How Offshore Drilling Works

Offshore Production Platforms

Offshore drilling platforms
Offshore drilling platforms

Once the exploratory drilling phase is over and geologists have determined that a petroleum reservoir is worth the massive expense, oil companies prepare to establish an offshore production platform. These rigs are designed to last decades, often far from land and in some of the most hostile waters on Earth.

Construction crews typically build the platforms on a nearby coast and then transport them as needed to the drilling site. Production costs for these vessels typically run in the hundreds of millions of dollars. There are currently seven different varieties of offshore platforms.

Fixed platform: This platform design tackles the challenges of offshore drilling in the most straightforward and industrial way imaginable. Need to fix production facilities to a position above your drilling site? Why not construct a gigantic tower of concrete and steel and mount your oil rig on top? To fully comprehend the amount of materials that go into constructing this underwater structure, consider that they operate at depths of 1,500 feet (457 meters) or less -- that's just a little taller than Chicago's Sears Tower. These platforms are extremely stable, despite the fact that the concrete base isn't even attached to the seafloor. It simply stays in place due to all the weight above it. However, at depths greater than 1,500 feet, the design begins to become more impractical due to material costs.

Compliant tower: These rigs take the basic idea of the fixed platform and make it viable to operate in depths of 1,500 feet to 3,000 feet (457 meters to 914 meters). The design achieves this by relying on a narrower tower of steel and concrete. But while fixed platform designs are rigid, compliant towers are designed to sway and move with the stresses of wind and sea -- even hurricanes. In this respect, they're much like modern skyscrapers that are built to sway with the wind.

Sea Star platform: The Sea Star platform is basically a larger version of the semisubmersible design we talked about in the last section. The production facilities sit atop a large submersible hull on a tower. When the lower hull fills with water, it sinks to a lower depth, providing stability while keeping the facilities high and dry. However, instead of giant anchors holding it in place, the Sea Star is connected to the ocean floor by tension legs. These long, hollow tubes remain rigid at all times, preventing any up-and-down motion on the platform. The legs are just flexible enough to allow side-to-side motion, which helps absorb the stress of waves and wind. These platforms operate from depths of 500 to 3,500 feet (152 to 1,067 meters) and are typically used to tap smaller reservoirs in deep waters.

Skip to the next page to learn about the four remaining types of offshore production platforms.