Types of off Shore Oil Rigs

Below the sea floors lie vast reserves of oil.  One 1996 estimate put the amount at 1.3 trillion gallons.  If this figure is accurate it is roughly half of the total oil reserves on the globe.  The Mineral Management Service of the U.S. Interior Dept has estimated that 30% of U.S. oil reserves exist under water in an area referred to as the Exclusive Economic Zone (EEZ) which extends 200 miles from all U.S. shoreline. 

Oil is the residual elements of previous living organism like plants and animals that existed many millennium before.  Over hundreds of thousands of years, as this previous marine life died, it has been covered with layers and layers of sediment.  The weight of this sediment has created pressure combined with heat from the earth’s core and transformed this decayed matter into this major sources of energy used around the globe.

This energy source sets under 320 million trillion gallons of water (give or take a trillion or two) and ocean depths on average of several thousand feet.  Yet this hasn’t stopped members at the top of the food chain from attempting to fulfill the laws of supply and demand for this product.  It may turn out that the price is too high to access these oil reserves, economically and environmentally.  To achieve this will require safe oil rigs that can effectively accommodate depths and underwater terrains that are difficult to work with under any condition.

For the deepest explorations semi-submersibles, or “Floaters” serve this type of operation best.  The deeper waters, like the North Sea between England and Europe, are rougher seas and require an oil rig structure that is not threatened by such forces. The pontoons or columns on Floaters, when filled with water, will partially submerge the unit to a depth required to get the job done.  When rough seas ensue the semi-submersibles are anchored to mooring cables at the sea floor to prevent it from drifting while simultaneously allowing it to “give” under these conditions; preventing it from breaking up.

In more stable waters Fixed Platform rigs are a better option.  Their designed steel or concrete legs anchor directly to the ocean beds.  They are usually massive structures, the largest being the Hybernia located in an oil field in the North Atlantic Ocean off the coast of Newfoundland.  Such large structures are most always built in-land then transported to their location where their built-in storage tanks served as flotation devices to be opened at the footings and filled with sea water when they arrive to their destinations.  Once vertically erect in waters as deep as 1700 feet, the rigs platform is constructed and facilities would be built atop of it to accommodate up to 150-160 drill and crew members.

Jack-Ups are off-shore rigs whose name pretty much describes its capabilities.  Their column-like legs reach the sea floor and can be “jacked up” or down to accommodate water depths up to 500 feet.  They are first moved to their location by tug boats then their legs are lowered, using a rack and pinion gear system on each leg, until they position themselves on the sea floor.

Drill Ship rigs are just that – ships designed to serve as oil rigs.  They have the mobility of a semi-submersible and Jack-Ups and do well at most sea depths but don’t handle well in rough weather.  They of course have no moorings to stabilize them under such conditions but most are equipped with a dynamic positioning system to enable them to maintain their position over the well.  Though used to extract oil from below the oceans floors they are more often used for oil and natural gas explorations and scientific drilling purposes.  They have drill depth capabilities of up to 12,000 feet

Spar Platforms are relatively new oil rig platforms that do well at deep sea depths.  Its concept originated from marker buoys that were used for gathering oceanographic data and storing oil.  Ninety percent of its body is under water and its design gives it very favorable motion characteristics.

Other variants of off-shore rigs are the Flotels and Floating production systems or FPSO (floating production, storage, and offloading system).  They are commonly used as support rigs serving as storage, leisure and catering facilities, attaching themselves to the bigger oil rigs.

Mankind’s ingenuity to reach into the oceans’ bowels and pull up these oil reserves are not without their hazards; but then neither was putting a man on the moon.  We have the capability but all efforts must insure that the costs are not prohibitive and the threat to our shorelines and underwater ecosystems are taken seriously when taking such risks.


Ocean Energy, U.S. Dept. of Interior

Energy Statistics

How much water is there on Earth?

Wikipedia – Oil Platform