STUDIES OF THE LARGE SCALE SEA TRANSPORTATION OF THE LIQUID HYDROGEN

ABSTRACT

1. Introduction

2. Organisation and Schedule of WE-NET hydrogen tanker program

Ishikawajima-Harima Heavy Industries Co. Ltd.(IHI)
Kawasaki Heavy Industries Ltd.(KHI)
Mitsubishi Heavy Industries Ltd.(MHI)
Mitsui Engineering and Shipbuilding Co. Ltd.(MES)

3. Outline of Phase�T

3.1.1 Overseas research on liquid hydrogen tanker

Firstly we have studied the report of EQHHPP(Euro-Quebec Hydro-Hydrogen Pilot Project) which are planning to transport liquid hydrogen produced in Canada with their ample electric energy to Europe. According to the report, transportation by barge with numbers of liquid hydrogen tanks on the deck(total volume 15,000m³ ) is mainly investigated.
Large liquid hydrogen tanker with the volume of 120,000m³ has reportedly been investigated in Germany for the future large scale transportation.
In addition to these, various ideas of liquid hydrogen tanker were reported, for example, SWATH tanker, hydrogen container ship, and so on.
However, details of tank and insulation system are not reported yet.

3.1.2 LNG ship technology

As the base of the development work of liquid hydrogen tanker, existing technology for LNG tanker was summed up and utilised for grouping the required technology for hydrogen transportation.
LNG tanker which have been developed since 1959 is a ship to carry LNG at boiling point of -163��, and its essential features exist in its cryogenic cargo containment system(cargo tank, insulation and tank support), secondary barrier concept, hull structural arrangement, boil off gas treatment, etc.

3.1.3 Comparison of Liquid Hydrogen and LNG

Among the various properties of hydrogen investigated, typical features are compared with LNG in Table-1, which are assumed to affect greatly on the design of hydrogen tanker.
Temperature of liquid hydrogen is lower than LNG by 90��, in which circumstance even oxygen and nitrogen are frozen. And the liquid density is as small as 1/6 of LNG, which makes the hull design difficult from the view point of keeping draft for propeller immersion and manoeuvrability.

Table - 1 Hydrogen Characteristics

Item	Liquid Hydrogen	LNG (methane)	Remark
Boiling Point Specific Gravity Latent Heat Explosion limit Higher Calorific Value	-253.0 deg.C 71.0kg/m3 447.0 kj/kg 4 - 76% 142,060 kj/kg 12,770 kj/m3 10,086,260 kj/m3	-163.0 deg.C 424.0kg/m3 510.0 kj/kg 5 - 15% 56,000 kj/kg 45,920 kj/m3 23,744,000 kj/m3	�@ Boiling point Boiling point Atmosphere �@ Gas(0deg.C,1atm) Liquid

These features seem not to be solved by extrapolation of LNG technology but will require the technical break-through as shown in the next section.

3.1.4 Difference from existing technology

The first issue is that the liquid hydrogen is extremely low in temperature that even oxygen and nitrogen are frozen. So its thermal insulation method requires higher performances than those of LNG carrier including vacuumed perlite insulation system, vacuumed multi-layer super insulation system and so on.
As thermal contraction is also greater than LNG carrier , more sophisticated support design is required.
The second issue is that liquid density is extremely light. This is the good feature of hydrogen for tank strength but makes the hull basic design much more difficult to provide the adequate draft with small displacement. Consequently it causes the problem of maneuverability and stability.
Considering that the hydrogen is 10 times easier to evaporate than LNG by the same heat intrusion, large tank volume and faster ship speed is preferable for the efficient and economical transportation. Then twin hull ship becomes one of the choice for hydrogen tanker.
As to the tank materials, stainless steel or aluminum alloy used in LNG tank are basically applicable because of its good resistance to the brittle fracture even in -253��.But in the future stage, detailed investigation and experiments will be indispensable with regard to the fatigue and fracture strength at -253��.
Taking the above as our common understandings, we had conducted the conceptual design of hydrogen tanker in the second year.

3.2 Conceptual Design

In order to clarify the technical problems more vividly, preliminary conceptual design of hydrogen tanker was carried out with prismatic and spherical tank systems.

3.2.1 Principal Particulars

Principal particulars were set as follows.

1) Capacity of cargo tank
When a 1,000,000KW class power plant consumes 1,200ton of liquid hydrogen per day, the required tank capacity for a tanker is approx. 14,000ton including transportation loss which is equal to 200,000m3, assuming that a round voyage is 20 days and 2 ships are utilized to load every 10 days.
2) Number of cargo tanks
Considering the arranging factor to keep ship balance in voyage, equipping factor for piping, stability in damaged condition, we applied 4 tanks for spherical system, and 2 tanks for prismatic system.
3) Endurance ,ship speed and engine power
6000 nautical miles was set to cover the almost of potential routes.
Consequently ship speed of 20-25kt was set to allow 6000n.mile per 10 days. Estimated engine power is approx. 80,000HP regardless of the tank system.
4) Ship type
Considering the required ship speed, normal mono-hull ship is also applicable.
Therefore mono-hull and twin-hull design are studied. Following four types of ship were designed.


	Spherical Tank-Mono-Hull and Twin Hull
	Prismatic Tank-Mono-Hull and Twin Hull

Twin hull ship design of the spherical tank and the prismatic tank are shown in Fig.1. In these design ,clearance between tank and hull was set as approx. 1.5m to accommodate the insulation system.
5) Boil Off rate
We assumed that the hydrogen burning engine would be applicable in the future and the boil off gas would be utilized as the fuel of the main engine same as forLNG ship. The design boil-off rate is set as 0.2-0.4%/day based on the normal burning ratio of 20-40% in case of main engine power of 100,000HP.

3.2.2 Results

Based on the above conceptual design, followings were confirmed.

1) With regard to the basic design elements of hydrogen tanker(hull form, powering, general arrangement, hull structure design etc.), we have confirmed that the conventional technology for LNG carrier can be used effectively.
2) If the high performance insulation and support system could be developed, 200,000m³ liquid hydrogen tanker can be built taking advantage of matured ultra-large ship building technology. At the same time, we have realized the significance to develop the effective insulation system for liquid hydrogen.
3) While the tank strength is not so critical because the liquid density is far smaller than LNG, unified design philosophy and criteria based on the risk analysis for hydrogen leak, accidental failure, collision and grounding to be established like a IMO TYPE-B Code for LNG carrier.
4) For the more realistic study of the hydrogen tanker, investigation of loading and unloading operation including dome and piping design is indispensable because the evaporation loss in such operation can be significant.

4. Fundamental Study for Cargo Containment System.

Results of our investigations are summarised as following table.

Supplemental explanation is described hereunder.

1) Conventional PUF panel system
In case of the hydrogen tanker, approx.1m thick PUF panel could theoretically provide the target boil off rate of 0.2-0.4%/day for hydrogen tank. But once tank is cooled down to -253��, not only the gas in the cell of PUF but even air or nitrogen gas in the hold space is frozen and could possibly destroy the PUF panel. In addition, the convection of gas in the gap between the adjacent panels could degrade the insulation performance.
In order to solve these problems, hold space can be vacuumed to some level. In this case, required panel thickness is 400-500mm including vacuum effect. Required vacuum level is set as 10-4 torr which is assumed to eliminate the effect of convection. In this system, it is to be solved how to make and maintain such a large hold space in vacuum condition. In addition, deterioration of vacuum by the gas in the cell of PUF is to be considered. Tentative answer is that vacuum pump provides 10-2torr, then it will be depressurized gradually to the level of 10-4 torr by the effect of cryogenic pump . This can be one of the advantage of this system together with its low material cost and residual insulating capability by PUF itself which prevents the sudden temperature increase and consequent bursting evaporation of hydrogen in tank in the case of accidental vacuum failure.
2) Vacuum panel system
Vacuum panel is composed by the light core material covered by the continuous membrane sheet and inside is kept vacuum. In order to attain the target boil off rate, this panel is required to have a thickness of between 250 and 500 mm. This panel reportedly provide the good insulation performance by the unit structures. But considering the application to large scale actual tanker, several problems are left to be solved. One is the convection problem in the gap of each panel, which is similar to PUF panel. Another is heat transfer problem through the metallic covering skin. These effects have to be adequately evaluated for the actual application. Further, the possibility that vacuum failure of the panel causes decrease of global insulation performance has to be carefully investigated .
3) Super insulation
Super insulation is installed in the evacuated space which is to be insulation layers so that thermal conduction and convection can be minimised. Then radiationshield, such as aluminized polyester evaporation film, is placed on the radiationsurface. In addition low conductivity material is placed as spacers between theradiation shield to prevent the shield from coming into contact with each other.For installation of superinsulation material, shape of the tank support is to be assimple as possible. Therefore, the tank and primary cover are independently supported by a cylindrical skirt. As a result of calculation of insulation performance,it is found that the boil off rate increases logarithmically as pressure in the vacuumspace increases. Also, we obtained a result that approximately 10-4 torr. is requiredin order to reach the required minimum rate.


In the fourth year, we have been investigating the tank insulation system in more detail using finite element method , etc.


4.2 Tank Support System


1) Spherical Tank
Although the tank shrinkage is bigger than LNG carrier, similar cylindrical support system to those of LNG carrier is considered to be basically applicable to hydrogen tank after the strength calculation is adequately carried out. This arrangement permit to minimise any thermal stress. Heat ingress can be minimised by using the thermal brake system.
2) Prismatic Tank
Prismatic tank system allows the free contraction of regardless the extent of shrinkage tanks to its support arrangement. Tank vertical load is supported by the horizontally slidable supporting blocks on the deck. Tank lateral movement is supported by chocks(key and key way guide) arranged in the centre line and midtank to form the anchor point at tank centre. This arrangement permit free contraction of hydrogen tank without causing any thermal stress. Heat ingress can be minimised by no metallic connection which is provided between hull and tank.





5. Conclusion

Outline of the preliminary study of large scale hydrogen tanker conducted in WE-NET program was reported. While we have to say that only a few concrete progress is available at this moment, some results or consideration are summarised as follows.

1) In order to meet demand of hydrogen energy network oriented by WE-NET program, large scale sea transportation of liquid hydrogen by tanker is necessary. Based on this requirement, conceptual design of 200,000m³ liquid hydrogen tanker is carried out introducing the LNG ship technology.
2) Independent tank system for LNG ship( spherical and prismatic) are basically applicable to the liquid hydrogen tank.
3) Twin hull ship is suitable to accommodate such very light cargo while keeping draft, stability and high speed performance.
4) Insulation system is most essential issue for hydrogen tanker because even the nitrogen gas and oxygen gas are frozen at hydrogen temperature. In addition, prevention of the deterioration of insulation performance due to the heat convection between each panel shall be the important problem to be carefully investigated.
5) According to preliminary study of the insulation system, following combinations can be the design alternatives and require further detailed investigations.

a) PUF Panel in Vacuumed or Non-Vacuumed Hold Space
b)Vacuum Panel Insulation in Vacuumed or Non-Vacuumed hold Space
c) Super Insulation in Vacuumed Hold Space

It is suggested that, in any selection, insulation system shall be provided with the essential redundancy to prevent the accidental temperature increase and consequent bursting evaporation of hydrogen in tank. In addition , in order to confirm efficiency and effectiveness of these insulation system ,further precise analysis, elemental experiment and model test are considered to be necessary.





6. Acknowledgement

The authors would like to express our gratitude to Professor Ohtsubo of the University of Tokyo for his supports and valuable comments on our research.
We also would like to express our hearty appreciation to Engineering Advancement Association of Japan for the promotion and co-ordination of this research program.





Literature Reference

1) G.Giacomazzi: Maritime Hydrogen Transportation, Symposium-Exhibition
"Hydrogen Within a Clean and Renewable Energy System"
2) Engineering Advancement Association of Japan:
"Development of Liquid Hydrogen Tanker", 1994(NEDO-WE-NET-9352)
3) Engineering Advancement Association of Japan:
"Development of Liquid Hydrogen Tanker", 1995(NEDO-WE-NET-9452)
4) Engineering Advancement Association of Japan:
"Development of Liquid Hydrogen Tanker", 1996(NEDO-WE-NET-9552)