HIGH-PRESSURE CO2 COMPRESSION

THE API 617 MECHANICAL TEST AND MAGNETIC BEARING EXCITER TEST DEMONSTRATE ROTORDYNAMIC STABILITY AT DESIGN OPERATING CONDITIONS

The Cidade de Angra dos Reis MV22 is a production, storage and offloading (FPSO) vessel owned by Petrobras and operating in over 2,000 meters of water above the Tupi Field in the Atlantic Ocean. It has to process a mixture of natural gas and CO2 in operating pressures that exceed 500 bar.

The process begins with a mixture of CO2 and natural gas that is compressed and delivered to a CO2 membrane. CO2 is separated fromthe gas streamand compressed to 310 bar. It is cooled and pumped to 550 bar to be injected back into the ground. Meanwhile, CO2-free natural gas is transported and compressed to 250 bar and either exported onshore or further compressed and re-injected back into the ground at 550 bar for continued oil production.

Petrobras selected a Dresser-Rand CO2 compression train to take the discharge pressure up to 310 bar instead of 250 bar as a means of reducing the number of CO2 pumps and minimizing the footprint. This multi-stage centrifugal compressor also has impellers arranged in a back-to-back configuration affording the capability of two sections of compression in one casing (Figure 1). These sections can be intercooled or nonintercooled as required by the application.

This two-case compressor train has a low-pressure (LP) compressor to boost the gas from an initial inlet pressure of 2.97 bara (43.1 psia) to a pressure of 35.7 bara, and a high-pressure (HP) compressor to compress the gas to a final discharge pressure of 308.9 bara (4,479 psia). It is driven by an electric motor through a speedincreasing gearbox (Figure 2).

Testing in advance

Petrobras requested full validation before implementation. This involved mechanical and aerodynamic testing, as well as rotordynamic modeling and numerical prediction methods. The high-pressure compressor received a “full load – full pressure” (FL-FP) inert gas test on a mixture of carbon dioxide and nitrogen closely approximating the actual aerodynamic cross-coupling force that would be experienced in the field.

The purpose was to evaluate the rotordynamic stability and mechanical integrity. The FL-FP test was conducted so that the MPACC number and volume reduction for each stage were within approximately 2%of design condition values. The MPACC number is the modified Wachel number used to quantify the aerodynamic cross-coupled stiffness force as described inAPI-617.

Establishing rotordynamic stability of the Tupi I centrifugal compressors was also critical in order to verify the long-termstable operation of the compressor in the field. High-pressure, high-density gases produce excitation and reaction forces on the rotor that may cause rotordynamic instability. As such, there is a need to reduce these forces.

The most significant forces usually arise at the impeller shroud and seals with highpressure differential. Technology, such as swirl brakes on labyrinth seals and damper seals, has been developed to reduce these forces and increase damping in the rotor.

Rotordynamic instability occurs when the forward driving forces exceed the resisting dissipation forces, leading to self-excitation of the whirling mode of the rotor. This, in turn, may lead to subsynchronous vibration that, in the worst case, is limited only by the rotor rubbing the stator.

Rotor natural frequencies can be estimated based on the measured frequency response. This permits the validation of rotor dynamic modeling techniques and demonstrates the compressor stability at both fulland part-load test conditions. Using rotordynamic stability testing, it is possible to validate the performance of damper seals, deswirl components and the bearing system. It also provides a significant risk mitigation tool,with quantifiable results, to demonstrate the reliability of a compressor.

Based on the above tests, the compressor for Tupi I achieved all aerodynamic and mechanical requirements. Rotordynamic stability was measured at full-load conditions and proven to be stable. These and similar tests for other Tupi units achieved the highest pressure ever recorded by a centrifugal compressor operating with a CO2-rich gas, and the highest discharge density for any gas compressed by a centrifugal compressor.