefined oil. Otherwise, oil tankers (huge ships with carrying capacity of 2 billion metric tons of oil) are used to transport the oil from the rig to the refinery.