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OWOE - Oil And Gas - What is a subsea development?
  Figure 1 - Illustration of Subsea Equipment (FMC)
Figure 1 - Illustration of Subsea Equipment (FMC)
What is a subsea development?
Topic updated: 2015-09-01

A subsea development refers to an oil or gas development that is physically located on the seafloor. Wells are drilled from the water surface using mobile drilling rigs, which are typically semi-submersible rigs in deepwater. The wells are then completed on the seafloor with what is call a "wet tree". In most subsea systems production flows through underwater flowlines to a surface production system for processing. A simple subsea system consists of a single well producing to a nearby platform. More complex systems consist of multiple wells producing through a subsea manifold and then via a common flowline system to a production facility, which could either be located offshore or onshore. See Figure 1 for an illustratin of various subsea production components tied back to floating facilities.

Although a subsea well is more expensive to drill than a conventional platform well, overall field development costs are lower either because an existing platform can be used for production or because a dedicated platform can be located in shallower water.

The very first subsea completions can be traced back to 1943 with the completion of a well in Lake Erie in 35-ft water depth. The first subsea well to be completed offshore was by Shell offshore California in 1961. In the US, after some early development for California and the Gulf of Mexico, interest in subsea waned as companies focused on fixed platform technology with dry trees. During that period, Norway moved to the forefront of subsea technology, starting with the Frigg field in the North Sea in 1982 and continuing through many developments over the next several decades. Then in the 1990s oil companies, particularly Shell in the US and Petrobras in Brazil, advanced development of sybsea systems for deepwater applications. Shell's Mensa field in the US Gulf of Mexico began producing in July 1997 in 5,376 feet of water and shattered the then water depth-record for production. The current deepest subsea development is Shell's Tobago in the Gulf of Mexico in about 9600 feet of water.

As subsea technology has advanced, operators have begun to place some processing equipment on the seafloor as well. The current state of technology involves gas-liquid separation, electrical submersible pumps, and some sand management equipment. Technology development is ongoing in a number of processing areas, with a focus on oil-water spearation and subsea disposal of produced water.

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