The evolution in the use of subsea technology has seen advancement -from I well in the Gulf of Mexico in 1961 to over 750 wells in a tide variety of locations by the end of 1993. Along with the growth in numbers, the industry has seen rapid advances in technology, 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 some of the milestone installations that shaped the technology advance.
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, as shown by Figure 1. By late 1993, a total of approximately 752 subsea wells have been completed worldwide, with over 440 of these wells still in service. This paper will provide an overview of subsea technology development by focusing on three areas which exemplify the technology used worldwide:
The Gulf of Mexico and West Coast of North America
The North Sea and
The Campos Basin of Brazil.
Subsea wells have been used in a variety of configurations. Typical arrangements shown by Figure 2 include single satellite wells consisting of subsea trees situated on their individual guidebases; subsea trees located 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 hybrid arrangements of them are usually produced back to platforms or to floating production vessels, although some have also been produced to shore. Over 50 floating production systems (FPS) have been deployed worldwide, with over 30 currently active.
Maximum water depth experience of subsea wells has reached 2562 feet in the Campos Basin and 2245 feet in the Gulf of Mexico. Figure 3 shows the water depth experience range for worldwide subsea wells. The deepest production experience to date is the Aquila extended well test by Agip in 2788 feet of water in the Mediterranean in 1993. 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 over 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 using reentry from a floating drilling unit or jackup, 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 chemicals can 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.
