Hydrogen Powered Propulsion for an Offshore Crane Vessel

A technical, environmental and economical evaluation from a shipowner’s perspective

Summary - This thesis performs a technical, economical and environmental feasibility study of dense hydrogen carriers as a fuel to power the largest semi-submersible offshore crane vessel in the world – Heerema’s Sleipnir. Three options are identified as feasible hydrogen carriers: liquid hydrogen (1), ammonia (2) and methanol (3). Given the boundary conditions of this thesis, an ammonia-fueled drive train with hydrogen co-combustion is the most preferred method to power the Sleipnir. This drive train is the most cost-effective solution and poses the least amount of technical challenges.

The below gallery is a graphic overview of the main results of the study. The subsequent paragraphs of this blog highlight the findings of the study in more detail, including elaboration on the drive trains, the main findings in terms of economics, conclusions to the research questions and finally the main assumptions used. Before we can elaborate however, we need to answer one question first. Why bother with hydrogen in the first place?

The maritime industry is facing a great challenge

Ship emissions need to be reduced significantly. The International Maritime Organization (IMO) is striving for at least 50% reduction of greenhouse gasses by 2050. For some, this is not enough. The US is now calling for shipping to have zero emissions by 2050. Such a call might be supported by many (Western) countries and organizations, but is unpopular for many other members of the IMO. Nonetheless - whether it is a 50% or a 100% reduction - the challenge remains almost daunting and will completely transform the maritime industry. One prominent player in the industry is paving the way in this transformation - Heerema.

> Why Heerema is facing the challenge head on

Heerema is a world-leading marine contractor that operates in the oil, gas and renewable energy industry. They own a fleet of offshore construction vessels, including the worlds largest and second-largest semi-submersible crane vessel Sleipnir and Thialf. In 2020, Heerema implemented a Carbon Neutral Roadmap, which made them the first carbon neutral marine contractor in the world. That is mostly achieved by means of purchasing carbon credits at the moment, but they aim to do more.

The Carbon Neutral Roadmap has been put in place to fully prevent, reduce and if needed compensate all CO2 emissions by 2025. A wide range of projects is currently being executed to decrease Heerema’s carbon footprint. A prime example of this is that Sleipnir is mainly sailing on LNG instead of conventional MGO. LNG has a low sulphur content, better hydrogen-to-carbon ratio and emits less NOx during combustion. Despite it being considered a cleaner fuel, it is still a fossil fuel however. That is why Heerema is looking into alternative fuels as well, one of them being hydrogen.

> Why hydrogen can help Heerema (and others)

Hydrogen is considered by many in the industry as a clean alternative to existing marine fuels. But how many shipowners have actually performed a techno-economic analyses for hydrogen of their vessels? Heerema is paving the way and has taken the largest crane vessel in the world - Sleipnir - as a case study. The study performed by Gilles Hagen aims to determine the most preferred way to drive a semi-submersible offshore crane vessel with hydrogen from a shipowner’s perspective.

It is not only Heerema who acknowledges that hydrogen might play a big roll in the future for powering their vessel. Heerema’s customers are also becoming increasingly motivated to do so. Companies such as Ørsted and Equinor have set ambitious targets to be CO2 neutral by 2040 and 2050. Within this ambition, they are demanding the companies within their supply chains to set the same targets. This creates a commercial incentive to look for alternative propulsion systems with renewable fuels. Hydrogen sounds promising in this regard, but poses serious technical, environmental and commercial challenges.

> Why Heerema looks into liquid hydrogen, ammonia and methanol

85% of all the hydrogen in the world is produced and consumed on-site, which is why long-term storage was never a big focus on innovation. This becomes an issue when it is applied to ships, as it would need to be stored for weeks or months on end. Because hydrogen gas is the lightest gas in existence, this raises some serious technical challenges.

If it were to be stored under ambient pressure, the fuel tank would need to be approximately 200 times larger to store the same amount of energy compared to LNG. Obviously, this is not possible on a vessel as space is a limiting factor. As a result, the storing conditions of hydrogen need to be changed. That is why the research performed by Gilles Hagen is looking into other methods of storing hydrogen. Grossly speaking there are two ways of doing so.

The first is to compress or cool the hydrogen. For example, hydrogen can be stored under a pressure of 700 bars, which is done for hydrogen automobiles. Or hydrogen can be cooled to -253 °C, at which point it becomes liquid. The hydrogen can then be led through a fuel cell to generate electricity. More information about the properties of hydrogen and fuel cells can be found on the insights section of Mr. Sustainability.

The second method is to chemically bind the hydrogen to a so called carrier. Which carrier would be best suited to chemically bind hydrogen is a much debated topic. For the maritime industry however, ammonia and methanol seem to be favoured as these are already widely available and used in many other chemical process. The hydrogen carrier can then be combusted in ‘regular’ engines, or in some cases used in fuel cells.

For this research, both these options for a drive train are investigated and their potential is applied to the largest crane vessel in the world, Sleipnir.

Technical Evaluation - Drive Train Configurations

The study aims to determine which hydrogen powered propulsion for an offshore crane vessel is most suitable from a shipowner’s perspective. This assumption automatically limits the amount of drive trains considered, as some are deemed undesirable from an operational (shipowner’s) perspective. Furthermore, the drive trains are all designed to provide so called hotel load for the vessel only. For Sleipnir, it is assumed that 6.5MW has to be continuously provided by each drive train. More assumptions on the drive trains, hydrogen carriers and others can be found at the end of this blog.

Given these assumptions, three distinct drive trains are considered and evaluated on technical feasibility. These are the liquid hydrogen fuel cell drive train - option 1 - the ammonia co-combustion drive train - option 2 - and the methanol combustion drive train - option 3. All options are considered technically feasible, but require retrofitting the storage tanks and piping, overhauling engines and installation of new equipment. This includes but is not limited to reformers, crackers, vaporizers and battery systems.

In terms of storage required, liquid hydrogen requires the most volume at 10.600 m3. Ammonia and methanol require 5.260 m3 and 4.100 m3 respectively. All these options require storage volumes on top of existing LNG and MGO storage. Storage space requirements (for liquid hydrogen in particular) thus impose serious operational limitations and is considered as the largest technical challenge that is to be resolved. More details on the different drive trains options are provided below.

> Liquid Hydrogen

Option 1 is a drive-train that powers the vessel using liquid hydrogen in an proton exchange mem-brane fuel cell. This drive-train design is mostly dominated by the regulations for gas fuelled ship installations and fuel cell installations. The proposed overall system has a theoretical on-board efficiency of 44%. As with all the other options, it is only able to provide hotel-load. The volumetric energy density of liquid hydrogen is more than twice as small as that of LNG, which will result in a fuel tank approximately twice as large to store the same amount of energy. This configuration consumes 7.300 m3 of hydrogen per fuelling cycle, but up to 30% of the hydrogen needs to remain in the fuel tank, resulting in a total tank capacity of 10.600 m3. At the time of writing, NASA has the largest LH2 storage tank in the world with a volume of 3.800m3. Additionally the hydrogen needs to be of very high grade purity. ISO 14687-2 specifies that hydrogen for fuel cells is to be 99.97% pure, which is almost ten times more than needed for hydrogen combustion (99.9%). Hydrogen requires at least 20% more energy to produce compared to LNG, ammonia and methanol due to the energy required for liquefaction.

Storage - The total required storage volume is approximately 10.600 m3 to ensure 7 weeks of operation.

Drive train details - The PemGen MT-FCPP-500 from Nedstack is chosen as modular fuel cell, of which a total of 60 are installed. Before liquid hydrogen enters the fuel cell however, it needs to undergo a regasification process. This process is comparable to that of LNG. Also similar to LNG is the dealing with boil-off gas from the liquid hydrogen. This gas is used to power the system in order to minimize losses. The system requires a set of converters, inverters and batteries to regulate electric power coming from the fuel cell.

Advantages

High efficiency - Above 20% load, fuel cells have a virtually equal energy efficiency. What this means is that even when the fuel cells are subjected to a low load - which is not expected - they provide a high conversion rate of energy.

Technically similar - Storage of liquid hydrogen bears many similarities to LNG, with similar sized tanks.

No emissions - Does not emit any tank-to-propeller greenhouse gasses.

Disadvantages

H2 Purity - The fuel cell requires high grade purity hydrogen, according to ISO 14687-2 a hydrogen purity of 99.97% is required.

Storage - Liquid hydrogen is the most challenging fuel to store, mainly due to its extremely low storing temperature. Because of the increased insulation standards, the storage tank is expected to have a 20% volume increase compared to the required fuel volume. A minimum of 10-30% of hydrogen needs to remain in the tank, similar to LNG. As this configuration consumes 7.300 m3 of hydrogen per fueling cycle, it will result in a total tank volume of roughly 10.600 m3. The size of the liquid hydrogen tanks is so such that it becomes almost undesirable to implement on the vessel.

> Ammonia

Option 2 is a drive-train that powers the vessel using ammonia in a spark-ignite dual fuel internal combustion engine. This drive-train design is mostly dominated by the regulations for gas fuelled ship installations. The overall system has an on-board efficiency of 43%. The energy density of ammonia and methanol are somewhat similar to LNG and therefore no major differences in tank volume are expected.

Storage - A total of 3.421 ton of ammonia needs to be stored in order to comply with the operational profile requirements. This leads to a total volume of approximately 5.260 m3 required including the space needed for insulation systems and leftover ammonia. Following the DNV-GL rules for classification, the ammonia will be stored in five 1.100m3 tanks under a pressure of 17 bar, each able to store approximately 750 tons of liquid ammonia. The DNV-GL rules for classification also restrict the locations of the cargo and service tanks. The service tank must be positioned on deck and have a venting system installed so direct venting of fuel gasses can be executed in case of emergency’s. The cargo tank can be installed below deck but must be positioned at least 760 mm from the hull for inspection access.

Drive train details - Ammonia is first vaporized and heated, after which 70%-volume is inserted into the engine and 30%-volume goes to the ammonia cracker for hydrogen reforming. The hydrogen is required for co-combustion, due to the poor combustion properties of ammonia. The 70%-volume ammonia and 30%-volume hydrogen mix results in a 96%-weight ammonia and 4%-weight hydrogen mix. Because the ammonia is mixed with the resulting hydrogen, a full hydrogen yield from the reformer is not required.

Exhaust - To comply with the IMO TIER III regulation, a Selective Catalytic Reduction (SRC) system needs to be installed located at the exhaust pipeline. This SCR system operates in the same way as the currently installed system on-board of the Sleipnir.

Advantages

No CO2 - Does not emit any CO2 but will require a Selective Catalytic Reduction system to minimize NOx emissions.

Storage - The energy density of ammonia and methanol are somewhat similar to LNG and therefore no major differences in tank volume are expected. It can be liquefied by storing it at -34 °C or at a pressure of 10 bar. Liquid ammonia has a volumetric energy density of 15.6 MJ/l.

Disadvantages

Fuel - Ammonia has a high resistance to auto-ignition and very slow combustion kinetics (also called low flame speed). These characteristics make the combustion of ammonia in an compression ignition engine challenging. That is why ammonia needs a fuel for co-combustion. For this study, hydrogen was chosen as co-combustion fuel.

Storage - Similar to LNG, all piping should be double walled and the complete fuel supply system has to be entirely separate from all other piping systems. All piping below deck must not only be double walled but also ventilated. This imposes serious technical demands when refitting existing vessels.

Exhaust - Internal combustion engines running on ammonia emit unburned ammonia in their exhaust gas, which necessitates the need for an SCR to comply with IMO TIER III regulations.

> Methanol

Option 3 is a drive-train that powers the vessel using methanol in a compression-ignite dual fuel internal combustion engine. This drive-train design is mostly dominated by the IMO regulations for low-flashpoint liquid fuels. The overall system has an on-board efficiency of 43%. The energy density of ammonia and methanol are somewhat similar to LNG and therefore no major differences in tank volume are expected. Methanol is easiest to store and does not require a large amount of cooling or expensive insulation and equipment.

Storage - A total of 4.100 m3 of liquid methanol under ambient pressure needs to be stored in order to comply with the operational profile requirements. As with ammonia, the DNV-GL rules for classification restrict the locations of the cargo and service tanks. The service tank must be positioned on deck and have a venting system installed so direct venting of fuel gasses can be executed in case of emergency’s. The cargo tank can be installed below deck but must be positioned at least 760 mm from the hull for inspection access.

Drive train details - Methanol is first vaporized and heated, after which 90%-volume is inserted into the engine and 10%-volume goes to the methanol reformer for hydrogen reforming. This hydrogen is used as pilot fuel, resulting in a 99%-weight methanol and 1%-weight hydrogen mix. Because the ammonia is mixed with the resulting hydrogen, a full hydrogen yield from the reformer is not required.

Exhaust - Upon combustion of methanol, CO2 is released. In order to remain carbon neutral, a feedstock for the methanol is to be chosen that ensures total carbon footprint of well-to-propeller emissions are zero.

Advantages

Fuel - Methanol presents excellent combustion properties: despite its energy density is less then half of the LNG energy density (20.1 MJ/kg for methanol, 50 MJ/kg for LNG) it has a comparable octane number LNG (108 for methanol, 130 for LNG). This allows for a higher compression ratio and an increase in combustion efficiency. The performance of a engine running on methanol is directly comparably to the engines running on LNG.

Storage - Methanol is liquid at ambient temperatures and has the largest volumetric energy density of the three options, making the storage installation for methanol easier compared to liquid hydrogen and ammonia. There are no limitations regarding the storage capacity of methanol tanks since the fuel is a liquid at ambient temperatures.

Disadvantages

Fuel - Methanol has a low flash point and needs a pilot fuel to operate.

CO2 emissions - Because methanol is made from CO2 and hydrogen, methanol combustion will always emit CO2. Since one of the boundary conditions of the new drive-train is that system should have net zero well-to-propeller emissions in its life-cycle, the methanol combustion engine can not use diesel as a pilot fuel.

Storage & corrosion - Methanol has corrosive properties which need to be mitigated by storing it in stainless- or carbon steel tanks. The low flashpoint also imposes other restrictions on the tanks. Similar to LNG, these are mostly focused on the extra venting- and gas detection systems to prevent gas build up in the tanks. All piping should be double walled and that the complete fuel supply system has to be entirely separate from all other piping systems. All piping below deck must not only be double walled but also ventilated. All pre-systems should be placed on the deck whenever possible and should not be positioned in the engine room.

Economic Evaluation

> What is the most cost effective configuration?

The ammonia fuelled drive-train is the cheapest configuration with a Levelized Cost of Energy (LCOE) of 0.154 €/kWh. Liquid hydrogen has a levelized cost of 0.195 €/kwh and methanol is the most expensive option at 0.209 €/kwh. Naturally, these levelized costs are subjected to assumptions that may significantly influence their outcome.

Fuel costs (OPEX) have by far the biggest influence on costs, between 71%-92% of total lifecycle costs (operating lifetime of 25 years). Ammonia and methanol are two of the most widely produced chemicals in the world, so the production costs of these fuels are expected to decrease little over time. Based on the price developments found in the literature, it can be expected that liquid hydrogen will eventually become cheaper than ammonia. This will happen when the production process is scaled up and economy of scale applies to liquid hydrogen.

It is expected a CO2 tax will make the business case for hydrogen carriers more attractive, which can be expected as soon as 2023 in the EU. A CO2 tax of €100 per mT is required to level the price of liquid hydrogen with current LNG on Sleipnir. It is expected liquid hydrogen will become cheaper than ammonia in the future, but this tipping point will only occur in 20 years time. With a CO2 tax of 75 €/ton, a drive-train running on LH2 will have the same LCOE as the current system. Methanol will become cheaper than the benchmark price at an CO2 tax of 150 €/ton, although it is unknown what a CO2 tax means for methanol considering a methanol-based combustion system still emits CO2.