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ENERGY TRANSITION

contaminants, usually done in HRSG through the References
continuous blowdown, so that water quality requi-
rements to feed OTSG is higher: as example, allo- [1] International Association of Oil and Gas Pro-
wed SiO concentration is from 600 to 8000 times ducers (IOGP). IOGP Environmental performance
2
lower in OTSG than in HRSG, as function of the Indicators – 2021 Data.
operating pressure [8]. This obliges to have some [2] Norwegian Petroleum Directorate. Emissions,
water treatment skids to make water suitable for discharges and the environment. https://www.npd.
feeding the OTSG. Nevertheless, in CCGT the wa- no/en/facts/publications/reports/resource-report/
ter quality requirements are driven by Steam turbi- resource-report-2019/emissions-discharges-and-
ne, regardless of the steam generator type. the-environment/
Another cons is the maximum allowed heat flux, up [3] Net Zero Technology Centre. https://www.
to 2.5 times higher for HRSG [9] due to the sudden netzerotc.com/news-insights/study-assesses-po-
and complete vaporization in the relevant section tential-routes-for-oil-and-gas-asset-electrification/
of OTSG tube bundle, that is compensated by the [4] Basha, Intieri, Mantini, Ermini, Del Puglia, Ba-
lower steel mass and water volume of the system, ker Hughes. Decarbonizing FPSO: Lifecycle As-
that means lower thermal inertia, leading to the fa- sessment of State-of-the-Art Rotating Equipment
ster start-up times as previously said. Solutions. OTC. 2023
Finally, pressure drops in OTSG are comparable [5] Voldsund, Reyes-Lúa, Fu, Ditaranto,
to those in HRSG on exhaust gas side, to remain Nekså, Mazzetti, Brekke, Bindingsbø, Grainger,
within allowable limits not to affect Gas turbine per- Pettersen, SINTEF, Equinor. Low carbon power ge-
formances, whereas on water-steam side they are neration for offshore oil and gas production. Energy
higher, to insure a steam stable flow along all the Conversion and Management. 2023
range of operative flowrates. This is achieved by [6] Følgesvold, Skjefstad, Riboldi, Nord, NTNU.
selecting smaller tube diameters than in HRSG, Combined heat and power plant on offshore oil
so improving the heat exchanging surface densi- and gas installations. Journal of Power Technolo-
ty again towards a higher compactness, with the gies. 2017
penalties of higher head feed water pumps selec- [7] STP-PT-066 Design Guidelines for Corrosion,
tion and their higher power consumption, of which Erosion and Steam Oxidation of Boiler Tubes in
impact on overall CCGT efficiency is anyway disre- Pulverized Coal-Fired Boilers, ASME. 2014
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ry installations - Part 12: Requirements for boiler
feedwater and boiler water quality, European Stan-
Conclusions dard. 2003
[9] RECOMMENDED PRACTICE 534 Heat Reco-
very Steam Generators, API. 2007
World’s renewed focus on decarbonization is af-
fecting the Offshore Upstream industry. A recent
trend is to consider CCGT solutions as an alterna- Acronyms
tive to the traditional OCGT for FPSO applications,
drastically improving electrical efficiency, reducing Capital Expenditure (CAPEX)
by 20% or more the overall emitted CO per MWh Combined Cycle or Cogeneration Gas Turbines
2
produced. (CCGT)
However, the increase of complexity and weights Conferences of the Parties (COP), Electrodeioniza-
determined by a CCGT requires careful considera- tion (EDI)
tion and the proper balance between performances Floating Production, Storage, and Offloading
and flexibility, while containing footprint and weight. (FPSO)
Therefore, a proper design and plan of the Offsho- Gas Turbine Generation (GTG)
re CCGT including the aeroderivative GT and the Greenhouse Gas Emissions (GHG)
associated selection of the steam generator type Heat Recovery Steam Generator (HRSG)
(OTSG), allows thermal and electrical power flexi- International Association of Oil and Gas Producers
bility production, reduced space and low weight, (IOGP)
availability and low water consumption. Levelized Cost of Electricity (LCOE)
Today, the CCGT represents a technically sound Operating Expenditure (OPEX)
and economically mature technology capable to Steam Turbine Generator (STG)
decarbonize deep water projects: in the medium Once Through Steam Generator (OTSG)
term, this solution can also represent a viable Waste Heat Recovery Units (WHRU)
option towards the future FPSO decarbonization
steps.

An extended content of this paper was presented
at the 16 OMC Med Energy Conference and Exhi-
th
bition in Ravenna - Italy, 23-25 May, 2023.

34 Impiantistica Italiana - Maggio-Giugno 2024