Integrated Wet Gas Compressor Test Facility

Abstract

The potential production increase from new and existing oil and gas fields worldwide is huge. In some areas, stringent requirements for field recovery specified in the production licence call for the development and utilisation of novel technology concepts. Enhanced recovery may be achieved with wellhead boosting. For specific systems, the booster is preferably installed subsea, either on a single production well or a cluster of these. Development of rotor-dynamic multiphase pumps for topside and subsea applications was initiated at the mid-1980s. A wide range of these pumps are currently installed and in operation worldwide. They typically cover the gas volume fraction (GVF) range from 0 to 0.70. The ability to increase pressure is limited above GVF 0.9, clearly restricting the area of application. In essence, the development of wet gas compressors covering GVFs from 0.95 to 1.0 has been limited to the centrifugal concept, although an axial contra-rotating concept is available. Two new subsea compression systems will be installed, commissioned and in operation from 2015 for the Gullfaks and Åsgard fields on the Norwegian continental shelf (NCS). Their compressors are based on centrifugal and axial technology respectively. Subsea compression is currently being evaluated for several other field developments. The centrifugal compressor has proved to be a robust concept and dominates in the oil and gas industry. Both inert low-pressure and high-pressure real hydrocarbon fluid tests have shown that understanding of the fundamental wet gas compression mechanisms is limited. Evaluating the ability of the centrifugal stage to handle wet fluids has therefore been of specific interest. A wet gas test rig has been designed and built at the NTNU. Its objectives are to validate a wet gas compression system and to determine capabilities and constraints related to the impact of impellerstage performance: • fluid behaviour and dynamics • corrosion and erosion tolerance • surge suppression and stall avoidance • transient operating conditions, including fluctuations in GVF • novel high-precision shaft torque control (static and dynamic) • electric motor and driver response and interactions • total system control. The article focuses on the ongoing test campaigns and related challenges, including test facility design. Understanding the challenges involved is essential for identifying concept constraints at an early stage and ensuring system reliability and availability.

System influence on the surge behaviour of a centrifugal compressor

“…This parameter compares the compressor pressure rise capability with the pressure rise required to induce mass flow oscillations: the conditions favouring surge are those reducing the pressure difference needed to oscillate the flow in the piping at the natural frequency of the system. (Pampreen, 1993) observed that surge initiation can happen at different locations -inducer, impeller, diffuser, and its frequency and magnitude are related to the volumes of inlet and outlet ducting, with this trend: a reduction of system volume increases the frequency and reduces the amplitude, while its flow-rate at inception is independent from them.…”

“…-physical components, such as piping, flow volumes as vessels and exchangers, fittings, and recycle and check valves; -compressor train inertias (fluid and compressor -driver inertias) and compressor performance characteristics; -anti-surge system strategy, details, and timing. An accurate surge margin definition is affected by uncertainties in prediction and transients during operation, as reported in (Cumpsty, 2004). A trade-off between safeguarding machine integrity and little impairment of performance is needed.…”