Powering Deepwater Production: Designing Gas Turbines for Modern FPSOs

Preventing unplanned downtime of rotating equipment assets, such as gas turbines and compressors, is important to success in the offshore production environment. This is particularly the case for fields located in harsh environments and highly remote locations, where the cost and complexity of performing maintenance and repairs is often magnified. At the same time, tightening environmental regulations, coupled with voluntary commitments from offshore operators to decarbonize, has increased focus on the fuel efficiency and emissions footprint of prime movers.

Companies have taken strategic steps in recent years to help the industry address these challenges by designing rotating equipment packages to meet the demands of modern FPSOs.

An example of this is the SGT-750 industrial gas turbine, which recently began operation on the Johan Castberg FPSO (Figure 1) in the Barents Sea, where it is being used to drive two DATUM centrifugal compressors in 1x100% service (i.e., with no backup). The compressors reinject gas into the reservoir, providing pressurization and eliminating the need for capture or flaring, the latter of which is not permissible in the region.

This article highlights several unique features of the compression train installed on Johan Castberg and discusses how the SGT-750 is supporting offshore requirements for low emissions and high availability in both direct drive and power generation applications.

Johan Castberg Project Overview

Johan Castberg is Norway’s most northerly Arctic field development project and is located roughly 240 kilometers northwest of Hammerfest. The development concept includes 30 wells distributed across ten subsea templates and two satellites that will be tied back to the FPSO. According to Equinor, production rates from the field could reach up to 220,000 barrels per day at the peak¹.

The reinjection compressors on Johan Castberg are being driven in tandem arrangement by a single SGT-750 twin-shaft industrial gas turbine. The frame features a free 2-stage power turbine with a nominal shaft speed of 6,100 rpm and output of 41 megawatts. In direct drive applications, the SGT-750 supports speeds ranging from 50 - 105% and does not require an intermediate gearbox. This reduces the turbine’s weight and footprint, making it well-suited for offshore applications.

The turbine permits both frequent and rapid starts and can reach full load in less than 10 minutes (under settle out conditions). On Johan Castberg, the centrifugal compressors and gas turbine share a common lube oil unit and other auxiliaries, including the starter motor and electrically driven back-up systems, all of which are mounted on the base frame.

A waste heat recovery unit captures heat from the gas turbine’s exhaust for distribution as heated liquid to prevent ice build-up on various parts of the superstructure. The heat can also be supplied to the HVAC plant or for processing crude from the wells.

Satisfying Stringent Emissions Requirements

Gas turbines specified for offshore applications are expected to operate reliability on a wide range of fuel compositions, often with calorific values considerably different from standard pipeline gas². For this reason, the majority of gas turbines installed offshore utilize non-dry low emissions (DLE) combustion technology, which generally is more tolerant to different fuel types.

Non-DLE systems, however, do come with trade-offs that operators must consider, including the acceptance of higher nitrous oxide (NOx) emissions, typically in excess of 250 parts per million (ppm).

In recent years, as the focus on environmental performance has increased (particularly in the North Sea and Barents Sea), more projects are specifying NOx emissions of less than 10 ppm. Gas turbine OEMs have responded to these requests by investing in the development of state-of-the-art DLE combustion systems.

DLE combustion systems differ from non-DLE in that they are not required to use injected diluents, such as water or steam, to quench the flame and reduce NOx production. Multiple DLE technologies have been developed over the years, including lean pre-mixed combustion; staged combustion; catalytic combustion; and rich-burn lean quench combustion. Of these, lean pre-mixed systems, which lower NOx formation by combusting fuel in an excess of air, have emerged as the technology of choice.

The SGT-750 used on Johan Castberg utilizes a 4^th generation, lean pre-mixed DLE system that can achieve single digit ppm NOₓ and carbon monoxide (CO) levels down to a 20% load.

Overall, the use of DLE combustion systems with weak and medium calorific value fuels has been subject to rigorous development programs. In addition to fundamental design and analytical work, comprehensive combustion rig testing along with core engine and full packaged unit testing has been completed. DLE engines are now capable of reliably operating on a wide range of fuel compositions and are considered an established technology for offshore production applications.

14 Days of Downtime Over 17 Years

Extending mean time between overhauls (MTBO) and reducing the duration of both planned and unplanned maintenance on rotating equipment is prioritized in offshore production applications. This is particularly the case on Johan Castberg, as the gas reinjection train, which is necessary for production, is operating in 1x100 configuration without backup.

When it comes to overhauling gas turbines, end-users generally have two options. The first option is to conduct all maintenance activities on the core engine, auxiliaries, and driven equipment onsite. The second is to perform a rapid onsite engine exchange using a customer owned spare and then transport the existing engine core to an onshore workshop or OEM facility for servicing.

For the SGT-750, the second option results in total planned downtime of 14 days over 17 years. The core engine exchange can be performed in 24 hours from load to load. Additionally, the compressors utilize modularized bundle concept, which minimizes the time needed to swap out spares and reduces overall maintenance costs.

Availability of the compression train on Johan Castberg is expected to be 99.5%. This is approximately 0.5% higher than what is typically specified for similar offshore applications.

To support high availability targets, the SGT-750 is equipped with online infrared monitoring of the turbine hot section. Each camera covers the pressure side, suction side, and the platform. Before and after each inspection, measurement of the turbine blades’ surface temperature in the compressor section can be performed while the machine is on load.

The cameras allow for early, non-destructive detection of possible issues, such as cracks or blockage of cooling holes, before they lead to failure events. Infrared images also speed up service activities by providing technicians with valuable diagnostic information. Other notable features of the SGT-750 include:

• Borescope ports for five of the 13 compressor stages.

• A door that allows access to the compressor at the front of the air inlet chamber.

• An overhead crane inside the gas turbine enclosure. The enclosure was designed with enough space for operating personnel to walk around the machine and exchange combustor components.

• A gas generator that can be removed from either side of the installation. The generator is then disconnected from the air intake and power turbine and removed sideways on a rail assembly.

Johan Castberg officially came on stream on March 31^th of this year. On June 17th, it reached its peak production capacity of 220,000 barrels of oil per day. This increases energy deliveries from the Barents Sea by 150%³. Since the installation, Siemens Energy has received three additional orders for the SGT-750 from FPSO projects for power generation.

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• ¹ Equinor. Johan Castberg Anchored on the Field. September 2024. url: https://www.equinor.com/news/johan-castberg-anchored-on-the-field

• ² Bulat, Ghenadie , and Matthew Rickert. "Making the Case for Dry Low Emissions Technology in Offshore Oil and Gas Applications." Paper presented at the Abu Dhabi International Petroleum Exhibition & Conference, Abu Dhabi, UAE, November 2019. doi: https://doi.org/10.2118/197830-MS

• ³ Equinor