Combined Cycle Power Generation: The Intersection of Innovation and Efficiency

With the strict demand of “Carbon Net Zero”, the oil and gas industry reinforce their facilities to provide assurance on power generation in one side and increasing efficiency to reduce the emission in other side. This is where the “Combined Cycle Power Generation” start to play their roles to answer this challenge to provide sufficient energy for the facilities with cleaner emission. Conventional FPSO applied open cycle gas turbines, power generation alone typically represents the source of more than 60% of the carbon emissions on recent-generation FPSOs. The combined cycle power generation can reduce the SOx and NOx emission by integrating Heat Recovery Steam Generator (HRSG) and Steam turbine to the power generation system.? In parallel the thermal efficiencies can be increase from 35-45% in the conventional open cycle to 50-65% for combined cycle.

Let’s start with an overview of combined cycle power plants. A combined cycle power plant is an assembly of heat engines that work in tandem to convert heat into mechanical energy and then into electricity. The most common type used for electricity generation onshore and relies on both gas turbines and steam turbines. Here’s how it works:

·?????? Gas Turbine: The process begins with the combustion of natural gas or oil in a gas turbine. This combustion generates heat, which is used to produce mechanical energy in the gas turbine.

·?????? Steam Turbine: The hot exhaust from the gas turbine still contains significant thermal energy. In a combined cycle plant, this heat is harnessed by passing it through a heat exchanger. The heat exchanger transfers the energy to a separate working fluid, typically water or steam. The steam is then used to drive a steam turbine, which further converts the remaining heat into electricity through a generator.

By combining these two thermodynamic cycles—the Brayton cycle (gas turbine) and the Rankine cycle (steam turbine)—a combined cycle power plant achieves higher overall efficiency compared to a simple gas turbine plant. In fact, the overall efficiency can increase by 50–60%, resulting in significant fuel cost savings.

One of the drawbacks of Combine Cycle power generation is the requirement of large footprint and their weight contributed by their heat recovery system to feed the steam turbines. For that reason, the implementation is mostly onshore instead of offshore. With the development Once Through Steam Generator (OTSG), the combined cycle plant is become more viable for offshore installation.? Once Through Steam Generator (OTSG) is a type of heat exchanger used in pressurized water reactors (PWRs) for boiling water into steam. The OTSG has a vertical-shell counter flow straight-tube heat exchanger design. In simple terms, it directly generates superheated steam as the feedwater flows through the steam generator in a single pass. Unlike traditional steam generators with boiler drums, the OTSG follows a continuous path without segmented sections for economizers, evaporators, and superheaters. With this approach the OTSG are requiring less footprint, less water and having less weight compared to the traditional Heat Recovery Steam Generator (HRSG). Although the unit price of OTSG is higher, the overall CAPEX and OPEX are significantly lower.

First offshore combined cycle application is Shell Appomattox Semi Sub platform (start up in 2019) with 4@27 MW GTG plus 40 MW OTSG+ Steam turbines. And nowadays Petrobras had ongoing 2 FPSO P-84 project and P-85 with all electric concept plus combined cycle power generation. MODEC also had BM-C-33 with Equinor ongoing for combined cycle implementation. Both projects are expected to present 20-30% carbon footprint reduction.

In the context of offshore oil and gas production facilities, the adoption of combined cycle power generation holds significant promise. While open cycle gas turbines (GTs) have been the go-to solution for electricity generation, the industry is now exploring more sustainable pathways. Combined cycle power plants offer several advantages over open cycle GTs, including increased efficiency, reduced greenhouse gas emissions, and improved overall performance. By integrating a Once-Through Steam Generator (OTSG), which extracts thermal energy from the GT exhaust to produce superheated steam, these facilities can achieve both power generation and heat supply for oil and gas processing. As the offshore industry strives for environmental footprint reductions, combined cycle technology emerges as a compelling choice for powering these critical facilities.