Richard Shultz reports on how a multiple coordinate control algorithm was used to stabilise all compressor rotor natural frequencies in frigid Siberian conditions
With decades of experience in the oil and gas industry, Lukoil, a Russian oil and gas business, aspired to expand its oil and gas fields. For the expansion, the company selected the Bolshekhetskaya depression in West Siberia, where large deposits of Cenomanian gas lay.
Developing and constructing a compressor station to connect fields would increase the total capacity of the pipeline. The compressor design had to produce a high discharge pressure. In addition to the pressure gain required to compensate for the piping frictional losses, the compressor’s discharge pressure would also support a refrigeration process applied to the gas at the plant outlet. This refrigeration process, based on a throttling principle, is required to cool the gas sufficiently to avoid the buried piping temperatures from thawing the permafrost. The permafrost helps maintain a stable support for the buried piping.
Equipping high-efficiency compressors
Lukoil partnered with Sumy Frunze, an oil and gas equipment manufacturer, to develop centrifugal compressors that could withstand the frigid Siberian conditions. The compressors would allow the station to pump 12 billion cubic metres of gas per year to another station, furthering the central gas pipeline of Russia. Providing such high compressor discharge pressures with reasonable drive power requires compressors with high efficiency.
The high efficiency was achieved with a compressor rotor shaft with a high slenderness ratio. This high slenderness ratio allows high efficiencies but results in a flexible compressor rotor shaft. The Lukoil compressors have highly flexible rotors equipped with magnetic bearings. The processing power and advanced control algorithms provided by the Waukesha Magnetic Bearings digital controller were required to make these machines a success. Waukesha’s multiple coordinate control (MCC) algorithm was used to stabilise all compressor rotor natural frequencies, with robust stability margins.
After installation, the technology performed successfully. Within the centrifugal compressor, the magnetic bearing technology provided savings in energy and the total cost of ownership for Lukoil.
The active magnetic bearing (AMB) system has an expected life of 25 years. The primary advantages for the end user throughout this time period are lower maintenance and operating costs and improved reliability compared to fluid film bearing solutions.
Increasing availability in harsh conditions
The Lukoil site is located in Western Siberia, isolated from all public utilities. All electric power must be generated on site, and losing all power is not unusual. During these outages, conventional equipment such as lube oil skids, required for fluid film bearings, will drop well below operable temperatures. The time required to heat this equipment and the oil reservoirs back to operable temperatures can be four hours or more.
This downtime risk was eliminated by using magnetic bearings for the compressors. The bearings can be reactivated within minutes after site power outages, greatly increasing the availability of the compressors. Following the 2013 installation, gas production and the total production of hydrocarbons were on track to increase by 42 billion cubic metres and 25%, respectively. It is now possible for 12 billion cubic metres of gas to be transported from the compressor station to the main station and ultimately to the central gas pipeline in Russia.
Versatility
Similar magnetic bearings may be used in other applications. In fact, the success of the six-stage centrifugal compressors led Lukoil to implement the Waukesha Magnetic Bearings technology on a booster compressor station in the same gas field.
Richard Shultz is chief engineer with Waukesha Magnetic Bearings.
