Session: 06-11 Innovations in Steam and Bottoming Cycles

Submission Number: 178533

Combined Cycle Power Plants Offshore: Comparison of Compact and Bulky Once Through Steam Generator Design and Operation in Terms of Weight, Transient Response, and Equipment Lifetime 

While several offshore operators have considered combined cycle power plants for their greenfield and brownfield production installations in the past, very few were implemented. However, with many organizations making formal commitments to curb emissions, interest in the concept is growing and new plants are being commissioned around the globe. The technology must be low weight and compact and offer operational flexibility over its lifetime. There is an R&D need to accelerate the learning curve of its operations. 
In this work, operation and load of a Once-Through Steam Generator (OTSG) in a combined cycle power unit offshore are investigated in terms of load changes for the gas and steam turbines. This is connected to remaining life-time analysis of OTSG components, focusing on the influence of the OTSG design parameters on dynamic operation and equipment fatigue.
The reference offshore combined cycle power plant configuration consists of two SGT750 gas turbines from SIEMENS Energy with a steam bottoming cycle for power production. At the nominal point, the combined cycle power plant is designed to provide a total power output of 92 MWel, where 36 MWel comes from each gas turbine and 20 MWel from the steam turbine. Two bottoming cycles were designed, one based on a compact OTSG design with a tube diameter of 1” (compact and light), and one based on an OTSG with a larger tube diameter of 1 ¾” (relatively heavier).
The design of the OTSGs employs an optimization model for the bottoming cycle with detailed process and heat exchanger modelling. The OTSG geometry parameters (like tube length, fin geometries, circuit configuration) are optimized for a minimum total weight of two units while achieving the target power output of the bottoming cycle. Practical constraints are also included, like the pressure drops of the exhaust gas and steam and minimum bending diameter of the tubes. The OTSGs’ weight is reduced by about 40% by using the 1” tube compared with the 1 ¾” tube.
The analysis of the OTSG dynamics using high-fidelity process models for both designs under normal operating conditions shows that, without controllers, the systems present considerably longer settling times for low gas turbine loads, which is explained by the overall lower flowrates through the system. The most compact system presented faster dynamics than the system with larger tube diameters due to its smaller thermal inertia.
Long-term dynamic simulations of the combined cycle systems were performed considering the variability induced by renewable energy integration. Here, we assume integration of the combined cycle with a 15 MW wind turbine, and we evaluate scenarios with average wind power production and high variability of produced power due to cut-out events. For these scenarios, temperature setpoint deviations did not exceed 3°C.
The fatigue analysis of the OTSG considers the changes in mechanical and thermal stresses due to pressure and temperature gradients along the tubes and for the header/tube connection. Stress cycles are counted using the Rainflow algorithm, and the cumulative damage is calculated so that the consumed lifetime can be estimated for each part of the OTSG. The number of cycles to failure is calculated based on both ASME and EN standards. Results indicate that a single load change event may consume 0.004% of the OTSG lifetime for the worst affected part. For all startups, the compact design presented smaller startup times due to having lower thermal mass. The hot startups caused the most damage, consuming 0.8% of the OTSG lifetime for both designs when the OTSG is started with its full flow rate, while a more conservative and slower startup consumed 0.009% of the OTSG lifetime. The thinner tubes from the compact design reduce the overall thermal stress, allowing for faster temperature and load changes.

Presenting Author: Ruben Mocholí Montañés SINTEF Energy Research

Presenting Author Biography: Dr. Rubén Mocholí Montañés is a Research Scientist at SINTEF Energy Research (Trondheim, Norway), specializing in thermal power plants and industrial decarbonization with and without carbon capture (CCS). He holds a PhD in Process and Energy Engineering from the Norwegian University of Science and Technology (NTNU), alongside a double Master’s degree in Mechanical and Industrial Engineering from Lund University (Sweden) and the Universitat Politècnica de València (Spain).
His research focuses on dynamic simulation and control of complex energy systems and thermal power plants. A recipient of the prestigious 2019 Green Talents Award from the German Federal Ministry of Education and Research (BMBF), Dr. Mocholí Montañés has published extensively on combined cycles operational flexibility and experimental testing at pilot and industrial facilities. He has lead several R&D projects in Norway and Internationally.

Authors:

Lucas Ferreira Bernardino SINTEF Energy Research
Håvard Falch SINTEF Energy Research
Magnus Kyrre Windfeldt SINTEF Energy Research
Han Deng SINTEF Energy Research
Geir Skaugen SINTEF Energy Research
Ruben Mocholí Montañés SINTEF Energy Research