LNG Carriers

MMA 167

Marine

Structural

Engineering

Assignment 1

LNG Carriers

Written by:

Hale Saglam

Ulrikke Brandt

Britta Wodecki

November 2012

MMA167- Marine structural engineering Assignment 1

Professor Jonas Ringsberg Hale Saglam - Ulrikke Brandt - Britta Wodecki

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Contents

1. Introduction ................................................................................................................. 2

2. Tank Types ................................................................................................................... 3

2.1. The membrane tank.............................................................................................. 3

2.1.1. GTT 96 Membrane System ............................................................................ 3

2.1.2. Mark III Membrane System ........................................................................... 4

2.2. The semi-membrane tank ..................................................................................... 4

2.3. Independent Types ............................................................................................... 5

2.3.1. Type-A Containment System ......................................................................... 5

2.3.2. Type-B Containment System ......................................................................... 5

3. Fatigue and Strength of LNG Ships .............................................................................. 7

4. LNG carrier design in the future .................................................................................. 8

4.1. Sloshing ................................................................................................................. 8

5. Reference .................................................................................................................... 9

Contents of Figures

Figure 1: Methane distribution [1] ..................................................................................... 2

Figure 2: World energy consumption by commodity [1] .................................................... 2

Figure 3: Diffination of elements ........................................................................................ 3

Figure 4: Gaz Transport System [3] ..................................................................................... 4

Figure 5: Mark III Membrane System [4] ............................................................................ 4

Figure 6: 78000 m3 LNG carrier with Type-A tanks [5] ....................................................... 5

Figure 7: Liquid Methane Carrier – Type A [3] .................................................................... 5

Figure 8: 137000 m3 LNG carrier with Type-B tanks (Kvaerner Moss system) [5] ............. 6

Figure 9: Kvaerner-Moss spherical tank [3] ........................................................................ 6

Figure 10: Critical Areas for Spherical Moss Tank LNG [7] .................................................. 7

Figure 11: Critical Areas for Membrane Tank LNG [7] ........................................................ 7

Figure 12: Typical LNG flows in membrane tank at high fillings & low fillings [8] .............. 8

Figure 13: LNG velocity fields in prismatic (membrane) & spherical (Moss) tank [8] ........ 8



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1. Introduction

Natural gas naturally occurs within the earth and it has been used as an energy source

since 1825. Natural gas was first discovered in Fredonia, New York. Later, sources have been

discovered all over the world. Almost 40% of the natural gasses are in the Middle East. The

distribution of the gas can be seen in Figure 1. IMO defines the liquefied gas as gaseous substance at

ambient temperature and pressure, but liquefied by pressurization or refrigeration – sometimes a

combination of both. All liquefied gasses are hydrocarbons and flammable in nature [1].

Figure 1: Methane distribution [1]

Figure 2: World energy consumption by commodity [1]

As the natural gas is used as an energy source all around the world, the need for the

transportation of the gas arises. The first LNG ship, Methane Pioneer, was used for transportation of

natural gas in 1959 from Louisiana to Canvey Island. About 5,000 cubic meters of LNG were

transported. The trade was a technical success because it demonstrated that it was possible to

transport the gas safely across the ocean, but from an economical view it was a failure. The Methane

Pioneer was very small and slow which resulted in a high unit cost. The failure helped to grow trade

as bigger ships were built and the design principle of LNG ships has been improved since 1965. As

shown in Figure 2, natural gas is the third major energy source transported by sea and with the

increase the fleet of LNG vessels follows. In 2007 the LNG fleet was about 240 vessels and the order

book consisted of 140 vessels [1].

The gas does not need to be liquefied when transported by inland transport since the

transportation is provided by pipelines. However, as the available cargo space is limited for sea

transport, the gas should be liquefied in order to carry as much as possible. Liquefying the gas is done

by reducing the temperature to -163 Celsius at atmospheric pressure. This decreases the original

volume by 99.8 %. The low temperature raises issues for the ship design. The material used for the

ship and the technology required to load and discharge need to withstand the very low temperature

and this makes the LNG carrier diverse from tanker vessels.

44%

29%

8%

9%

5%4% 1%

Middle East

Russian Federation

Africa

Asia

North America

South America

EU



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2. Tank Types

During the development of LNG carriers there have been different tank designs. The

main purpose of the cargo containment system is to keep the gas below its boiling point and

maintaining the adequate insulation. For this purpose it is important to select the appropriate

containment system which is also determined by taking into consideration the tank’s capability to

withstand sloshing loads. The tank designs can be divided into two main categories; the membrane

or the independent tanks. The membrane category consists of two tank design types used for LNG,

namely the membrane tanks and the semi-membrane tanks. Even though the tank types have

different designs, some characteristic elements are present in all of the tanks. The double bottom

and the secondary barrier are very important in case of leakages of the LNG to prevent pollution of

the ocean. These elements are shown in Figure 3.

Figure 3: Diffination of elements

2.1. The membrane tank

The membrane tank system consists of a very thin primary layer (membrane)

supported by insulation. The system is directly supported by the ship’s inner hull. The membrane

containment systems must always have a secondary barrier in case of a leakage in the primary

barrier. The membrane is designed so that the thermal expansion or contraction is compensated

without stressing of the membrane. Generally, two types of membrane tanks are used. [2]

Gaz Tranport and Technigaz, the two leading companies developing the two main

types of membrane tank types, fusioned to GTT.

2.1.1. GTT 96 Membrane System

The GTT 96 Membrane System, formerly known as the Gaz Transport system, consists

of two identical Invar membranes and two independent thermal insulation layers. The primary and

secondary Invar membranes are made of a 0.7 mm thickness of 36% nickel-steel alloy, which has a

very low coefficient of thermal expansion. Both thermal insulation layers consist of prefabricated

plywood boxes filled with perlite as shown in Figure 4. [4]



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Figure 4: Gaz Transport System [3]

2.1.2. Mark III Membrane System

The Mark III Membrane System, formerly known as the Technigaz system, consists of a

primary corrugated stainless steel membrane and a prefabricated insulation panel including the

secondary triplex membrane as shown in Figure 5. The primary membrane is 1.2 mm thick. The

polyurethane foam insulation is made of prefabricated panels. It contains the primary and secondary

insulation and the secondary membrane. The secondary membrane is made of a thin sheet of

aluminum between two layers of glass cloth and resin. To anchor the insulation and spread the loads

evenly, the panels are bonded to the inner hull by resin ropes. [4]

Figure 5: Mark III Membrane System [4]

2.2. The semi-membrane tank

This tank type is similar to the membrane tank design. The primary barrier is much

thicker than in the case with membrane tanks. The corners have large radius and are not supported,

so this part of the tank can withstand expansion and contraction. The bottom and the sides of the

tanks are straight plates and are supported through insulation by the adjacent hull structure. It is

through these connections the weight and the dynamic loads are transferred to the hull.



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The tank is self-supported in empty condition, but only in the empty-condition and not

in the loaded condition. Self-supported means that the primary barrier is stiff enough to carry its own

weight, but when the additional weight from the gas puts pressure on the tank the primary barrier is

not strong enough and the tank needs support from the hull structure. In other words, the tank is not

designed to be placed on the deck, it must be inside the vessel. [2,3]

2.3. Independent Types

Independent types of containment systems are divided into 2 categories, as Type-A

containment system and Type-B containment system. These tanks are generally designed as

spherical but there are examples of prismatic ones.

2.3.1. Type-A Containment System

The Type A system is designed as box shapes or prismatic tanks. This type is used in early

LNG ships and the design is carried out using the traditional methods of general ship structural

analysis. An example of LNG ships with box shaped tanks is seen in Figure 6.

Figure 6: 78000 m3 LNG carrier with Type-A tanks [5]

Early LNG ships such as the ‘Methane Princess’ and ‘Methane Progress’ were fitted

with self-supporting tanks of aluminum alloy having center-line bulkheads. The balsa wood insulation

system was attached to the inner hull (secondary barrier) and each insulated hold contained three

tanks. The midship section drawing is seen in Figure 7. Later vessels built with tanks of this category

have adopted a prismatic tank design. [3]

Figure 7: Liquid Methane Carrier – Type A [3]

2.3.2. Type-B Containment System

Type-B cargo containment systems are the most used types since they are improved in

terms of fatigue and crack propagation and the main principle is based on the crack detection before



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failure, which allows the usage of a partial secondary barrier. [6] The independent B type tanks are

generally categorized as spherical moss and self-supported prismatic tanks. An example of a

spherical moss system is shown in Figure 8 as Kvaerner Moss system.

Figure 8: 137000 m3 LNG carrier with Type-B tanks (Kvaerner Moss system) [5]

Kvaerner type tank systems are used in most LNG ships. The tanks are made of

aluminum alloy or where the only connection between tanks and hull is made of 9% nickel steel

which is seen in Figure 9.

Figure 9: Kvaerner-Moss spherical tank [3]

Type-B tank systems are designed to provide sensors to detect leakage and ability to

repair itself periodically before any critical cracks occur. This type of cargo containment systems have

comparatively less cargo space, for instance the cargo capacity of 5 large spheres is approximately

125.000 m3.

Independent Type-B containment systems are not resistant to sloshing - which will be

explained in detail in the next section- and they have some disadvantages. The only way to increase

the cargo capacity is to increase the diameter of spherical tanks and this will be gained by increasing

the ship length which is not desirable for stability and global strength aspects. Besides, the ship will

have less hull volume efficiency and a high area affected by wind and a limited deck area for the

installation of regasification equipment. [3,5].



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3. Fatigue and Strength of LNG Ships

The growth in the size of LNG ships has produced a problem of global strength and

fatigue problems. The stress levels are increased compared to the old conventional LNG ship types.

Wide openings in the strength deck and non-continuous tank covers generate extreme torsional

response of the hull in large spherical Moss type LNG carriers. The critical areas, as a result of fatigue,

are shown in Figure 10.

Membrane type tank containment systems are more exposed to the vertical bending

moment. Besides, the stiffness between longitudinal and bottom transverse girders and the increase

in length of web frames have a big impact on the fatigue life of the inner hull hopper knuckles and

the foot of the cofferdam as seen in Figure 11. The other parts such as cofferdam stringers,

cofferdam girders in the double bottom and the liquid cover dome have an influence on fatigue.

Figure 10: Critical Areas for Spherical Moss Tank LNG [7]

Figure 11: Critical Areas for Membrane Tank LNG [7]



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4. LNG carrier design in the future

The demand for LNG is still increasing. Forecasts for LNG show that the demand will

double during the next decade, which results in the LNG carriers’ need to be more efficient. The size

of the ship needs to increase, but the vessels also need to be faster, have a higher operational

flexibility and be more efficiently. Most of the propulsions today are steam turbines, which have low

efficiency and by replacing those turbines with e.g. gas turbines the ship will be more efficient. From

a structural view, to meet the high demand is to increase the size of the ship. By increasing the size of

the tanks a new issue occurs; the sloshing. [9]

4.1. Sloshing

Sloshing loads are as important design parameters as the fatigue consideration, since

LNG ships carry huge amounts of energy, it is important that the tanks and the ship’s hull have an

adequate resistance to sloshing forces. The sloshing effects can be alleviated by using large chamfers

and strengthened insulation systems in the upper part of the tanks. This system allows the increase

of the containment limit of up to 80%.

Many studies have been carried out since the 1970s and there are examples from

Bureau Veritas numerical CFD analysis, exhibiting different types of sloshing flows and LNG velocity

fields generated in tanks of large LNG Carriers, are given in Error! Reference source not found. and

Error! Reference source not found.. [8]

Figure 12: Typical LNG flows in membrane tank at high

fillings & low fillings [8]

Figure 13: LNG velocity fields in prismatic (membrane)

& spherical (Moss) tank [8]



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5. Reference

[1] M. Stopford, Maritime Economics (2009), 3rd edition. Oxon: Routledge

[2] International Safety Guide for Inland Navigation Tank-barges and Terminals, chapter 33:

http://www.isgintt.org/files/Chapter_33en_isgintt_062010.pdf

[3] Eyres, D. J., Ship Construction (12/2006), Butterworth-Heinemann

[4] GTT, 11/2012 http://www.gtt.fr/content.php?cat=34&menu=60

[5] T. Miller, The carriage of liquefied gases, UK P&I Club

[6] Liquefied Gas Carrier, DNV Part 5 Chapter 5, January 2012

[7] T. Lindemark, F. Kamsvåg and S. Vlasgård, Fatigue analysis of gas carriers, Maritime Technical

Consultancy – Det Norkse Veritas AS

[8] M. Zalar, S. Malenica & L. Diebold, Selced hydrodynamic issues in design of large LNG carriers,

Bureau Veritas

[9] J. WANG, A study on Technical Development on LNG Vessel, IEEE Conference in Systems, 2009

Documents

LNG Carriers