Subsea Production Systems

W. B. Bleakley

Lockheed Petroleum Services Ltd.

Evolutionary processes in the oil industry are slow since there is a natural resistance to change. But these processes are always at work and certain trends forecast the need or desire for subsea completions for oil and gas wells.

As the potential for onshore oil and gas discoveries diminishes, oil operators and governments are moving offshore. Many countries with shorelines have already granted exploratory permits, and a large number of these have drilling and production concessions (same as U.S. lease) in force.

With this trend worldwide the total offshore acreage under lease has increased rapidly. The Gulf of Mexico sale in October 1974, for example, offered more than 500,000 acres (2,023 square kilometers) to the oil industry.

As the total area under lease increases so does the average water depth. The natural course to follow is to start with near-shore areas and to progress seaward. This seaward progress carries with it the liability of deeper and deeper water. In the Gulf of Mexico sale referred to above, 89 tracts were in 400 to 2,000-foot water depths.

□eepwater Drilling Trends

Over the last few years, the number of floating drilling Tigs —both ship-shape and semisubmersible—has increased at a steady pace. Drilling contractors, who are responsible for the design of almost all new Tigs, are aware of the trend toward deep water developments and have continually increased the depth capabilities of new design rigs.

As of October 1, 1974, there were 32 drillships and 49 semi-submersible rigs in operation worldwide. At that time, 34 drill-ships and 70 semisubmersible rigs were under construction, and all were due for completion within a 3-year period. Some of these drillships have unlimited depth capabilities because of dynamic positioning and many of thesemisubmersibles are able to operate in 3,000 feet of water.

Many oil companies and research groups already have deep water drilling experience, as numerous exploratory wells have been drilled in water deeper than 600 feet. Shell Oil Company has drilled in 2,150 feet of water off the coast of Africa and the Glomar Challenger, a research vessel, has drilled in water well beyond the 2,000 foot mark.

The oil companies have shown confidence in the industry's ability to cope with deep water problems through their heavy financial commitments for deep water leases.

Offshore Production and Reserves

Production data show that 18.9% of the world's oil supply was produced from offshore fields in 1973. This was an increase over the 15% from offshore fields just 2 years earlier. This is even more impressive when realizing that the first offshore oil was produced in the late 1940's. Offshore productive capacity has grown from that meager beginning to more than 10 million barrels/day in 1973. The estimate for productive capacity in 1985 is 25 million barrels/day from offshore fields.

Offshore oil reserves in 1974 were estimated at 137 billion barrels compared to 453 billion barrels onshore. Thus, the offshore area, with 25% of the world's reserves, is responsible for 18.9% of the world's daily production. Estimates for future reserves—those yet to be discovered—show a marked increase for offshore areas. British Petroleum Company's outlook for the future shows ultimate offshore reserves to be 571 billion barrels with onshore reserves to reach 1,038 biilion barrels. Thus, while onshore reserves will ultimately double, offshore reserves will increase by a factor of four.

With this well established trend toward deep water development of oil and gas fields, the need for subsea operations becomes clear. This does not imply that all operations will be carried out on the ocean floors, because even the most sophisticated subsea production system requires surface support and facilities. However more and more emphasis will be placed on ocean floor techniques as oil operators move farther from land and into deeper and deeper waters.

Even at this early stage in deep water development, operators have a choice in the approach to take, Subsea production systems are currently available on a commercial basis, and improvements in these systems are almost a daily occurrence. The systems which are in operation now and those which are in advanced stages of development will be discussed separately.

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