Floating LNG - Cost and Safety Benefits of a Concrete Hull

8/11/2019 Floating LNG - Cost and Safety Benefits of a Concrete Hull

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FLOATING LNG : Cost and Safety Benefits of a Concrete Hull

Denis MARCHANDBouygues Offshore

France

Christophe PRAT

Technigaz

France

Pierre BESSEBureau V ritas

France


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Introduction

The origin of this work is in the European Union Thermie project called AZURE: Full floating LNG Chain.

It involved a consortium of nine European Engineering Companies and Classification Societies, among

them BOUYGUES OFFSHORE and BUREAU VERITAS. The aim of the project is to demonstrate theindustrial viability of a full floating LNG chain from production well to natural gas distribution network.

One of the project work packages was the design and validation of a concrete hull for the LNG Floating

Production, Storage and Offloading vessel (FPSO).

To date concrete has been used offshore mainly in the North Sea for gravity base platforms. However

concrete floating units also exist in other parts of the world, the best known examples being the Ardjuna

LPG barge for Arco in Indonesia (ref 1) and the NKossa FPU for Elf in Congo (ref 2). Concrete is also

used for onshore LNG storage tanks in association with membrane or 9% nickel containment systems

(ref3). As a materiel, concrete is well adapted to a floating LNG plant because of :

its good cryogenic and insulation properties (ref 4)

its resistance to fire, heat radiation, impact loads, cold splash, seawater corrosion.

its stiffness: there is limited hull deformation and cost effective LNG containment systems can beused.

The concrete hull developed in the AZURE project shows several innovative aspects:

Hull "multi-vaults" shape, made of three horizontal semi-cylinders, to take advantage of the seawater

hydrostatic pressure. LNG tanks shape and size are such that resonance between vessel motion and liquid cargo is avoided.

Continuous concrete deck for segregation of storage and process parts.

The design criteria specified for this concrete hull is the liquefaction and storage of the associated gas

from deep-water oil fields offshore West Africa. Plant capacity is medium scale: around one million tons

of LNG per year. LNG is stored in three longitudinal tanks, with a conventional Technigaz type

membrane containment system. Total LNG storage capacity is 110,000 m3. Although only nitrogen is

used as refrigerant for liquefaction, there is a need for separation and storage of LPGs, due to feed gas

composition. The barge is spread moored, head to the prevailing wave direction. Export of LNG and LPG

is done by tandem offloading to dedicated carriers, through the Boom-to-Tanker system being developed

by FMC.

The hull is made of high performance, lightweight concrete. Thorough structural analysis has been

performed for the various loading configurations. Special consideration was given to accidental scenarios,

such as damage stability, collision, fire, explosion, dropped object, cold leaks etc. Extensive testing was

performed on the membrane and on the concrete in abnormal cryogenic conditions. Hydrodynamic

behavior is being tested by basin tests and liquid motion tests. The concrete hull design has proved to be a

very robust concept.

The barge can be built in a conventional dry dock or in a graving dock. Its draft is small enough to allow

concrete hull construction and topsides integration to be carried out at the same location. Containment

system integration is performed on a non-critical path basis, and the overall construction schedule is quite

attractive compared to land-based projects, or to LNG carrier building.

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A cost effective hull design

General architecture

The main principles governing the design of the hull are:

a) A monohull concept with a large deck area. A rather shallow draft was selected for the barge so as to

provide a large deck area for the liquefaction process plant and to allow the complete construction of the

FPSO in a single location; either in an existing dry-dock or in a new-build graving dock. The benign

environmental conditions of the Gulf of Guinea, with 100-year significant sea state of 4.2 m, allow the

use of such floaters.

b) Use of vault effect. In order to reduce concrete volume, choice was made to use a semi-cylindrical

shape design for the floating support. A good compromise between concrete volume, radius of the semi-

cylinders and ease of construction has been achieved by dividing the hull into three 11 m radius semi-

cylinders. The main interest of this shape is that semi-cylinders have a good structural behavior under

combined external and stored fluid pressure loading, as the curved shape greatly reduces the flexion and

take the advantage of the excellent behavior of the concrete when compressed. This principle has beenused in multi-vaults dams for many years.

Figure 1: a multi-vault dam

The upper part of the hull is composed of:

- vertical bulkheads and shells joining two adjacent semi-cylinders

- six diagonal slabs- a single deck slab protecting the tanks against the process plant hazards.

The interest of the three previous components of the structure is the reduction of the spans in order to

avoid stiffening the structure and to reduce the thickness of these elements.

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Figure 2: Transverse section of the concrete barge

Thus, this structural configuration of the hull minimizes the volume of concrete:

- by using cylindrical shapes for the bottom slab,- by reducing the spans of the straight walls and slabs.

Another reduction of concrete volume has been obtained by limiting the number and the thickness of the

transverse bulkheads of the hull. Thus, only three LNG tanks have been designed to store the 110,000 m3

of LNG. LPG is stored into four tanks (two for propane, two for butane). Fore and stern tanks arededicated to ballast. Ballast requirements are kept minimal as LNG loads are centralized. In addition, no

ballast is necessary to compensate the variation of draft when loading or offloading the cargo, except for

trim and heel control. This symmetrical configuration limits the overall bending moments and thus the

quantity of longitudinal pre-stressing required.

c)Longitudinal storage tanks.For the LNG FPSO, where tanks are partially filled most of the time, tanks

are arranged in a longitudinal manner. There are 3 longitudinal storage tanks in the middle of the barge,each about 100 m long and 22 m wide (see figure 3). This arrangement allows a high transverse stability

with negligible free surface effect. Moreover, as a good engineering practice, the resonant period of the

liquids inside the tanks have been placed outside the wave energy spectrum to limit possible sloshing

occurrence (ref 7). Figure 4 shows that the tank transverse resonant periods (in roll) are kept below the

wave periods for all filling levels of the tank. In a similar manner, the longitudinal resonant periods of the

tanks are always above the wave periods.

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Figure 3: plan view of the hull

Figure 4: resonant periods of the tank Vs filling level

Lightweight high performance concrete

A concrete hull provides some major advantages for offshore applications. These features have been

validated by current industrial offshore applications of concrete, which include several concrete platforms

and floaters for oil and gas applications worldwide.

Particular advantages may be summarized as follows:

- The long tradition of concrete as a construction material for the marine environment with well

established design and construction techniques.

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Wave energy spectrum

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- Reinforced concrete is fatigue resistant and durable. Therefore, a concrete hull will have a highresidual value at the end of the field life.

- Concrete requires virtually no maintenance, and thus operating costs are minimized.

- High potential flexibility of the topsides loads during the field life and for later re-use.

- The sides of the concrete hull are able to withstand significant collision loads from supply boats.- The concrete deck is not susceptible to damage from dropped objects.

The following features make concrete especially suitable for LNG storage applications compared with an

equivalent steel hull:

- A concrete hull is stiffer than an equivalent steel hull, leading to less stress on the longitudinal pipingand on the cryogenic containment system. It allows also the optimization of the structures of the

topsides.

- Concrete structures have much better fire resistance than steel structures, which is of major

importance for LNG production and storage vessels.

- The excellent cryogenic behavior of the high performance concrete makes it particularly safe for

LNG applications, and thus provides a higher overall safety.

- The pre-stressed concrete structure permits the use of the membrane containment system that is used

for land based LNG tanks. This is simpler than the one used for steel membrane LNG carriers.- The membrane containment system within the hull provides a clear deck for the topsides. This greatly

improves topsides design flexibility and safety. No process equipment is installed within the hull.

- The ability to keep the hull stable even when storing relatively light fluids such as LNG, and more

particularly in damage conditions.

The concrete used for the design of the barge has a compressive strength equal to 70 MPa on cylinder.

The interest of the high performance concrete for this application has been further enhanced by the use of

lightweight concrete. As high performance concrete density is 2.4 t/m3, the use of 2.1 t/m

3 allows an

important reduction of concrete volume and length of the hull by limiting the volume of void tanks

required in the hull for buoyancy. Thus, this reduction of 12.5% in the concrete density allows the hull

length to be decreased by 11% and concrete volume by 9%, the criteria being to fulfil the ILLC free-board criterion. In addition to concrete volume reduction it should be noted that the reduction of the hull

length allows 25% reduction of the longitudinal pre-stressing quantity, which is dependent on the

maximal values of the overall bending moments.

It should be noted that 2.1 t/m3density has been chosen as a good compromise between lightness and

material validation.

Structural analysis

A structural analysis has been performed to check the resistance of the hull to the environmental and

internal loads. This analysis has been based on the same standards and computer software as those usedfor the structural analysis of the NKossa concrete barge built for Elf Congo in 1995. In fact, the design is

checked by performing two analyses:

- A longitudinal analysis to design the longitudinal pre-stress by calculating the stress in the hull due to

overall hydrostatic and hydrodynamic bending moments.

- A transverse analysis to check the design of the transverse section of the hull submitted to local loads.

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Verification of reinforced concrete sections are made using the Norwegian Standards (NS 3473 E) (ref.5).

The main criteria to be fulfilled guarantee the resistance of the concrete and the water tightness of the

section.

Longitudinal analysis

The static overall bending moments have been calculated for the following load cases, which summarize

the main load cases the hull is likely to experience during its whole life:

- All tanks empty,- Full storage,

- LNG tanks empty, others full,

- LNG tanks full, others empty,

- One LNG side tank empty, other tanks full,- Full laden except butane tanks.

The dynamic overall bending moments has been evaluated using Bureau Veritas Regulations.

Transverse analysis

The principle has been to model a third of the hull (equivalent to 80 m long), including the whole width of

the hull, from this centerline to the transverse bulkhead separating water ballast and LPG tanks. Software

used is Hercule, developed by the French classification society Socotec.

The supports defined in the model are:

- conditions of symmetry at the centerline of the barge- no displacements or rotations of the whole transverse section at the end of the model.

Figure 5: view of the structural model

End of the

model

Plane of

symmetry

LNG tanks

LPGtanks

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Load cases are described below. They are typical of the loads that hull will have to resist through its life-

cycle:

- self weight

- external static and dynamic pressures during towing and in site- internal static and dynamic pressures (WB, LPG, LNG)

- deck loads during installation of the process modules- topsides vertical loads at the end of the deck spans.

These load cases have been associated into six combinations:

- 3 in service,

- 1 for modules installation,

- 2 for towing.

Pre-stressing has been designed to guarantee that the structure fulfills the Norwegian Standards criteria.

Reinforcement has only been designed for the most loaded sections.

Longitudinal and transverse analysis lead to a ratio of pre-stressing steels equal to 90 kg per m3 ofconcrete, and 220 kg for reinforcement steels.

Construction

The construction philosophy is based on reduction of the overall schedule. Thus, one of the main

objectives is to complete LNG tanks construction as early as possible in order to start to install the

containment system. Thus, the construction of the hull is performed as indicated in thefigure 6.

Figure 6: construction philosophy

Construction of the concrete hull is made easier by the regularity of its geometry, thus improving the

planning schedule. Transverse shells and bulkheads are built as soon as the adjacent longitudinal elementsare built.

The hull is constructed by 8 m wide slices. Each section is divided into different elements, which are built

in the following sequences:

- Installation of the sub semi-cylinders supports,

- Construction of bilge keels,

1stphase

2n

phase

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- Construction of semi-cylinders- Construction of vertical walls

- Construction of triangle junctions and first slabs of roof

- Construction of last slabs of roofs.

- Construction of blisters for pre-stressing anchors.

These sequences are illustrated in thefigure 7.

Figure 7: sequences of construction of the concrete hull

Following sequences in hull construction are:

- erection of the transverse shells and bulkheads,- installation of containment system of LNG tanks

- realization of pre-stressing.

Installation of the containment system begins when concrete parts of LNG tanks have been completed. It

includes insulating panel erection and membrane erection, both being performed using dedicated

scaffoldings rolling along the length of the tank (see figure 8). Membrane installation can continue during

topsides integration.

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Figure 8: sequences of installation of the containment system

Strains of the hull due to pre-stressing allow its completion during erection of the containment system. As

longitudinal pre-stressing is more important at the center of the hull than at fore and stern, pre-stressing

installation can be started as soon as the concrete part of the LNG tanks is completed.

The resultant reduction of the construction schedule is significant, as concrete completion can be achieved

in 13 months. Consequently the hull is ready to receive topsides 13 months after the start of concrete

pouring.

The construction site of the concrete hull can be of three types:

- dry-dock- new build dedicated graving dock

- existing graving-dock.

This choice increases the opportunities and the competitiveness between sites and thus may lead to costsavings.

Safety benefits of the concrete hull

A good resistance to collision

Use of a single shell hull means that it is necessary to check its resistance to boat impact, particularly in

the area of the LNG and LPG tanks. The study has been performed according to DNV rules, assuming a

supply boat of 5,000 tons out of control at a speed of 1 m.s-1

. 3 headings of the supply boats have been

studied: bow, broadside and stern impact. Results shows that overpressures on the concrete hull shells

plus static and hydrodynamic pressures are much less than pressures supported by hull during towing.This guarantees the resistance of the hull to supply boat impact.

In case of impact with a shuttle carrier operating at the stern of the hull, the presence of water ballast and

void tanks guarantees the integrity of the LNG and LPG tanks. In addition, the great stability of the hull

allows the flooding of two adjacent compartments, in accordance with the International Gas Carriers

(IGC) code.

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A perfect water tightness

For in-ground LNG tanks it is important to limit hygrometry between the concrete and the insulation, to

prevent condensation that is likely to freeze, thus slightly increasing the thermal conductivity. For the

Azure barge, a study has been performed to check the tightness of the hull, to check that no water ingress

occurs from outside that could increase hygrometry within the insulation.

The first step was to determinate the nature of water transfer in concrete. Two phenomena could be likely

to occur: percolation or diffusion.

Water permeameter testing was performed on NKossa samples giving an intrinsic permeability of

2.10-20

m_. Under those conditions, the calculated water speed of filtration through the concrete hull has

an order of magnitude of 0.1 millimeter a year. From this result it is evident that the water percolation

phenomenon through the studied concrete is negligible, therefore the water ingress through the concrete

hull is governed by a liquid diffusion process described by Ficks law.

The percolation phenomenon has been studied through the finite element code CESAR-LCPC. This code

allows the calculation of the solution of the non-linear diffusion equation.

The study took into account the following parameters:

- evolution of concrete humidity content- internal (0%, 50% and 90%) and external humidity (100%)

- external pressure (1.7 bar)

- internal (-5 C) and external temperatures (20C)

- widths and density of concrete cracks

Calculations took into account the non-linear nature of the diffusion phenomenon:

- hydrous diffusivity variation with humidity content

- boundary exchange conditions variations with humidity content

- temperature dependent diffusion process

- diffusion process modeling with cracked concrete

The model represents a 60 cm thick concrete section (bottom slab). The finite element mesh is divided

into several stripes to take into account the concrete crack and the temperature distribution within the

concrete for steady state conditions.

The calculations performed (a period of time of one hundred years was simulated) indicated that the

concrete undergoes a dessication from the face exposed to 0%, 50% or 90% relative humidity, and that

there is a diffusion controlled water ingress from the face exposed to 100% relative humidity. Both of

these processes are very slow and reach a steady state resulting from an equilibrium between dessication

on one face and water ingress on the other. The figure 9 gives the water penetration depth for a givenconcrete section, for 0%, 50% and 90% internal humidity.

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0

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60

0 20 40 60 80 100

Years

Waterpenetrationdepth(cm)

HR = 90 %

HR = 50 %

HR = 0%

Figure 9: water penetration depth Vs time

For the studied concrete, the mass transfer process is diffusion controlled rather than a percolation process

(speed of percolation 0.1 mm a year). After one hundred year the water ingress by diffusion through theconcrete hull has reached a depth of around 34 cm for 50% internal humidity. As far as leakage is

concerned, the studied concrete (N Kossa high performance concrete) performs well, assuring no leakage

during the service life provided that the concrete does not include any cross-sectional cracks. Therefore,

for our structure, moisture content between concrete and insulation remains the same as for an in-ground

LNG tank.

A good resistance to cold spot

The storage of LNG imposes temperatures of about 170C. For this range of temperature, a specific

containment system including an efficient thermal insulation must be installed.

For the FPSO for LNG, the concept developed is based upon a concrete hull - single external walls and

tanks covered with the Technigaz membrane, including corrugated stainless steel membrane and

insulation panels. These panels maintain the temperature of the internal face of the concrete shell at about

0 / -5 C. This temperature is really acceptable for the concrete material, and does not represent a

significant loading.

Membrane leakage is a very unlikely scenario. However, what can be the consequences in that case? LNG

is directly in contact with the concrete slab and causes a thermal shock. Such a loading may last several

days before emptying the tank and decommissioning it. As the concrete acts as a secondary barrier, in

addition to the Technigaz membrane, the concrete slab must withstand this thermal shock without

significant damage. The demonstration of keeping this integrity has been demonstrated through a finite

element calculation in order to establish the temperature of the concrete and to check the resistance of thetank for a local cold spot.

The model represents the bottom corner of the Azure barge LNG tank, insulation and concrete being

modeled (see model in figure 10). LNG leakage is represented through nodal heat of 5 m x 2 m extension

on the upper face of the concrete slab in the corner of the tank. The choice has been made to model the

bottom corner of a LNG tank, as it is considered the most critical area of the tank in case of LNG leakage.

The model represents the whole width of the tank, with the half height of the transverse bulkhead plus

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Figure 11: results of cold spot analysis

Results shows that the integrity of the concrete sections is maintained if some vertical prestressing cables

are added in the transverse bulkheads or by adding corner protection (LNG tight facing below the steel

membrane). Then the hull is able to resist the effects of the local cold spot until the LNG tank has been

emptied. In addition, concrete repair is unnecessary.

An excellent behavior in lower temperatures

Concrete is mainly composed of aggregates, cement, water, and some additive products. During

hardening of the paste, water and cement react to precipitate and form a gel that guaranties the cohesion

of the whole mixture. All the water content will not react with the cement, as it is necessary to add some

water for the workability of the fresh paste of concrete. Some water will remain in the concrete pores, atvarious scales of the microscopic structure.

If concrete is subjected to a very low temperature, as could happen in the improbable case of breaking of

the containment system, the water contained may freeze. Consequently ice formation may begin in the

biggest pores. This formation could implicate some water migration in the concrete. If the structure

remains in water, as it is the case for offshore constructions, migration of water toward the cold face

implicates some absorption of water at the warm face. This phenomenon is called cryosuction.

As a result of the cryosuction, the migrating water could form ice crystals and the expansion of these

crystals in the pores could induce an important over-pressure that creates micro cracking and the structure

expands irreversibly.

Various phenomena have to be considered to explain cryosuction: thermal gradient, saline gradient,

internal pressure of residual water, thermodynamic effects in the hydrate paste etc. Up to now, these

explanations have been developed to justify the observed cryosuction for freezing / thawing tests adapted

to harsh winter conditions with temperatures about 25C. The purpose here was to test concrete, one face

being submitted to very low temperatures corresponding to cryogenic storage, the other face being in

contact with sea water at 25C.

Cylindrical pieces of concrete were prepared and subjected to a testing bench as indicated in Figure 12.

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Figure 12: testing bench

Samples tested are:

- High Performance Concrete B 70 density about 2.4 t/m3(NKossa concrete),

- High Performance Concrete B 70 modified density about 1.8 t/m3.

2 samples of each concrete composition have been tested, plus one as reference.

Duration of cryogenic testing were about six days per samples, concrete temperature being stabilized

within a few hours. Temperatures in the concrete, cryogenic tank and seawater were monitored bythermocouples. Minimal temperature measured in the concrete was about 140C.

Figures 13shows the general arrangement of the testing bench.

Gaseous LN2Liquid LN2

at 196

Cryostat (LN2)

Concrete

Insulation

Seawater 25

Tightness protection

(triplex)Diameter: 450 mm

Thickness: 120 mm

PlywoodSeal

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Figure 13: general view of the testing bench

After testing, the aspect of the concrete samples remains unchanged, no cracks have been observed.Currently, measures of concrete characteristics are in progress to confirm the good ability of high

performance concrete to resist this type of thermal shock, which may permit the conclusion that the

cryosuction effect is not important in our case. The aim of the tests in progress is to measure the following

parameters of the concrete:

- Water content

- Porosity

- Permeability

- Compressive strength- Dynamic module

- Spacing factor L , qualifying the average spacing of air bubbles in concrete, which is linked to the

durability of concrete cold environments.- Volume increase

- Microstructure analysis

- Thermal expansion coefficient

- Diffusion constant for chloride ions

An unmatched deck resistance to process hazards

In order to guarantee the structural integrity and to prevent any damage to the LNG tanks, a study ofconsequences of explosion on the deck has been performed. Results shows that the concrete deck is able

to resist the following overpressures:

- 8 t/m_ on the whole width of the deck,

- 10 t/m_ on a single span of the deck,

- higher pressures on smaller lengths than one span.

Longitudinal extension of the explosion overpressure has no influence on the results.

Recording of

temperature data

Testing benchLiquid nitrogen

tank (6 m3)

Sea water tank to maintain

water level in bench test

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Conclusion

Studies and tests performed during the Azure project have led to a reliable design of a concrete hull for

the production, storage and offloading of LNG. Cost effectiveness of the design is associated with a highlevel of safety with regard to the hazards likely to be faced by the FPSO.

Cost effectiveness is due to reduction of concrete quantity and ease of construction, in terms of realization

and sites.

Safety benefits are due to inherent properties of high performance concrete.

The validation of the concrete hull for the LNG FPSO is an important contribution to the demonstration

of the industrial viability of the full floating LNG chain.

Figure 14: artistic view of the concrete barge

References

1 "Design and construction of a 375,000 bbl prestressed concrete floating LPG storage facility for

the Java sea", by A.R Anderson (Concrete Technology Corp.) OTC 2487, Dallas, 1976

2 "The NKossa concrete oil production barge" by C.Valenchon & R.Nagel (Bouygues Offshore),J.P.Viallon & H.Belbeoch (Bouygues), J.Rouillon (Elf Congo) DOT 1995, Rio

3 "LNG storage: adaptability of the membrane containment system to existing and future concepts

along the LNG chain" by P.Genoud (SN Technigaz) IBC 1999 Conference, London, October1999

4 "Liquid gas storage using high-performance concrete: a way to improve safety and reduce costs"

By C.Valenchon (Bouygues Offshore), N.Roux & M.Cheyrezy (Bouygues) GASTECH 1993

Conference, Paris, February 1993

5 Norwegian Standard NS 3473 E "Concrete structures, Design rules", 1998

6 IGC code 93, "International code for the construction and equipment of ship carrying liquefied

gases in bulk"

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7 "Membrane LNG FPSO and FSRU Methodology for sloshing phenomenon" by L.Spitta l

(Gaztransport & Technigaz), M.Zalar (Bureau Veritas), P.Laspalles (Bouygues Offshore) and

L.Brosset (IRCN) GASTECH 2000 Conference, Houston, November 2000

Acknowledgements

- The European Commission

- The sponsors (ELF, SHELL, CHEVRON, TEXACO, CONOCO)

- The partners of the AZURE project (Bouygues Offshore, MW Kellogg Ltd, les Chantiers de

lAtlantique, Fincantieri, FMC Europe, Gaz Transport et Technigaz, Bureau Veritas, RINA, IRCN are

acknowledged for financial and technical support provided.

Documents

Floating LNG - Cost and Safety Benefits of a Concrete Hull