Text 2. Offshore Drilling Platforms

Implementation of operations offshore such as field exploration, well drilling and oil and gas production requires application of offshore platforms designed for well drilling and production, separation of produced well fluid into oil, gas condensate, gas and water with further pumping of the first three components through pipelines to onshore terminals.

Classification of offshore platforms includes fixed and floating platforms. Fixed platforms can be applied only in shallow waters and are installed with their basement against sea bottom. Such platforms are not mobile and are designed for long-term operation in a perma­nent location.

However fixed platforms are technically difficult and economically inefficient to use for drilling of oil and gas wells in deep waters of seas and oceans. Floating drilling platforms ca­pable independently or by means of towboats change drilling locations are designed for opera­tions in deep waters.

Floating offshore platforms can be divided into jack-up drilling platforms, semi-submersible drilling platforms and gravity-based drilling platforms.

Drilling, completion and production of offshore wells has its specifics requiring application of special equipment. For example, a marine riser is used to protect drill pipe string against sea environment in the course of well drilling and production operations and prevent environmental contaminations. Marine riser is a pipe of large diameter connecting moon pool on a drillship or semi-submersible platform with preventer stack installed on subsea wellhead. In the process of drilling drill pipe string rotates inside the marine riser.

Marine riser is also used for drilling mud circulation and guiding of drill pipe string to the subsea wellhead. In the process of oil and gas production marine riser is used to protect production string located inside it.

Fluid produced from wells cannot flow through pipelines to the shore pushed only by energy of its own pressure, and existing technologies cannot provide for its pumping. That is why separation of produced fluid is required. As long as installation of full-scale processing plant on an off-shore platform is not cost-efficient, only primary treatment is performed.

Oil treatment plant includes such major components as field separators, pumps for oil transportation, centrifugal and piston compressors, gas stripping and dehydration system, and also condensate treatment system.

Well fluid goes to separators where it is separated into oil, gas condensate, gas and wa­ter. After that the oil is pumped to the shore terminal through export pipeline. Gas is com­pressed with centrifugal compressors, stripped and dehydrated. Part of the gas is used on the platform for well injection and power generation, most part is transported to the shore, and only small portion of gas is flared.

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