Mostly oil is found buried beneath the seabed. Oil rigs are hug floating devices that are anchored to the seabed while wells are drilled into the oil bearing rocks.
These rigs contain the drilling machinery and a helicopter pad for receiving supplies. When the oil is extracted from the rock, it contains a large amount of gas, which has to be burnt off at the surface. The gas, coming out of the oil well is in great force and can get ignited resulting in fire.
Fire fighters are used to put off the fire by using explosion. This explosion uses the surrounding oxygen in putting the fire out.
Below Oil Rigs info from Wikipedia (Read More):-
An offshore platform, often referred to as an oil platform or an oil rig, is a large structure used to house workers and machinery needed to drill wells in the ocean bed, extract oil and/or natural gas, process the produced fluids, and ship or pipe them to shore. Depending on the circumstances, the platform may be fixed to the ocean floor, may consist of an artificial island, or may float.
Most offshore platforms are located on the continental shelf, though with advances in technology and increasing crude oil prices, drilling and production in deeper waters has become both feasible and economically viable. A typical platform may have around thirty wellheads located on the platform and directional drilling allows reservoirs to be accessed at both different depths and at remote positions up to 5 miles (8 kilometers) from the platform.
http://en.wikipedia.org/wiki/Oil_rigs
