An Oil Rig

What you see in the background is an oil rig. That's what most people see - what they dont see is the amount of pipework and subsea structures (PLEM, PLET, christmas trees, valves, pig launchers and so forth) that lie on the seabed.

The oil rig is definitely the most famous structure in the whole oil extraction system and its the most dangerous place to be where heat and oil sometimes tend to mix too closely. The purpose of an oil rig is to drill oil. A rig is usually constructed directly above an oil well and drills are sent deep into the ground to extract the oil. Once oil has been extracted, some basic processing is done on the rig where oil and impurtities are separated before the oil is sent to shore vis subsea pipes or to a nearby FPSO where further processing is done.

Here's a more interesting explanation about an oil rig.