Projected Reductions in CO₂ Emissions by Using Alternative Methanol Fuel to Power a Service Operation Vessel

Abstract

Due to increasingly stringent IMO and European Commission requirements for greenhouse gas emissions, the present study analysed the projected reductions in CO₂ emissions achieved by using methanol as an alternative fuel to power custom service operation vessels (SOVs) serving wind platforms in the Baltic Sea. Methanol is a relatively new fuel, approved for use as a safe marine fuel in the late 2020s. In these analyses, reference was made to the current interim guidelines, supplementing the IGF Code in the form of MSC.1/Circ.1621. The SOV type was chosen because of the current growing demand for these ships (the dynamic development of offshore wind power) and the lack of analyses of this type of small craft. The importance of assessing CO₂ emissions in this case is due to the specifics of the vessel’s operation in different modes, and thus the variable load on the propulsion system and the area of operation close to the coastline. A computational research method was used to evaluate CO₂ emissions, as well as the cost of methanol fuel, using current regulations and technical data. A comparison was also made between conventional MDO and LNG fuels. The first results of the analysis showed that methanol fuel is only competitive with MDO (a few-percent advantage) in terms of the average estimated index value EIV. Economically, it will require a higher investment, despite the favourable unit price of methanol compared to LNG and MDO.

1. Introduction

1.1. Research Background

At present, the transportation of goods by sea accounts for more than 80% of global trade. As a result, this form of transportation, mainly driven by fuel, contributes to global pollution through the high emissions of greenhouse gases, i.e., carbon dioxide (CO₂), sulphur dioxide (SO₂) or nitrogen oxides (NO_x) (Chen et al., 2018) [1].

According to [2], in 2022, the total greenhouse gas emissions from the global fleet increased by 4.7%, and according to the UN [3], in April 2022, CO₂ emissions were 847 million tons, an increase of 23% over the last 10 years, as shown in Figure 1.

As a result of these conditions, the International Maritime Organization (IMO) is issuing increasingly stringent demands for reducing greenhouse gas emissions. The latest report, dated February 2023 [4], shows that zero emissions should already be achieved by 2050 (the current plan is for 50% by 2008 and 0% by 2100). IMO’s long-term strategy is shown in Figure 2.

Likewise, the European Union, through the introduction of the “Fit for 55” package in 2021, plans to reduce greenhouse gas emissions by at least 55% by 2030 compared to the 1990 levels, and to increase the use of renewable energy, as well as a greater share of ecological fuels in the maritime economy [5].

In response to the strict restrictions and limitations, there is an increased determination in the maritime industry to search for and use ecological fuels to power ships (i.e., methanol, ammonia, hydrogen and biofuels), as well as a determination in many countries, including Poland, to search for and use developing renewable energy.

Currently, an increasing number of transport ships powered by one of the alternative fuels are in operation. The largest group is ships powered by LNG (a total of 840 ships, consisting of container ships, car carriers, and tankers), then LPG-fuelled (84 ships) and further by methanol and hydrogen [4]. However, alternative fuels constitute only 0.5% of the entire operational commercial fleet.

In new ship orders, however, it is noted that the world’s largest shipping companies, such as Maersk, are increasing their orders for methanol-powered vessels, e.g., 16,000 TEU container ships. According to the report [6], a record number of orders for methanol-powered ships were placed throughout 2022, i.e., 35 vessels, equivalent to 13% of all orders, and in February 2023 orders for another 22 vessels were reported [6]. According to forecasts, the current fleet of these vessels will be tripled by 2028, as shown in Figure 3 [4].

Marine engine manufacturers, i.e., MAN, Wartsila and Rolls Royce, draw much attention to the development of methanol technology as a future green fuel.

The first ship to use methanol fuel was the retrofitted ro-pax Stena Germanica, which has been in operation since 2015. Adapting the ship for methanol fuel required rebuilding the main propulsion machinery and installing all the required auxiliary systems in accordance with IMO regulations.

This important issue is scarcely discussed in the literature, especially such topics as the impact of methanol fuel use on:

• Design and structural modifications to the ship’s hull;
• Volume of the fuel tanks, and their effect on the size and layout of the engine room area and adjacent compartments;
• Carrying capacity and displacement of the ship and related operational parameters, such as the ship’s speed.

A number of scientific studies perform economic or environmental evaluations of ships powered by a selected alternative fuel, or examine exhaust gas reduction, mainly in cargo ships operating on a specific shipping route. For example, Solakivi et al. (2022) [7] present a price forecast for alternative fuels (including LNG, ammonia, and methanol) and their cost-effectiveness analysis relative to conventional fuels. Jeong et al. (2023) [8] undertook analyses of the profitability and cost of investment in building a new container ship fuelled by alternative fuels, such as LSFO, LNG and ammonia. The publication of Oloruntobi et al. (2023) [9] extensively characterizes the physical and chemical properties of methanol (as a clean fuel), presents its advantages and disadvantages, and analyses its impact on exhaust emissions and compliance with the applicable IMO regulations.

Using a ro-ro vessel as an example, (Ammar, 2017) [10] presents solutions that benefit the environment as a result of the use of LNG for propulsion and exhaust gas reduction measures for diesel fuel. Using a container ship as an example, (Ammar, 2019) [11] demonstrated the cost-effectiveness of using a methanol dual-fuel engine, presenting the percentage reduction of exhaust gases SOx, NOx or CO₂. Similar topics are addressed by many authors, including (Ammar & Seddiek, 2020) [12] and (Ivica A., 2015) [13].

One of the publications that deal with the design of cargo ships in terms of hull modelling to improve propulsion efficiency, and thus reduce exhaust emissions by calculating the EEDI index, is (Szelangiewicz & Żelazny, 2014) [14]. The authors mainly refer to changes in the ship’s hull when using conventional fuel to improve the EEDI index.

The literature review leads to the following conclusions:

• The use of alternative fuels is mainly discussed in reference to cargo vessels;
• There is a lack of information on the influence of fuel type on the design and construction of a ship, as well as on its operational parameters, i.e., main dimensions, hull shape, functional-spatial arrangement of the hull, deadweight and gross displacement of the ship;
• For special vessels such as offshore vessels, there is a lack of scientific publications on the effects and feasibility of alternative fuels, including emission reduction.

Each type of alternative fuel poses a different technological challenge to hull design, dictated by the physico-chemical properties of these fuels, such as density and weight, low energy density, toxicity and explosiveness.

In an effort to meet environmental challenges, the subject of this analysis is one of the future fuels, namely (CO₂-neutral combustion) methanol, which will be used to propel a less-common specialized vessel that serves offshore wind farms: a service operation vessel (SOV).

Methanol (CH₃OH) is produced from a variety of feedstocks. It can be of fossil origin (produced from coal or gas) or renewable (made from biomass or captured CO₂ combined with green hydrogen). To reflect the greenhouse gas reduction potential of methanol fuel, a variety of colours are used (from brown and grey to blue and green).

Based on [15,16], methanol made from coal and natural gas (black and grey) currently accounts for the largest share, and it is this type that increases the total greenhouse gas lifecycle and emissions in the production process. While all types of methanol can lead to a reduction in CO₂ emissions of about 7% [17] compared to conventional fuel, blue and green methanol are actually becoming the future alternative. The greatest benefits of methanol come from its ability to convert into green and blue methanol. Green methanol is the most environmentally sustainable, while blue methanol significantly reduces CO₂ emissions over its life cycle. The use of green methanol made from biomass, for example, is currently a challenge due to its limited availability and economic considerations. Hence, the type of methanol used is a big test related to decarbonization, as its degree of “purity” depends primarily on the type of methanol, and this in turn is a direct result of how it is obtained.

This study considered methanol fuel—known as “grey methanol”—which, according to the Methanol Institute [15], currently accounts for 95% of the total methanol that is used in the marine shipping industry. It is this type that produces 80% less NO_x, 99% less SO_x, 95% less particulate matter and about 20% less CO₂ [15]. In this study, the type of methanol did not affect design considerations.

Methanol, of all liquid fuels, is the simplest alcohol with the lowest carbon content and the highest hydrogen content [6]. It is a liquid under ambient conditions with a temperature range of −93 °C to 65 °C at atmospheric pressure [6]. This makes it easier to store and transport, but it has a low volumetric energy density compared to conventional fuel, resulting in a requirement for much larger storage space in the ship’s hull (Vedachalam et al., 2022) [18]. This can lead to some cargo loss, which is particularly important with respect to cargo vessels. Additionally, methanol is biodegradable and water-soluble, making it less of an environmental hazard and a safer option in the event of a fuel spill (Verhelst et al., 2019) [19]. If spilled or leaked in the hull space, it should be handled with care. At high concentrations, it also becomes poisonous and toxic to humans. Additionally, it has a low flashpoint, making it a greater fire hazard.

An important question is whether the parameters of methanol fuel will have a beneficial effect on the design and construction parameters of an SOV vessel, which is a more diversified and functionally different type of vessel compared to large, traditional cargo vessels. The use of methanol is particularly important for those vessels where hull space is not a special design constraint and those that operate close to the shoreline and have access to the infrastructure.

This type of vessel has been chosen for analysis here due to the current dynamic demand for them, which was caused by an apparent revival in the offshore wind sector. The largest number of wind farms operate in the North Sea. In the near future, there are plans to build wind farms in the Baltic Sea, in the Polish Exclusive Economic Zone. The plan of the Polish Energy Group (PGE) assumes the construction of offshore wind farms in three locations near the coastline (the farthest will be 80 km away, i.e., Baltica 1) [20]. Therefore, a fleet of service vessels will be needed to serve the wind farms.

SOV vessels are used not only to carry technical and service personnel or spare parts for an offshore wind farm, but also primarily to stay offshore for a certain period of time, carrying out their service profile. When performing service work, the most important task is the reliable and safe operation of the propulsion system to ensure manoeuvrability and precise positioning with the DP system.

The difficulty of assessing CO₂ emissions in this case is due to the ship’s specific operation in different regimes, i.e., the variable loads on the propulsion system when performing tasks in the area of offshore wind platforms, as well as sailing to the destination and back to port. SOV vessels are operated in close proximity to land, usually on bi-weekly operating cycles. This results in frequent port calls and stays in port, which can consequently negatively affect the environment. Hence, methanol fuel that burns cleanly and has low emissions, ensures a reduction in CO₂ emissions, completely eliminates sulphur oxide SOx emissions and reduces nitrogen oxide emissions by nearly 60% seems to be a good alternative for use in this particular case.

There are currently 29 typical SOV vessels in service, and another 21 are under construction [21]—all operating in Europe—as shown in

Table 1. Global SOV vessel fleet [21].

Global SOV Fleet
	2021	2022	2023	2024	2025
In Service	29
Under Construction	2	5	4	3
Possible				2	5

.

Notably, two shipyards specialize in the construction of SOV ships: the Norwegian Ulstein shipyard and the Netherlands Damen shipyard. The Polish shipyard Crist in Gdynia is building the second of a series of hulls of SOV ships, to be delivered in 2024.

The increasing use of methanol as ship propulsion fuel is due to the better availability of this fuel, but also to the increasing bunkering infrastructure, e.g., in Rotterdam—the largest port in Europe—a methanol bunkering station for marine vessels is currently being built [22], and in the Port of Gothenburg, one has already been in operation since 2015. According to experts, the current infrastructure for ship bunkering requires minor modifications to adapt it for ships with methanol fuel. Methanol utilization technology is currently available in 125 ports around the world [23], as shown in Figure 4.

Based on the literature review, this study is the first of its kind and uses an innovative approach to assessing the CO₂ emissions of an SOV vessel potentially intended for operation in the Baltic Sea, in combination with a construction-design approach.

1.2. Research Aim

As a result of the literature review and the lack of published data on the analysis of the use of alternative fuels in offshore vessels, the purpose of this research is to analyse the feasibility of using methanol for the propulsion of an SOV vessel, to assess the amount of CO₂ emissions and the impact of this fuel on the design and construction elements and cost-effectiveness.

To achieve the main objective of this research, it is necessary to:

–

Develop a design concept for an SOV vessel, including a propulsion system adapted for different fuels;

–

Conduct an analysis of the main operating regimes of the SOV vessel and the associated variable power requirements;

–

Perform a design analysis of the hull with tanks for different fuels;

–

Apply technical requirements for the safe operation of fuel in accordance with current IMO guidelines;

–

Identify the benefits (via the hull volume resulting from fuel tanks) to be obtained by using different fuels;

–

Calculate fuel consumption and CO₂ emissions for different regimes of ship operation;

–

Conduct an economic cost analysis for several fuel options (methanol, LNG, MDO).

2. Initial Assumptions and Methodology

2.1. Technical and Operational Parameters of the SOV

The starting point for the analysis was an SOV vessel;its silhouette is shown in Figure 5, and its basic design parameters are shown in

Table 2. Technical and operational parameters of the SOV vessel adopted for further analysis [24].

Parameter	Value	Symbol	Unit
Length between perpendiculars	72.0	Lbp	m
Breadth	18.0	B	m
Hull depth	8.9	H	m
Draught	5.3	T	m
Displacement	5004	D	t
Deadweight	1005	DWT	t
Cargo weather deck area	280	Awd	m2
Endurance	14	A	days
Technical crew number	60	nc	people

Ship is equipped with a DP2 dynamic positioning system.

. Several variants of the main engine were hypothetically assumed for the vessel, which will be powered by different fuels, i.e., methanol and, comparatively, LNG and conventional MDO fuel.

The following types of main engines manufactured by Wärtsilä were used:

• Two-stroke dual fuel engine (Wartsila 6L 32M), fuel: Methanol (M) and Marine Diesel Oil (MDO);
• Diesel engine (Wartsila 6L 32), fuel: Heavy fuel oil (HFO);
• Dual fuel engine (Wartsila 6L34DF), fuel: Liquid natural gas (LNG) and MDO.

The use of several engine solutions will make it possible to study the potential impact on the spatial layout of the ship, especially concerning the size and location of the fuel tanks, the engine room region, the impact on the ship’s size, displacement and carrying capacity.

2.2. SOV Working Area

The calculation analysis carried out for the ship’s data is shown in Table 2 and includes the following assumptions:

–

A service vessel for the planned Baltica 1 wind farm in Poland in the Baltic Sea (Figure 6), located 80 km (approx. 43 Nm) from the mainland [25].

–

Annual operation time TR = 250 days, service operation time T_s = max.14 days (mission)—Figure 7.

–

Marine environment conditions in the area under consideration—worse weather parameters were used for calculations, i.e., average significant wave height Hs = 2.5 m, average wind speed v_w = 10 m/s [24].

Figure 6. Areas of planned offshore wind farms in the Baltic Sea, by PGE [25].

Figure 6. Areas of planned offshore wind farms in the Baltic Sea, by PGE [25].

Figure 7. SOV Operating Profile.

Figure 7. SOV Operating Profile.

2.3. Operational Profile and Phases of SOV Service Work

The SOV vessel performing its tasks can be in various temporary operating states, during which it exhibits variable energy requirements. The ship’s operating time in the analysed case consists of four stages called the operational profile (numbered from 1–4), which is shown in Figure 5.

During each stage of the ship’s operation, and mainly during service work (numbered 2. in Figure 7), there are different propulsion power demands depending on the operating regime. In Figure 8, (2. Service work) has been divided into several different regimes, which have been assigned appropriate numbers from I to V. For each regime, hourly operating hours were assumed and the percentage of one day was determined.

Each of the service operation phases in Figure 8 is characterized by variable power demand.

Table 3. Power required for individual service work phases.

Service Work Phases		Power Requirement PR′ [kW] **	Time T [hours/day]	Energy Required
				ER [kWh/day]	EMIS * kWh/Mission
I	Operation with active working platform and DP2 dynamic positioning system	2851	3.5	9978.5	139,699
II	Maneuvering	1526	1.5	2289	32,046
III	Operation with DP2 dynamic positioning system	1976	3.5	6916	96,826
IV	Low operation—sailing vs = 6 kn	773	1.0	773	10,822
V	Night break using DP1	1426	14.5	20,677	289,478
Sailing to/from the Baltica 1
Normal operation v = 12 kn		2190	2 × 4	n/a	17,523 only round trip

* Mission—14 days, ** P_R′—power requirement for individual phases acc. [24].

shows the preliminary assumed values of energy demand for each service phase and operating regime during one mission. This is necessary for further calculations herein of stock and fuel consumption, and consequently the level of CO₂ emissions. The level of exhaust emissions will be estimated first for each of the service phases carried out, and then as an average value for the entire 14-day mission.

From the data in Table 3, it can be seen that the highest power demand occurs in the (I) mode of operation—with DP2 and the working platform active.

2.4. Research Method

The analysis of the impact of using an alternative type of marine fuel on a relatively new type of SOV ship (existing for a decade), and checking its advantages in terms of safety and design (space in the hull), can be carried out with a computer-aided design, using CAD methods.

To evaluate the benefits achieved (the reduced carbon dioxide emissions and the cost of methanol fuel compared to the cost of LNG and MDO fuels used during the service work in the Baltica 1 area by an SOV vessel), a computational research method was used. The method was based on technical and operational data of the potential SOV vessel—acc. to [24] (Table 2), the latest IMO guidelines (MSC.1/Circ.1621) [26] relating to methanol fuel and, most importantly, the Estimated Index Value (EIV).

In Figure 9, in the block diagram form, the main stages of the computational analyses, resulting from the initial assumptions and research background, are presented in a hierarchical manner.

3. Design Analysis of the SOV for the Use of Alternative Fuels for Propulsion

Referring to the technical and operational data of the SOV, presented in Section 2 (Table 2), a design analysis of the vessel was carried out in terms of the fuel tank modifications, regarding their volume and weight depending on the fuel used. For comparison, in addition to methanol, LNG and conventional MDO fuels were used.

3.1. Types of Fuels Used

To study the size of fuel storage, it is necessary to characterize the parameters of these fuels. The most relevant physical and chemical properties of the above-mentioned fuels are presented in

Table 4. Comparison of the physico-chemical properties of methanol, MDO and LNG fuels [27,28,29].

Property	Fuels
	Methanol (65 °C)	MDO	LNG (Liquid—163 °C)
Density ${\gamma }_{fuel}$ (kg/m3)	787–792	830–850	410–500
Emergency content LHV (MJ/kg)	19.9–20	42.0–43.0	50.0
Boiling Point (°C)	65.0	150–370	−162.0
Flashpoint (°C)	9–11.0	min. 60	−188.0
Auto ignition (°C)	385.0–464.0	240.0	537.0
Viscosity cSt w 20 °C	~0.6	~13.5
Fuel tank size relative to MDO	2.3	1.0	1.7

.

For each type of fuel, a suitable Wärtsilä main engine was assumed, as mentioned in Section 2. For design purposes, the technical parameters of these engines were also relevant, especially regarding the dimensions and weight, hence,

Table 5. Main technical data of marine engines [30,31,32].

No.	Parameter	Engine Type
		Two-Stroke Dual Fuel Engine (Wärtsilä 6L 32M)	Diesel Engine (Wärtsilä 6L 32)	Dual Fuel Engine (Wärtsilä 6L34DF)
1.	Rated power PR [kW]	3480	3480	3000
2.	Speed [rpm]	750	750	750
3.	Dimensions [mm]	5570 × 2380 × 3490	5255 × 2389 × 3498	5352 × 2389 × 3498
4.	Weight ME [t]	35	35.4	35.2
5.	Fuel type	Methanol fuel (M) Diesel fuel (MDO)	Heavy fuel oil	Liquid natural gas (LNG + MDO)
6.	Specific fuel consumption * [g/kWh]	acc. to Formula (2)	acc. to Formula (2)	acc. to Formula (2)
7.	IMO	Tier II or III	Tier II or III	Tier III

* Due to a lack of published data (commercial confidentiality) on specific fuel consumption, this parameter was estimated on the basis of the Formula (2).

shows the technical data of the marine engines used.

As low-carbon fuels, methanol and LNG have lower carbon intensity compared to conventional petroleum-based marine fuels. For easier storage and use, LNG is liquefied by cooling it to −163 °C. LNG-fuelled ships use their own engine architecture, cannot be combined with conventional marine diesel engines, and require specialized fuel storage systems that include, for example, double-wall fuel tanks and pipes [33]. LNG fuel and methanol have a lower volumetric energy density and yet higher specific density MJ/kg compared to conventional marine diesel (Table 5), which requires more tank volume and leads to some cargo losses.

This study focuses on the methanol-fuelled engine solution. The Wärtsilä 32 6L32M engine is suitable for methanol, HFO, MDO and liquid biofuels. According to the manufacturer [31], the engine has high reliability, high power density, low fuel consumption over a wide load range, and operates efficiently and economically.

Due to the properties of methanol, and to maintain the safety of the entire combustion process, the engine uses necessary equipment that prepares the fuel before it is delivered, i.e., methanol fuel transfer pumps, a nitrogen generator for neutralizing and purifying the atmosphere and for ventilating the doubled fuel pipelines, a low-pressure pump with a cooler, a fuel valve system, methanol fuel pumps and pumps for sealing oil. The process of fuel preparation and supply is described by the engine manufacturer [32], therefore, its description here is omitted.

In the engine compartment, it is necessary to include both the engine dimensions specified in Table 5 and space for all the equipment and components according to the regulations, which increases the usable area.

3.2. Technical Requirements for Fuel Tanks on Ships

3.2.1. Current Regulations

Methanol, as a new alternative fuel, does not have complete regulations yet. At present, there are only interim guidelines from the International Maritime Organization (IMO) to supplement the International Code of Safety for Ships Using Gases or Other Low-flashpoint Fuels (IGF Code) [34], i.e., MSC.1/Circ.1621 [26]. The purpose of the regulations is to provide an international standard for ships using fuel in the form of gas or low-flashpoint liquids. They specify criteria for the arrangement of equipment, machinery and installations on ships using a low-flashpoint liquid or gas for propulsion, such as methanol. There are also class regulations, including DNV regulations (Pt.6 Ch2. Sec.8 Fuel Ready Ships, ABS) [4], a Guide for Methanol and Ethanol Fuelled Vessels or Lloyd’s Register, and Rules for the Classification of Methanol Fuelled Ship, which include regulations for methanol-fuelled ships.

3.2.2. Fuel Tanks with Methanol in the Ship’s Hull

Compared to LNG fuel operation, the requirements for methanol are much less complex. Tanks can be made of ordinary carbon steel coated with epoxy/galvanized coating or stainless steel of the appropriate grade (to avoid corrosion), and do not require refrigeration.

This article presents some of the most relevant design-related guidelines of Circular MSC.1621 [26] that affect the location of methanol tanks in the ship’s hull, i.e.:

–

Methanol tanks can be integrated into the hull structure, but require more space in the ship’s hull compared to standard fuel or LNG. This limits the payload capacity of the vessel, which can be a technical challenge for some vessel types, especially in those that would be converted;

–

Unlike a traditional fuel tank, an MDO does not need to have a double bottom or a cofferdam-separated side to prevent leaks—tanks require additional cofferdams only above the waterline, and separating the machinery compartment to prevent potential leaks;

–

An inert atmosphere must be maintained in the cofferdams and in the tank itself, filled with an inert gas—nitrogen;

–

Necessary space is needed where fuel preparation equipment (preparation room) is located, i.e., fuel pumps, fuel valves, heat exchangers and filters; the rooms should be located outside the engine room (outside category “A” machinery compartments).

LNG fuel tanks were laid out in the ship’s hull in accordance with the guidelines of the IMO regulations (IGF Code) [34]. The general requirements of these regulations, in the context of the layout of LNG tanks in the ship’s hull, have been omitted due to their presentation in other publications by the author, including [35,36].

3.3. Calculation of Fuel Reserve and Arrangement in the Ship’s Hull

The fuel supply for the SOV vessel for one mission (T_s = 14 days) and the design assumptions from Section 2.1 were calculated according to the following relationships:

▪

Total fuel reserve $M_{Tfuel}$:

$$M_{Tfuel}[\mathrm{t}]=P_R[\mathrm{k}\mathrm{W}]·{\rho }_{fuel}[\frac{\mathrm{k}\mathrm{g}}{\mathrm{k}\mathrm{W}\mathrm{h}}]·T_s[\mathrm{h}\mathrm{o}\mathrm{u}\mathrm{r}]$$

(1)

▪

Specific fuel consumption ${\rho }_{fuel}$:

$${\rho }_{fuel}[\frac{\mathrm{k}\mathrm{g}}{\mathrm{k}\mathrm{W}\mathrm{h}}]=\frac{P_R[\mathrm{k}\mathrm{W}]}{LHV[\frac{\mathrm{k}\mathrm{J}}{\mathrm{k}\mathrm{g}}]}$$

(2)

▪

Fuel volume $V_{fuel}$:

$$V_{fuel}[{\mathrm{m}}^3]=\frac{M_{Tfuel}[\mathrm{k}\mathrm{g}]}{{\gamma }_{fuel}[\frac{\mathrm{k}\mathrm{g}}{{\mathrm{m}}^3}]}$$

(3)

The calculation of the required fuel mass and the resulting volume of tanks for the three types of fuel made it possible to determine and compare the necessary area in the ship’s hull, especially in the engine room and fuel tanks. The layout of the tanks for the alternative fuels, the main engine with components taken into account the protective compartments required by the regulations, the fuel preparation room, the preparation room for methanol, and the airlocks for both alternative fuels, as well as the guidelines of Circular MSC.1621 [26], are all presented in Section 3.2.2.

Figure 10, Figure 11 and Figure 12 show an overview of the SOV vessel—with the longitudinal, vertical and transverse views, and with the main engine, fuel tanks and the approximate size of the space required visible.

The estimated area required for the methanol fuel tanks and required compartments was 347 m² without engine room; for the LNG it was 264 m², and for the MDO it was 216 m², as indicated in

Table 6. Estimated total fuel reserve and fuel volume for a mission.

No.	Parameter	Methanol	LNG	MDO	The Ratio of Methanol to
					LNG	MDO
1.	Fuel reserve $M_{Tfuel}[\mathrm{t}]$	204	82	101	2.5	2.0
2.	Fuel volume Vfuel [m3]	258	220	112	1.2	2.3
3.	Emergency content LHV [MJ/kg]	20	49.6	40.2	-	-
4.	Specific fuel consumption ${\rho }_{fuel}$ [g/kWh]	174	70.2	81.5
5.	Density ${\gamma }_{fuel}$ [kg/m3]	789	435	991	-	-
6.	Area required (tanks + compartments required) [m2] acc. to design analysis	347	264	216

.

3.4. CO₂ Emission and Fuel Costs

Various criteria for CO₂ emissions are used in maritime transport, mainly through EEDI or EEOI indicators, as mandatory rules for implementing GHG reduction measures, (Resolution MEPC.308(73)—2018 [37]. However, small vessels such as SOVs are not subject to these requirements and calculation guidelines do not exist. Therefore, the Estimated Index Value EIV—which is a simplified form of the EEDI index [37]—was used to estimate CO₂ emissions for the assumed fuel types.

CO₂ emissions from ship power systems depend on the type and amount of fuel consumption.

Several simplifying assumptions were used in the EIV Formula (4), which are due to the lack of reference in [37,38] to the group of offshore vessels that include SOV.

▪

Estimated Index Value EIV:

$$EIV=C_F·\frac{{SFC}_{ME}·\sum _{i=1}^{NME}{P}_{ME}+{SFC}_{AE}}{Capacity·V_{ref}}[\mathrm{g}{\mathrm{C}\mathrm{O}}_2/\mathrm{g}\mathrm{M}\mathrm{m}]$$

(4)

▪

Reference line values V_ref:

$$V_{ref}=107.48·{\left(DWT\right)}^{-0.216}$$

(5)

–

C_F (g CO₂/g fuel)—determines the conversion factor, i.e., the carbon content of the fuel for the specific type of fuel used. According to Table 7 [38,39], the following CO₂ emission factors were adopted, for methanol: C_F = 1.375 g CO₂/g, for LNG: C_F = 2.75 g CO₂/g, and for conventional fuel: C_F = 3.114 g CO₂/g.

–

SFC_AE—omitted due to the assumption that the generators will use batteries recharged at the port, which is beneficial for the size of the fuel tanks and their weight, but most importantly for environmental reasons.

–

P_ME (kW)—the power of the main engine, representing 75% of the total installed main power, which affects the amount of fuel consumed as well as the type of fuel used (as shown in Table 5).

–

V_ref—reference line values—due to the lack of data for ships in the offshore group, it was adopted for the general cargo ship [34].

Table 7. Emission factor values C_F [38,39].

Fuel Type	CF in [g CO2/g]	Carbon Factor in g/MJ
Methanol	1.375	69
Liquefied Natural Gas (LNG)	2.75	57
Heavy Fuel Oil (HFO)	3.114	77
Marine Diesel Oil (MDO)	3.206	75

Table 7. Emission factor values C_F [38,39].

Fuel Type	CF in [g CO2/g]	Carbon Factor in g/MJ
Methanol	1.375	69
Liquefied Natural Gas (LNG)	2.75	57
Heavy Fuel Oil (HFO)	3.114	77
Marine Diesel Oil (MDO)	3.206	75

Unfortunately, the C_F emission factor or carbon content—as shown in Table 6—does not specify the type of methanol to be grey, blue or green, whose percentages vary. Green methanol is, of course, a priority because it is based on biomass or hydrogen (renewable sources), but, unfortunately, it is the most costly because the technology is only in the developmental stages.

To verify the cost-effectiveness of using the appropriate fuel, its costs were also calculated for the ship’s 14-day mission and annually, taking into account the number of voyages performed.

For the economic analysis, it was necessary to take into account the unit price of individual fuels. For this purpose, a compilation was made of the levels of prices formed in recent times, in order to forecast possible volatility. On the basis of global fuel prices, their characteristics were drawn up for the period of 2021—mid-2023—shown in Figure 13. The unit price of methanol in the last two years was stable, and was not as volatile as the price of LNG—as shown in Figure 13.

The study considered the April 2023 price of $P_{fuel}$:

P_methanol = 500 [$/mt]—unit price of methanol,

P_LNG = 846 [$/mt]—unit price of LNG,

P_MDO = 508 [$/mt]—unit price of MDO.

• Fuel costs $K_{fuel}$:

$$K_{fuel}=M_{fuel}·P_{fuel}$$

(6)

$P_{fuel}$—unit price (depending on the type of fuel assumed)

• Number of voyages per year $n_M$:

$$n_M=\frac{T_R}{14days\left(mission\right)}$$

(7)

• Annual fuel costs $K_{Tfuel}$:

$$K_{Tfuel}=n_M·K_{fuel}$$

(8)

4. Results

In this chapter, the results of the analyses are presented in tabular and graphical form. These are the collective results of the partial analyses, for which the methodology and initial assumptions from Section 2.1 and the calculation formulas from Section 5 were used.

Table 8. Results of fuel demand calculations for individual service phases.

Service Work Phases		Fuel Demand ${{\mathit{M}}_{\mathit{f}\mathit{u}\mathit{e}\mathit{l}}}^{\prime }$ [kg]
		Methanol	LNG	MDO
I	Operation with active working platform and DP2 dynamic positioning system	1997	805	935
II	Maneuvering	458	185	215
III	Operation with DP2 dynamic positioning system	1384	558	648
IV	Low operation—sailing vs = 6 kn	155	62	72
V	Night break using DP1	4137	1668	1938
Shipping to/from the Baltica 1
	Normal operation v = 12 knots	3506	1414	1642

shows the partial results of the required fuel according to the phase of service work.

After calculating the total fuel demand and its consumption, the approximate EIV was calculated for each phase of service operation first—the results are shown in Figure 14—and then the average value of the EIV, considering the entire mission—which is shown in Figure 15.

The estimated fuel costs referring first to one mission and then to all missions performed in a year are shown as bar charts with numerical values—illustrated in Figure 16 and Figure 17.

5. Summary and Conclusions

This study appears to be innovative in that it represents the first preliminary results from the design and computational analyses for an unusual vessel such as the service operational vessel (SOV).

The conclusions from the use of grey methanol fuel versus conventional fuel and LNG on the SOV are as follows:

• Methanol fuel tanks in the hull of an SOV need 120% more volume, resulting in a 250% increase in fuel weight, which will consequently increase the vessel’s displacement and lead to higher fuel consumption compared to conventional fuel.
• The 35 t methanol fuel engine is dimensionally slightly larger than the two engine types compared, which ultimately did not affect the design layout changes. However, it needs additional space for methanol fuel preparation equipment outside the engine room. Methanol needs about 25% more area in the ship’s hull than LNG and about 38% more area than conventional fuel. It is presumed that this additional space can be a challenge on vessels where conversion to methanol is considered and on large vessels with a longer cruising range. In the short-range SOV vessel presented herein, the problem is irrelevant. Economically, a methanol-fuelled engine adds about 10% to the cost of a new ship, while an LNG engine adds 22%, according to MAN Energy Solutions.
• The design analysis of the SOV was limited to identifying and comparing different engine and fuel options in the available hull space and identifying the differences in spatial layout, in accordance with current regulations. The dimensional parameters of the SOV have not been modified, whereas in the next iteration of the project, it would be necessary to verify and define certain relationships linking the type and volume of fuel to the vessel size, mass and speed.
• The design analysis, based on an SOV with methanol fuel, represents only one aspect of the design issue in the overall design spiral model. A certain approach to the design of a specialized vessel such as SOV, as presented in this paper, needs to be elaborated on according to a general iterative design process.
• This analysis proved that the use of methanol fuel on an SOV (in the Baltic Sea area), in terms of CO₂ emissions, offers advantages only over conventional fuel. The average EIV values for the individual fuels were: 12.7 (Methanol), 10.24 (LNG) and 14.3 (MDO). As a result, methanol has an advantage over MDO in an amount of about 11%, while LNG proved to be better by almost 20% compared to methanol. The EIV values for the different service phases are detailed in Figure 14.
• As a result, the degree of CO₂ emissions will depend on the type of methanol used, i.e., how it is obtained.
• Despite the lowest unit price of methanol ($500/mt), the calculated cost of this fuel at $90,346/year is 30% higher than LNG ($846/mt) and 50% higher than MDO ($508/mt), which cannot be regarded as a benefit for methanol use. To fully assess the economic benefits of using methanol, further studies should cover the entire supply chain from production, to transportation, storage, disposal, as well as environmental protection.
• The next few years will show whether methanol fuel technology, especially when used on smaller vessels, will become a permanent fixture in the ship’s energy supply and improve air quality. Methanol is a future technology in line with current decarbonisation strategy and is being considered particularly in emission-controlled shipping areas, such as the Baltic Sea, with its planned wind farms.