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

Abstract

Offshore oil and gas installations are mostly powered by simple cycle gas turbines. To increase the efficiency, a steam bottoming cycle could be added to the gas turbine. One of the keys to the implementation of combined cycles on offshore oil and gas installations is for the steam cycle to have a low weight-to-power ratio. In this work, a detailed combined cycle model and numerical optimization tools were used to develop designs with minimum weight-to-power ratio. Within the work, single-objective optimization was first used to determine the solution with minimum weight-to-power ratio, then multi-objective optimization was applied to identify the Pareto frontier of solutions with maximum power and minimum weight. The optimized solution had process variables leading to a lower weight of the heat recovery steam generator while allowing for a larger steam turbine and condenser to achieve a higher steam cycle power output than the reference cycle. For the multi-objective optimization, the designs on the Pareto front with a weight-to-power ratio lower than in the reference cycle showed a high heat recovery steam generator gas-side pressure drop and a low condenser pressure.Keywords: black-box optimization, multi-objective optimization, genetic algorithm, combined cycle, process simulation, heat recovery

Lifetime Assessment of Combined Cycles for Cogeneration of Power and Heat in Offshore Oil and Gas Installations

“…Similar considerations apply to explain those outputs. For case D, it can be noticed that the values became closer to what showed by a previous work [11], which referred to an offshore combined cycle but not in a cogeneration mode. This confirmed the effectiveness of the analysis.…”

“…Thus, lower steam pressures had to be used, even though they led to a decrease of the ST performance. With regard to the superheated steam temperature, the optimum values obtained were relatively lower from what shown in the literature [11]. The high level of steam pressure demanded a reduced superheated steam temperature in order to ensure a sufficient steam generation and a better exploitation of exhaust gases.…”

“…The GA algorithm does not require the objective function to be differentiable or continuous [31] and has proved itself to be a reliable method for problems of the same type as the one of this paper [32,33]. The GA parameters for the constrained multi-objective optimization process were initially selected in accordance with the indications given by previous similar works [11,13,32] and following corrected with the experience gained with the first runs. The values used in the paper represent the chosen compromise between the likelihood of finding the global minimum and the time necessary to solve the optimization problem:…”

“…The steam evaporation pressures obtained from the optimization process were rather high, in many cases over 30 bar. A previous analysis on offshore combined cycles showed that the optimal pressure was around 25 bar [11]. The mentioned paper investigated a combined cycle without cogeneration of heat.…”

“…), while other were modified according to the values of the decision variables tested (number of tubes, nozzle areas, etc.). Other works defined a single composite variable (e.g., weight-to-power ratio [11]) in order to have a single-objective optimization problem and, thus, a single optimum point. However, in this work it was preferred to define the optimization problem with two objective functions in order to have a broader overview on the possible designs that could be defined and to evaluate the tradeoffs associated with the different solutions (also related to off-design operations).…”