It is most valuable when integrated from the design stage. Tank spaces, safety zones and control logic must be cleanly incorporated.
Built-in reserves, pre-planned interfaces or modular energy architecture. Not maximum openness, but targeted preparation.
Every form of flexibility costs space, weight and CAPEX. It must be weighed against the probable benefit.
Evaluate future pathways realistically, clarify conversion depth and factor in residual value.
Fuel flexibility does not begin with the engine but with the vessel’s general arrangement. The most fundamental decision is the placement and sizing of fuel tanks. Conventional heavy fuel oil bunkers are typically located in the double bottom or side tanks. Alternative fuels – whether methanol, LNG or, in future, ammonia – have different requirements for tank geometry, insulation and safety distances.
A flexible design must account for these differences from the outset. In concrete terms: spaces are dimensioned to accommodate various tank types. Bulkhead penetrations are positioned at locations usable for methanol lines, LNG cryogenic piping or future ammonia installations. The basic structure must be capable of absorbing the additional loads of the heavier or larger tank systems of alternative fuels.
At the engine level, dual-fuel concepts offer a natural starting point for flexibility. A MAN ME-LGIM can switch between methanol and conventional fuel. A Wärtsilä 32M can do the same. But true fuel flexibility goes beyond the dual-fuel approach: it asks whether the engine or engine concept could be converted to a third fuel in 10 or 15 years that is not yet commercially available today.
Control technology is a key element here. Modern engine controls operate on a software basis, which theoretically enables adaptation to new fuels through software updates. In practice, however, this is limited: different fuels require different injection parameters, cylinder pressure profiles and safety monitoring. A control system optimised for methanol cannot simply be switched to ammonia by update – the physical differences are too great.
A further technical aspect is energy efficiency. Every form of flexibility involves compromises. A tank suitable for both LNG and methanol is optimal for neither fuel. An engine with maximum fuel flexibility may not achieve the peak efficiency of a dedicated design. The art lies in identifying the range of sensible compromises – flexibility where it offers the greatest strategic value, and specialisation where efficiency is critical.
Auxiliary engines and generators deserve particular attention. Whilst the main discussion frequently focuses on the main engine, converting auxiliary diesels to alternative fuels can reduce a significant portion of total emissions. Methanol gensets already exist across various power classes and offer an entry point for fuel flexibility that is less complex than converting the main propulsion.
A flexible vessel design costs more than a conventional one. The question is how much more – and whether these additional costs amortise over the vessel’s lifetime. Experience from current newbuild projects shows premiums of 5 to 15% on the newbuild price, depending on the scope of built-in flexibility.
A pure space reservation concept – keeping spaces free, preparing structural reinforcements but installing no hardware – sits at the lower end of this range. A complete dual-fuel-plus-ready concept – dual-fuel engine installed, third fuel pathway prepared – sits at the upper end or above.
For yard planning, fuel flexibility means additional complexity in the design process. Each flexibility option must be verified not only technically but also regulatorily. Classification societies require separate evidence for each possible fuel pathway – which extends the approval process.
A practical tip from project work: the most valuable flexibility arises when the shipowner sets clear priorities already at the design stage. Not “keep everything open” but rather “primarily methanol, secondary preparation for ammonia” or “primarily LNG, with methanol retrofit option”. This prioritisation reduces costs whilst simultaneously improving the quality of preparation.
The sensible depth of flexibility varies strongly by vessel segment. Container ships with long deployment periods and high fuel consumption benefit most from dual-fuel concepts, because fuel costs represent a substantial proportion of operating expenses. Ferries and RoPax vessels with fixed routes can rely on the specific bunkering infrastructure of their operating area and need less broad flexibility.
Offshore supply vessels (PSVs) and tugs frequently operate in regions with good electrical infrastructure and can therefore benefit from hybrid solutions with batteries – a form of flexibility that focuses less on fuel and more on energy source. Bulk carriers and tankers with irregular routes face the greatest challenge because they do not know where they will be bunkering in five years.
When deciding on the degree of fuel flexibility, operators should answer these questions:
Which fuel is the most probable primary pathway? Flexibility should be built around this pathway, not as equal openness to all options.
Which retrofits are the most expensive to avoid? Structural works (tanks, bulkheads, foundations) are the most complex to undertake retrospectively. These should be prepared as a priority.
How long is the planned service life? The longer the vessel is intended to operate, the more valuable flexibility becomes – because the fuel landscape will change significantly over 25 years.
What does the charter market say? Charterers with decarbonisation targets are already paying premiums for flexible tonnage. This market factor can alter the business case.
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