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 CO2 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 CO2 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.