Since the advent of offshore oil drilling in 1887, offshore oil drilling platforms (drill rigs) have evolved along an exponential trajectory. The first offshore oil rig was built by H. L. Williams on a large pier known as a wharf in 1887.
It stretched 300 ft from shore and set the stage for an industry which would grow continually throughout the next century. Williams’ wharfs were a predecessor in many ways to jackup rigs – the most popular offshore drilling platforms in the world today.
The oil industry was eager to keep pace with the industrialized world’s thirst for oil. In order to increase production, offshore drilling rigs have needed to increase their distance from shore as well as the depth of water through which they can reach. These demands affect each of the five primary concerns of offshore oil drilling:
- Exploration requires technologies that can determine the wealth of oil deposits hundreds or thousands of meters below the surface of the sea. The capabilities of modern drill ships have enabled exploratory drilling at rapidly increasing depths.
- Drilling requires meticulous attention to competing physical and technical details. Namely, the platform must remain stable during a constant onslaught of waves, tides, and winds. Jackups provide an air gap between the surface of the water and the hull of the platform.
- Fabrication is increasingly specialized as depth (and consequently, pressure) increases. Semi-submersibles float atop pontoons which are located below the surface of the water.
- Transportation difficulties arise as the amount of oil and distance from shore increase.
- Production capacities must continue to increase in accordance with the quantities of oil extracted from the earth.
Jackup Rigs are Mobile Platforms Equipped with Jack and Pinion Systems
Jackup rigs consist of a barge and several legs which dig into the sea floor supporting the barge. The barge is often towed or pushed by a tug boat. Alternatively, it may be self-propelled into its position. Once in place, the rig utilizes its jack and pinion system in to lower the legs snuggly into the sea floor.
During “preloading” the barge takes on water in order to increase weight. This “ballast water” is used to aid in driving the legs into the sea floor with enough force as to ensure that waves, tides, and currents do not budge the legs from their position during drilling.
The jack and pinion system works much like the action of a car being jacked up to change a wheel. In the case of a drill barge, the rig must be jacked up by several enormous legs which penetrate several meters into the sea floor. In most cases, these legs lift the barge hull above the surface of the water leaving an air gap between the hull of the barge and the surface of the water. In the case of “mat-type jackups,” however, the hull is partially submerged.
Jackups are especially popular due to their endurance at sea. Certain rigs are rated with normal holding capacities in excess of 600 tons with storm holding capacities reaching 720 tons.
While jackups have long been used in the offshore drilling industry, they have only recently been incorporated into the offshore wind energy industry. This is just a single example of the technological advances of offshore drilling leaking across into other fields.
Drill Ships are Primarily Used as Exploratory Vessels
In order to drill in water deeper than the 120 meter reach of jackups, the platform must float on the water. Drill ships have the advantage of drill depths up to ten times that of jackups.
The first drill ship, CUSS 1, built in 1961 used four underwater propellers to keep it in place. It was able to stay in position while drilling a well off the coast of California at a depth of 948 meters.
While CUSS 1 used a complicated system of radar and buoys to stay in place, modern drill ships are equipped with dynamic positioning systems. Because of the dual advantages of loose tether to any point and their ability to self-propel, drill ships travel to and from oil fields at a relatively quick pace.
Generally a drill ship will only be used in specific circumstances. They may be utilized in drilling new deep water wells, maintenance, or finish work. Drill ships are robbed of their mobility advantage if they are maintained over one well for too long a period. They are the cavalry as the oil business fights its war against the seas.
Semi-Submersibles are Useful in On-sight Production
The pontoons of a semi-submersibles (also known as a semi-sub or simply a semi) are ballasted – loaded with water – so that the pontoons provide buoyancy from below the surface of the water. The actual operation deck is built high over the pontoons with a sufficient air gap so as to keep wave activity from affecting operations. Because the pontoons are located below the surface of the water, wave activity does not affect them as much as a vessel that floats above the water.
Like submarines, semi-submersibles can take on or release water from pontoons in order to raise or lower the vessel. This process is known as drafting. When the pontoons of the semi-sub are below the surface, the vessel is in a “deep-draft” when they float above the water, a “shallow-draft.”
The advantage of a semi-submersible over a drill ship is its stability. Semi-subs are not so prone as drill ships to be jostled by waves. After drilling semi-subs are useful for on-sight production. This is the oil industry’s version of killing two (sometimes more) birds with one ship.