EEXI requirements coming into force by the end of 2022 can be met by converting to LNG, with container ships being prime candidates
Upcoming IMO mandates concerning the efficiency of existing ships is driving a surge in LNG retrofits. Subject to adoption in June 2021, the Energy Efficiency Design Index (EEXI) will be applicable for all vessels above 400 gt and will enter into force in 2023. While LNG retrofitting can potentially apply to all vessels, container ships seem to be more suitable candidates for LNG retrofitting to meet EEXI mandates. As these ships have higher fuel consumption overall, they are also obliged to cut emissions more. Additionally, container vessels represent one-third of the global merchant fleet, making emission reductions from the sector crucial to shipping’s overall carbon footprint.
To leverage the market, many firms in the LNG newbuild sphere are offering LNG retrofit solutions. While the equipment may be similar, project management is often different from that of a newbuild. Much engineering work needs to be done before the vessel is pulled out of service. Many of the details can only be finalised after accessing the vessel, adding to the pressure on schedules. Further, tight schedules demand pre-fabrication and skid mounts, thereby affecting technology choices.
For a boxship of 200,000 dwt, the reduction has to be 50% with reference to the EEDI baseline, for 120,000 to 200,000 dwt it is 45%, for 80,000 to 120,000 dwt, 40%, and for 40,000 to 80,000 dwt, 35%, according to a study by C-LNG Solutions. “In our opinion, EEXI is the direct driving force because it is a measurable indicator,” says C-LNG Solutions researcher Fan Hongjun.
A dual-fuel retrofit project on main engines can provide between 20-25 % lower EEXI, according to MAN Energy Systems. MAN’s head of projects and PVU sales Klaus Rasmussen says the payback for LNG retrofits at pre-Covid oil and gas prices is five years.
LNG retrofitting may well be among the most effective ways to achieve EEXI targets, says Mr Rasmussen. “De-rating is very efficient to meet new EEXI targets – however, as many vessels today already slow steam, de-rating will not necessary affect the real fuel consumption in tonnes,” he adds.
The Carbon Intensity Indicator (CII) is a measure that will be reported on a yearly basis based on total fuel used in tonnes divided by total distance sailed in nautical miles (nm), and a dual-fuel conversion will provide lower fuel consumption in tonnes due to the energy content in gas versus oil. Therefore, vessels that go for dual-fuel conversion can expect a 10 – 15 % lower yearly fuel consumption in tonnes, which is positive for CII index, says Mr Rasmussen.
MAN has introduced a full retrofit solution that includes not just the engine conversion to dual-fuel, but the supply of a fuel gas supply system, bunkering skids, besides EPC, shipping, installation, project management and so on. The MAN solution features two bi-lobe Type C tanks with 3,500 m3 volume and four deep-well pumps (two per tank). The boil off gas (BOG) is estimated to be 400 kg/hour.
Each tank connection space contains two tank domes, access to LNG pumps, tank safety bunkering and vapor connections besides tank instruments for temperature, pressure and level. The fuel preparation room contains the high-pressure pump and vapouriser (PVU) unit, BOG compressor including heater and cooler, low-pressure vapouriser for the dual-fuel auxiliary boiler, and an optional gas valve train (GVT).
All the systems are proven, approved designs. Pre-fab equipment and skid mounts ensure shorter turnarounds. The total project schedule is 15 months, from contract signature to everything delivered. Commissioning and sea trials will take 20 days.
Choosing the tank type
While LNG retrofits, including that of the 2014-built, 15,000-TEU ultra-large container ship (ULCS) Brussels Express (ex Sajir) have featured membrane tanks, several players seem to be favouring Type C tanks. “Type C tanks have high pressure-bearing capacity, and their own pressure-bearing capacity can comply with the requirement of the 15 days BOG holding time by the IMO IGF Code. Higher the design pressure, higher the cost,” says Mr Hongjun. He adds that if lower pressures are used, then the C-LNG Solutions system uses a BOG recondenser.
Mr Hongjun says some 23% of small LNG carriers had Type C tanks in 2000; that has now climbed to 67%. “There is a trend to move towards Type C tanks in small LNG carriers,” he says, “That decision can provide references when it comes to LNG-fuelled ships.”.
Further, pressure vessels involve fewer patented technologies and patent fees are less or none at all, he says. The Type C tank can be completely pre-fabricated outside the vessel. Though the Type C adds weight, it will not significantly affect the light weight of a large-scale LNG-fueled ship, says Mr Hongjun.
The MAN system adopts pressure accumulation in Type C tanks as the main method of BOG management and, if possible, thermal oxidation of vapours in the gas combustion unit (GCU)/dual-fuel boiler as backup. A Type C tank with 4 bar design pressure can keep control of gas pressure for a long time, more than the 15 days specified in the IGF code. BOG from tanks will be consumed in the main engine while in operation.
Membrane tanks have design pressures of 0.7 bars, but make the optimum use of space. Gloryholder LGM Europe managing director Jonas Bergström calls them atmospheric tanks. He adds, if membrane tanks can provide design pressures of 2 bars then it will facilitate BOG management. If the bunker tank requirement is 10,000 m3 or 15,000 m3 then it makes sense to look at membrane tanks, otherwise Type C tanks are best suited, he says.
LGM Engineering received approval in principle (AiP) from ABS for its multi-body, stacked ‘LGM–MMC’ tank last year. The tank is suitable for installation in higher, longer and narrower hold spaces and could also be used on ore carriers and other vessel types. The number of tank bodies incorporated into the design can be varied to meet specific space requirements.
The new tank has been under development since 2018 by China-based Gloryholder Liquefied Gas Machinery (LGM), which specialises in the design and turn-key delivery of marine LNG fuel gas supply systems and cargo-handling systems for gas carriers and LNG bunkering vessels.
The new LGM-MMC tank will be made from the same materials as conventional Type C tanks and will not require special consideration to achieve the same level of safety in that regard, says Mr Bergström. The design achieves a space utilisation of over 79%, which is more than conventional Type C tanks, he adds. As only one set of equipment and instruments is required, the new design also saves money, he says.
The AiP is a preliminary review, and the load will be different depending on ship size and installation location; strength calculations, including yielding, buckling and fatigue analysis, will be done based on the details of specific projects.
Mr Rasmussen says the ability to pre-fab Type C tanks and shorten project schedules were key factors in choosing them for the MAN systems.
Retrofitting the engine means rebuilding the combustion chambers so they match an already existing engine design approved by class. “This means ensuring that all existing technical files, including NOx and other emission values developed on testbed engines, can be applied to the retrofitted engine. The retrofitted engine will match an already existing DF-fuelled engine,” says Mr Rasmussen.
In the MAN project, the engine retrofitting cost occupies some 25% of the entire project cost. Container ship engines are typically bigger and weigh more and are therefore more expensive to retrofit.
For a retrofit project, bidding and preparation time are relatively short. Many details are determined on board the vessel, or in close interaction with the shipowner or operator. The skid-mounted design can save time for the process and facilitate quality control. Parallel fabrication is possible, installation on board is convenient, and skid-based product approval by classification societies is also possible.
Yard competence is another factor in LNG retrofitting. The difference between newbuilding yards and repair yards is most often their competences in engineering and design work and also their ability to set up processes that support lower costs during repeat projects. Repair yards may have limited engineering and design work competences, which is why it is important that naval architects are hired in to develop the detailed design works, enabling a cost effective dual-fuel retrofit project, says Mr Rasmussen. “Piping, cabling engineering, together with the engineering tasks connected to the LNG bunker tank, are the main focus areas,” he says.
In C-LNG Solutions’ view, the shipyard should have at least the following capabilities: deep understanding of the IMO IGF code; familiarity with relevant guidelines issued by ISO, SGMF and other organisations; and experience in building gas ships.