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18

January 2020

Long established in the LPG and

ethylene trade

During the early days of marine liquefied gas trades, it

became clear that, for small and medium sized gas carriers,

pressure vessel type containment systems are the most

economic system. Obviously, for fully pressurised LPG

carriers, transportation at ambient temperatures requires

the gas to be transported at up to 18 bar pressure. With

the increase in tank size in the handysize segment, due

to economics for the tanks and trading requirements, a

reliquefaction plant is added and the design pressure is

reduced to 3 – 6 barg. For LPG, this leads to a breakeven

between semi-refrigerated carriers with pressure vessel

Type C tanks and fully refrigerated carriers with atmospheric

Type A tanks at approximately 22 000

23 000 m³ total tank

volume.

Most economic solution for

small scale LNG and LNG as fuel

For large LNG vessels of 150 000

210 000 m³, the most

established tank types are membrane and Moss type tanks.

A noticable market for smaller LNG carriers only developed

during the last decade. During the last decade, growth in

the small scale LNG trade and the arrival of LNG as fuel

have lead to a significant growth of the small scale LNG

fleet of vessels of 30 000 m³ capacity and below. In 2010,

a fleet of merely 13 vessels was mainly controlled by Asian

interests. By 2020, the fleet size will have been quadrupled

to more than 40 vessels. A strong factor in the growth of

the fleet are the LNG bunker vessels, which form the basis

of a worldwide LNG fuel infrastructure. Over 85% of these

newbuilds built between 2010 and 2020 are equipped with

Type C tanks. The technical superiority, as well as economic

considerations, form the basis of the domination of this

tank type.

Advantages of Type C tanks

From a design and project execution standpoint, the

benefits are as follows: Type C tanks have been established

under the IGC Code since the early days, with a design that

is considered inherently safe. Due to this safe design, a

secondary barrier is not required as no leakage can occur

over the lifetime of the vessel. The hold space around

the tanks is considered a gas safe area. With decades

of operation, this approach has proven to be a safe and

reliable design. The fact that there is no secondary barrier

needed is obviously also favourable to the economic

comparison to other tank types, such as Type A and

membrane type tanks. From a project execution point of

view, the main benefit is that the tanks can be completely

fabricated, insulated, tested and calibrated outside of the

hull. This decouples the tank fabrication schedule from

the building schedule of the vessel, and saves valuable

docktime. TGE Marine has even transported its Type C

tanks from the fabrication site in China to building sites

in Europe. For some more recent projects, as well as for

fuel gas projects, the tanks have also been delivered

fully outfitted to reduce outfitting time at the yard. With

TGE Marine’s expertise, small LNG carriers and bunker

vessels can even been placed at yards with little prior

references in the small scale LNG market.

Good examples are the

Coral Methane

(built in 2009) or the

first three newbuild bunker vessels (

Cardissa

,

Coralius

and

ENGIE Zeebrugge

).

Early adaption from ethylene to

LNG

On the technological side, the adaption of Type C tanks

from the previous application of LPG and ethylene to LNG

required some modifications in terms of both material and

design. LPG is usually transported at -48°C and ethylene

at -104°C, while LNG is transported at -163°C. This means

that instead of steel grades with nickel contents between

0.5% and 5%, for LNG, 9% nickel steel or stainless steel

has to be applied. Due to the low temperature, shrinkage

Figure 1.

Largest cylinder Type C LNG tank (14 000 m3).

Figure 2.

Trilobe Type C tank for container vessel retrofit.

Figure 3.

Type C tanks in various applications

35 000 m3 ethane transport, 2000 m³ LNG fuel, and 5800 m³

LNG bunker.