## 5 Conclusions  
The obligation to provide CI facilities and refueling points for LNG in EU ports by 2025 (Directive 2014/94/EU) involves institutional support to boost these technologies to advance port sustainability. However, their effectiveness is highly conditioned on port features that include the hinterland population size and the share of renewable sources in the energy mix of the regions where the ports are located. This concern is especially justified when SSS is considered, since this type of traffic is characterized by short call times, shipping close to the coast, and frequent operation in insular scenarios with an electricity generation mix that offers few renewable sources.  
In light of the foregoing, this paper contributes to knowledge about the port performance of LNG and CI alternatives for SSS traffic, by introducing a calculation method that is able to evaluate the environmental advantages provided by their use. The method was applied to three routes with ports located in regions with varying sustainability in their land electricity grids: Peninsular Spain-France, South Spain-Canary Islands and interisland (Canary Islands). Simultaneously, two different SSS fleets were analyzed: feeder and Ro-Pax vessels.  
From the application cases, a number of findings can be drawn. One important influence is the connection/disconnection lag time on CI performance for SSS. Specifically, this is inversely proportionate to berthing time. Thus, the feeder vessels with longer loading times than Ro-Pax vessels show lower sensitivity to this variable by achieving greater environmental benefits by CI use. Likewise, SSS routes with ports located in regions where low-emission sources predominate in electricity generation clearly benefit more from CI use than the LNG alternative in port. On the other hand, on those routes whose ports offer an electricity supply with high dependence on fossil fuel generation, the convenience of CI versus LNG use is highly conditioned on the circumstances of the regions involved. The latter has been exemplified in this paper through the analysis of interisland traffic (Las Palmas de Gran Canaria-Sta. Cruz de Tenerife); the results show LNG performance is clearly greater in environmental terms, regardless of the fleet type.  
Thus, the results obtained in this paper quantitatively confirm prior warnings about the benefits of CI in the insular framework, especially with short call traffic. This highlights the need for additional measures to ensure the sustainability of the on-shore electricity supply for the application of Directive 2014/94/EU in the insular territories. Otherwise, Directive 2014/94/EU will not only involve considerable port investment but also will not bring about the expected health benefits. These measures necessarily require a prior increase in the share of renewable sources in the port electricity supply.  
Conceptualization, A.M.-L., A.R. and J.A.O.; methodology, A.M.-L. and A.R.; validation, A.M.-L. and J.A.O.; formal analysis, A.M.-L. and J.A.O.; resources, A.R. and J.A.O.; data curation, A.R.; writing--original draft preparation, A.M.-L.; writing--review and editing, J.A.O.; visualization, A.R. and J.A.O.; supervision, J.A.O. All authors have read and agreed to the published version of the manuscript.  
This publication has been supported by the H2020 project FLOTANT, which is coordinated by the Oceanic Platform of the Canary Islands (PLOCAN). The project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 815289.  
Not applicable.  
The authors declare no conflict of interest.