In the petroleum industry, offshore drilling refers to the extraction of oil and gas from a water-covered area such as the open sea or a large lake. The drilling equipment used in offshore drilling is the same used for drilling oil wells on land with some exceptions. The primary difference is offshore drilling depends on the use of one of many types of artificial platforms. The type of platform used depends on the characteristics of the oil well being drilled and how far down the drill site is located under the water.

The drilling rig may include a mobile floating vessel such as drillship or drilling barge. Other types of mobile drilling rigs include submersible rigs and semisubmersible rigs. Jack-ups are self-propelling rigs featuring a triangular shaped platform and three to four supporting legs that get jacked down to the subsea floor. Other types of rigs built with large stationary platforms are used in the drilling of development wells over longer extended periods of time, while mobile floating vessels are more suited for drilling wildcat wells.^[1]

The single most important piece of equipment used in offshore drilling is called a drilling template.^[2] A drilling template connects an underwater well to the platform resting above the water at the surface. Depending on the actual number of wells being drilled at a site, the drilling template consists of a steel box with multiple holes. The drilling template is then positioned directly over the drill site using satellite or GPS technology, a shallow hole is dug and the drilling template cemented into place. Once secured, the template is connected to the drilling platform above via multiple cables. This system allows for drilling to be accurately carried out below water while permitting some movement of the platform above.

Another important piece of equipment used in offshore drilling is called a blowout preventer. Installed on the bottom of the ocean floor, a blowout preventer is a system designed to stop oil or gas from seeping into the water. A marine riser, part of the blowout system, extends from the ocean floor up to the drilling platform above and houses the drill bit and drill string. It is designed to be flexible and sturdy enough to withstand any movement from the drilling platform situated above. In addition, slip and ball joints positioned in the marine riser allow the subsea well to sustain any movement coming from the drilling platform as well.

[edit] History

The first attempts at offshore drilling date back to 1869 when one of the first patents for an offshore drilling rig design was awarded to T.F Rowland. The rig was built to be operable only in very shallow water and was fixed to the bottom of the ocean floor by four legged tower bears. It wasn’t until after World War II that the very first offshore oil well was drilled in the Gulf of Mexico. Since then, offshore drilling has become a dynamic segment of a booming global oil industry.^[3]

[edit] Process

Tapping into possible reservoir traps underwater is a very difficult process, especially in open water where conditions are often turbulent and volatile. Sourcing out a potential reservoir trap underwater is carried out with the same types of technology used to source a reservoir trap on land—using seismic survey data to locate and drill exploratory oil wells and then conduct further testing for oil.

Once oil is located the drilling rig is usually replaced with a production platform assembled at the site with the assistance of a heavy barge and lifting cranes. These platforms are made form heavy steel and get fixed to the ocean by huge steel piles. All platforms feature accommodations for crew and some are even built to provide oil storage. A number of smaller monopod platforms remote controlled and located adjacent to a main platform may also be used. Other systems include a floating platform tethered to a structure called a floating production storage and offloading vessel (FPSO). A tension leg platform is another type of production platform built either of steel or concrete and moored to the sea floor with vertical tendons.

When a permanent platform has been assembled, drilling is carried out. In offshore operations the initial first stages of drilling are referred to as “spudding.”^[4] Spudding occurs as soon as the drill bit, either a roller cone bit or rock bit, is lowered into the seabed. The drill bit, attached to a drill string, is rotated via a turntable located on the platform floor above. As the well hole is drilled and deepens, extra sections of 30 foot drill pipe are attached and added to the existing drill string.^[5] Drilling the well can take weeks and sometimes even months before oil is reached.

Drilling mud is used in offshore drilling similar to drilling an oil well on land. The drilling mud is used to bring rock cuttings up to the surface and cool down and lubricate the drill bit. Once used it gets recycled through a circulation system and passed through equipment mounted on the drilling rig that disposes of the cuttings overboard before clean fluid is pumped back down into the hole again.

As oil is drawn up from the reservoir it also draws up water. This water must be separated from the oil and returned to the ocean. This process is called “produced formation water” (PFW) and is carried out with the aid of mechanical separation machines and chemical treatments.^[6] There is much emphasis placed on ensuring the water returned to the ocean is clean and free of oil and chemicals. For example, some countries such as Australia have very strict regulations on how much petroleum hydrocarbon can be contained in PFW.

Not all offshore oil wells are drilled straight down. One method of drilling called directional drilling has been developed to drill oil wells at a slant or angle. The drill bit can be steered laterally over a distance spanning many kilometers until the petroleum reservoir is reached. Directional drilling is particularly effective for drilling multiple oil wells located in one central area in various directions and angles.

Once a well has reached its final depth production casing is set and cemented into place. Tubing is lowered down into the hole with packers which seal any space between the tubing and casing. When the end of the well is reached, the casing is perforated to a predetermined depth with the use of small explosive charges detonated remotely. The casing, full of small holes, permits the oil to flow up to the surface under the weight of its own pressure.

When an oil well has reached the end of its productive life cycle it is considered dry. At this stage, the well is either plugged or abandoned. This involves capping the well with cement plugs.^[7]

[edit] References

1. ↑ Industry Glossary. Society of Petrophysicists and Well Log Analysts. 26-05-2009.
2. ↑ Offshore Drilling. Natural Gas. 26-05-2009.
3. ↑ Offshore Drilling. Natural Gas. 26-05-2009.
4. ↑ Offshore Drilling. World Petroleum Organization. 26-05-2009.
5. ↑ Offshore Drilling. World Petroleum Organization. 26-05-2009.
6. ↑ Offshore Drilling. World Petroleum Organization. 26-05-2009.
7. ↑ Offshore Drilling. World Petroleum Organization. 26-05-2009.