Application Of DC Motor Drives To An Offshore Production Platform

ABSTRACT DC drives are attractive because speeds are infinitely variable and power plants can be sized to equal ultimate loads. Space, fuel and maintenance savings, as well as greater equipment reliability have resulted versus engine drives. Special features, the design process, construction practices, testing, training, start-up and operations experience are discussed. INTRODUCTION DC motor drive technology for drilling rigs has been applied to Exxon's South Pass-93 B production facilities. This platform is a combination, simultaneous drilling and producing platform which is designed for 15 kbod and 50 Mscfd throughput. Facilities design was initiated in early 1983 and production began in April 1985. A 3.1 MW power plant (engine limited) supplies eight, 850 Bhp rated motors driving two oil shipping pumps, two sales gas compressors and an injection gas compressor (not simultaneously) through a silicon controlled rectifier (SCR) system. The oil shipping pumps have single motors and the gas compressors have tandem motors. A simplified partial layout of production facilities, which are mainly on the lower deck of the two-level 90'x205', 8-pile platform, is shown in Figure 1. Most of the top level of the platform is required for the drilling rig. Three 1100 KW, dual-fuel engine driven generators, switchgear, SCR units and motor control centers (MCC's) are located in the 40'x90' south deck module. The two 100%, 790 Bhp multi-stage centrifugal oil shipping pumps are located on the lower level of the 90'x125' primary or central deck unit. The two, 1040 Bhp sales gas compressors and the 1200 Bhp injection gas compressor are located on the top level of the 40'x90' north deck module. Oil production is separated in three stages and free water is disposed offshore. Oil is not treated to 1% BS&W and gas is not dehydrated prior to export from the platform. Machinery and gas cooling is by a seawater/freshwater cooling system. Several types of electric motor drive systems were considered for the production facilities, including variable speed AC motors and two-speed AC motors. However, the DC motor drives utilizing drilling rig type DC traction motors were selected because of their operating characteristics and proven service. Versus direct engine driven equipment, layout, operational and maintenance advantages have resulted from this application. The selected motor drive system resulted then in the application of drilling rig DC motor drive technology to an offshore oil production facility. The use of standard drilling rig components as much as possible was felt necessary to obtain field acceptance. Standard drilling rig SCR units were modified with automatic controls, operating limits and safety shutdowns for the driven production equipment. Special forced air purging/pressurizing/cooling systems for the motors were designed to satisfy the electrical area classifications and motor cooling requirements. Multiple generators and driven equipment units, and tandem motor configurations were utilized to improve the overall platform power factor and fuel efficiency. Motor and driven equipment dynamic analyses were made for designing couplings and determining operating limits.