Evolution of Subsea Production Systems: A Worldwide Overview
Authors: Hansen R.L. , Rickey W.P. 
Published: December 2007

This paper is SPE 29084. Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are notspecialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose:to inform the general readership of recent advances in various areas of petroleum engineering. A softbound anthology, SPE Distinguished Author Series:Dec. 1981-Dec. 1983, is available from SPE's Book Order Dept. Journal of Petroleum Technology, August 1995.

Summary
The evolution in the use of subsea technology has seen advancement from one well in the Gulf of Mexico in 1961 to more than 750 wells in a wide variety of locations by the end of 1993. Along with the growth in numbers, the industry has seen rapid advances in technology, increased distances from the host facility, and water depth records. This paper gives an overview of the evolutionary changes in subsea applications, with emphasis on the most active regions and on some of the milestone installations that shaped the technology advance.

Introduction
In the 33 years since the first subsea well was completed in the Gulf of Mexico in 1961, the use of subsea wells has spread to most offshore producing areas of the world (Fig. 1). By late 1993, a total of approximately 752 subseawells had been completed worldwide, with more than 440 of these wells still in service. This paper will provide an overview of subsea technology development by focusing on three areas that exemplify the technology used worldwide:the Gulf of Mexico and west coast of North America,the North Sea, andthe Campos basin of Brazil.

Subsea Technology Overview
Subsea wells have been used in a variety of configurations. Fig. 2 shows typical arrangements, including single satellite wells consisting of subseatrees on their individual guide bases; subsea trees on steel-template structures with production manifolds; and clustered well systems, which are single-satellite wells connected to a nearby subsea manifold. These various design layouts and their hybrid arrangements are usually produced back to platforms or to floating production vessels, although some have also been produced to shore. More than 50 floating production systems (FPS's) have been deployed worldwide, with more than 30 currently active.
Maximum water depth of subsea wells has reached 3,369 ft in the Campos basin, 2,788 ft in the Mediterranean, and 2,245 ft in the Gulf of Mexico. Fig.3 shows the water-depth range for worldwide subsea wells. Maximum producing distance to the host facility is 30 miles for a gas reservoir and 12 miles for an oil reservoir, both in the North Sea. Most subsea wells have produced by natural flow, but more than 110 wells have been produced by gas lift. Pressure maintenance with subsea water injection wells is used where needed.
Well servicing or workovers can be performed by use of re-entry from a floating drilling unit or jack up. Also, specialized techniques, such as through-flowline (TFL) operations, can be performed downhole by pumping tools from the surface host facility through the flowlines and down the tubing. Chemicals can be pumped into the formation through the flowlines, and chemical scan be injected into the subsea tree or downhole by pumping from the surface host facility through hydraulic hoses in the subsea control umbilical. Pressure and temperature can be monitored at the tree or even downhole.
Subsea technology was first developed and used commercially in the Gulf of Mexico and offshore California in the early 1960's by various operators. Although subsea wells were installed only in shallow water accessible to divers for many years, shallow-water diver-assist technology evolved simultaneously with deepwater diverless technology from the very first applications of subseawells. Much early subsea technology focused on diverless technology in anticipation of future deepwater requirements, even though deepwater-exploration success was yet unknown. However, operators wasted no time in using diver-assist technology for commercial development of shallow water fields, even though subsea technology was in its infancy. By using surface hardware adapted to diver-assist underwater use, subsea field developments proceeded off North America in the decades 1960-70. Subsea technology gradually evolved as refinements and improvements were made based upon field experience.
The world's first subsea completion was installed in 1961 at West Cameron192 in 55 ft of water. This system was designed for remote installation and operation to demonstrate deepwater capability. Numerous TFL operations were performed during the 4 years that the well was produced, establishing the feasibility of TFL technology. When the well was later abandoned, the tree was in good condition, with no significant corrosion damage after 17 years on the seafloor.
Subsea technology development continued in the 1960's with development of two wells in the Grand Isle Block 16 field. Although located in shallow water, these wells were designed to simulate remote, deepwater installation and operation. Fig. 4 shows one of the trees used. TFL technology was used on one well to perforate, consolidate sand, and bring the well on stream with no rig intervention.
The first full-field subsea development took place in the early 1960's with20 subsea satellite wells with gas lift and multiple-zone completions producing the Conception field to a platform offshore California. At about the same time, the Molino gas field was developed off California with 10 subsea satellite wells producing through flowlines to shore. Evaluation of a Molino tree after20 years of subsea service found that it was still serviceable and in good overall condition with little deterioration. Although located in shallow water accessible to divers, Molino trees were designed for intervention by a special robot called MOBOT, which was another indication of future deepwater expectations.
In the Gulf of Mexico, eight multiple zone subsea wells were installed in the mid-1960's at Eugene Island 175 to restore oil production lost when a platform was destroyed by a hurricane. Numerous TFL operations were conducted, including washing of wellbore sand back to the platform from downhole, performing sand consolidation, and cutting paraffin. Subsea maintenance was performed by divers in the shallow water depth of 85 ft.
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