THE DESIGN OF LIQUEFIED GAS CARRIERS

Ships that are designed to carry liquefied gas have become more significant and increased
in number in the recent years, with the increasing need for alternative fuel.

The two main types of liquefied gas carriers are

1. LPG (Liquefied Petroleum Gas) Carriers, and

2. LNG (Liquefied Natural Gas) Carriers.

To understand the design characteristics of these two types of ships, we first need to know
a few notable details about the composition and properties of LPG and LNG.

Liquefied Petroleum Gas (LPG):

Petroleum hydrocarbon products such as Propane and Butane, and mixtures of both have
been categorised by the oil industry as LPG. It is widely used in domestic and industrial
purposes today. The most important property of LPG is that it is suitable for being
pressurised into liquid form and transported. But there are conditions related to pressure
and temperature that need to be maintained for the above to be carried out without
posing threat to life, environment, and cargo. At least one of the following conditions need
to be complied with, for transportation of LPG:

• The gas should be pressurised at ambient temperature.

• The gas should be fully refrigerated at its boiling point. Boiling point of LPG
rangers from -30 degree Celsius to -48 degree celsius. This condition is called
fully-refrigerated condition.
• The gas must be semi-refrigerated to a reduced temperature and pressurised.

We will see, at a later stage, how the above conditions affect the design of different types
of LPG tankers.

Other gases such as ammonia, ethylene and propylene are also transported in liquefied
form in LPG carriers. Ethylene, however, has a lower boiling point (-140 degree celsius)
than other LPGs. Hence it must be carried in semi-refrigerated or fully-refrigerated
conditions.

Liquefied Natural Gas (LNG):

Natural gas from which impurities like sulphur and carbon-dioxide have been removed, is
called Liquefied Natural Gas. After removal of impurities, it is cooled to its boiling point (-
165 degree Celsius), at or almost at atmospheric pressure. Note here, that unlike LPG, LNG
is cooled to low temperatures but not pressurised much above atmospheric pressure. This
is what makes the design of LNG carriers slightly different from LPG carriers. LNG, at this
condition is transported as liquid methane.

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Design of Different Types of Gas Carriers:

In this article, we will understand the general arrangement, and other design details of gas
carriers as and when we look into the different types of vessels based on their functionality
and type of cargo being carried. The most important feature of gas carriers is the cargo
containment system. It is according to this criteria that LPG carriers are categorised into
types.

Integral Tanks:

These are the tanks that form a primary structural part of the ship and are influenced by
the loads coming onto the hull structure. They are mainly used for cases when LPG is to be
carried at conditions close to atmospheric condition, for example – Butane. That is
because, in this case, there are no requirements for expansion or contraction of the tank
structure.

Independent Tanks:

These tanks are self-supporting in nature, and they do not form an integral part of the hull
structure. Hence, they do not contribute to the overall strength of the hull girder.
According to IGC Code, Chapter 4, independent tanks are categorised into three types:

Type ‘A’ Tanks: These tanks are designed using the traditional method of ship structural
design. LPG at near-atmospheric conditions or LNG can be carried in these tanks. The
design pressure of Type A tanks is less than 700 mbar. The following figures show the
general arrangement of a liquid methane carrier with Type ‘A’ tanks.

Figure 1: General Arrangement of Methane Carrier with Type A Tanks.

The general arrangement of an LPG ship is almost same as that of an oil carrier, with the
cargo tanks spread over a certain length forward and abaft the midship, the machinery
and superstructure at the aft. A forecastle is fitted at the bow so as to prevent green
waters on deck. Ballast water cannot be carried in the cargo tanks, hence spaces for ballast
are provided by incorporating double hull spaces (note the double hull in the midship
section), bilge and upper wing tanks.

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The most notable and distinguishing feature of Type ‘A’ tanks is that the IGC Code specifies
that Type ‘A’ tanks must have a secondary barrier to contain any leakage for at least 15
days. The secondary barrier must be a complete barrier of such capacity that it is sufficient
to contain the entire tank volume at any heel angle. Often, this secondary barrier
comprises of the spaces in the ship’s hull as shown in the figure below.

Figure 2: Secondary Barrier for Type ‘A’ Tank.

One important question that could arise, here, is that the tank in the midship section view
seems to be an integral part of the hull. Why then, is this type of tanks categorised under
Independent Tanks? To find the answer we need to have a closer look at how the tank is
installed in the hull.

Figure 3: Integration of Type-A tank with hull structure.

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The above figure shows how the aluminum tank structure is not integrated to the inner
hull of the methane carrier by means of any metal contact. The inner hull plating and
aluminum tank plating are separated by layers consisting of timber, glass fibre, and balsa
panels for insulation from external temperatures. The balsa panels are held together by
plywood on both faces which are sealed using PVC foam seals. An inert space of 2 or 3 mm
separates the inner glass fibre layer from the aluminum tank plate. This space is provided
for insulation and also allows expansion and contraction of the tank structure. This type of
non-welded integration makes this tank structurally independent in nature. (Anti rolling
chockes, anchors, hold inspections)

Type ‘B’ Tanks: The concept behind the design of such tanks is to have such a structure in
which a crack can be detected long before the actual failure. This allows a time margin
before the actual failure occurs. The methods used for design of such tanks include
determination of stress levels at various temperatures and pressures by first principle
analyses, determination of fatigue life of tank structure, and study of crack propagation
characteristics. This enhanced design of such tanks requires on a partial barrier, that we
will look into, soon.

The most common arrangement of Type ‘B’ tank is Kvaerner-Moss Spherical Tank, as
shown below in Figure 4.

Figure 4: Kvaerner-Moss Spherical Tank

The tank structure is spherical in shape, and it is so positioned in the ship’s hull that only
half or a greater portion of the sphere is under the main deck level. The outer surface of
the tank plating is provided with external insulation, and the portion of the tank above the

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main deck level is protected by a weather protective layer. A vertical tubular support is led
from the top of the tank to the bottom, which houses the piping and the access rungs.

As evident from the layout, any leakage in the tank would cause the spill to accumulate on
the drip tray below the tank. The drip pan and the equatorial region of the tank are
equipped with temperature sensors to detect the presence of LNG. This acts as a partial
secondary barrier for the tank.

LNG is usually carried in this type of tanks. A flexible foundation allows free expansion and
contraction according to thermal conditions, and such dimensional changes do not
interact with the primary hull structure, as shown in Figure 5.

Figure 5: Expansion and Contraction of Spherical Tanks.

The following are the advantages of Kvaerner-Moss Spherical tanks:

• It enables space between the inner and outer hull (see Figure 4.) and this can
be used for ballast and provide protection to cargo in case of side-ward
collision damages.
• The spherical shape allows even distribution of stress, therefore reducing the
risk of fracture or failure.
• Since ‘Leak before Failure’ concept is used in the design, it presumes and
ensures that the primary barrier (tank shell) will fail progressively and not
catastrophically. This allows crack generation to occur before it propagates
and causes ultimate failure.

Type ‘C’ Tanks: These tanks are designed as cryogenic pressure vessels, using conventional
pressure vessel codes, and the dominant design criteria is the vapour pressure. The design
pressure for these tanks is in ranges above 2000 mbar. The most common shapes for these

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tanks are cylindrical and bi-lobe. Though Type ‘C’ tanks are used in both, LPG and LNG
carriers, it is the dominant design in LNG carriers.

The following figures show the arrangements of cylindrical and bilobe tank arrangements
in midship view. The cylinders can be either vertically or horizontally mounted, depending
on the dimensions and spatial constraints of the ship. Note, in Figure 6, that the space
between the two cylinders is rendered useless. Due to this, the use of cylindrical tanks is a
poor use of the hull volume. In order to circumvent this, the pressure vessels are made to
intersect, or bilobe tanks are used (Figure 7).

Figure 6: Horizontal Cylinder Tanks in LNG carrier.

Figure 7: Bilobe tank arrangement in LNG carrier.

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These types of tanks do not require a secondary barrier. Rather, to detect the leakage of
cargo from the tanks, the hold space (refer to Figure 6) is filled with inert gas or dry air.
Sensors placed in the hold space can detect the change in composition of the inert gas or
dry air due to fuel vapour, and leakages can hence be detected and prevented. Bilobe tanks
at the forward end of the ship are tapered at the end.

Membrane Tanks:

Unlike independent tanks, membrane tanks are non-self-supporting structures. Their

primary barrier consists of a thin layer of membrane (0.7 to 1.5 mm thick). The membrane
is supported to the inner hull structure through an insulation that can range upto 10 mm
thickness as per IMO IGC Code. Due to their non-self-supporting nature, the inner hull
bears the loads imparted onto the tank. This way, the expansions and contractions due to
thermal fluctuations are compensated by not allowing the stress to be taken up by the
membrane itself. Membrane tanks are primarily used for LNG cargo.

Often, there are two layers (primary and secondary) of insulation and membranes placed
alternatively. The most common types of membrane tanks are the ones designed and
developed by two French companies Technigaz and Gaz Transport. The Tehnigaz system
makes use of a stainless steel system that is constructed with corrugated sheets in such a
way that one sheet is free to expand or contract independent of the adjacent sheet. The
Gaz Transport system uses Invar as the primary and secondary membranes. Invar has low
coefficient of thermal expansion, which makes corrugations unnecessary. The insulation is
usually made of materials like Reinforced Polyurethane. In GTT membrane tanks, the
primary membrane is made of Corrugated SUS 304, and the secondary membrane is made
of Glued Triplex. Figure 8 illustrates the anatomy of twin-membrane tanks.

Figure 8: Parts of a membrane tank.

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 Figure 9: Interior (Primary membrane) of a Membrane tank on an LNG carrier

Some of the advantages of membrane tanks are as follows:

• They are generally of smaller gross tonnage, that is the space occupied within
the hull is lower for a given cargo volume.
• Due to the above reason, maximum space in the hold can be used for cargo
containment.
• Since the height of tanks above the main deck is significantly lesser compared
to the cases of Moss tanks, membrane tanks provide allow visibility from the
navigational bridge. This also allows a lower wheelhouse. This can be
compared in Figures 10 and 11.

Figure 10: An LNG Carrier with Moss type tanks.

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Figure 11: A membrane type LNG carrier under construction in a shipyard. Note the height
of tanks above the main deck and the height of the wheelhouse.

LPG Containment Systems:

Unlike LNG, LPG cargo requires storage at conditions that are different from atmospheric
conditions. The LPG containment systems are classified into three types, and each LPG
carrier is designed according to any one of them.

Fully Pressurized Tanks:

Propane, Butane and Anhydrous ammonia are carried in fully pressurized tanks. The
capacity of these tanks is usually less than 2000 cubic meters. They are usually uninsulated
cylindrical pressure vessels that are arranged partly below main deck level. Since these are
Type C tanks, they often prevent complete utilization of under deck volume.

Semi Pressurized or Semi Refrigerated Tanks:

Though the cargo carried by semi-pressurized ships are same as that of fully-pressurized
ships, the volume of semi-pressurized ships is about 5000 cubic meters. These use
Independent Type C tanks, and are constructed with ordinary grades of steel. The outer
surface of these tanks are insulated, and refrigeration or reliquefication plants are installed
on these ships to maintain the working pressure of the cargo. The most common types of
tanks used for this purpose are cylindrical and bi-lobe type.

Fully Refrigerated Tanks:

Fully Refrigerated gas carriers have a capacity of 10,000 to 1,00,000 cubic meters. The ships
in the smaller size range are used to carry multiple types of cargo, whereas the larger ones
are designed for a single type of cargo to be transported on a permanent route. The tanks

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used for this purpose is usually Type ‘A’ prismatic tanks that are sloped at the top end to
reduce free surface effect, and sloped at the bottom to suit the shape of the bilge
structure. They are usually divided longitudinally by a liquid-tight bulkhead, in order to
reduce free surface effects further. These tanks are constructed with notch ductile steel,
in order to be provided with maximum notch toughness at temperatures as low as -48
degrees Celsius, at which cargo like Propane is transported.

The number of gas carriers have increased drastically over the last ten years, owing to the
increasing need for alternative fuel. These are usually high speed ships with fine hull-form,
which makes it possible for extensive research opportunities to improve on hull
efficiencies in order to achieve more power efficiency. A lot of research is also being
carried out to design advanced cargo containment systems and concepts of adjoining
bunkering systems are being developed by various countries that are opening themselves
to extensive use of natural gas. Today, not all shipyards are equipped to design and build
specialised ships like LPG and LNG carriers. This leaves a wide scope for designers and
shipbuilders to develop skills and infrastructure to specialise in building these ships.

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