Fuel Supply Chain Prospects in the On-Going Transition to Sustainable Ship Propulsion: A Multifaceted Paradigm Ahead

Abstract

Shipping is urgently exploring alternative vessel energy sources across a wide range of options—from other fossil fuels to renewables—with a view to more sustainable ship propulsion. Based on processing of publicly available data, the authors discuss the prospects of the supply chains for 16 vessel power sources alternative to oil, comparing descriptive statistics across respective fuel supply chain key performance indicators (KPIs) to evaluate potentiality along with hidden vulnerabilities. While finding marked differences across calculated mean, standard deviation and coefficient of variation values, the authors do not preclude the development of parallel ship fuel supply chains, unlike the case of previous fuel transitions in shipping. To support this scenario, already formed in practice, they emphasize the enabling attributes of today’s world fleet in terms of total capacity and of size of each of the main shipping sectors which could eventually sustain nowadays multiple fuel supply chains. Concluding on limitations and challenges that such an energy-source multitude can create, the authors underline the need to consider in the Life-Cycle Assessment (LCA) of shipping fuels their total impact, including necessary ship hardware changes for a more thorough assessment of fuels’ impact across the entire shipping services’ supply chain.

1. Introduction: Ship Energy in the Sustainable Era

Historically, shipping has relied heavily on fossil fuels —abundant and also inexpensive until the oil shocks of the 1970s—transitioning from coal to oil in the years to World War II. The adoption of a novel vessel power source on each occasion had been commonly driven by market considerations, such as availability, cost, operational efficiency and alignment with worldwide technological advancements. In the last two decades, however, rising regulatory pressures and market-driven sustainability goals have driven cost concerns largely to the sidelines, driving themselves instead of the adoption of alternative fuels. As a consequence, industry began exploring new options, such as LNG, methanol, biofuels, ammonia or hydrogen to significantly reduce emissions [1]. This was together with either the consideration or the practical implementation of other main or supplementary energy sources for ships, stretching from the most debated nuclear power to wind-assisted propulsion systems (WAPS) to address heightened environmental pressures and fast-evolving regulatory demands [2]. At present, a number of new fuels have already had practical applications, either as sole sources of energy for vessels or on the basis of dual-fuel ship engines. However, technological, environmental and regulatory aspects are changing fast with, as an example, the most popular fuel replacement choice: liquefied natural gas (LNG), emerging itself as a target of greenhouse gas (GHG) regulation due to its composition from methane. As a result, this popular alternative to oil is treated anymore as “transition fuel”, albeit with the final duration of the transition phase eventually longer than currently considered [3].

This transition is not so different from the transitional stages of past fuel shifts as there are already de facto parallel supply chains for alternative fuels. However, the current transition is induced by regulation with the absence of clear direction, exacerbated delays in the relevant definitive decisions in the context of the International Maritime Organization (IMO) [4], justified as these may be due to lack of sufficient research, increases investment risks. In this context, the pattern of past transitions by which a unique vessel power source prevails for about a century until another one completely dominates seems less likely. At the same time, any sudden policy shifts due to intensifying environmental pressures could lead to rapid obsolescence of certain fuels, effectively causing “sudden death” scenarios for some of the now emerging new energy source supply chains.

The research question of this paper explores the sustainability of supply chains of alternative fuels within an evolving regulatory and technological landscape. As a key part of this process, the authors proceed as follows: A: Firstly, an analysis of descriptive statistics of KPI values which have been put forward regarding ship fuel supply chains with the aim to explore differences in fuel supply chains overall readiness and in the level of their vulnerability which may remain hidden behind high average scores. B: Secondly, the authors discuss how the—massively different today—key attributes of the world fleet can support the co-existence of a system of multiple supply chains. C: Thirdly, the authors highlight key challenges that a multifuel new paradigm may create for shipping operations. D: Fourthly, an alternative proposal for a more efficient fuel transition framework is put forward for future policy interventions involving a more holistic Life Cycle Assessment concept, if alternatives are considered in this context from the perspective of the replacement of hardware systems they involve.

The paper is divided into five sections. Following the introduction, Section 2 covers the background to the emergence of new vessel energy sources as policy action on shipping emissions accelerated. Section 3 calculates descriptive statistics from available metrics of KPIs of shipping fuel alternatives pointing to supply chains with high average scores but also to cases where high variability may imply challenges for implementation. Section 4 lays out the changed attributes of the world fleet which may allow the successful coexistence of parallel fuel supply chains and points to new operational needs including required new skillsets on board and in ports. The concluding fifth section summarizes prerequisites for establishing a sustainable multi-fuel system, highlighting aspects such as resource-sharing potential, exchange of complementary know-how and the necessity for common specifications to ensure scalable and resilient fuel supply chains.

2. The New Energy Challenge for Shipping: The Background

The 21st century is marked by accelerating and intensive efforts to mitigate environmental pollution including from the maritime industry [5] through, initially partial, measures intended to improve the GHG footprint of fuel oil. Despite assessed discrepancies of current strategies [6] the direction of policy points to the abolition of the use of fossil fuels in a not very distant future. According to the International Maritime Organization (IMO), global shipping contributes roughly 2–3% of annual CO₂ emissions worldwide with later estimates verging more to the lower end of the range [7].

As part of the IMO’s Initial GHG Strategy, the industry has been tasked with reducing annual greenhouse gas (GHG) emissions by at least 50% by 2050, compared to 2008 levels. This ambitious target, along with other regional efforts such as the EU’s “Fit for 55” package, aiming to cut emissions by 55% by 2030, accelerated the search for viable alternative fuels while carbon pricing, tax incentives, and subsidies encouraged early adoption to meet climate targets [8].

Today, technological advancements like dual-fuel engines enable ships to switch between traditional marine fuels and alternatives such as LNG or methanol, providing added flexibility to shipowners. As of 2024, more than 900 LNG-capable vessels are either in operation or in order worldwide [1,9]. Engine manufacturers are also investing heavily in ammonia and in hydrogen-capable propulsion systems, aiming to achieve commercial readiness by 2025/2026 [10,11].

While forecasts for adoption of specific alternative fuels remain highly uncertain, several energy sources, previously considered only as distant possibilities, have now been approaching commercial viability/feasibility. In parallel, the number of vessels capable of utilizing alternative fuels has seen a significant increase with a 50% rise in orders for alternative-fueled ships in 2024. The 600 new vessels added to the orderbook over the past year brought the total to 1737, reflecting the industry’s commitment to decarbonization [12]. From the side of ship operators, Maersk had already estimated in 2022 that its ordered methanol-powered vessels would be cost-comparable to fossil-fuel operations, particularly with the expected rise in carbon taxes on traditional fuels [13]. Alongside many shipping companies, major port operators have also committed to Zero Emissions Roadmap strategies [14].

However, rapid developments on both the technological and regulatory fronts keep fueling uncertainty about the sustainability of available solutions. While for some energy sources such as methanol and LNG, there has been direct applicability and commercial applications some others—including ammonia and hydrogen—remain in a more exploratory research phase, in view of safety, technological, and financial concerns. Additionally, there is as yet no agreement on how comparative prices of alternative fuels will evolve [15] nor whether market alignments will ensue to facilitate their broader adoption.

The question at the center of this multi-billion-dollar stake that the market for marine fuel represents, is a strategic one involving substantial financial risk: setting up any fuel supply chain for alternative fuels requires significant capital investment not only by fuel producers and providers but from maritime infrastructure stakeholders as well. For example, to position Rotterdam as a central hub in the emerging hydrogen economy, the Port of Rotterdam has allocated €140 million for hydrogen bunkering facilities with operations expected to start by 2025–2026 [16]. Other countries, such as Norway, which are pioneers in sustainable technological innovation and in the adoption of LNG as a marine fuel, have been able to develop bunkering networks since the early 2000s [17], supported by government incentives and/or strict environmental policies. Despite developments, the global readiness of ports for handling next-generation fuels—particularly ammonia and hydrogen—remains limited. Drawing on the lessons from the introduction of LNG as shipping fuel, it appears unlikely that full-scale solutions will be available soon, at least not without close collaboration among key stakeholders, including vessel energy source providers, producers, port authorities, engine manufacturers, classification societies and shipowners [18].

With safety being always the first Litmus test to be passed in shipping, as alternative fuels are advancing, safety protocols, classification rules and operational standards are introduced, or evolve quickly. The International Code of Safety for Ships using Gases or other Low-flashpoint Fuels Code (IGF Code)—originally created for LNG—is now being expanded to encompass additional low-flashpoint fuels such as methanol, ammonia and hydrogen with classification societies following quickly to provide additional rules [19].

Unlike past transitions, the current shift, driven by sustainability imperatives, leaves little room for any “trial and error” approach, which cannot be tolerated anymore. The specter of the explosion on Brunel’s “Great Eastern” shortly after setting to sea for the first time, killing several stokers [20] and, from afar, its own designer, still lingers to remind how haunting journeys to shipping energy innovation may be.

At a fundamental level, a system of multiple supply chains of ship fuels seems to have already emerged. For nearly two decades, oil and LNG supply chains co-existed and in recent years they have been gradually complemented by some alternative fuels. However, these emerging supply chains differ significantly in terms of their Technological Readiness Level (TRL), a key criterion used to assess their status as viable and sustainable alternatives [21].

Ref. [22] proceeded to a GHG evaluation of main alternative fuels highlighting the potential in the process of shipping’s decarbonization of green hydrogen, FAME biofuel, and bio-methanol with biofuel presenting the highest stable reduction among the three. Ref. [23] also developed a structured, data-driven framework to help policymakers and maritime stakeholders assess the potential of alternative fuels for achieving shipping decarbonization. They identify 24 sustainability criteria, spanning engineering feasibility, infrastructure readiness, economic cost, environmental performance, and socio-political. Using the Best-Worst Method (BWM), and then Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS), they rank five leading fuel options: methanol, hydrogen, biofuels, ammonia, and LNG. The superiority attributed to methanol is based mainly on compatibility with existing infrastructure, low retrofitting requirements, and regulatory readiness. Safety concerns result in hydrogen and ammonia ranking lower, despite being promising for long-term decarbonization potential. Biofuels scored moderately across all criteria with future scalability remaining the basic moot point. Liquefied natural gas ranks last due to methane slip and uncertainty on future regulations towards it. The authors of [23] consider that system-level implications, such as ship conversion costs, port bunkering capabilities, fuel supply stability, and stakeholder perception, are brought forward to build a framework positioned to serve as a practical decision-support tool to guide/assist investments and regulatory planning. The review of marine fuels by [24] thoroughly, but concisely, presents their differing properties including, critically, engine and infrastructure availability. Such recent studies confirm the ample evidence on the varying characteristics of alternative fuels along with the existence of dispersion of levels of readiness and the various criteria strengthening the motivation to explore such disparities in order to estimate the sustainability of future ship fuel supply chains as these continue to emerge even more clearly following the initial approach of this prospect by the authors in [25].

3. Comparative Analysis of Alternative Fuel Supply Chain KPIs

While alternative fuels are mainly discussed in terms of their potential to reduce greenhouse gas (GHG) emissions, the production pathway has a decisive impact on their actual carbon footprint. To distinguish between various routes and levels of sustainability, a color-coding approach for the various alternatives has emerged in both academic literature and industry practice. The color associated with a specific fuel (e.g., “green,” “blue,” “grey,” “pink”) indicates the feedstock used, the energy source, and whether carbon capture or other mitigation measures are involved. For instance, “green” denotes production via renewable electricity (e.g., solar, wind); “blue” typically indicates fossil-based production coupled with carbon capture and storage (CCS) to reduce net emissions; “grey” (or “brown”) implies reliance on fossil resources without CCS. This categorization clarifies the life-cycle emissions and can help stakeholders—shipowners, ports, regulators—assess the true environmental benefits of different fuels within a clear taxonomy [26,27]. This combination of generic alternative fuels with the specific method of production (whether already achieved or still at experimental level) produces the colorful variability of

Table 1. Vessel energy sources by color-coding of production method.

Fuel	Blue	Green	Turquoise	Grey/Brown	Pink
Oil	Theoretically possible if large-scale CCS is applied during extraction/refining [28]	Not applicable (no common renewable/oil pathway)	Not applicable	Typical fossil-based (extraction and refining without CCS)	Not applicable (nuclear for possible energy production savings [29]
Lng	“Blue” LNG if from fossil gas with CCS [2]	Green if derived from biomethane or synthetic methane using renewable energy [2,30,31]	Not applicable	Grey LNG from fossil natural gas (no CCS) [32]	Not applicable
Methanol	Blue methanol if fossil-based but CO2 is captured or sequestered [26]	Green if produced from renewable hydrogen + captured CO2 (biogenic or direct air) [26,32]	Limited references to turquoise methanol (methane pyrolysis → H2 → methanol)	Grey methanol from fossil sources (without CCS) [32]	Pink, if produced via nuclear-powered hydrogen plus captured CO2
Batteries	Blue if grid electricity is fossil-based with CCS [2]	Green if electricity from renewable sources (solar, wind, hydro)	Not typically classified as turquoise, since term applies to methane pyrolysis	Grey if electricity used for charging is mainly fossil-based (no CCS)	Pink, if electricity comes from nuclear (low-carbon, but not renewable)
Hydrogen	Blue from fossil feedstock with CCS [2,31]	Green via electrolysis powered by renewables [32]	Turquoise from methane pyrolysis, yielding solid carbon instead of CO2 [33]	Grey from natural gas (steam methane reforming) or coal without CCS	Pink, if nuclear-powered electrolysis [33]
Biofuels	Blue is rare but could apply if biomass-to-fuel process captures & stores CO2 [34]	Green, if from sustainably sourced biomass with low net carbon footprint [12,35]	Not generally referred to as turquoise	Brown/Grey, if feedstock or process has high net emissions	Pink is theoretically possible, if biomass is processed with nuclear-derived energy [35]
Ammonia	Blue from fossil-based hydrogen with CCS [36]	Green, if synthesized from renewable hydrogen + air-derived nitrogen [36]	Turquoise, if hydrogen feedstock would be from methane pyrolysis [33]	Grey from fossil-based hydrogen (no CCS)	Pink if hydrogen feedstock is nuclear-powered [37,38]

A. Blue: Production from fossil sources (oil, natural gas, etc.) with CCS technology implemented to reduce net emissions. B. Green: Production from renewable sources (e.g., wind, solar, sustainable biomass) resulting in very low or near-zero net GHG emissions. C. Turquoise: Typically associated with methane pyrolysis, where the carbon byproduct is solid rather than CO₂, reducing GHG emissions—though the upstream energy source still matters. D. Grey/Brown: Indicates fossil-based pathways without carbon capture; can also specifically refer to coal-based as “brown”. E. Pink: Production powered by nuclear energy (low carbon, but not classified as renewable), often through electrolysis for hydrogen and derivative fuels. Source: [2,12,26,27,28,29,30,31,32,33,34,35,36,37,38].

, next.

3.1. Alternative Fuels: Current Applications and Prospects

Current market and specialist projections vary. A significant shift towards hydrogen-based fuels and biofuels is projected while blue ammonia and e-ammonia, also included in Table 1, are optimistically expected to grow from 20% to 60% of shipping fuels by 2050. Similarly, liquefied bio-methane (LBM) is projected to account for an average of 34% of shipping fuels by 2050, with production having also been predicted to fall short of vessel needs [11].

The expected balance between demand and supply for alternative fuels is critical. Tight supply may trigger more energy efficiency measures in shipping operations which are estimated that by themselves could reduce fuel consumption to between 4% and 16% by 2030, lowering emissions by 120 million tons of CO₂ [2]. While potentially contributing to decarbonization targets, such efficiency measures can only be considered as a partial strategy, as are further reductions in average service speed which are already the rule in the market and have been even incorporated in market instruments around the world [39].

The question of choosing an alternative fuel remains, thus, urgent, both at the level of current day-to-day vessel operations and, most critically, at the level of long-term investment decisions. On the one hand, efficient energy sources in shipping are adopted primarily to optimize energy output per unit of fuel consumed, while minimizing emissions and reducing operational costs to comply with new regulations [40]. On the other hand, however, fluctuating fuel costs and the need for cost-effective alternatives remain critical factors in investment and decision-making [41,42]. Ultimately, applicability is the most decisive factor for selection as it is the first ground criterion for the other two to play their role.

To provide a comprehensive, yet pragmatic, overview of the potential of the supply chains of alternative fuels, Table 2 below encapsulates key benefits and existing applications of candidate fuels based on recent sources mainly in reports of organizations around shipping such as classification societies. The table highlights two core evaluation criteria along with real-world application examples whereby fuels have already been tested and deployed in operational shipping contexts:

A.

Main emissions benefits

B.

Infrastructure availability, assessing whether the necessary fuel supply chains, bunkering facilities, and logistics networks are in place.

Table 2. Focused criteria highlighting prospects of alternative energy sources for vessels.

FUEL	Main Emissions Benefits	Infrastructure Availability	Notable Real-World Examples
LNG	Up to 20–25% lower CO2 vs. HFO Significant reduction in Sox and	As of late 2023, ~200 ports globally offer LNG bunkering [16,43]	(a) Norwegian Ferries (Widespread adoption since early 2000s) [44]
	Nox especially if produced as e-LNG via mild energy [2]	Mature in certain regions (Norway, Netherlands, U.S., Gulf)	(b) CMA CGM (30+ LNG vessels in operation/on order) [45]
		Hundreds of LNG-capable vessels in operation or on order [21,46]	(c) Various cruise lines (e.g., RCL, Carnival, MSC) have adopted LNG for new cruise ships [47]
Methanol	Lower CO2 than HFO if produced from renewable sources	Emerging bunkering hubs in N. Europe, Asia [48,49]	(a) Maersk: Orders for 18+ methanol-fueled container ships. First deliveries from 2024 [13]
	Minimal SOx and particulate emissions [13,26]	Several pilot projects for renewable methanol production [13]	(b) Stena Line: Conducting trials for ro-pax vessels [49] (c) Growing interest from Waterfront Shipping (Methanex Corp.) in methanol-powered tankers [50]
Ammonia	Zero CO2 at point of use.	Limited bunkering infrastructure in 2023; focus on demo projects (EU, Asia) [16]	(a) Wärtsilä & MAN ES developing ammonia-capable engines, targeted for commercial release by 2025–26 [51]
	No SOx emissions Potentially low lifecycle GHG, if produced via green methods (electrolysis + renewables [36]	Ports of Rotterdam, Singapore, and Japan exploring ammonia corridors [16]	(b) H2Ships initiative exploring both hydrogen and ammonia bunkering [52]
Hydrogen	Zero CO2 at point of use, if ‘green hydrogen’ which produces no SOx, NOx, or particulate matter	Very limited; pilot-scale bunkering ferry projects in Norway & EU [53,54]	(a) H2Ships: European program for hydrogen fueling solutions [52]
	[55]	Infrastructure mostly in demonstration phases [56]	(b) Pilot ferries in Norway testing LH2 propulsion [57] (c) AquaVentus in Germany: hydrogen production/usage projects aiming for maritime applications [55,58]
Biofuels	Potentially net-zero CO2 if sustainably sourced	Can use existing bunkering in most cases; compatibility depends on engine specs [59]	(a) GoodFuels trials with MSC reducing up to 80–90% net CO2 [47]
	Reduction in SOx, NOx depends on feedstock and blend [60]		(b) Maersk and Shell collaborating to test advanced biofuel blends [59]
OIL	Traditional fossil-based fuel with high CO2 and pollutant emissions [60]	Extensive global bunkering infrastructure established over decades [16,44]	Dominant fuel for the majority of the global shipping fleet [44]

Source: [2,13,16,18,21,26,36,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60].

Table 2. Focused criteria highlighting prospects of alternative energy sources for vessels.

FUEL	Main Emissions Benefits	Infrastructure Availability	Notable Real-World Examples
LNG	Up to 20–25% lower CO2 vs. HFO Significant reduction in Sox and	As of late 2023, ~200 ports globally offer LNG bunkering [16,43]	(a) Norwegian Ferries (Widespread adoption since early 2000s) [44]
	Nox especially if produced as e-LNG via mild energy [2]	Mature in certain regions (Norway, Netherlands, U.S., Gulf)	(b) CMA CGM (30+ LNG vessels in operation/on order) [45]
		Hundreds of LNG-capable vessels in operation or on order [21,46]	(c) Various cruise lines (e.g., RCL, Carnival, MSC) have adopted LNG for new cruise ships [47]
Methanol	Lower CO2 than HFO if produced from renewable sources	Emerging bunkering hubs in N. Europe, Asia [48,49]	(a) Maersk: Orders for 18+ methanol-fueled container ships. First deliveries from 2024 [13]
	Minimal SOx and particulate emissions [13,26]	Several pilot projects for renewable methanol production [13]	(b) Stena Line: Conducting trials for ro-pax vessels [49] (c) Growing interest from Waterfront Shipping (Methanex Corp.) in methanol-powered tankers [50]
Ammonia	Zero CO2 at point of use.	Limited bunkering infrastructure in 2023; focus on demo projects (EU, Asia) [16]	(a) Wärtsilä & MAN ES developing ammonia-capable engines, targeted for commercial release by 2025–26 [51]
	No SOx emissions Potentially low lifecycle GHG, if produced via green methods (electrolysis + renewables [36]	Ports of Rotterdam, Singapore, and Japan exploring ammonia corridors [16]	(b) H2Ships initiative exploring both hydrogen and ammonia bunkering [52]
Hydrogen	Zero CO2 at point of use, if ‘green hydrogen’ which produces no SOx, NOx, or particulate matter	Very limited; pilot-scale bunkering ferry projects in Norway & EU [53,54]	(a) H2Ships: European program for hydrogen fueling solutions [52]
	[55]	Infrastructure mostly in demonstration phases [56]	(b) Pilot ferries in Norway testing LH2 propulsion [57] (c) AquaVentus in Germany: hydrogen production/usage projects aiming for maritime applications [55,58]
Biofuels	Potentially net-zero CO2 if sustainably sourced	Can use existing bunkering in most cases; compatibility depends on engine specs [59]	(a) GoodFuels trials with MSC reducing up to 80–90% net CO2 [47]
	Reduction in SOx, NOx depends on feedstock and blend [60]		(b) Maersk and Shell collaborating to test advanced biofuel blends [59]
OIL	Traditional fossil-based fuel with high CO2 and pollutant emissions [60]	Extensive global bunkering infrastructure established over decades [16,44]	Dominant fuel for the majority of the global shipping fleet [44]

Source: [2,13,16,18,21,26,36,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60].

Several criteria have been advanced by specialist organizations to monitor fuel progress. These include criteria related to TRL [9], which are commonly applied in emerging products [61]. Each alternative fuel may differ in terms of technological maturity level, in terms of resources, production readiness, port infrastructure capabilities and in terms of vessel adaptability readiness either to handle and store or to use a specific fuel for propulsion.

KPIs included in the main sources consulted for this analysis can be grouped as follows:

i.

Well-to-Wake Emissions—Assessing total emissions from fuel production to onboard combustion [40].

ii.

Energy Density & Storage Requirements—Analyzing storage for different fuels [42].

iii.

Fuel Scalability & Infrastructure Readiness—Evaluating global bunkering supply network expansion [44].

iv.

Economic Feasibility—Considering lifecycle costs, including production, transportation, and onboard utilization [62].

v.

Technology Readiness Level—Used not only at a composite level but also at a more specific one which may be composed by TRL for specific components along the supply chain, e.g., TRL of propulsion (TRL P), or for Handling and Storage (TRL H & S) as in [9].

vi.

At the level of efficiency of the supply chain of an alternative fuel, main criteria include the ability to deliver the required energy sources to vessels in a cost-effective manner [63], reducing environmental impact [5], overcoming logistical challenges [64], aligning with sustainability goals and regulatory requirements [34], suitability for fuel adoption by major shipping lines [65], reducing logistical bottle-necks and supply chain risks [40] and enhancing long-term price stability and regulatory compliance [34]. In constructing a roadmap for alternative fuels [66], criteria proposed have been fuel availability, cost, R&D, safety regulations, propulsion technology, port infrastructure for the fuel, stakeholder support, carbon tax, emissions’ public awareness and early adopters.

3.2. Supply Chain Prospects for Alternative Shipping Fuels: An Exploration of KPIs

Investigating prospects and challenges for the supply chains of the most well-known alternative fuels is at the core of this research. For consistency, one main source conducting its own surveys on the basis of two Likert-types has been selected among various available options such as in [9,65,67,68] or in [40], where KPIs ranging from the Energy Efficiency Design Index (EEDI) and the Carbon Intensity Index (CII) to directly assess fuel risks have been used.

Descriptive statistics have been calculated by the authors for a specific selected range for KPIs that have been advanced in [9] for each of the alternative supply chains of fuels considered. The selection was based on the data source being from the side of technical and advisory services to the shipping industry, having access to a wide range of stakeholders. The purpose of the authors’ calculations on the basis of the selected original KPI values has been to show the following:

a.

How strong is the evaluation of the prospects of a specific shipping fuel supply chain both in absolute and relative terms, i.e., compared with other fuel options?

b.

Is there any strong variability among the components of such evaluations which may imply hidden risks for the sustainability of the respective supply chain?

As the mean of KPI values for a specific supply chain can be overall high or low but does not provide information on specific weaknesses,

Table 3. Descriptive statistics of KPIs related to alternative fuels’ supply chains.

ENERGY SOURCES	AVG	STD	CV	AVG	STD	CV
	Original Values			Transformed Values
E-AMMONIA	3.62	2.02	56%	2.9	1.36	46%
BLUE AMMONIA	3.23	1.79	55%	2.63	1.1	42%
BIOFUEL (FAME)	6.46	1.98	31%	5.57	1.19	21%
BIOFUEL (HVO)	6.62	1.85	28%	5.49	0.5	9%
ELECTRIFICATION (BATTERIES)	4.77	2.65	56%	3.79	1.45	38%
E-HYDROGEN	3.77	2.28	60%	3.02	1.4	47%
BLUE HYDROGEN	3.85	2.41	63%	3.04	1.4	46%
LIQUIFIED BIO-METHANE	4.54	3.15	69%	3.64	2.07	57%
LIQUIFIED E-METHANE	4.69	3.04	65%	3.8	1.97	52%
BIO-METHANOL	4.08	2.69	66%	3.24	1.5	46%
E-METHANOL	4.38	2.53	58%	3.52	1.38	39%
NUCLEAR (PWR)	5.15	2.67	52%	4.14	1.61	39%
NUCLEAR (MICRO-REACTORS)	3.23	2.31	72%	2.77	1.76	63%
NUCLEAR (MOLTEN SALT)	3.38	2.29	68%	2.87	1.72	60%
NUCLEAR (LMCRs)	4	2.2	55%	3.34	1.59	48%
NUCLEAR (HTGRs)	3.62	2.18	60%	3.1	1.65	53%

Note: AVG = Average, STD = Standard Deviation, CV = Coefficient of Variation. Biofuel (FAME): Fatty Acid Methyl Ester. Biofuel (HVO): Hydrotreated Vegetable Oil. Nuclear (PWR): Pressurized-water reactor. Nuclear (LMCR): Liquid metal cooled reactors. Nuclear (HTGR): High-temperature gas-cooled reactors. Biofuel (FAME): Fatty Acid Methyl Ester. Biofuel (HVO): Hydrotreated Vegetable Oil. Nuclear (PWR): Pressurized-water reactor. Nuclear (LMCR): Liquid metal cooled reactors. Nuclear (HTGR): High-temperature gas-cooled reactors. Source: Calculated by authors based on the original data as in [9]. In the right sub-table were re-calculated through linear transformation of the two scales involved. Supplementary Materials data are provided in the online material: https://docs.google.com/spreadsheets/d/1xp4RZjEXAo3MYyTrRcaDpTBkUAGU08rhaAMEDMuwDRs/edit?usp=sharing (accessed on 16 February 2026).

, below, includes—beyond the average of KPI values in the second column—the standard deviation and the coefficient of variation as well for Technology Readiness Level (TRL), Investment Readiness Level (IRL) and Community Readiness Level (CRL) provided for Resources, Production, Bunkering and Ports, plus four KPIs for ships, which are TRL Handling and Storage, TRL for Propulsion, the Investment Readiness Level (IRL) and the Community Readiness Level (CRL) on the basis of the values included in the original source. A full explanation for each value of the relevant scale awarded is provided through the publicly accessible electronic document [9].

As is the case with the simple use of the mean, the value of the standard deviation fails to reveal by itself the relative cohesiveness of the key aspects of alternative fuel supply chains. To further investigate the resilience of these chains, the coefficient of variation, CV, has been also calculated, shown in the last column of Table 3. Table 3 ensues from the original data which were measured in two scales by the data source: 1 to 9 and 1 to 6. Table 3 also ensues from the conversion of the five 1–9 series (which were fewer in the original data) to the scale of 1 to 6 by the original data source for the remaining 8 series, through linear transformation.

Across both the right and the left sub-tables of Table 3, shipping fuel supply chains with potential high sustainability emerge clearly through the obvious case of biofuels, a result which matches long-voiced market opinions. Top and worst achievers in terms of the mean and of CV are the same across both tables along with some minor changes in the respective ranking—mostly by one place—with the notable exception of the last two nuclear supply chains.

As shown in Figure 1, below, the CV values for both FAME and HVO biofuels highlight them as the most stable performers with the highest means of KPI values as well. In terms of the latter, the worst performer among the 16 alternatives evaluated by the primary data source, emerges to be blue ammonia. This fuel is ranked last positioned, even lower than the other dismal KPI mean performer based on the original data, i.e., nuclear propulsion through micro-reactors which scores very low in terms of its poor valuation regarding reception in ports. It is also interesting to note that, in the original data, the only fuels that achieve a “top level” mark from the side of the ship in terms of community acceptance are the two biofuels with the worst performers including blue ammonia and all types of nuclear which are scoring near the bottom in this regard having obviously failed hitherto to convince as valid alternatives at the community level continuing on a previous path [69].

Beyond the indications provided by delving into these data on the current prospects of supply chains of alternative fuels, the question remains as to whether multiple chains can indeed be supported today or whether there are analogies with the time of the introduction of diesel engines and oil as shipping fuel: in that era, a number of alternatives to coal had already emerged [70], yet they were short-lived with the transition to oil bunkers being solidified, rapidly constituting the standard for about the next 100 years. As multiple fuel systems co-existing at terminals can create additional costs, and technical and safety constraints as well, developments in shipping which may allow such a successful parallel existence are discussed next.

4. A Larger World Fleet: Challenges and Opportunities for Alternative Fuels

The sheer scale of the current fleet is several times larger than a century ago when diesel engines were introduced to establish oil as the dominant maritime fuel for the next one hundred years (cf. Figure 2). While the size and the continuous growth of the world fleet might seem at a first level as sustainability challenges, they could eventually support sourcing energy from parallel—and through market size increase now viable—supply chains. By itself, the massively increased tonnage of the world fleet rising from almost 800 million to well over 2.1 billion dwt [71] can facilitate the development of parallel and viable energy supply chains.

It must be noted that the increase of the world fleet also creates larger segments in which ships are classified by sector of shipping in which they operate, e.g., in ocean-going or in short-sea routes, with the specialization also being related largely to their size. In this respect

• Ocean-going vessels are considered the most likely candidates to transition to ammonia or hydrogen in the long run, although significant technological and safety barriers remain [72,73]; nevertheless, hydrogen is currently considered as holding significant potential in the clean energy transition being an effective pathway for large-scale deep decarbonization [74].
• Short-sea vessels, particularly prevalent in northern Europe already benefit from LNG-ready bunkering ports with, for example, the port of Zeebrugge already handling over 150 LNG bunker operations annually [75,76].
• Cruise operators have already been increasingly interested in methanol and LNG to match strict emission controls near coastal, tourist and ECA areas [77].

Hence, both the absolute increase of the total tonnage and this of the number of vessels in each specific segment further corroborate the hypothesis that multiple supply chains of vessel energy sources may be supported in the future.

4.1. World Fleet Growth and Its Features: Another Scale for a Different Type of Transition?

During 2024, shipowners kept investing in alternative fuel-capable vessels for a future of lower emissions. As a result, the total orderbook has grown by 50%, creating a massive total of 1737 vessels to come into service in the forthcoming years [20]. While the prospective viability of the commercial operations of any of the emerging supply chains cannot be correlated to their current uptake, the growth trends of vessels already in service and on order, which use or will use alternative fuels, provide a strong indication of the overall alignment of the industry to sustainability requirements. Ultimately, this alignment will shape the market ground for survival, success, or disappearance of related fuel supply chains, a phenomenon not unknown in the history of shipping [70].

As illustrated in Figure 3, next, the cumulative number of alternative fuel vessels has reached about 5% of total numbers in service and on order, with LNG-fueled vessels still accounting for most. The percentage may appear modest; however, a critical milestone has already been achieved while the overall estimate for the use of alternative to diesel fuels earlier in the decade by [2] as mentioned also in [24] is significantly higher by at least a factor of two. It should also be noted that, by late 2024, the number of alternative-fueled vessels on order reached a 1:1 ratio with those already in service, demonstrating a significant dynamism as shown through the evolution of data in [21].

4.2. Incorporating the Full Decarbonization Impact: The Next Steps

Such developments at the level of orders for new ships directly highlight the issue of compatibility of new fuels and of the requirements of their operations with the current hardware both in terms of hulls and in terms of equipment; they also highlight the potential impact of the acceleration of shipbuilding and the environmental impact of a faster decommissioning process if the full “cradle to grave” impact of the decarbonization process is taken into account. While scope 3 emissions estimations, which broaden the scope of calculation of environmental impact have made clear inroads into other industries [78], there had been until recently [79] rather limited mention in the literature of this broader shipping services’ system boundary. More holistic approaches have been either too recent and tested on specific ship types, such as LNGs by [80], which include in the evaluation the shipbuilding (Cradle-to-Gate), the vessel operations Gate-to-End and, finally, the scrapping process (End of Operations) as the three constituent stages of such a lifecycle. Such extended approaches succeed earlier partial extensions of the traditional well-to-wake and well-to-tank LCA approaches such as in [81] for Panamaxes or “cradle-to-propeller” ones as in [82]. Such an approach should include the environmental impact of ship replacements required detailed in GHG emissions which in the case of a required newbuilding replacement would require the following:

a.

Production of shipbuilding materials along all their own supply chain;

b.

Shipbuilding energy for the construction per se;

c.

Recycling of ships to be replaced, if technically or financially no candidates for retrofits; the latter could be a sustainable alternative for fuel options requiring less drastic retrofitting.

In view of the delay, in late 2025, of the IMO deciding on definitive shipping decarbonization steps ahead, the opportunity arose for embracing such more encompassing LCA frameworks, including the key for ship-fuel transition “shipyard-to-scrapyard” life-cycle of the vessel. Otherwise, the specter of a repetition of the adoption of diesel oil and not of bio-fuel—as presented initially by the inventor of the reigning for about a century diesel engine himself [83]—may wield a “trial and error” introduction of new energy sources which may require additional corrections for which there is neither time or any margin to error.

5. Conclusions: Operational Challenges in a Multi-Fuel Future

In this paper the authors estimated the current prospects of a significant number of alternative fuel supply chains on the basis of recent data. The results highlighted the relative superiority of readily available vessel energy sources such as biofuels compared to a number of much discussed but, apparently, not so highly evaluated, more innovative and novel shipping fuels; the picture emerging from the descriptive statistics was clear in terms of the low evaluation of alternatives such as blue ammonia and in terms of vulnerability across most of the fuels considered apart from biofuels.

The biofuel prospect is one that the airline industry was quick to embrace [84], also securing rather preferential access at European level [85] to a resource the availability and the competitive uses of which have been largely debated, with the airline industry seeming better poised currently in terms of future fuels than shipping. Although the dim prospect of the production of biofuel being relatively expensive is still being put forward as a major obstacle, it would be a dissonance if this would be the only alternative to diesel sources not to benefit by fast accelerating new technologies, especially the ones not directly involving sources competing with food, as so many other fuels already have. However, with no clear givens overall, the shipping sector will have to navigate the challenges of a multi-fuel future which—apart from the more or less obvious solutions as these emerge through their overall evaluation—remain uncertain in terms of the number and the nature of energy sources which will ultimately be composing it.

5.1. Alternative Fuels—Alternative Challenges

The challenges emerging in the prospect of a multi-fuel future relate to economic, managerial, training and policy aspects, with the list not being exhaustive:

(a)

In terms of economic repercussions, it is possible that an intensification of the climate crisis may result in an acceleration of measures which may render obsolete relatively new hardware elements of alternative fuels’ propulsion systems; in the case of older vessels this may render the ship itself obsolete due to the limited amortization period creating an additional investment challenge for shipowners and related stakeholders [41,86].

(b)

Managerial aspects primarily involve the need to secure a properly serviced network of energy sources for the fleet having to now take into account alternative fuels as well; this is especially relevant to network-based operations such as liner shipping [87] where, for instance, management challenges include optimizing bunkering logistics in a port offering only two of five potential fuels.

(c)

Electricity/battery-based solutions—currently mostly chosen in the case of small ferries/passenger ships which operate in many, though not all, ways as in the container case discussed by [15]—seem to point to additional constraints for ocean-going vessels. Large freight ships have reduced autonomy considering the length of their standard voyages and the current lack of global density of fuel supply chain networks introducing a challenge in terms of adoption of most efficient alternative fuels without well-developed supply chains.

(d)

The repercussions of parallel fuel supply chains—and of new fuels in general—for shipping operations may be substantial, thus hindering the optimization of the latter which is put forward as a complementary strategy [88]. The issue of necessary skills [17] required for each fuel and respective propulsion system is a crucial aspect if the industry quickly evolves into a “multi-vessel energy source” future, with potentially multiple fuel distribution networks. While prioritizing training is proposed for fuels which would likely dominate in the near term, such as LNG and methanol, to ensure seafarers are adequately prepared for the evolving energy landscape, the overall shortage of seafarers, especially at the level of officers [89] may accentuate gaps further. Such shortages could affect optimal alternative fuel choices, creating an often not prioritized, yet critical challenge.

(e)

Finally, any need for significant modifications in ship hardware, e.g., engine(s)—or eventually for replacement of vessels themselves due to new fuel requirements—should be considered in the context of any Life-Cycle Assessment of alternative fuels. Any such approach should extend beyond methods of production, treatment, distribution etc., of fuels through any classic LCA methodology—even an all-encompassing and detailed well-to-wake one [12]—and should include the hardware dimension as well for all ships, a view shared, along with the initial suggestion of the authors in [25], by [90] when analyzing the case of ammonia.

(f)

Such a “shipyard-to-scrapyard” (StS) more holistic perspective would secure a more efficient fuel transition framework within which future policy interventions should take place adding depth to current LCA methodologies. This more encompassing assessment should at least include i. main and auxiliary engine energy and materials involved, ii. shipbuilding energy and construction materials or eventual retrofitting costs and iii. end-of-life recycling/disposal impacts at the withdrawal of the ship of life. This methodology could also eventually result in different rankings of specific fuels depending on the time-horizon adopted each time when including all stages—and not just specific ones—of the life of a ship itself and not only of the fuel it uses. Differences in the ranking order could also ensue, if onboard carbon capture and storage is taken into account, whereby even traditional fuels may even be a surviving option for the latter beyond currently forecasted dates, especially if the price of alternative fuels does not reduce [91].

Points (a) to (d), above, highlight the areas of innovation in adoption, testing, data collection, impact assessment and of validation as priorities for shipowners and collaborating organizations such as engine makers, classification societies and shipyards. On the port authorities’ side, the areas of risk–safety, as well as of resilience–recovery, require an early focus which may already have been delayed in some instances as a multi-fuel future increased the complexity of related issues. However, as will be emphasized again in the final part of these conclusions, the most difficult task falls on policy makers having to prioritize the maximization of impact on climate effects versus the potential risks of too early a solution—or several ones—which may prove as deleterious as the replacement of coal by what was considered the more efficient oil. In this respect, the maturity and proof of solutions should be high on the policy makers’ priority list.

Although the analysis of data that preceded shows that some mature solutions already exist, it seems unlikely that, at the current stage, the wisdom of conservation will win over the enthusiasm and the potential gains of innovation. The much larger world fleet of today enables economies of scale and, also, the optimization of fuel distribution networks, making investment in multiple alternative energy sources more feasible. Moreover, the widespread adoption of new fuels like LNG, methanol, and ammonia can be supported by shared infrastructure investments and collaborative industry initiatives, thereby reducing individual costs and enhancing overall fleet sustainability. While definitive regulation is delayed, resource-sharing potential, exchange of complementary know-how and the necessity for common specifications thus emerge as critical to ensure scalable and resilient fuel supply chains in a timely energy transition process.

5.2. Limitations Research of the Research

Using one specific set of KPIs—no matter how extensive, measurable and varied—for reasons of consistency, has its limitations in terms of additional or even alternative components of fuel supply chains that can be considered. The security of fuel supply, for instance, is not explicitly considered in terms of geopolitical interferences on fuel trade routes of a further concentration of hydrogen-exporting countries. It should also be emphasized that, while highlighting the need for a holistic “shipyard-to-scrapyard” LCA, this research does not address the engineering constraints related to ship modifications.

Also, results obtained and highlighted in the paper might change as alternative maritime fuel supply chains develop in the future due to scalability by growing economies of scale and scope at varying degrees for each fuel. As data become available, economic sensitivity analysis may provide a more informed ranking of the relevant fuel supply chains.

Equally, fuel supply chain rankings may change when time-horizon is to be considered when looking at fuels [92] along with changing economic and regulatory frameworks [93] which may accelerate or decelerate the current growth paths of specific fuels.

Critically, alternative fuel supply chain sustainability was focused on GHG emissions only and could thus be open to criticism, although the StS proposition for the LCA can be expanded to include competition for other uses (such as food or aviation in the case of biofuels) and emerging constraints such as freshwater use in the case of specific fuels such as hydrogen which, however, can be addressed partly through evolving technologies and alternative water sources [94].

5.3. The Need for Further Research and Smart Policy Response Amidst the Climate Crisis

The challenge of new supply chains for novel energy sources is currently acknowledged more or less explicitly. There are scant margins for delaying more coordinated actions and guidance, although 2030 targets have remained unchanged—as the slow speed of progress was realized [6]—with 2050 now being the next net-zero time-horizon target. However, with climate phenomena continuously adding to new, devastating events across most continents, there may be additional pressure to move the goalposts, turning faster towards Zero Near-Zero GHG (ZNZ) energy sources for ship propulsion systems. Global population growth projections being close to the “ten billion mark”, within the next fifty years [95], make the prospect of a regulatory push for quicker introduction of ZNZ fuels quite probable. In this context there is need to support the more fragile economies so that they equip themselves with the necessary infrastructure for accommodating new fuels [9].

From the perspective of shipping supply, the continued growth of vessel order books is fueling the expansion of the world fleet at about three per cent annually, as per the latest measurement [71]. However, the direction and technological feasibility of the shift to alternative fuels are far less predictable; in the case of accelerated introduction, risks may emerge, including legal liability risks, not only for fuel providers, but for shipowners/managers as well [96], areas where future research will need to focus. The impact of green corridors [97] as defined in [98], i.e., a major shipping route where net-zero fuel solutions are provided, and of regional initiatives which have a role to play [99] in supporting multi-fuel supply chains must also be considered in estimates of alternative fuel prospects and when proceeding to rankings where this factor should be also addressed.

Within this perspective, future research should emphasize more on the analysis of regional versus global supply-chain analysis, proceeding further to a dynamic modeling of fuel transitions while also considering the potentially significant role of digitalization in multi-fuel logistics and, further, of AI based tools to evolve in this direction; data-driven machine learning techniques for fuel economy prediction are already a path undertaken by recent research [100].

Future policy interventions must take place within workable and efficient frameworks. The SMART-Specific, Measurable, Attainable, Relevant, and Time-bound framework [101] is one suggestion in this respect with emphasis on the timeframe implied by the last term in the acronym, which has been a suitable framework in this respect in the case of other nodes of maritime transport such as ports [102].

Overall, the next wave of policy measures should, at long-last, focus more on a holistic Life Cycle Assessment, and thus on conservation, at least equally to the currently prevailing focus on innovation. An extension of current research on the cost of alternative fuels for specific types of vessels [81,103,104] to include more ship categories, and the combination of such research with readiness KPIs would provide valuable guiding for operators but also for shipyards, for engine makers and for policy makers as the cost element is critical when proceeding to fuel choice.

Overall, if chances to advance mild, and not costly long-term alternatives, such as biofuels [105] which do not require massive replacement of vessel hardware, are missed, then the environmental impact of forced replacement of engines or entire vessels may prove hard to balance. A biofuel allowed-use quota for existing, older, but still sustainable if operating on alternative fuel vessels, may be seen as an incentive to avoid their premature replacement which would be a negative outcome from an LCA vantage point. Fuel shifts without a specific time-path of transition may also put infrastructure of terminal points under more pressure. Additional training for personnel onboard and on shore, including ports, is also emerging as a priority.

Research resources should urgently be dedicated to such areas of concern, especially if a net zero shipping fuel future—and the constraints that come with it—may come sooner than we currently think.