Genesis of a Three-Phase Subsea Metering System

Abstract

Summary. This paper describes the development of a three-phase metering system for installation on seabed templates at or near subsea wellheads. It tracks the development of the system from its conception through continuing tests on a production platform in the North Sea. The paper also reviews modification of the original design as a result of 1989 offshore tests. Introduction Current challenges facing offshore operations include developing technologies that economically maximize production from existing offshore platforms and allow reservoirs in ever-increasing ocean depths to be produced. Considerable attention is being given to the use of single- and/or multiwell subsea satellites and the development of equipment and production systems that can be placed at these satellites. In 1981, for example, a series of subsea developments began to boost production from the Tartan Alpha platform in the North Sea. Initially, simple hydraulically controlled, nearplatform, single-well satellites were developed. Then, in 1985, the sophisticated Highlander subsea oil production template was installed 8 miles from Tartan. The Petronella field development followed in 1986 with an offset of 6.8 miles. Such work on long offset subsea developments highlighted the high cost of providing metering facilities for flow testing remote subsea wells. Periodic well flow testing is necessary to monitor well and reservoir performance over time to optimize decisions on well production rates and new well requirements through improved reservoir models, to determine the timing of well workovers, and to identify when wells become uneconomical to produce. A dedicated "test separator" conventionally is used to meter individual wells. Fluids from a well are separated into the three component phases (oil, gas, and water) in a large vessel, and the flow rate of each phase is measured on the respective outlet lines from the vessel. The same method currently is used for subsea satellite developments by providing a dedicated "test pipeline" from the subsea field to carry a selected well's production to a test separator for metering on the host platform. The capital cost of these systems rises rapidly with distance. Greater distances between the wellhead and flow test system increase the cost of the test pipeline and require larger and hence more expensive slug catchers and risers. Clearly, a subsea-based well-test system could result in large capital cost savings by eliminating the need for conventional test systems (Fig. 1). This paper tracks the development of one subsea well test system from conception to field testing on the Tartan A platform in the North Sea. This work defines the design requirements of the system, reviews system development and fabrication, describes modifications made as a result of initial field tests, and reports the results of topside tests completed through Dec. 1990.

Design and Performance of a Three-Phase Subsea Metering System

“…The system's operating conditions and flow measurement range were based on the design requirements of the Highlander template and on a reasonably broad range of oil, gas and water production rates for a single well. The specific criteria are ( 1) Deployment subsea to depths of 1000 ft. (300 meters).…”