Technical and Economic Feasibility Analysis of a Conceptual Subsea Freight Glider for CO₂ Transportation

Abstract

This study analyses the technical and economic aspects of a novel subsea freight glider (SFG). The SFG is an excellent replacement for tanker ships and submarine pipelines transporting liquefied CO₂. The main aim of the SFG is to ship CO₂ from an offshore facility to an underwater well where the gas can be injected; as an advantage, the SFG vehicle may be used to transport all kinds of cargo. The SFG travels below the sea surface, making the vessel weather-independent. The research is divided into two steps. Firstly, the technical feasibility analysis is performed by designing a baseline design with a length of 56.5 m, a beam of 5.5 m, and a cargo volume of 1194 m³. The SFG is developed using DNVGL-RU-NAVAL-Pt4Ch1, which was initially created for military submarine designs. Two additional half-scaled 469 m³ and double-scaled 2430 m³ models are created when the baseline design fulfils the technical requirements. Secondly, the economic analysis is carried out using the freely accessible MUNIN D9.3 and ZEP reports. The economic feasibility analysis is illustrated through a case study with a CO₂ transport capacity range of 0.5 to 2.5 mtpa (million tons per annum) and a transport length range of 180 km to 1500 km. The prices of CO₂ per ton for the SFG, crew and autonomous tankers, and offshore pipelines are comprehensively compared. According to the results, SFGs with capacities of 469 m³, 1194 m³, and 2430 m³ are technically possible to manufacture. Moreover, the SFGs are competitive with a smaller CO₂ capacity of 0.5 mtpa at distances of 180 and 500 km and a capacity of 1 mtpa at a distance of 180 km.

1. Introduction

1.1. General Background

Carbon capture and storage systems (CCSs) require the development of transportation infrastructure to enhance their safety and economic efficiency [1]. The capture system should be interconnected with a storage system to complete the CO₂ transport operation. However, these systems are usually located hundreds of kilometres apart. Carbon dioxide should be transported under appropriate pressure conditions, which depend on different transmission methods. The transport of CO₂ via the pipeline will occur under different temperature and pressure conditions compared to transport by vessels.

Transporting CO₂ via underwater pipelines is the most commonly used method [2]. This is because by implementing this method, the products can be transported continuously, effectively enhancing the transportation efficiency. Additionally, pipelines can transport carbon dioxide in three states: liquid, gaseous, and solid. They can also take shortcuts and be installed anywhere, including underwater or underground. Moreover, it is a closed type of transportation, which effectively avoids loss, meaning it is safe, reliable, and clean, with no pollution. However, this solution has some limitations. Impurities such as water or hydrogen sulphide in the CO₂ stream can cause corrosion in the pipelines. In the case of pipe cracks in populated areas, the unexpected release of carbon dioxide can lead to severe environmental and human threats. Furthermore, installing and maintaining a subsea system for transporting gases is very expensive. In addition, steel prices are increasing yearly [3], which means that the design and construction processes are costly, often being unprofitable in the case of small reservoirs.

Liquefied hydrocarbon gases are transported in very large LNG or LPG tanks. However, it was proven that these carriers could also be used for CO₂ transportation. The largest LNG carriers have a capacity of 266,000 m³, which means they can carry 230,000 t of CO₂. The transport efficiency is maximised when the density of the liquid CO₂ is as high as possible. The density increases sharply with the decreasing pressure in the triple point region, reaching 1200 kg/m³ [4]; however, it is essential to avoid the formation of dry ice. The optimal conditions for transportation of CO₂ are at a temperature of 218.15 K and a pressure of 7 bar.

Transporting CO₂ by vessel tanks allows massive quantities of goods to be carried over long distances. However, sometimes it is impossible to perform a marine operation due to the inclement weather conditions. Factors such as wind and rain may prevent or delay the performance of maritime operations.

Pipelines seem to be a perfect solution if continuous transport for a relatively short distance of CO₂ is needed. Vessel tanks should be utilised when the transportation distance is long. However, there is a gap between these two solutions. Currently, no transportation method can transfer carbon dioxide for medium distances without continuous delivery. Pipelines and marine transportation leave a carbon footprint that negatively impacts the environment. Many countries and oil and gas companies have decided to reduce their absolute emissions to near zero by 2050 [5]. Therefore, an alternative CO₂ transportation method is needed.

1.2. Previous Work

The idea of using submarines to transport cargo is not new. The proposal to use subsea vessels for commercial transportation was first proposed in the 1970s when the idea of using large nuclear-powered submarines for transporting crude oil in the Artic regions was proposed. Nuclear-powered submarines of 20,000 DWT to 420,000 DWT were proposed by Jacobsen [6] and Taylor et al. [7]. In the 1980s, Jacobsen et al. [8] proposed using giant nuclear and non-nuclear submarines of 660,800 and 727,400 DWT to carry liquefied natural gas. In more recent years, a 3500 DWT multi-purpose submarine was proposed by Brandt et al. [9] for a wide range of operations in the Arctic regions. These research works described above have so far, in general, not proceeded beyond initial concept proposals and evaluations.

Three studies have involved more detailed research work on subsea cargo drones in recent years. In 2019, Equinor ASA proposed the concept of an underwater drone to transport CO₂ [10]. The subsea shuttle is an autonomous 135 m vehicle that can transport carbon dioxide back to the reservoirs, replacing the pipeline carrying CO₂. Even though the concept has been presented, minimal studies have been carried out. Xing et al. [11] gave a detailed description of the baseline design and conducted a finite element analysis for the ring-stiffened cylinders in the design. Santoso et al. [12] presented and compared three different models of the subsea shuttle tanker and proved that they are technically feasible. Additionally, an economic analysis was performed. In the study by Xing et al. [13], a new type of underwater vehicle for CO₂ transportation was proposed. The concept is an autonomous Subsea Freight Glider, a novel cargo submarine equipped with large hydrodynamic wings that allow gliding underneath the sea surface. The gilder does not have a propeller, and the only driving force is buoyancy force. This solution covers the gap with the previous studies and enables transporting vast amounts of cargo autonomously over long distances. In the study by Ahmad [14], a control methodology was proposed based on the feedback from the developed glider model and the obtained glide path.

These studies show that the concept of the subsea shuttle tanker can compete with different methods of CO₂ transportation. However, the performed analyses are insufficient, and some limitations exist, namely that they were not performed for the Subsea Freight Glider. The previous studies showed that large submarines can transport an enormous amount of CO₂. On the other hand, some small offshore facilities are close to land where CO₂ can be stored, and the conventional way of transporting CO₂ is too expensive.

This paper presents a methodology for designing a small underwater gliding vehicle (presented in Figure 1) and compares it with traditional methods of CO₂ transportation. The research will cover the gap in shipping small amounts of carbon dioxide for small distances. With this, the authors hope this will engage the research and engineering community to consider further and evaluate the concept, resulting in a final working concept that will eventually be a preferred alternative for low-volume and short-distance transport. The research into subsea CO₂ cargo drones started at the University of Stavanger in 2019 in collaboration with Equinor [11]. It has its roots in Norway, where Equinor is a large operator and has focused so far on the Norwegian Continental Shelf, where there are many marginal fields within 200 km of the coast. Even so, the authors expect that the vessel can also be applied to suitable fields in other regions of the world.

2. Description of a Novel Subsea Freight Glider

The baseline design of the Subsea Freight Glider is a 1224 ton underwater vehicle, specified with a beam measuring 5.5 m and a length of 56.5 m that can carry 1194 m³ of CO₂ (Figure 1). The distance that the SFG can reach at 1 m/s (2 knots) is 400 km.

It is possible to create models of different sizes based on the baseline design. In this paper, two more designs were performed:

• A half-scaled version of the baseline SFG;
• A doubled-scaled version of the baseline SFG.

The design methodology is displayed in Figure 2, and it is described as follows. The design starts by establishing the mission requirements (cargo capacity, operation range, operating depth, and environmental data) and detailed specifications of the SFG, such as the demotions, speed, depth, and design loads. Then, the external hull design process is performed, which includes determining the stiffeners’ dimensions and pressure. Next, the internal hull arrangement is conducted, including the design of the main cargo, auxiliary cargo, compensation, trim, and buoyancy tanks. All structural calculations are based on DNVGL-RU-NAVAL-Pt4Ch1 [15] and the American Society of Mechanical Engineers Boilers and Pressure Vessels Code ASME BPVC VIII-2 [16].

Furthermore, a wing was added and the complete method of computing the reference area was introduced in the study by Ahmad and Xing [17]. Finally, a stability check is performed. If the vessel is unstable, dimension adjustments are needed. The design loop is iterative to satisfy all criteria. The power consumption is determined when the design is finished. A more detailed description of the design’s steps can be found in the study of Ma et al. [18].

One assumed condition is that the payload should be around 45% of the displacement. A double-hull design with an active pressure compensation system should be used to satisfy this condition because it can limit the external pressure loads on the hull structure. Thus, it is not necessary to design the external hull for its operating depth.

2.1. Mission Requirements and Subsea Freight Glider Specifications

The design process starts with establishing the requirements of the mission. Based on these data, it is necessary to determine the SFG properties and specifications that will allow the mission to be fulfilled. This information provides the basis for the whole design process.

Table 1. Operating specifications.

Properties	Value	Units
Operating depth (nominal diving depth)	200	[m]
Collapse depth	400	[m]
Operating speed	2	[knots]
Cargo pressure	35–55	[bar]
Current speed	1	[m/s]
Cargo temperature	0–20	[°C]
Maximum range	400	[km]

presents the baseline SFG operating parameters.

The SFG baseline has a cargo capacity of 1223 tons, which allows for 510 tons of CO₂ to be transported. The half-scale and double SFG can carry approximately half and double the payload of the baseline capacity, respectively.

To prevent a collision or any possible impacts from the vessels or floating structures, the safety depth is defined as 40 m. The safety depth also reduces the dynamic loads on the SFG from the waves, making the submarine weather-independent. The nominal diving depth is introduced based on the recoverable depth, which refers to the limit of the loss of control. While carrying CO₂, the SFG has a nominal diving depth of 200 m. The collapse depth and test diving depth are defined in DNVGL-RU-NAVAL-Pt4CH1 [15]. According to the standard, the test diving depth is equal to 1.25 times the nominal diving depth, which is 250 m, and the collapse depth is 2 times the nominal diving depth, which is 400 m. The depths of the CCS field in which the SFG transfers CO₂ are displayed in Figure 3.

The purpose of the SFG is to transport CO₂ in the Norwegian Sea. In that region (0–10° E, 60–70° N), the temperatures in the seawater are in the range of 2–12 °C [15]. The SFG is designed to work in temperatures between 0 and 20 degrees Celsius. The design current’s velocity is 1 m/s, characterising the largest average current speed for the North Atlantic and Norwegian coastal areas. Nevertheless, the noted mean current velocity in the Norwegian Sea is about 0.2 m/s [19].

The baseline of the SFG has a range of 400 km. This range allows transporting the CO₂ back and forth between the Snøhvit and Troll fields. Moreover, the SFG can travel one way between Sleipner and Utgard.

2.2. Layout of SFG

The general arrangement is shown in Figure 4. It displays both the external hull and the internal hull modules. The external hull of the SFG has a torpedo shape to minimise the drag resistance. The external hull consists of a hemispherical bow, a conical aft, and a cylindrical mid-body make-up. In the baseline design, the mass of the aft and the bow modules is around 25% of the overall weight of the external hull. A double hull structure is adopted for the cylindrically shaped mid-body to avoid collapse under pressure. By implementing this solution, the external hull of the mid-body is not exposed to any differential loading, i.e., hydrostatic pressure. The cargo and buoyancy tanks are designed to resist collapse and burst pressure. The SFG is also equipped with four bulkheads that support the internal cargo and buoyancy tanks and isolate the accessible flooding compartment from the flooded mid-body compartment.

The external hull of the SFG consists of free main different sections:

• A flooded mid-body compartment is in the centre of the SFG. It is the largest part of the vessel, and it carries the cargo tanks, buoyancy tanks, and piping;
• A free flooding bow compartment carries the front compensation tank, front trim tanks, radio, control station, sonar, and offloads pumps;
• A free flooding aft compartment carries sensitive equipment, i.e., the battery, gearbox, motor, aft trim, compensation tank, and steering controls.

The internal compartment of the SFG contains five different internal pressure modules, the main cargo, the auxiliary cargo, the compensation tank, the trim, and the buoyancy tanks.

• Cargo tank: Seven main and six auxiliary tanks are placed symmetrically in the SFG, as shown in Figure 4. The tanks have a rounded shape with hemispherical heads.
• Buoyancy tanks: To make the SFG neutrally buoyant, eight buoyancy tanks are distributed in the upper part of the mid-body to make the SFG neutrally buoyant. All tubes have the same volume and are attached to the front and back bulkhead.
• Compensation tanks: Two compensation tanks provide the weight and trimming moment to give the SFG neutral buoyancy under different hydrostatic loads. One of the compensation tanks is placed in front of the vessel and another is placed in the back, as presented in Figure 4.
• Trim tanks: There are two trim tanks inside the SFG. These tanks make the vessel neutrally trim by placing the centre of gravity below the centre of buoyancy. The trim tanks are located in the front and back of the SFG. Both tanks are inaccessible flooding compartments. They do not interact with the open sea, so they are free from external hydrostatic pressure and only have to deal with the internal hydrostatic pressure.

2.3. Structural Desing

2.3.1. Materials

All of the material types for each compartment of the SFG and their mechanical properties are shown in

Table 2. Materials selected for the SFG.

Properties	Material	Yield Strength	Tensile Strength
Bulkhead	VL D37	360 MPa	276 MPa
External hull—Bow compartment	VL D47	460 MPa	550 MPa
External hull—Aft compartment	VL D47	460 MPa	550 MPa
External hull—Mid-body	VL D47	460 MPa	550 MPa
Internal hull—Main cargo tank	SA-738 Grade B	414 MPa	550 MPa
Internal hull—Compensation tank	SA-738 Grade B	414 MPa	586 MPa
Internal hull—Auxiliary cargo tank	SA-738 Grade B	414 MPa	586 MPa
Internal hull—Buoyancy tube	SA-738 Grade B	414 MPa	586 MPa
Internal hull—Trim tank	SA-738 Grade B	414 MPa	586 MPa

. The selection is based on the international standard DNVGL-RU-NAVAL-Pt4CH1 [15].

2.3.2. External Hull

The SFG is a torpedo-shaped vessel with a length-to-diameter ratio (a slenderness ratio) of 10:1. This design was chosen for the production simplicity of the vessel and to adjust the structure’s slenderness to obtain the largest cargo capacity with lowered drag resistance. The external hull of the SFG is reinforced by stiffeners, which prevent the external hull from having a buckling effect. The stiffeners are designed following the procedures provided in DNVGL-RU-NAVAL-Pt4CH1 [15]. The dimensions of the stiffener are shown in

Table 3. Stiffener dimensions for the SFG design.

Component	Symbol	Value	Units
Frame web thickness	sw	30	[mm]
Frame web height	hw	165	[mm]
Inner radius to the flange of the frame	Rf	2532	[mm]
Flange width	bf	80	[mm]
Frame spacing	Lf	1000	[mm]
Flange thickness	sf	30	[mm]
Frame cross-sectional area	Af	73,500	[mm2]

.

The external hull compartments of the SFG are as follows:

• The acceptable stresses in the nominal diving depth are 203 MPa, in the test diving depth are 418 MPa, and in the collapse depth are 460 MPa;
• The pressure hulls in the free-floating compartment are subjected to hydrostatic pressure. Stress at the collapse depth, nominal diving depth, and test diving depth for the flooded and free-flooding sections are computed and compared to the permissive stresses required in the DNVGL Rules for Classification for Naval Vessels, specifically in Part 4 Sub-Surface Ships, Chapter 1 Submarines (DNVGL-RU-NAVAL-Pt4Ch1) [15];
• The design of the flooded mid-body module uses the same procedure as for the free flooded compartments. Nonetheless, the flooded mid-body does not handle the hydrostatic pressure. Therefore, this section uses 20 bar (200 m) for the collapse pressure to avoid mechanical failure in unintentional load cases.

The external hull of the SFG is presented in

Table 4. SFG baseline design of the external hull.

Parameter		SFG 469 m3	SFG 1194 m3	SFG 2430 m3	Units
Free-floating bow compartment	Thickness	0.025	0.030	0.036	[m]
	Length	6.625	8.750	11.50	[m]
	Steel weight	21.899	43.877	87.510	[ton]
	Material	VL D47	VL D47	VL D47
	Design collapse pressure	40	40	40	[bar]
Flooded mid-body	Thickness	0.009	0.013	0.026	[m]
	Length	25.00	33.75	42.00	[m]
	Steel weight	34.049	80.850	222.749	[ton]
	Material	VL D47	VL D47	VL D47
	Design collapse pressure	20	20	20	[bar]
Free-floating aft compartment	Thickness	0.025	0.030	0.036	[m]
	Length	10.625	14.00	18.00	[m]
	Steel weight	27.412	54.581	105.942	[ton]
	Material	VL D47	VL D47	VL D47
	Design collapse pressure	40	40	40	[bar]

. Overall, the SFG mid-body of the external hull is the largest part of the submarine and accounts for 44% of the total structural weight of the baseline SFG design.

The detailed calculations and stress results for the external hull of the SFG baseline design are presented in Appendix A.

2.3.3. Internal Hull Design

The design of the internal tanks is based on the ASME BPVC Chapter 4, Section VIII, Division 2 [12]. The internal tank is defined as follows:

• The cargo tanks that are used for the storage of CO₂ are subjected to internal and external hydrostatic pressure. The tanks are designed for a burst pressure of 55 bar, which is the worst case scenario if the SFG must emerge from the water. Due to the external pressure being 0 bar gauge, the pressure difference equals 55 bar. A PCS (pressure compensation system) can be used to avoid failure caused by the collapse pressure. Detailed work on the PCS can be found in the studies by Ma et al. [18] and Xing et al. [12];
• The buoyancy tubes are designed to handle 20 bar of hydrostatic pressure. The pressure corresponds to the 200 m nominal diving depth;
• The compensation and trip tanks are located in the free flooding section, and they do not have particular requirements to withstand external pressure. For this reason, they are called soft tanks. Accordingly, they only need to resist the internal pressure caused by the flooding of the mid-section in the SFG. The tanks can have various shapes to use the space as much as possible. However, the calculation uses a cylindrical shape for both the compensation and trim tanks.

The details of the SFG’s internal design are displayed in

Table 5. The SFG’s internal tank properties.

Parameters		SFG 469 m3	SFG 1194 m3	SFG 2430 m3	Units
Main Cargo Tank (Total No. = 7)	Diameter	1.20	1.62	2.20	[m]
	Length	25.00	33.75	42.00	[m]
	Thickness	0.014	0.018	0.025	[m]
	Hemisphere head wall thickness	0.007	0.009	0.012	[m]
	Steel weight	68.688	168.998		[ton]
	Total volume	190.376	468.395	1073.411	[m2]
	Allowable burst pressure	55.0	55.0	55.0	[bar]
	Material	SA-738 Grade B	SA-738 Grade B	SA-738 Grade B
Auxiliary Cargo Tank (Total No. = 6)	Diameter	0.45	0.70	0.80	[m]
	Length	24.25	32.83	40.60	[m]
	Thickness	0.005	0.008	0.009	[m]
	Hemisphere head wall thickness	0.003	0.004	0.005	[m]
	Steel weight	8.153	26.670	43.115	[ton]
	Total volume	22,478	73.568	118.894	[m2]
	Allowable burst pressure	55.0	55.0	55.0	[bar]
	Material	SA-738 Grade B	SA-738 Grade B	SA-738 Grade B
Compensation Tank (Total No. = 2)	Diameter	3.0	3.5	5.5	[m]
	Length	1.7	2.5	5.0	[m]
	Thickness	0.006	0.010	0.014	[m]
	Steel weight	20.224	72.063	96.214	[ton]
	Total volume	17.34	61.25	75	[m2]
	Allowable burst pressure	8.0	8.0	8.0	[bar]
	Material	SA-738 Grade B	SA-738 Grade B	SA-738 Grade B
Trim Tank (Total No. = 2)	Diameter	1.6	1.8	3.00	[m]
	Length	2.24	2.50	6.3	[m]
	Thickness	0.006	0.008	0.022	[m]
	Steel weight	12.479	14.702	78.728	[ton]
	Total volume	10.0	45.00	60.0	[m2]
	Allowable burst pressure	10.0	10.0	10.0	[bar]
	Material	SA-738 Grade B	SA-738 Grade B	SA-738 Grade B
Buoyancy Tube (Total No. = 8)	Diameter	0.30	0.35	0.40	[m]
	Length	24.1	32.5	40.2	[m]
	Thickness	0.003	0.004	0.005	[m]
	Hemisphere head wall thickness	0.002	0.002	0.002	[m]
	Steel weight	4.816	8.845	14.300	[ton]
	Total volume	13.572	24.910	40.279	[m2]
	Allowable burst pressure	20.0	20.0	20.0	[bar]
	Material	SA-738 Grade B	SA-738 Grade B	SA-738 Grade B

.

The detailed procedure of calculations for the internal hull of the SFG baseline design is displayed in Appendix A.

2.4. Wing Design

The procedure for the design of the wings is illustrated in Figure 5. The nominal operating depth of the SFG defines the vessel class, which provides the foundations for selecting an actual angle of the glide path [13]. It is possible to compute the SFG velocities, lift, and yield drag forces based on the gliding angle. Next, the reference area of the hydrofoil and lift/drag ratio can be estimated. The parameters of the glider are shown in Figure 6, where W is the weight of the SFG and F_b is the buoyancy force [13].

The detailed procedure of derivation reference area for the wing is provided in Appendix A.

2.5. Weight Calcuation

The weight computation of the SFG is performed when the structural design is finished. The following requirements are used for the SFG:

• The machinery mass is 2% of the displacement;
• The permanent ballast is 2% of the displacement;
• The aimed payload is 40% of the displacement;
• The trim ballast is 0.7% of displacement.

Table 6. Weight compensation for the individual SFG design (CO₂-filled condition).

Component	Weight (Tons)
	SFG 469 m3		SFG 1194 m3		SFG 2430 m3
Machinery	9.610	2.00%	24.474	2.00%	49.798	2.00%
Permanent ballast	9.610	2.00%	24.274	2.00%	49.798	2.00%
Structure	187.999	39.12%	476.361	38.93%	962.768	38.67%
Mid-body seawater	69.051	14.37%	179.381	14.66%	277.306	11.14%
Compensation ballast	0.804	0.17%	0.999	0.08%	11.994	0.48%
Payload	200.082	41.64%	509.446	41.63%	1120.768	45.01%
Trim tank	3.364	0.70%	8.566	0.70%	17.429	0.70%
SUM	480.520	100%	1,223,700	100%	2489.914	100%

presents the weight composition for the SFG design filled with CO₂.

2.6. Hydrostatic Stabiliry

Once the structural design and weight estimation are completed, the hydrostatic stability is checked based on the requirements specified in DNVGL-RU-NAVAL-Pt4Ch1 [11]. For a submarine with a displacement of 1000–2000 tons, the distance between the centre of gravity G and the centre of buoyancy B must be greater than 0.32. Moreover, the location of the metacentric height GM must exceed 0.20 [18]. Four cases of hydrostatic stability are considered as follows:

• Submerged (CO₂ filled): Seven main tanks and six auxiliary tanks are fully submerged with liquified CO₂. In this case, the SFG is fully loaded.
• Surfaced (CO₂ filled): Seven main tanks and six auxiliary tanks are fully submerged with liquified CO₂. In this case, the SFG is floating on the surface of the sea and ready to dive to the nominal operating depth.
• Submerged (seawater filled): Seven main tanks and six auxiliary tanks are fully flooded with seawater. This case occurs after the SFG offloads the CO₂ at a well.
• Surfaced (seawater filled): Five primary and three auxiliary submarine tanks on the bottom side are filled with seawater. This case occurs when the vessel starts or finishes its mission.

The stability check results are presented in

Table 7. The stability check for the three different designs of the SFG.

SFG 469 m3 (Half-Scaled)
	Submerged (CO2 Filled)	Submerged (SW Filled)	Submerged (CO2 Filled)	Submerged (SW Filled)
CoG(x,y,z)	[−0.58, 0.00, 0.33]	[−0.52, 0.00, 0.33]	[−0.52, 0.00, 0.33]	[−0.60, 0.00, 0.51]
CoB(x,y,z)	[−0.67, 0.00, 0.00]	[−0.67, 0.00, 0.00]	[−0.67, 0.00, 4.10]	[−0.67, 0.00, 3.50]
M(x,y,z)	[0.00, 0.00, 0.00]	[0.00, 0.00, 0.00]	[0.00, 0.00, 0.00]	[0.00, 0.00, 0.00]
BG	0.330	0.330	3.770	2.990
GM	0.330	0.330	0.330	0.510
Result	BG > 0.32 == OK	BG > 0.32 == OK	GM > 0.2 == OK	GM > 0.2 == OK
SFG 1194 m3 (baseline design)
	Submerged (CO2 filled)	Submerged (SW filled)	Submerged (CO2 filled)	Submerged (SW filled)
CoG(x,y,z)	[−0.60, 0.00, 0.35]	[−0,54 0.00, 0.36]	[−0.70, 0.00, 0,37]	[−0.66, 0.00, 0.36]
CoB(x,y,z)	[−0.83, 0.00, 0.00]	[−0.84, 0.00, 0.00]	[−0.84, 0.00, 5.50]	[−0.84, 0.00, 4.20]
M(x,y,z)	[0.00, 0.00, 0.00]	[0.00, 0.00, 0.00]	[0.00, 0.00, 0.00]	[0.00, 0.00, 0.00]
BG	0.347	0.361	5.126	3.844
GM	0.347	0.361	0.374	0.356
Result	BG > 0.32 == OK	BG > 0.32 == OK	GM > 0.2 == OK	GM > 0.2 == OK
SFG 2430 m3 (doubled-scaled)
	Submerged (CO2 filled)	Submerged (SW filled)	Submerged (CO2 filled)	Submerged (SW filled)
CoG(x,y,z)	[−0.60, 0.00, 0.37]	[−0.56, 0.00, 0.39]	[−0.56, 0.00, 0.39]	[−0.65, 0.00, 0.39]
CoB(x,y,z)	[−1.00, 0.00, 0,00]	[−1.00, 0.00, 0.00]	[−1.00, 0.00, 5.10]	[−1.00, 0.00, 7.30]
M(x,y,z)	[0.00, 0.00, 0.00]	[0.00, 0.00, 0.00]	[0.00, 0.00, 0.00]	[0.00, 0.00, 0.00]
BG	0.372	0.388	4.712	6.13
GM	0.372	0.388	0.388	0.387
Result	BG > 0.35 == OK	BG > 0.35 == OK	GM > 0.22 == OK	GM > 0.22 == OK

.

2.7. Three SFG Schemes

In

Table 8. The main parameters of the final derived SFG design.

Parameter	Value
	SFG 469 m3	SFG 1194 m3	SFG 2430 m3
Lightweight [ton]	197.609	500.835	962.819
Lightweight [m3]	192.789	488.619	939.336
Deadweight [ton]	282.911	722.866	1527.094
Deadweight [m3]	276.011	705,235	1489.848
Length [m]	42.25	56.50	71.50
Beam [m]	4.00	5.50	7.00
Displacement [ton]	480.520	1223.701	2489.914
Displacement [m3]	468.800	1193.854	2429.184
Total power consumption [kW]	6450	9545	14,533
Power consumptions [kWh/day]	1381	2044	3112
Speed [knots]	2.00	2.00	2.00
Travel distance [km]	400.00	400.00	400.00

, the critical parameters of the final design are displayed.

3. Methodology for the Economic Feasibility Analysis of the Novel SFG Concept

The costs of a project for developing a subsea project are generally referred to as the capital expenditure (CAPEX) and operational expenditure (OPEX). The capital expenditure is the total investment required to put a project into operation. It includes the initial design, engineering, and construction of the facility. The term OPEX refers to the expenses incurred by a facility or component during its operation. These expenses include labour, materials, and utilities. Aside from these, other costs such as testing and maintenance are also included in the OPEX [12].

The economic analysis was performed based on two publicly available cost models from the MUNIN [20] and ZEP [21] reports. The MUNIN D9.3 [20] report presents a complete study of autonomous ship development, economic effects, security and safety effects, and relevant areas of law. In this paper, the data from the MUNIN report [20] related to autonomous ships are used for the economic impact assessment cost analysis. The ZEP report [21] shows an analysis of CO₂ transportation in the deployment of carbon capture and storage (CCS) and carbon capture and utilisation (CCU) systems. Data provided in the ZEP report [21] were provided by members of maritime organisations, including stakeholders and essential players in marine transportation, such as Teekay Shipping, Open Grid Europe, and Gassco. The analysis is very detailed and covers all components. For instance, the cost of the actual coating is specified and considered for the offshore pipeline. The present work uses cost models from the ZEP report [21] for the cost estimations, including the OPEX and CAPEX, ship capacities, and electricity prices, for offshore pipelines.

An outline of the MUNIN D9.3 [20] and ZEP [21] reports is listed in

Table 9. Cost models presented in the MUNIN D9.3 and ZEP reports.

The Cost Model Shown in MUNIN D9.3	The Cost Model Presented in ZEP
Autonomous ship capital expenditure	Offshore pipeline capital expenditure
Fuel price	Offshore pipeline operating expenditure
Ship fuel consumption	Electricity price
Discount rate	Discount rate
	Ship capacity
	Vessel loading and offloading durations
	Vessel transport velocity
	Ship capital expenditure
	Ship operating expenditure
	Transport distance cases
	Transport volume cases
	Liquefaction price
	Project lifetime

.

This paper considers transport distances of 180, 500, 750, and 1500 km with CO₂ shipping capacities of 0.5, 1, and 2.5 million tons per annum (mtpa). The CO₂ is carried from a capture plant at ambient temperature and a pressure of 110 bar. The following assumptions for the CO₂ transportation are made:

• The SFG and ship offload straight to the well without the usage of an intermediate buffer storage space;
• In cases of large CO₂ volumes or long distances, there is a need for more than one transportation vessel; for instance, in a 180 km transport distance and 1 mtpa transport volume scenario, 6 SFGs measuring 2430 m³ or 11 SFGs measuring 1194 m³ are required;
• The cost of the subsea well-head is not considered in the following study;
• The rate of currency exchange is 0.87 EUR/USD;
• The discount rate is 8% and the project lifetime is 40 years.

The procedure and computations for the economic study of autonomous/crew tanker ships, offshore pipelines and SFGs are presented in Appendix B.

3.1. SFG, Crewed and Autonomous Tanker Ship

The ship transporting the CO₂ is equipped with semi-refrigerated liquefied petroleum gas (LPG) tanks. The liquefied gas is transported at a temperature of −50 °C. During the transportation, the tanker ship requires refrigeration and liquefied gas, which is transported at 7–9 bar and close to −55 °C to avoid the risk of the formation of dry ice. During transportation, the temperature of the CO₂ will increase, initiating a boil-off and increasing the internal pressure of the ship. Therefore, the cargo pressure at the end of the loaded journey will typically be around 8–9 bar.

The properties of the tanker ship are displayed in

Table 10. Properties of the autonomous and crewed ships.

Crew and Autonomous Tanker Ship Properties
Liquefaction 2.5 mtpa	5.31	€/ton
Liquefaction 10 mtpa	5.09	€/ton
Loading/offloading time	12.00	h
Speed	14.00	knots
Fuel consumption, ship 22,000 m3	9.13	ton/day
Payload	80.00	%

.

The minimum number of SFGs and tanker ships required to fulfil the mission is calculated using the following equation:

$$\mathrm{N}=\mathrm{roundup}\left(\frac{{\mathrm{V}}_{{\mathrm{CO}}_2}}{{\mathrm{V}}_{\vartheta }{\mathsf{\rho }}_{{\mathrm{CO}}_2}\frac{365}{2{\mathrm{L}}_{\mathrm{t}}{\mathrm{U}}_{\vartheta }+2{\mathrm{T}}_{\mathrm{L}}}}\right)$$

(1)

where N is the number of vessels, Vv is the total capacity for a vessel, U_v is the velocity of the vessel, T_L is the time of loading or offloading, ${\mathsf{\rho }}_{{\mathrm{CO}}_2}$ is the CO₂ density, L_t is the distance of the transportation, and ${\mathrm{V}}_{{\mathrm{CO}}_2}$ is the total CO₂ capacity per annum.

The calculated parameters to find the number of SFGs (baseline design—1194 m³) needed to complete the mission of transporting 2.5 mtpa of CO₂ for a distance of 180 km are displayed in

Table 11. Required number of SFG vessels (baseline design).

Parameters	Value	Units
Total CO2 capacity	2.5	[mtpa]
Transport distance	180	[km]
Loading and offloading time	4	[hours]
The velocity of the SFG	2	[knots]
Cargo volume	723	[m3]
CO2 density	940	[kg/m3]
Number of required vessels	27

.

The capital expenditure (CAPEX) is calculated based on the price per ton of structural steel weight. According to the ZEP report [21], the maximum and minimum costs for a ton of steel are calculated at 11,631–28,888 € per ton. As presented in

Table 12. CAPEX inputs for crew and autonomous tanker ships.

Inputs to CAPEX of Crew and Autonomous Tanker Ships
Steel price (max) in ZEP report	28,888.50	€/ton
Steel price (min) in ZEP report	11,631.45	€/ton
Steel price (average) in the ZEP report	18,896.04	€/ton
Residual value	0	€
Autonomous ship price	110% crew ship price

, it is assumed that an autonomous tanker ship has a CAPEX of 110% of a crew tanker ship. The vessels should be modern and equipped with submerged turret offloading buoy capabilities and dynamic positioning.

The CAPEX values of the SFGs and the tanker ships are calculated using the following equation:

$$\mathrm{Annuity}=\frac{\mathrm{CAPEX}\times \mathrm{discount} \mathrm{rate}}{1-{(1+\mathrm{discount} \mathrm{rate})}^{-\mathrm{lifetime}}}$$

(2)

The discount rate is estimated to be 8% and the lifetime to be 40 years. Based on these assumed parameters, the annuity is calculated using the below equation.

The tanker ship is powered by marine diesel oil or LNG. For both fuels, the price per ton is the same. The data used to calculate operating expenditure (OPEX) are displayed in

Table 13. OPEX inputs for the SFG and crew and autonomous tanker ships.

Inputs to the OPEX for Crew and Autonomous Tanker Ships
Fuel price	573.33	€/ton
Electricity price	0.11	€/kWh
Crew Price	640,180.80	€/year—20 crews
Maintenance	2	%

.

The OPEX values of the SFGs and tanker ships are calculated using the following equations:

$${\mathrm{OPEX}}_{\mathrm{CS}}=\mathrm{Maintenance}+\mathrm{Crew}+\mathrm{Fuel}+\mathrm{Liquefaction}$$

(3)

$${\mathrm{OPEX}}_{\mathrm{AS}}=\mathrm{Maintenance}+\mathrm{Fuel}+\mathrm{Liquefaction}$$

(4)

$${\mathrm{OPEX}}_{\mathrm{SFG}/\mathrm{SST}}=\mathrm{Maintenace}+\mathrm{Electricity}$$

(5)

Based on the data provided in the ZEP report [21], the crew tanker ship’s capital expenditure is approximately 60–149 m€. Accordingly, the CAPEX for the autonomous tanker ship is about 66–164 m€.

3.2. Offshore Pipelines

Overall, the offshore pipeline costs are controlled by the CAPEX, and they are proportional to the pipe’s length. In the design of offshore pipelines, the essential factors are the pipeline diameter, wall thickness, transport capacity, outlet and inlet pressures, and steel quality. Additionally, factors such as the corrosion protection, design against trawling, installation method, dropped object protection, and bottom stability should be considered.

In this study, the manifold cost for the well and the injection well drilling are not considered. The capital expenditure is estimated based on the steel market price, pipeline installation cost, trenching, and pipeline coating. The CO₂ is sent through the pipelines at 55–88 bar in the supercritical phase. In this case, the pressure boosters and the related costs are required, and they are contained in the calculation of the CAPEX. Furthermore, the pressure of the CO₂ in the pipeline is determined by the storage conditions. In this analysis, the cost of the pre-transport CO₂ compression is included in the price of the capture facility.

In this study, the lowest volume (1 mtpa of CO₂) for the offshore pipelines is not considered. This is because offshore pipelines are too expensive due to their small transportation capacity, and it is not economical to use this method of transfer.

The components’ properties and pricing for the offshore pipelines are displayed in

Table 14. Properties of the offshore pipelines.

Properties of the Offshore Pipelines
Pressures	250	[bar]
Outlet pressure	60	[bar]
Inlet pressure	200	[bar]
Pipeline internal friction	50
External coating	3	[mm]
Pipeline material	Carbon steel
Concrete coating (pipeline above 16”)	70 mm; 2600 kg/m3

and

Table 15. Pricing of the offshore pipeline components.

Component Pricing of the Offshore Pipelines
Trenching cost	20–40	€/meter
Installation cost	200–300	€/meter
Pipeline OPEX for 2.5 mtpa	2.35	m€/year
Contingency	20%
Steel price for pipeline 16”	160	€/meter
Steel price for pipeline 40’	700	€/meter
External coating for pipeline 16”	90	€/meter
External coating for pipeline 40”	200	€/meter
Trenching cost	20–40	€/meter

.

The CAPEX values for an offshore pipeline are shown in the ZEP [21] report. The maximum and minimum values are expected to be 120% and 80%, respectively.

The average OPEX values for an underwater pipeline are shown in the ZEP [21] report. The minimum and maximum OPEX values are approximately 80% and 120%, respectively. The pipeline’s CO₂ volume is expected to be around 2.5 million tons per year.

The offshore pipeline annuities are calculated based on the design definitions, and the related costs are included in Table 14 and Table 15, respectively. A 2.5 million tons of transport capacity offshore pipeline will take the cost to about 20.986–126.961 m€. All calculated data are displayed in

Table 16. The offshore pipeline annuities.

CO2 Volume	Offshore Pipeline Length
	180 km	500 km	750 km	1500 km
0.5 mtpa	-	-	-	-
1 mtpa	-	-	-	-
2.5 mtpa	20.986 m€	48.688 m€	69.412 m€	126.961 m€

. All operational costs and aspects of maintenance are included in the OPEX. The operating expenditures equal 2.35 m€/a.

4. Results and Discussion

The technical and economic feasibility analysis results are discussed in this section. The analysis includes detailed technical–economic studies of modern transportation submarines, namely the SFGs, used for CO₂ transport with comparisons with crew and autonomous tanker ships, offshore pipelines, and SST values.

4.1. Number of Vessels

The minimum number of vessels required to perform the mission is illustrated in Figure 7.

4.2. CAPEX and OPEX Results

The CAPEX results for all transportation methods are displayed in Figure 8. Overall, can obviously be seen that the SFG CAPEX increases significantly with the size. This implies that the SFG is not an economical solution if large transportation capacities are needed.

The SFG was designed based on DNV-RU-NAVAL-Pt4Ch1 [15], and was initially created for military submarine designs. Due to the high safety requirements, the SFG has a very heavy structural weight, making the CAPEX value very high. For a specific SFG, a potential solution to reduce the weight and CAPEX is to reduce the design safety factors that are suggested in the design code for general SFGs.

The OPEX/CAPEX ratios are displayed in Figure 9. It can be seen that OPEX dominates among the costs for crew and autonomous tanker ships. For these vessels, the CAPEX/OPEX ratios range between 2.59 and 7.28. On the other hand, the highest CAPEX and lowest OPEX results are for offshore pipelines, and the OPEX/CAPEX ratio range is 0.06–0.38. For the SFGs, the OPEX is comparable with the CAPEX, and their CAPEX/OPEX ratio range is 1.07–1.39.

4.3. Economic Analysis

Figure 10 displays the results for the average cost per ton of CO₂. Overall, the subsea shuttle tanker and offshore pipelines have the lowest costs for short distances with large capacities. In contrast, the tanker ships (crew and autonomous) have the lowest prices for longer distances. The SFG is economical for small CO₂ volumes of 0.5–1 mtpa and short distances of 180–500 km. It is noted that with increasing CO₂ volumes, the cost per ton of CO₂ decreases. This is because of the better economies of scale.

4.3.1. Short Distances (<180 km)

For the small CO₂ capacity of 0.5–1 mtpa, the SFG has the lowest cost. The major reason for having the lowest price is the small number of vessels needed to complete the mission. In contrast, the crew tanker ship with the smallest capacity is oversized. As a result, the SFG has a lower CAPEX and OPEX than the other vessels.

In the 0.5 and 1 mtpa volume cases, the offshore pipelines are not considered. Overall, offshore pipelines are not economical for transferring small volumes of CO₂. They are most profitable for large transport volumes (10–20 mtpa).

4.3.2. Intermediate and Long Distance (500–1500 km)

Due to travelling at low velocity, the SFG requires more vessels to meet the requirements for transporting CO₂ at larger than 1 mtpa capacity. This results in higher capital expenditures and a significantly higher cost per ton of CO₂ than for a crew or autonomous tanker ship. For instance, if the amount of CO₂ is 2.5 mtpa for 516 or 1500 km, the SFG approach requires 103–530 ships. The SFG CAPEX range is 1827.33–1982.95 million euros, while 298.71 million euros is the CAPEX for crew ships. As a result, the average cost of the SFG per ton of CO₂ is in the range of 82.93–94.13 million euros, while it is 15.47 million euros for crew ships. Nevertheless, the SFG is economical for smaller CO₂ volumes (0.5 and 1 mtpa).

Table 17. Transport methods with the lowest costs for various distances and volumes.

	180 km	500 km	750 km	1500 km
0.5 mtpa	SFG	SFG/SST	SST	CS/AS
1 mtpa	SFG/SST	SST	AS/CS/SST	CS/AS
2.5 mtpa	OP/SST	CS	CS	CS/AS

presents the economic feasibility analysis results, along with the lowest costs.

5. Conclusions and Future Work

This study deals with a technical–economic feasibility analysis for a novel subsea freight glider, which consists of two steps. The first step involves investigating the design limits of the SFG, while the second step focuses on performing an economic analysis. The SFG is designed based on the procedure provided in international standards of the DNV-RU-NAVAL-Pt4Ch1 and ASME BPVC. The Marine Unmanned Navigation through Intelligence in Network (MUNIN D9.3) and Zero Emission Platform (ZEP) cost models are used for the economic analysis.

The presented research demonstrates that the SFGs with a cargo volume of 469 m³, 1194 m³, and 2430 m³ are able to fulfil the mission requirements. The scenarios considered for this study involve the transport of CO₂ volumes of 0.5, 1.0, and 2.5 million tons per year over distances of 180, 500, 750, and 1500 kilometres. The cost per ton of CO₂ for the SFGs is compared with the cost of transporting the same volume on a tanker ship or via offshore pipelines. This study indicates that the use of SFGs is technically feasible for short distances of up to 500 kilometres and smaller CO₂ volumes of less than 1 million tons per year. The SFG approach is also a cheaper solution than the use of crew and autonomous tanker ships due to the lower OPEX and CAPEX. Additionally, because of its slow-moving speed and the advantage of having no liquefaction cost, the SFG can transport CO₂ in a saturated state, which significantly reduces the total price.

The performed technical–economic analysis of the SFG shows that the small underwater vehicle is technically feasible and economically profitable to complete the mission presented here. Moreover, the research shows that the SFG is an attractive alternative for CO₂ transportation. It is shown that the solution is cost-competitive for low-volume and short-distance transport. With this, the authors hope this will engage the research and engineering community to further consider and evaluate the concept, resulting in a final working concept that will eventually be a preferred alternative for low-volume and short-distance transport. However, there is still work that can be done in the future. In this study, the design-by-rules method was applied. There is still a need to perform a design-by-analysis approach with an elastic and plastic stress analysis. In addition, this analysis included only small sized submarines. It is essential to carry out an economic study for SFGs with drastically increased sizes.

Lastly, this research started at the University of Stavanger in 2019 in collaboration with Equinor [11]. This research has its roots in Norway and has focused so far on the Norwegian Continental Shelf, where there are many marginal fields within 200 km of the coast. Even so, the authors expect that the vessel can also be applied to other suitable fields in other regions in the world.