Mitsubishi Heavy Industries Technical Review Vol. 61 No. 2 (June 2024)
1

Liquefied CO2 Carrier "EXCOOL"

Mitsubishi Shipbuilding Co., Ltd.
Business Department
Marine Engineering Center

As an effective means to realize a carbon-neutral society, CCUS (Carbon dioxide Capture,Utilization and Storage) is attracting attention. In order to realize a CCUS value chain, a means oftransporting the captured CO2 to the storage site is necessary. Liquefied CO2 carriers are
considered to be a safe and low-cost means of transportation and their demand is predicted toincrease in the future, so there are high expectations for the technology to be established.
EXCOOL (hereinafter referred to as "the vessel") is a liquefied CO2 (LCO2) and liquefied
petroleum gas carrier built at Shimonoseki Shipyard & Machinery Works of Mitsubishi HeavyIndustries, Ltd. for the purpose of being offered to the New Energy and Industrial Technology
Development Organization (NEDO) project "Research, Development and Demonstration of CCUS
Technology/Large-scale CCUS demonstration testing at Tomakomai/Demonstration testing on CO2Transportation". The vessel was delivered in November 2023 and will be utilized for theaforementioned NEDO demonstration testing.

|1. Features of the vessel
1.1Specifications and general arrangement

Table 1 and Figure 1 shows the specifications and general arrangement of the vessel,respectively. The vessel is a pressurized liquefied gas carrier with a forecastle and a poop. The main
hull has a single-bottom structure with two cargo holds, each of which is equipped with one
horizontal cylindrical cargo tank. The propulsion system is a single-engine, single-shaft, controllable
pitch propeller, and a bow side thruster is provided for berthing and unberthing. The vessel'snavigation area is classified as the greater coasting area (non-international), so there are lessrestrictions on the navigation area where demonstration testing can be conducted. In addition, since
the main route is between Maizuru and Tomakomai, cold weather protection measures are also
provided.
Table 1

Specifications of EXCOOL

Length overall
BreadthDepth
draft
Gross tonnage (Japanese)
Complement
Total cargo tank capacity
Speed
Maximum output of main engine

(m)
(m)
(m)
(m)
(persons)
(m3)
(kn)
(kW)

Abt. 71.99
12.50
5.50
4.60
996
10
Abt. 1,450
12.01,471

Mitsubishi Heavy Industries Technical Review Vol. 61 No. 2 (June 2024)
2Figure 1 General arrangement

1.2

CO2 characteristics and requirements for liquefied CO2 carrier

At room temperature and atmospheric pressure, CO2 is a colorless, odorless gas that is heavier
than air. When transporting CO2 by ship, it is desirable to liquefy it to reduce its volume, but asshown in the phase diagram in Figure 2, keeping CO2 in a liquid state requires maintaining low
temperatures and high pressures. For this reason, the temperature and pressure of liquefied CO2 tobe transported by sea needs to be in the range of approximately -55 to -20°C and just below 1.0
MPaA to 2.0 MPaA, respectively, thus the tank to contain the CO2 needs to be a pressure vessel madeof a material with high strength and excellent low-temperature characteristics.
In addition, if the pressure and temperature of CO2 fall below the triple point during marinetransportation or cargo handling, solidification may occur, i.e., dry ice may be formed. Therefore, indesigning cargo tanks and cargo handling equipment of the liquefied CO2 carrier, these factors need
to be taken into consideration.
Generally, to reduce the risk of dry ice formation during operation, it is desirable to maintaina relatively high pressure and a sufficient margin with respect to the triple point shown in Figure 2.
In this case, however, the cargo tank needs to withstand a relatively high pressure, thus its size islimited from the viewpoint of structural strength. On the other hand, to improve transportation
efficiency, it is desirable to keep pressure conditions as low as possible and increase the volume of
the cargo tank. In this way, these two conflicting factors need to be carefully taken into considerationin designing cargo tank structures and cargo handling equipment and setting cargo handlingconditions is an important technological point for the realization of safe and efficient marinetransportation by the liquefied CO2 carrier.

Figure 2 CO2 phase diagram

1.3

Cargo tank

The vessel is equipped with two horizontal cylindrical cargo tanks, which are the independenttank type C as specified in the regulations of the International Maritime Organization (IMO) on theInternational Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk

Mitsubishi Heavy Industries Technical Review Vol. 61 No. 2 (June 2024)
3(IGC Code). In order to satisfy the requirements of the IGC Code and the rules of the Classification
Society, appropriate structural design and material selection for the cargo tanks were made as an
LCO2/LPG carrier. In addition, necessary countermeasures are taken against sloshing phenomena,
which occur when the period of ship motion and the natural period of the liquid free surface in thetank are in sync, by calculating the detailed fluid loads using computational fluid dynamics (CFD)
calculations and evaluating the strength of the tank structure using finite element analysis (FEA).
Figure 3 shows an example of the flow of liquefied CO2 in the tank based on CFD analysis.
The cargo hold is divided into two sections, the front section and the rear section, and the tankcovers are placed independently on the No. 1 and No. 2 cargo tanks. To minimize the clearance
between the cargo tanks and the tank covers and maximize the tank volume at the same time, thestructural reinforcement members of the tank covers are placed outside the hold. This measure allows
the 996-gross tonnage vessel to secure a total volume of approximately 1,450 m3 for the two tanks.
Furthermore, since low-temperature liquefied CO2 is stored in the cargo tanks, their outersurfaces are covered with urethane foam insulating materials to suppress heat gain from the outside.
Figure 4 shows the external appearance of the cargo tank.

Figure 3 CFD sloshing simulation of liquefied
CO2 cargo tank

1.4

Figure 4

Cargo tank

Cargo handling equipment

The vessel's cargo handling equipment consists of shore manifolds located in the middle of thevessel between the two tanks on both sides of the hull, and two cargo tanks equipped with cargo
pump, piping, instrumentation, and tank dome with access manhole. Two cargo gas compressors forcargo handling are installed in the cargo gas compressor room under the forecastle deck.
For these cargo handling equipment, equipment selection and piping design as LCO2/LPG
carriers were conducted considering various temperatures and densities due to cargo conditions, aswell as changes in the state of liquefied CO2 due to pressure and temperature changes during
operation.
Furthermore, to ensure safe handling of CO2 on board, experts from within the MHI Group
have conducted repeated risk assessments in order to take the necessary safety measures. Prior to thedelivery of the vessel, the gas trial (testing with cargo liquids) required by the IGC Code wasconducted to confirm the performance of the cargo tanks and cargo handling equipment.

|2. Future prospect
In order to achieve carbon neutrality by 2050, which is the reduction target of the Japanese
government, it is considered important to implement CCUS in society, so liquefied CO2 carriers areexpected to be in greater demand in the future as a means of safe and economical mass transport ofCO2 separated and captured at the source to storage sites.
Mitsubishi Shipbuilding Co., Ltd. Is working to “Realizing Decarbonization of the Maritime
Economy" in its "Marine Future Stream" development strategy, and as part of this, we are actively
promoting the commercialization of the "LCO2 handling system”, which consists of cargo tanks andcargo handling equipment for liquefied CO2 carriers. Through such approaches, we will continue tocontribute to the realization of a decarbonized society as a marine system integrator.