Floating Liquefied Natural Gas (FLNG) vs. conventional Natural Gas (NG) supply chains: energy and environmental sustainability perspectives

In the current energy scenario, natural gas (NG) represents at least 20 % of the global primary en-ergy consumption and more than 30 % of the overall fossils consumption. After the 2020 pandem-ic, amid a first reduction in the demand of c. 3%, the growth of the NG sector surpassed other fos-sil fuels reaching up to 5 %. Indeed, NG has been envisaged as an enabler for the transition to-wards a low greenhouse gas (GHG) emissions economy, due to its improved environmental per-formance compare to other fossil feedstock. The environmental impacts of full supply chains are gaining relevance for the current climate change mitigation goals. Shedding light on the indirect emissions (and not only on the direct ones resulting from fossil fuels combustion) can revel important environmental footprints to be ad-dressed. Therefore, both, the upstream and midstream phase (i.e., production, processing and transportation) of the NG supply chains, are investigated, in order to highlight possible bottlenecks to be optimized. In the first instance, this thesis aims to compare the conventional production and processing tech-nologies (i.e., NG and LNG facilities) with the emerging Floating Liquefied Natural Gas (FLNG) one, in terms of the engineering requirements of each of them. FLNG is the main focus of this work as a growing technology aiming to exploit stranded NG reservoirs, hence providing flexibil-ity to the supply chain and likely improving the security of the energy supply. In particular, the Eni-Coral facility is evaluated in detail as a model case study for the generation of an accurate inventory of the main exchanges with the technosphere and biosphere, taking into account all life stages and the production patterns reported in the literature for this type of facili-ties. Its full supply chain includes operation such as reservoirs exploitation, raw fluids treatments (e.g., water, acid gases and mercury removal and NGL fractionation), liquefaction, LNG transpor-tation, regasification and distribution. The inventory phase was carried out by combining primary and secondary data (obtained from the Ecoinvent v3.9 cutoff database). The environmental impact was evaluated through the life cycle assessment (LCA) technique, and the energy performance through the energy sustainability analysis (ESA). For these, dedicated datasets were generated us-ing the Activity Browser of the python brightway2 framework, and 1 Sm3 of NG is used as the functional unit (FU). The LCA impact assessment phase was conducted using the CED and CML v4.8 2016 methods. In particular it was found that lower GWP associated to the upstream and midstream phase of FLNG can be obtained for distances over 2500 kilometers, where trade-offs with compressed NG pipe-lines occur. On the other hand, due to the massive amount of steel employed for its construction, the floating facility chain resulted in considerable higher values both for Human toxicity and Ter-restrial ecotoxicity, than the LNG and NG ones. The ESA, despite methodological differences, showed that the EROI found for Eni-Coral FLNG and its entire chain, is in line with the most re-cent values assessed for both the LNG and NG chains. Finally, the Energy Payback Time (EPT) re-sulted in the 0,6-1,3 yr range, for Eni-Coral (for which an activity level of the 70% was considered) and its supply chain, respectively.