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ASME FEATURE

Flexible combined heat and power systems for offshore oil and gas facilities with CO2 bottoming cycles By Marit J. Mazzetti, Yves Ladam, Harald T. Walnum, Brede L. Hagen, Petter Nekså and Geir Skaugen, SINTEF Energy Research, Trondheim, Norway

Improved energy efficiency is the only “fuel” that simultaneously meets economic, energy security and environmental objectives, according to the 2013 IEA report “World Energy Outlook.” [1] This also is the case for oil and gas production, where it is gaining importance. Particularly as offshore fields are aging, the energy needed to produce a barrel of oil and gas increases significantly. Offshore facilities are normally designed for maximum production (or “plateau”) rates.[2] In many cases, declining production results in increased power demand. One example is water injection in order to maintain reservoir pressure. This is a common energy intensive process, which is often necessary as the platform goes into tail production. Improved energy efficiency will lead to reduced fuel consumption and resulting CO2 emissions and help meet the world’s climate goals, as well as improving offshore process economics by reducing fuel cost and CO2 taxes where applicable. This is the case for Norway, where the government introduced an offshore CO2 tax to accelerate the implementation of CO2 reduction measures. Offshore oil and gas platforms are in most cases generated by gas turbines operating in a simple cycle. However, on three offshore installations on the Norwegian continental shelf (NCS) a steam bottoming cycle has been installed that recovers the heat from the hot exhaust of the gas turbine, increasing the efficiency of the electricity production on the platform [3]. Alternative concepts for more compact bottoming cycles saving weight and footprint on the platform have been discussed by Walnum et al. [4]. Those cycles are based on the use of CO2 as working fluid. Lately, supercritical CO2 cycles have received

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attention, especially for nuclear applications. [2,3,5] The main reason is the potential for weight, size and cost reduction. These characteristics also are transferable to bottoming cycles for gas turbines [6]. For most oil and gas production platforms, part of the heat from the gas turbines is recovered for use in the energy intensive onboard oil/water separation process, among others. Depending on the plant layout, compressor driver concepts and power production strategy, a waste heat recovery unit (WHRU) might be Figure 1. Layout of the two concepts for combined needed on multiple gas heat and power (CHP) production. Upper: Dual waste heat recovery unit (DWHRU). Lower: The turbines. The ratio of heat to power demand internal heat recovery unit (IHRU). will typically vary and heat recovery systems are needed that can combine heat integration and power production (CHP) across a wide range of operating conditions. For steam systems this is typically performed with steam extraction or backpressure turbines, where steam is extracted at an intermediate pressure and utilized for process heat. For CO2 systems this is not an option, since the CO2 is typically expanded in a single-stage turbine and far away from the two-phase boundary. In this work, several concepts for CHP will be evaluated at different operating conditions. The concepts are modeled using in-house tools, enabling the use of detailed component models taking real off-design effects into account. This is necessary due to the large load

ASME Power Division Special Section | APRIL 2015

Profile for Energy-Tech Magazine

April 2015  

Heat Exchangers – Retrofit/Rebuild/Equipment Upgrade – Bearings – Turbine Tech: Steam – ASME: Combined-Cycle Plants

April 2015  

Heat Exchangers – Retrofit/Rebuild/Equipment Upgrade – Bearings – Turbine Tech: Steam – ASME: Combined-Cycle Plants

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