Design and off-design simulations of combined cycles for offshore oil and gas installations

Abstract

Combined cycles applied on offshore oil and gas installations could be an attractive technology on the Norwegian continental shelf to decrease costs related to CO 2 emissions. Current power plant technology prevailing on offshore oil-and gas installations is based on simple cycle gas turbines for both electrical and mechanical drive applications. Results based on process simulations showed that net plant efficiency improvements of 26-33% (10-13%-points) compared to simple cycle gas turbines can be achieved when the steam bottoming cycles are designed for compactness and flexibility. The emitted CO 2 could be decreased by 20-25% by opting for a combined cycle rather than a simple cycle gas turbine. A clear disadvantage for offshore applications is that the weight-to-power ratio was 60-70% higher for a compact combined cycle than for a simple cycle gas turbine based on results in this study. Once-through heat recovery steam generator technology can be an attractive option when designing a steam bottoming cycle for offshore applications. Its flexibility, the avoidance of steam drums, and, with the right material selection, the possibility to avoid the bypass stack while allowing for dry heat recovery steam generator operation are all advantages for offshore applications. All process models, that were developed for offshore installations in the study presented, included once-through technology. A combined cycle plant layout for an offshore installation with both mechanical drive and generator drive gas turbines was included in the study. This setup allows for flexibility related to changes in demand for both mechanical drive and electricity. With the selected setup, designed for 60 MW shaft power, demand swings of approximately ±10 MW could be handled for either mechanical drive or electrical power while keeping the other drive-mode load constant.Keywords: once-through, Rankine cycle, steam bottoming cycle, process simulation, steady-state

Weight and power optimization of steam bottoming cycle for offshore oil and gas installations

“…1. This setup is explained in more detail in [5] (layout c). The GT was equipped with dry low emission burners and variable guide vanes (VGVs).…”

“…The HRSG designed for an offshore oil and gas installation including design parameter selection is discussed in [1]. Model validation of the knowledge-based design, which was the starting point for the optimization, was performed in [5]. For the gas turbine, the exhaust mass flow rate was constant at 90 kg/s for all design cases, whereas the turbine outlet temperature varied slightly (530-534 • C) due to changes in HRSG pressure loss.…”

“…While the design criteria for maximizing the HRSC power and efficiency are well-known [e.g., see 4], those for minimizing the weight-to-power ratio are still unclear. Previous studies of off-shore HRSC installations are based on knowledge-based designs relying on previous experience, literature search, and experts' opinions as exemplified in [5]. The knowledge-based design methodology is described in [6].…”

Oil and gas platforms with steam bottoming cycles: System integration and thermoenvironomic evaluation

“…The second route has been considered in works that suggested to integrate a bottoming cycle, either to the gas turbines [16][17][18] or to the processing plant [19]. The integration of steam cycles on oil and gas platforms in the North Sea region is not common, as it is believed that the additional investment costs related to the supplementary weight and space would outweigh the financial gains of exporting a higher amount of gas.…”

“…The engineering challenges of installing these power cycles are emphasised in Nord and Bolland [16,17], and a power-to-weight of about 10 tonnes per MW was estimated. For the case studies presented in the works of Kloster et al [11,12], the integration of a steam cycle was performed as a retrofit option on existing facilities, and the steam cycle was implemented on either one or two gas turbines.…”