The Development of a Subsea Power Transmission System for Deep Water Boosting Applications

Abstract

This paper presents the development of a subsea power transmission in medium voltage and variable frequency, as a key system for application of Boosting Technology and, more particularly, for Electrical Submersible Pumping in deep water wells. The focuses of this paper are mainly on the design and manufacture of subsea power cables and transformers for 1,000 m water depth. The production from a subsea well equipped with ESP's is a fact since October/94, with the first installation in Campos Basin, Brazil. The development of subsea power transmission in medium voltage and variable frequency will allow the installation of a Boosting System in deep water at long distance (25 km or more) from the production platform. The design and manufacture of subsea power cables and subsea power transformers, as well as the integration of the complete power system is a result of a Technological Cooperation Agreement with TRONIC, PIRELLI, SIEMENS A.G. and SIEMENS BRAZIL. As result from this agreement subsea power cables up to 12/20kV voltage level, conductor sizes from a 35 to 150 mm2. oil filled subsea power transformer rated at 759 kVA, nominal voltage ration 10,000/3,000 V and the electrical connectors to X-tree will be developed and manufactured. Brazillian Offshore Oil Field Scenario In the last few years, PETROBRAS has been making important oil discoveries in deep water, over 400 meters. The reserves in the fields located in deep water, over 400 meters, account for 64% of the total reserves in Brazil. The importance of deep water technology is also by the fact that 65% of the potential oil discoveries will be in deep and ultra deep water. The development of deep water oil fields in Brazil has been made by the use of floating production systems. In platforms, the oil flows from the subsea wells by gas lift injection. However, the distance from well to platform is limited by the reservoir pressure, which decline with depletion, requiring water injection. Gas list becomes less efficient when horizontal distance increase and BSW rises. Therefore, it's necessary to use another more efficient method for adding energy to the produced oil. PETROBRAS has chose the Boosting System drived by electric motors, but the development of a power transmission system in medium voltage and variable frequency for a long distance (25 km) is necessary. This method will allow to reduce the number of platforms to drain giant fields and will allow to supply power to electric motors installed in marginal wells. Electrical Submersible Pumps in Subsea Wells The Electrical Submersible Pump in a Subsea well is one of the projects of PROCAP 2,000. This project installed the first electrical submersible pump in subsea well in October/94. This installation was in 4-RJS-221 well, located at Carapeba Field, Campos Basin, Brazil. The success of this installation is the greatest encouragement for new installations in deep water wells. This new installation should be designed with power transmission in medium voltage under several conditions and drive of variable frequency at long distance from the production platform.