Transient analysis of wet gas compression systems

Abstract

The demand for oil and gas remains high, and changing market conditions require innovative solutions. Development of compression technology tailored for subsea oil and gas production can contribute significantly to profitability of both new and existing fields. Installation close to the well-head assures enhanced extraction and extended operation from gas/condensate fields. Additional benefits are: Exploitation of more remote fields, increased process simplicity and profitable extraction from fields with lower wellhead pressures. Wet gas compression is challenging due to the complex behavior of multiphase flows, which impacts the thermodynamics and fluid dynamics of the compression process. Moreover, subsea compressor systems demand high availability owing to the difficulty and cost related to maintenance. Hence, knowledge regarding compressor behavior in transient conditions is pivotal to maintain production. In the present work, attention has been given to wet gas compressor system behavior in transient conditions. The work presented here is a combination of experimental research and model simulation. The majority of the experimental research is obtained from a single-stage centrifugal compressor operating in air/water flow, located at the Norwegian University of Science and Technology (NTNU). The test facility is an open loop configuration operating at ambient conditions. A dynamic process simulation model has been built to replicate the test facility and to conduct detailed engineering studies. The main objective of the work has been to contribute to development of the dynamic simulation tool, to include modeling of wet gas compressors. Accurate and applicable modeling techniques are important with respect to design and operation of wet gas compressor systems. The experimental results reveal that the pressure ratio increases and the polytropic efficiency decreases when the compressor is subjected to wet gas flow. Care should be taken when analyzing stability and surge margins at different gas mass fractions (GMFs). Further, the interaction between the impeller, diffuser and volute is vital to understand with respect to compressor performance and stability. The compressor performance has been analyzed when subjected to inlet slugs down to a GMF of 0.185, which imposed a marked impact on the gas flow rate, compressor shaft torque and compressor pressure ratio. The wet gas impact on the control valve performance has been studied and analyzed. The results reveal that wet gas not only alters the compressor performance, but also the system behavior. The combined effect is important to understand for development and operation of future wet gas compressor systems. In cooperation with Aspentech R&D, Hysys Dynamics has been extended to support modeling of wet gas compressors. This enables the user to input multiple performance curves at different GMFs as well as different compressor speeds. The compressor performance methodology has been validated when operating with several wet performance curves. Further, the applicability of the affinity laws in wet conditions has been studied. Finally, the simulation model has been validated against several transient test cases and provided a close correspondence to these. Continued studies should involve the influence of different multiphase flow characteristics on the compressor and system performance. Further studies on flow behavior through the impeller and diffuser would also be of great interest, and potentially give insight into new design principles. Based on my work a redesign of the volute is suggested to physically remove the possibility of volute backflow. More studies are necessary to continue development of wet gas compressor modeling, i.e. additional parameters, like the fluid density ratio, are needed to accommodate the performance shift related to changes in composition and inlet conditions. Finally, emphasis should be given to system behavior and system response to contribute to design and operation of future subsea production systems.