FPSO Decarbonization with Combined Cycles: The Case of High Heat Demand
Authors: Gennari G. , Amidei S. , Nasini E. , Intieri A. , Betti D. 
Published: April 2024

It is well known how climate change is a priority for the global agenda, across public and private sectors. Energy intensive industries, like the Oil&Gas, are the most affected. While moving towards a Decarbonized world, hydrocarbons lying in Deep Waters are still expected to remain important sources for the energy-thirsty world for years to come. Therefore, the renewed focus on Decarbonization creates opportunities to deploy more efficient, alternative technologies to mitigate carbon emissions from Offshore.

Looking at the emissions of a Floating, Production, Storage, and Offloading (FPSO) vessel, more than 60% of the total greenhouse gas (GHG) emitted come from energy use and specifically fuel combustion. Such system is usually including a set of Open Cycle Gas Turbines (OCGT), for power generation or mechanical drive, with advantages of low weight and compact footprint with electrical efficiency up to 40%, depending on ambient conditions and gas turbines size.

A recent trend is to introduce Combined Cycle Gas Turbines (CCGT) and Combined Heat and Power Gas Turbines (CHP), a solution not traditionally adopted in Offshore due to the impacts of weight and footprint. On top of an OCGT, the CCGT/CHP foresees a Steam Bottoming Cycle with a once thru steam generator (OTSG) and a steam turbine for additional electricity generation. The heating process demand, instead, can be supplied either by low pressure steam extraction or bleed, or by an additional Waste Heat Recovery Unit (WHRU), or by a combination of both solutions. The CCGT/CHP drastically improves electrical efficiency as it can reduce by 20% or more the overall emitted CO2 per MWh produced, and at same time eliminates the need for additional pieces of equipment (e.g. fired heaters) to satisfy the heat demand.

Leveraging its competences as both rotating equipment original equipment manufacturer (OEM) and plant and modules solutions provider, in this paper Baker Hughes describes the key topics to design and operate a CHP plant on a FPSO when the thermal demand is high due to multiple reasons: e.g. for flow assurance purposes (to prevent wax formation, inhibit hydrates, etc.), for oil processing (crude oil stabilization in storage tanks, oil de-salting, etc.) and for gas treating (dehydration, sweetening, etc.). In certain projects, heating requirement can exceed 100% of the total vessel electric power requirement. Baker Hughes is currently developing a number of improvements, under a family of "Next Generation" combined cycle solutions, which allow to proper optimize the needs of this type of projects.

This paper will show four alternative architectures for FPSOs with high thermal power demand: OCGT and WHRU with Gas Turbines for mechanical drive applications,Multiple pressure levels in the OTSG,OTSG with back-pressure STG.WHRU and OTSG integrated design.

For each alternative, pros, cons and technical considerations will be provided. While improving efficiency, a CCGT/CHP plant increases the complexity of the FPSO: therefore, multiple aspects need to be considered to find the proper balance between performances and overall flexibility, cost, weight and footprint.

Moreover, this paper will also introduce some of the key enhancements which can be introduced to the above configurations, to optimize their performance, as part of "Next Generation" combined cycle solution.