How to (possibly) create the largest zero-emission crane vessel in the world

Summary - This blog sketches a vision on how to convert the largest crane vessel in the world - Sleipnir - owned by Heerema Marine Contractors, to a zero-emission vessel. Several promising carbon reduction measures are combined which are technically viable and based on matured technology, although scaling of existing technologies and cooperation with key partners is required. Key technologies include electrification, on-board batteries, solar panels, synthetic fuels, carbon capture and storage and possibly hydrogen.

Disclaimer - This blog is not a step-by-step engineering guideline on how to convert Sleipnir into a zero-emission vessel. It is a potential vision to inspire a future for Sleipnir, where it will operate with virtually zero emissions. It originates from Heerema’s ambition, who recently announced they are the first marine contractor to go carbon neutral. All data concerning Sleipnir is either taken from public sources such as the Heerema website, or is ‘generalized’ to avoid potential commercial conflict or IP infringement. It should be noted that all measures proposed are under investigation; no guarantees on the actual performance of these technologies can be given at the time of writing.

"What, sir, would you make a ship sail against the wind and currents by lighting a bonfire under her deck? I pray you, excuse me, I have not the time to listen to such nonsense."

This was the reaction from Napoleon Bonaparte when he was told of Robert Fulton’s steamboat, signaling the age of the steam engine in the early 1800s. The world of shipping was soon to transition from sail to steam, but this was not recognized by even the greatest men of this era.

I believe that we are at a transition point in history with regards to ship propulsion. Once again, we will (and we must) transition from fossil fuels to into a zero-emission world. All the technology that is required exists and is matured. Scaling of technology for marine purposes, in close cooperation with key partners, is required to ensure economic viability.

The exact design and economic viability of each vessel will differ, as each vessel is unique. The resulting options might be confusing for the average shipowner, and the possibilities posed in this blog might not be suitable to many other vessels.

Nonetheless the author chooses to elaborate on the vessel it is familiar with. This is Sleipnir, the flagship of Heerema Marine Contractor’s fleet and the largest crane vessel in the world. It is believed that it is technically feasible, economically viable and desirable to convert Sleipnir into a ‘zero-emission construction vessel’.

In the subsequent chapters, the current technical and operational framework of Sleipnir will be elaborated upon, after which a detailed case is made consisting of five elements. The case argues to convert Sleipnir into a ‘Solar-Synthetic’ vessel, combined with Carbon Capture and Storage (CCS) for remaining emissions.

But first, why should we convert to zero-emission shipping in the first place?

Why Do We Need Zero-Emission Vessels?

The issue is that, as most readers are aware, the carbon clock is ticking. Fast. There are only about seven years remaining to stay below the 1.5°C stated in the Paris Agreement, according to the IPCC Special Report on Global Warming and Mercator Research Institute. Zero-emission shipping is sustain needed to sustain our planet as well as our economy, and time is of the essence.

But it is not just about carbon emissions…

According to most estimates however, international shipping worldwide is responsible for ‘only’ 2-3% of all global greenhouse gas emissions. The International Maritime Association has recently stated that maritime emissions could rise 50% by 2050, raising it to 3-5%. Such a small number might lead you to think: why the urgency? Why push for zero-emission vessels, if the marine industry is only a small part of the total problem?

That is because the fuel generally used by the marine industry is the most toxic substance imaginable: Heavy Fuel Oil, commonly referred to as HFO or IFO. It is a category of fuel oils with a tar-like consistency identified as a "worse case substance". It is essentially all the leftovers of the fossil fuel and chemical industry combined, which is then joyfully incinerated in engines on-board our vessels, after which the toxic fumes are inhaled by not just the vessel crew but everyone living within 400 kilometers of the sea. The resulting emissions consist of, among others, CO2, NOx (nitrogen oxides), SOx (sulphur oxides) and Particulate Matter (PM or smog). These emissions are a bigger problem than you might think.

According to the World Health Organization, more than 200,000 deaths in Europe every year can be attributed to particulate matter alone. Nitrogen dioxide was responsible for around 78,000 deaths in Europe in 2014, according to the European Environment Agency. 

Though it is hard to link the exact source of the particulate matter and NOx emissions - cars and other industries also pollute - it is a fact that outdoor air pollution is a major cause of death and disease globally. The marine industry contributes significantly to these numbers. According to some estimates, the NOx and SOx emissions from international shipping around Europe are expected to equal or even surpass the total emissions from all land-based mobile, stationary and other sources in the 25 EU member states combined by this year.

You read that correctly: the shipping industry is set to surpass all NOx and SOx of all other sources in the EU combined.

… It is also about the costs

If you are not interested in creating zero-emission vessels for the environment or your health, you could be concerned over your wallet as a shipowner. Fuel costs represent as much as 50-60% of total ship operating costs, depending on the type of ship and service. A single 28-day round trip voyage across the Pacific costs several millions of dollars for a large modern container vessel, consuming more than 200 tons of heavy fuel oil per day.

Over the entire 25-year lifetime of a vessel, it is even more astounding. Even with the costs for marine fuels at an all-time low, you would still have to pay more than $400,000,000 (almost half a billion), assuming 25% idle time of the vessel during its life. Reducing only half the amount of fuel during a vessel’s lifetime with zero-emission technology will save millions and millions of euros for the shipowner. This is one of the strongest tangible arguments for shipowners to convert to zero-emission shipping, at least partly.

The shear diversity in vessels however makes this task extremely challenging. In 2017 alone, there were about 93,161 vessels in existence. Each one of them is unique, has a different drive train configuration, crew and fuel consumption. Detailing them all would require a book, if not several, to explain and goes beyond the scope of this blog.

So let us start with the biggest one out there and work our way down - the Semi-Submersible Crane Vessel Sleipnir of Heerema Marine Contractors.

Current State - Technical and Operational Framework of Sleipnir

Heerema is a marine contractor that owns some of the largest construction vessels in the world, operating globally. The largest crane vessel is Sleipnir, which has an installed power generation capacity of 98MW. All Heerema’s vessels are diesel-electric, meaning power is provided by a diesel engine, which spins an electric generator, which in term provides electricity for the entire vessel.

It can readily be understood that the tremendous amount of power these vessels require, result in tremendous fuel consumption. Although heavy fuel oil is conventionally used in the shipping industry, Sleipnir operates on Liquefied Natural Gas (LNG) with Marine Gas Oil (MGO) as pilot fuel and as a back-up. This is already a much cleaner fuel-mix compared to heavy fuel oil in terms of CO2, NOx, SOx and particulate matter (smog).

The carbon footprint of the Sleipnir is approximately 100 metric tonnes a day, depending on the operational mode. For the sake of simplicity, three operational profiles for the Sleipnir are proposed: Low, Regular and Heavy Duty. Each mode has a corresponding power profile, energy demand and drive train configuration, of which the latter is currently LNG/Diesel-Electric for each mode. Below are shown the different operational modes’ power profiles, with an indication of the amount of kilo-Watt-hours (kWh) required to operate.

Low Mode - When the vessel is moored to the quayside for maintenance, repairs or simply waiting for the next project the begin. This is a low power mode, when there is a limited amount of activities on board.

Regular Mode - During regular operations, working on projects offshore and usually on Dynamic Positioning. The activity level on board is high, but power demand is less when compared to sailing. It is also intermittent: there are peaks when operating the thrusters for dynamic positioning or when using the cranes.

Heavy Duty Mode - When sailing to and from projects, or when there is a continuous high power demand during projects. This could be the case with heavy and long lifts, or when weather conditions are demanding.

The Million Dollar Question

You might wonder: how in God’s Earth can such a large vessel ever be made into a zero-emission vessel, without impacting its operations? Most of my peers answer that it is not technically feasible to do, at the very least not any time soon.

Stating that it is not technically feasible is in the author’s opinion an argument of convention. It is an argument based on existing convention or belief, but not grounded in- or prohibited by the laws of physics. It is perfectly feasible, even in the (very) short term, though it is perhaps not always economically viable or generally desirable to do so.

Instead of thinking in black and white terms, or wondering if it can be done ‘yes or no’, one can simply ask the following question: How can it be done? In which ways can it be done? What would we need to do in order to get it done?

Before answering these questions for a large crane vessel, let us start with two easier questions: which emission reduction technologies for the marine industry are out there? Which assumptions do we need to make for Sleipnir?

An Overview of Carbon Emission Reduction Technologies

To become truly zero-emission and not rely on carbon offsetting, one would need to adopt and scale several zero-emissions technologies. So which ones are out there? Where can we choose from? The below figure is from the literature review by Evert A. Bouman, Elizabeth Lindstad, Agathe Rialland, Anders H. Strømman (Transportation Research Part D, 2017). It provides a graphical overview using boxplots of the CO2 emission reduction potential. From the below overview, a combination of emission reduction measures can be applied to any vessel. This has been done for Sleipnir, though this ‘cafeteria model’ methodology of cherry-picking measures can be applied to any vessel.

Assumptions for Sleipnir

1. The focus is on carbon emissions. Firstly because CO2 is a ‘leading indicator’ for emissions in general. When carbon emissions are zero, other emissions are usually also zero. Secondly, the solutions that are presented all incorporate ‘clean’ fuels, such as synthetic fuels and hydrogen. These do not contain the plethora of pollutants and stray elements that fossil-fuel based hydrocarbons contain, making them cleaner by default. Less pollutants in the fuel also mean the relative carbon content increases, henceforth the focus on carbon emissions.

2. Focus is on the so-called ‘tank-to-propeller' emissions. This means that the entire supply chain behind the vessel is not taken into consideration. It is assumed that emissions made from ‘the well to the tank’ are zero.

3. The timeline to achieve a zero emission vessel is, unless stated otherwise, 2025 to 2030 at the earliest. This can only be achieved with intense cooperation with key partners, as well as scaling of existing technologies for offshore purpose.

4. The definition of ‘zero-emissions’ is to reduce emissions with at least 99.9% compared to current levels.

A Possible Future for Giant Zero-Emission Vessels

Given the current technical and operational framework of Sleipnir, as well as the current state of zero-emissions technology, there are several options in which Sleipnir can operate with zero emissions. Naturally, this depends heavily on the operational mode, as well as the assumptions made of course. Therefore it should be readily understood that the proposed measures in the subsequent paragraphs are possibilities, chosen out of many options. Even though all these measures are currently under investigation, no guarantees on the actual performance can be given at the time of writing. New insights might yield (totally) different results in the future.

Despite the uncertainties inherent to this process, it is believed that solar panels can be combined with on-board batteries for regular mode of operations. For the heavy duty mode, when sailing for example, energy would be provided by a ‘synthetic’ fuel. This is made from either fully-synthetic fuels, biofuels, hydrogen, or a combination of these. On-board Carbon Capture and Storage (CCS) will ensure any remaining emissions to the environment are virtually eliminated. During low power mode, all the energy can be provided by means of shore power. This has already been committed to by Heerema, which is why focus will be on regular and heavy duty mode. The rationale of these measures is elaborated in the subsequent sections.

Reduced Energy Demand by Full Electrification

Transforming to fully electric operations will virtually eliminate energy losses and reduce on-board power demand significantly. Currently, the base load requirement or ‘hotel-load’ for Sleipnir is 7-9 MW on average, with occasional peaks of up to 15MW or more. These peaks are of an intermittent nature and occur only briefly, usually lasting only several minutes. Generally speaking, these peaks can be covered with current-day technology batteries. Therefore one should not be concerned about the maximum power demand, but the energy required to keep the vessel running. For the low and regular power mode, this is between 170 and 230 MWh.

Now consider the fact that for any diesel-electric system, but especially for a vessel the size of Sleipnir, there are significant energy losses in the engine, drive-train and auxiliary equipment for power generation. As illustration, energy losses for vehicles in general are in the range of 75-82%. A significant portion of the power generated is dissipated into heat or used to generate the energy. This is not the case for a fully electric system, in which all you need are batteries and a means to provide electricity. All the equipment in the engine room required for power generation can essentially be shutdown. All the fuel pumps, heating and cooling systems, fuel treatment systems and more: these would not be needed.

To determine the exact power demand reduction through full electrification would require further detailed engineering. Nevertheless, given the results from literature review by Evert A. Bouman, Elizabeth Lindstad, Agathe Rialland, Anders H. Strømman, it is assumed only half the amount of energy is required with a fully electric system. This results in a daily energy requirement of 85 and 115 MWh for low and regular operational mode. An example of the daily energy demands for regular modes are shown below.

The required energy can be provided by solar cells, also called solar PV.

Solar PV for On-Board Power Generation

You might wonder how solar cells can power the largest crane vessel in the world, when sometimes it is hard to power your own house with solar cells. This can be argued because of one main reason: there are tenths of thousands of square meters of surface area on Sleipnir which can all generate power.

Imagine the entire vessel, all the sides and surface areas where possible and practical, covered in solar cells and generating energy. To get an estimate of the electricity production from solar PV on board Sleipnir, a conservative and a high estimate are made using the peak sun hour methodology. As a rule of thumb, it is assumed 10.000 square meters produce 1 mega-Watt of (peak) power. Additionally, a 50% efficiency loss over half the surface area is assumed for areas not directed towards the sun.

Conservative Estimate Solar - 1.6 MW - 6.4 MWh

Estimates based on surface area might be over-estimating significantly. We know that the surface area of Sleipnir is not always directed perfectly towards the Sun. Furthermore, the deck is subject to significant loads during the execution of projects, welding takes place on it frequently and there are people, even small cranes driving over it. Obviously not the best place for solar cells and should thus be omitted. This yields a total surface area of approximately 21,000 square meters, resulting in a peak power of 1.61 MW. Assuming an average of 4 peak sun hours, a total daily energy production of 6.4 MWh is obtained.

High Estimate Solar - 4.6 MW - 28.0 MWh

In case solar PV turns out to be more practical than expected and parts of the deck, cranes and accommodation are used, the total surface area can increase to approximately 46,000 m2. Deck usage can only be achieved when some sort of flexible system is used that can be removed for projects when required. Furthermore, it is claimed that solar PV could have a 15% higher yield offshore than traditional PV systems. When assuming a 15% higher yield, the total peak power production becomes 4.6 MW. Assuming optimal conditions during summer times with 6 peak sun hours, a total daily energy production of 28.0 MWh is obtained.

These back-of-the-hand calculation show that, although the energy output from solar PV can be significant, it is currently not sufficient to power the entire vessel all the time. The total surface area for solar PV needs to be expanded to generate enough power for the entire day (and night). This could be achieved by creating a floating solar island next to the Sleipnir of just a few hundred square meters for additional energy generation. Solar PV for marine purposes not only exists, but also it is matured and it available at scale from companies such as Wattlab. There are even several pilots ongoing regarding floating solar, although such an options remains highly theoretical for Sleipnir at the moment.

It is believed however that by 2025 or 2030, more ‘exotic’ solutions will become available. Improvements in solar paint or (flexible) solar cells are potentially so far advanced by then, that it is conceivable that virtually every surface of the vessel generates power. This would increase the total power production to levels sufficient to provide energy during the day and store excess energy for the night. The trick in the long term therefore is not to generate the power, but to store the energy when less is available, which (lithium) battery cells can provide.

On-Board Batteries for Energy Storage

A sizeable portion of the produced energy from solar cells would need to be stored to provide energy during the night and for peak demands. Assuming the current energy demand on board the vessel is approximately 115 MWh per day for regular mode, and assuming 6 sun peak hours, 86.25 MWh would have be stored on-board. This is equal to just over a thousand Tesla model 3s, which are not really suitable to be stored on deck. To keep with nautical terms, this amount of storage would be equivalent to at least eighteen 20ft containers for the lower range estimates. Such an amount can be stored on deck, where they can potentially be swapped with recharged batteries if needed. It is noted that this is the most optimal case, in all likelihood the amount of batteries (using current day technology) would need to be increased.

Current day lithium-ion batteries do have some drawbacks though, among others the safety concerns, the substantial environmental footprint of mining, plus recycling challenges. New generation lithium ion batteries will solve many of these issues, where solid-state lithium metal batteries might just be the key. Samsung recently revealed a new solid state lithium metal battery with 900 Wh/l density, an increase in energy density by about 50% compared to current batteries while drastically improving the safety of the battery. Other companies, such as Northvolt, have as mission to develop the world’s greenest battery cell and establish one of Europe’s largest battery factories. 

The biggest advantage of batteries when compared to fuels however, is that they are a technology. Their maximum energy storage is not fixed, but growing exponentially. For fuels, their chemical storage limit are chemically determined. Innovation can therefore only increase battery storage, often dramatically, as is the case with Samsung. With costs plummeting, and energy density for batteries doubling or almost tripling per decade, it is only a matter of time before batteries become the standard energy storage method.

Find more interesting battery news in Tesla’s biggest announcements on battery day.

Despite the amazing developments in battery technology and solar PV however, the amount of generated electricity provided by solar energy will not suffice to provide all the power all the time. When energy demand is high for longer periods, for example during sailing or in case of heavy weather, it can be argued the amount of required energy remains in the same order of magnitude, i.e. 800 MWh.

This begs the question: given the current time-frame, what kind of zero-emission technology is available at the scale that we need it? It is believed the answer is a mix of either synthetic- and/or biofuels, combined with hydrogen combustion.

Combine Synthetic and Biofuels…

Synthetic fuel can be both liquid or gaseous and is obtained from syngas, a mixture of carbon monoxide and hydrogen. The syngas is derived from gasification of solid feedstocks such as coal, biomass or by reforming of natural gas. Common ways for refining synthetic fuels include the Fischer–Tropsch conversion. From a chemical standpoint, this process ensures both ‘synthetic’ and ‘biofuels’ are the same product (for example Gas-to-Liquids or GTL and Hydrotreated Vegetable Oil or HVO). These fuels, or a mix of these, are both referred to as ‘synfuel’ for this blog. The only requirement for these types of fuel would be that the carbon feedstock is a circular carbon, from certified feedstocks labelled as waste, residue or fully synthetic altogether. The latter case - a fully synthetic fuel - is exemplified by Heliogen, a company which aims to produce fuel directly from water and air.

For any fuel however, the requirement is that there should be no land-use issues and no competition with food production or deforestation.

What would be the reason to use a synfuel for combustion, and not straight hydrogen then? One of the biggest advantages of circular synfuels is the fact that they make use of existing infrastructure and supply chain. No adjustments or retrofitting to engines or equipment is required. It appeals to current fleet personnel with decades of experience, as no extensive (re)training is required to handle the fuel as opposed to full hydrogen systems. Additionally, GTL and HVO can be readily mixed, both with one another and conventional marine gas oil. It will therefore not interfere with current operations and can be added to the existing fuel-mix once it become more readily available, proportionally reducing carbon emissions.

… Mix it up with Hydrogen …

Nonetheless, even with a circular synfuel where the tank-to-propeller emissions would be zero, there would still be emissions associated at the exhaust pipe. To reduce the emissions even further, synfuel can be mixed with hydrogen for combustion in diesel engines. A mix of up to 30% synfuel and 70% hydrogen can be injected in existing diesel engines, which reduces the emissions by about 70% as well.

The main reason for using hydrogen combustion as opposed to hydrogen fuel cells, is the fact that one can use ‘dirty’ hydrogen in diesel generators. Dirty hydrogen is still 99.9% pure hydrogen, but still almost ten times less refined compared to high-grade hydrogen for fuel cells. ISO 14687-2 specifies that hydrogen for fuel cells is to be 99.97% pure. Additionally, conventional diesel engines have no issue with common pollutants that are detrimental to hydrogen fuel cells. In other words, there is no need for added fuel cells or high grade (and expensive) hydrogen systems or retrofitting.

Hydrogen is accompanied by challenges on its own however. One risk for existing equipment would be hydrogen embrittlement, of which the impact is as of yet unknown. This is why the exact synfuel-hydrogen mix ratio cannot be stated with confidence. Another, less tangible risk is the fact that the knowledge on hydrogen (co-)combustion in larger diesel engines is limited.

One of the leading companies in this field is CMB, who have been developing hydrogen combustion engines and systems for more than 10 years. Heerema is currently conducting research on the use of hydrogen combustion on smaller diesel generators. Though detailed engineering is still to be done, at the time of writing it is believed that converting the existing generators of Sleipnir should be technically feasible. Whether it is also economically viable or even desirable from a safety standpoint remains to be seen.

Nonetheless, even in the best case with a mix of 70% hydrogen and 30% synfuel, there would still be emissions remaining at the exhaust coming from the synfuel that is left. To become a truly zero-emission vessel, any carbon emissions coming from the exhaust would have to be captured and stored on-board.

… Top it off with On-Board Carbon Capture and Storage

That is why Heerema is part of the DerisCO2 consortium, investigating the technical feasibility of capturing CO2 directly on-board LNG-powered vessels. The DerisCO2 consortium’s goal is to prepare this technology for a pilot on-board a large vessel within several years. It is based on existing, matured land-based technology used for carbon capture and storage in power plants. Waste management company AVR has constructed a large-scale CO2 capture system at one of their locations in Duiven. The CO2 is used for the growth of crops, although the captured CO2 can be used for many purposes.

One of them is to re-use it to create synfuel. This could drive a circular business model for zero-emission vessels. You read that correctly: the CO2 that would be emitted from the Sleipnir can be used to create the synfuel which would be combusted again and again in an infinite loop. This is one of the fields of expertise of Professor Earl Goetheer, Principal Scientist Process Technology TNO, CO2 Utilisation & Mechanical Engineering, who works at TNO and the TU Delft to make this a reality. Given the current momentum of the energy transition, and the sheer amount of investments being made by the energy industry into clean-energy infrastructure, this might become a reality sooner than we might think.

A Possible Future for Sleipnir - Result

All of the above proposed measures to eliminate carbon emissions for the Sleipnir require a tremendous amount of engineering still. Nonetheless all of these measures are incorporated in the carbon neutral ambitions of Heerema and backed by scientific research. As stated before, the exact performance of these technologies cannot be guaranteed at the time of writing, which is exactly why Heerema is investigating all options. The below figure shows the resulting daily energy and power needs for Sleipnir in low, regular and heavy duty mode when all zero-emission technologies are used as described in the previous sections.

Low Mode - When moored, shore power (100% electrical) provides all power.

Regular Mode - Solar PV in combination with battery systems provide power. A floating solar field or improvements in solar PV and/or batteries are required. When more redundancy is needed, or during prolonged periods of higher power demand.

Heavy Duty Mode - A mix of synfuel and hydrogen is used in combination with carbon capture and storage. The synfuel consists of either circular biofuels or fully synthetic fuels. The exact synfuel/hydrogen ratio, as well as the carbon capture and storage facility on-board requires further detailed engineering.

Closing Remarks

We will sail once more with zero-emissions, as we have done for so long in the past, but it will take time. It took almost a hundred years to transition from sail to steam. It then took about fifty years to transition from steam to fuel oil.

How much time it will take to transition from fuel oil to zero-emission shipping, whichever form it will be, is unknown. We can be sure that for most vessels it will not be any time soon. Not in the very least because currently, virtually no shipyards are building any zero-emission vessels and the average lifetime of a large vessel is about 25 years or more.

Though be not mistaken! It took only seven years for Robert Fulton to prove Napoleon wrong, and change the face of the shipping world forever. Who would you rather be? A Napoleon, or Fulton?

Personal Note

I graduated in 2013, in a world of bright prospects regarding fossil fuels, where not a single soul at the TU Delft was concerned about sustainability. Except for the few odd-balls, who proved to true visionaries. The change that followed in the years to come is even difficult for me to comprehend. The oil crisis of 2014 and the signing of the Paris Agreement in 2015 sowed the seeds for a generation of critical, creative and brilliant young minds to blossom and think different. We will soon bring home that harvest, when those bright minds enter board rooms around the world and promote sustainable thinking in them.

To me, the incredible bold action of Heerema Marine Contractors going carbon neutral shows that they have already dedicated to a sustainable future. It makes me proud to work for such a company, and the many exciting opportunities it will bring.

References & Further Reading

Data from Sleipnir is either taken from the Heerema website or ‘generalized’ to avoid potential commercial conflict or IP issues. All measures proposed in this blog are under investigation; no guarantees on the actual performance can be given at the time of writing.

Researchgate - Bouman, Evert & Lindstad, Elizabeth & Rialland, Agathe & Strømman, Anders. (2017). State-of-the-art technologies, measures, and potential for reducing GHG emissions from shipping - A review. Transportation Research Part D Transport and Environment. 52. 408. 10.1016/j.trd.2017.03.022.

ZESTAS - Zero-Emission Ship Technology Association

Professor Earl Goetheer - Principal Scientist Process Technology TNO, CO2 Utilisation/mechanical engineering

Jasper Vos, TNO - CO2 Capture on the Sleipnir

SAFT - Battery technology

Forward Ships - Shipping Pollution

Neste - Renewable Diesel Handbook

Shell - GTL Knowledge Guide

Transport & Environment - Shipping’s impact on air quality

Ivan Komar and Branko Lalić - Sea Transport Air Pollution