The case for electrifying brownfield compressor stations
Special Focus: Pipeline, Infrastructure and Transport
N. J. van RENSBURG, Siemens Energy, Houston, Texas
Gas transmission system operators around the world are under intense pressure to reduce emissions, lower costs and ensure profitability in a future defined by market volatility. Upgrading legacy drive systems in compressor stations can play a key role in achieving these objectives.
For compressor stations that rely on older gas turbine mechanical drives (and where a grid connection is available), converting to an electric motor drive (e-drive) offers several advantages (FIG. 1), including lower operational expenditures (OPEX), increased operating flexibility and a reduced carbon footprint. Significant emissions savings are possible in regions like the Permian Basin (Texas, U.S.), where opportunities exist to harness clean power from onshore wind farms.
Despite these benefits, pipeline operators often perceive e-drive conversion projects as costly and complex, and therefore are not considered as an option for many revamp projects where an electricity grid is available.
Over the last several years, the author’s company has completed multiple e-drive conversion projects for customers in the midstream sector, along with numerous pre-front-end engineering design studies. This article will highlight the advantages of electric motor-driven (EMD) compression and outline key factors operators should consider when evaluating options for a brownfield modernization.
E-drives vs. gas turbines. E-drives are becoming increasingly prevalent across oil and gas applications as operators seek to capitalize on renewable energy sources and achieve decarbonization targets. For pipeline compression stations, they can provide numerous benefits vs. mechanical drives, including:
- Higher efficiency. The typical efficiency of a gas turbine-driven compression train ranges from 35%–45%, whereas state-of-the-art e-drives offer efficiencies up to 96% across the entire operating envelope. Even if a gearbox is incorporated, the efficiency of an e-drive is higher than that of a direct-drive turbine. In addition, electric motors are largely unaffected by changes in ambient temperature. Conversely, gas turbines can experience large efficiency losses when temperatures increase as mass flowrate through the machine decreases.
- Lower emissions. Total lifecycle emissions savings by converting from a gas turbine to an e-drive are difficult to quantify, as it is dependent on the source of electricity generation. The savings can be substantial if there is contribution from renewables, such as solar or wind.
In terms of direct onsite emissions, the replacement of a 5-MW gas turbine with an e-drive can result in a decrease of as much as 25,000 tpy of carbon dioxide (CO2) emissions. Nitrogen oxide (NOx) and sulfur oxide (SOx) emissions are eliminated, as well. For projects in regions where carbon taxation frameworks exist, these savings can shorten the payback period of the conversion, creating more favorable economics and a higher return on investment.
The reduced noise and emissions from EMD compression can also potentially help secure an operating license in locations where gas turbines are prohibited (or will be in the future)—e.g., near a residential development or environmentally sensitive area.
- Reduced OPEX. In areas where low-cost electricity is available, e-drive OPEX are typically much lower than that of a direct-drive gas turbine (even with natural gas prices at historically low levels).
Maintenance requirements are also reduced. A typical gas turbine has an availability of roughly 95%. After 2 yr in operation, anywhere from 10 d to 3 wk is required for scheduled maintenance.
E-drives can achieve 99% availability. It is common for motors to go 5 yr–6 yr without scheduled maintenance. Most of the routine service on motors can be performed by non-specialized electrical maintenance technicians. Overall, the maintenance costs of an e-drive can be up to 90% lower than that of a gas turbine.
- Greater operating flexibility. Electric motors used with variable speed drives (VSDs) can easily adapt to a broad range of torque and speed requirements for different operating points, providing broad compressor map coverage without a loss in power (efficiency).
In the case of stations with single gas turbine-driven trains, if there is a requirement for flexibility, the turbine will likely have to be larger (i.e., oversized) than what is optimal given the baseload of the station, which results in extended periods of part-load operation and reduced efficiency. EMD compression also provides the ability for noticeably short ramp-down and startup times without any adverse effects.
In 2019, the author’s company completed an e-drive conversion for a pipeline station in the U.S. The station, which utilized multiple gas turbine- and motor-driven compression strings, was experiencing overload of its baseload unit for multiple extended periods throughout the year. Replacing one of the gas turbines with a 14,000-hp (10,444-kW) electric motor (and speed-increasing gearbox) solved the issue.
Modifications were also made to the compressor to accommodate new flow conditions, including the installation of a new rotor and aerodynamic stationaries that featured modern, high-efficiency flow paths. The original casing, head, bearing housing and seal system components were all reused to minimize the impact to the piping layout. The baseload unit is now more centrally located in the efficient part of its compressor performance map, improving overall station efficiency and lowering CO2 emissions.
- Lead time. In many cases, the lead time for an e-drive is shorter than that of a gas turbine. This is particularly the case if standardized components are specified (as short as 6 mos). Operators can also utilize a single spare motor for multiple sites. The spare can easily be swapped out if an existing motor must come offline for an extended period. This is typically not feasible with gas turbines, as they require highly specialized services for disassembly and repair.
Hybrid drives. In recent years, an increasing number of pipeline operators have explored the possibility of installing hybrid (gas-electric) drives to meet compression requirements (FIG. 2). Configurations can vary depending on the application. However, a typical design features an electric motor installed on one side of the compressor shaft end and a gas turbine on the other. For centrifugal compressors with an axial inlet, which only have one end for connecting a driver, the motor can be installed between the gas turbine and the compressor.
Hybrid drives provide added flexibility, as they allow the operator to switch between using electricity or gas (or both) to drive the compressor train. This alleviates concerns of a disruption in the electrical supply or a spike in congestion charges. The motor can even be designed to dually function as a generator, enabling the compression station to turn into a net exporter of electricity if there is spare capacity in the gas turbine after station compression duties are met.
Hybrid drives can be an especially attractive option in cases where there is an opportunity to connect to a grid with high intermittent renewables penetration. In such cases, EMD compression alone may be too risky due to the potential for service disruptions or price spikes.
Another possible application is in sites with large ambient temperature variations that might impact the efficiency of the gas turbine.
Considerations for project execution. The technical feasibility of an e-drive conversion project is dictated by unique factors related to the station. Several different configurations of e-drives are possible depending on the requirements of the operator.
For example, with a low-speed electric motor, a gearbox and two couplings are required. With a high-speed electric motor, no gearboxes are necessary and only one coupling is required, which further improves the overall train efficiency and reduces maintenance components. Avoiding the use of the gearbox also eliminates the need for lubricating oil.
While there are cases where the installation of a new compressor package and foundation, piping, ancillary systems, etc., are unavoidable, it may be possible for operators to save on capital expenditures (CAPEX) and associated downtime by modifying or upgrading the existing compressor. Potential upgrades can include (but are not limited to):
- Complete re-aero of the compressor internals and conversion of the gearset, including stationary and rotating parts, without changes to the casing, external process connections and/or existing footprint
- A dry gas seal (DGS) retrofit and the potential elimination of a seal gas booster compressor
- Upgrades to the lube oil or control system (i.e., to enable remote monitoring)
- Material upgrades to O-rings (upgrade to polymer or Teflon) or labyrinth seals [from aluminum to polyetherketone/polyetheretherketone (PEK/PEEK)].
Once an equipment arrangement has been selected, an electrical solution must be developed. Conversion cannot be viewed as a simple driver swap.
Larger e-drives for mainline stations (> 15 MW) may require a comprehensive grid analysis. Electrical studies model the impact of the additional load in the upstream supply network by providing analyses of parameters, such as power factor correction and harmonic filtering requirements. These studies can be provided as either a high-level overview or as high-fidelity dynamic models.
The electrical system must be analyzed to ensure safety and availability under all scenarios. Developing a safe and optimized solution requires the evaluation of many different variables across the entire electromechanical system, including available footprint, vibrations, the foundation and available power, among others.
In most cases, a torsional analysis is required, as gas turbines tend to have an inherent positive damping effect on the compressor or pump shaft. While this has traditionally been an area of added complexity, advances in modern drive technology have successfully resolved many of the issues that are of concern. The author’s company has experience performing torsional analysis.
Overall, operators often view e-drive conversions as overly complex and costly. However, medium-voltage (MV) EMD technology has advanced significantly in recent years. For most pipeline applications, motors within the voltage source inverter (VSI) drive range can be utilized. This reduces the complexity of the conversion, as there is less of an impact on the electrical grid (i.e., no harmonics up to 35 MW).
Execution is further simplified with single skid and single lift configurations, which can reduce the onsite work scope. For existing stations with limited space, auxiliary equipment—such as the VSD—can be placed away from the compression train in an optimized location. The resulting package footprint is then very compact. Moving MV equipment out of the explosive zone also reduces equipment costs and lead time.
Reducing interface risk. Numerous variables must be considered when evaluating the viability of a brownfield e-drive conversion. No two projects are the same, and the choice of equipment will ultimately be determined by site-specific factors and operator objectives regarding CAPEX, OPEX, emissions and footprint, among others.
From a project management standpoint, operators can benefit by engaging with a single original equipment manufacturer for a substantial portion of the supply scope. While a multi-supplier approach for critical components—such as the motor, compressor and VSD—is feasible, it typically requires extensive coordination to ensure successful system design and project execution.
Aside from avoiding common vendor interfacing issues, sole-sourcing electrical and rotating equipment from a single supplier is advantageous, as it allows for technical integration issues to be resolved early in the project timeline. This includes problems related to torsional dynamics and harmonics, which often present significant cost and schedule risk.
The author’s company is uniquely capable of supporting pipeline operators on brownfield electrification projects, as the company can provide all equipment for the train(s), including the motor, VSD, compressor, and, if necessary, the switchgear and transformer.
In addition, the author’s company can support the complete train evaluation, including the mechanical and torsional analysis, which allows for the development of an integrated solution that begins with a redesign of the compressor and extends all the way to the electrical substation. GP&LNG
LITERATURE CITED
1 Langlitz, K. and J. Washington, “Weighing modernization options for legacy turbocompressors,” Gas Compression, September 2023.
ABOUT THE AUTHOR
Nico Jansen van Rensburg is the Head of the LNG Taskforce for Process Industries at Siemens Energy. Prior to his current role, he was the Head of Oil & Gas Solutions for the Americas Onshore business for Siemens Energy. He has been with Siemens (Siemens Energy as of 2020) for 18 yr. He earned a BS degree in electronic engineering and information technology from the University of Potchefstroom (NWU).
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