FLOATING LNG : Cost and Safety Benefits of a Concrete Hull

Denis MARCHANDBouygues Offshore

France

Christophe PRATTechnigaz

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, amongthem 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 FloatingProduction, Storage and Offloading vessel (FPSO).

To date concrete has been used offshore mainly in the North Sea for gravity base platforms. Howeverconcrete floating units also exist in other parts of the world, the best known examples being the ArdjunaLPG barge for Arco in Indonesia (ref 1) and the N’Kossa FPU for Elf in Congo (ref 2). Concrete is alsoused for onshore LNG storage tanks in association with membrane or 9% nickel containment systems(ref˚3). 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 be

used.

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 seawaterhydrostatic 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 gasfrom deep-water oil fields offshore West Africa. Plant capacity is medium scale: around one million tonsof LNG per year. LNG is stored in three longitudinal tanks, with a conventional Technigaz typemembrane containment system. Total LNG storage capacity is 110,000 m3. Although only nitrogen isused as refrigerant for liquefaction, there is a need for separation and storage of LPG’s, due to feed gascomposition. The barge is spread moored, head to the prevailing wave direction. Export of LNG and LPGis done by tandem offloading to dedicated carriers, through the Boom-to-Tanker system being developedby FMC.

The hull is made of high performance, lightweight concrete. Thorough structural analysis has beenperformed 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 wasperformed on the membrane and on the concrete in abnormal cryogenic conditions. Hydrodynamicbehavior is being tested by basin tests and liquid motion tests. The concrete hull design has proved to be avery robust concept.

The barge can be built in a conventional dry dock or in a graving dock. Its draft is small enough to allowconcrete hull construction and topsides integration to be carried out at the same location. Containmentsystem integration is performed on a non-critical path basis, and the overall construction schedule is quiteattractive 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 toprovide a large deck area for the liquefaction process plant and to allow the complete construction of theFPSO in a single location; either in an existing dry-dock or in a new-build graving dock. The benignenvironmental conditions of the Gulf of Guinea, with 100-year significant sea state of 4.2 m, allow theuse of such floaters.

b) Use of vault effect. In order to reduce concrete volume, choice was made to use a semi-cylindricalshape 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 undercombined external and stored fluid pressure loading, as the curved shape greatly reduces the flexion andtake 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 toavoid 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 thetransverse 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, noballast is necessary to compensate the variation of draft when loading or offloading the cargo, except fortrim and heel control. This symmetrical configuration limits the overall bending moments and thus thequantity of longitudinal pre-stressing required.

c) Longitudinal storage tanks. For the LNG FPSO, where tanks are partially filled most of the time, tanksare 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 stabilitywith negligible free surface effect. Moreover, as a good engineering practice, the resonant period of theliquids inside the tanks have been placed outside the wave energy spectrum to limit possible sloshingoccurrence (ref 7). Figure 4 shows that the tank transverse resonant periods (in roll) are kept below thewave periods for all filling levels of the tank. In a similar manner, the longitudinal resonant periods of thetanks 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 beenvalidated by current industrial offshore applications of concrete, which include several concrete platformsand 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 wellestablished design and construction techniques.

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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 anequivalent 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 thetopsides.

- Concrete structures have much better fire resistance than steel structures, which is of majorimportance for LNG production and storage vessels.

- The excellent cryogenic behavior of the high performance concrete makes it particularly safe forLNG applications, and thus provides a higher overall safety.

- The pre-stressed concrete structure permits the use of the membrane containment system that is usedfor 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 greatlyimproves 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 moreparticularly 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 oflightweight concrete. As high performance concrete density is 2.4 t/m3, the use of 2.1 t/m3 allows animportant reduction of concrete volume and length of the hull by limiting the volume of void tanksrequired in the hull for buoyancy. Thus, this reduction of 12.5% in the concrete density allows the hulllength 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 hulllength allows 25% reduction of the longitudinal pre-stressing quantity, which is dependent on themaximal values of the overall bending moments.

It should be noted that 2.1 t/m3 density has been chosen as a good compromise between lightness andmaterial validation.

Structural analysis

A structural analysis has been performed to check the resistance of the hull to the environmental andinternal loads. This analysis has been based on the same standards and computer software as those usedfor the structural analysis of the N’Kossa concrete barge built for Elf Congo in 1995. In fact, the design ischecked by performing two analyses:

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

Longitudinal analysis

The static overall bending moments have been calculated for the following load cases, which summarizethe 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 ofthe hull, from this centerline to the transverse bulkhead separating water ballast and LPG tanks. Softwareused 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 themodel

Plane ofsymmetry

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 mainobjectives is to complete LNG tanks construction as early as possible in order to start to install thecontainment system. Thus, the construction of the hull is performed as indicated in the figure 6.

Figure 6: construction philosophy

Construction of the concrete hull is made easier by the regularity of its geometry, thus improving theplanning 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 builtin the following sequences:

- Installation of the sub semi-cylinders supports,- Construction of bilge keels,

1st phase2nd 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 the figure 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. Itincludes insulating panel erection and membrane erection, both being performed using dedicatedscaffoldings rolling along the length of the tank (see figure 8). Membrane installation can continue duringtopsides 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. Aslongitudinal pre-stressing is more important at the center of the hull than at fore and stern, pre-stressinginstallation 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 achievedin 13 months. Consequently the hull is ready to receive topsides 13 months after the start of concretepouring.

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 inthe area of the LNG and LPG tanks. The study has been performed according to DNV rules, assuming asupply boat of 5,000 tons out of control at a speed of 1 m.s-1. 3 headings of the supply boats have beenstudied: bow, broadside and stern impact. Results shows that overpressures on the concrete hull shellsplus 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 andvoid tanks guarantees the integrity of the LNG and LPG tanks. In addition, the great stability of the hullallows 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, toprevent condensation that is likely to freeze, thus slightly increasing the thermal conductivity. For theAzure barge, a study has been performed to check the tightness of the hull, to check that no water ingressoccurs 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 likelyto occur: percolation or diffusion.

Water permeameter testing was performed on N’Kossa samples giving an intrinsic permeability of2.10-20˚m_. Under those conditions, the calculated water speed of filtration through the concrete hull hasan order of magnitude of 0.1 millimeter a year. From this result it is evident that the water percolationphenomenon through the studied concrete is negligible, therefore the water ingress through the concretehull is governed by a liquid diffusion process described by Fick’s law.

The percolation phenomenon has been studied through the finite element code CESAR-LCPC. This codeallows 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 (20¡C)- 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 dividedinto several stripes to take into account the concrete crack and the temperature distribution within theconcrete for steady state conditions.

The calculations performed (a period of time of one hundred years was simulated) indicated that theconcrete undergoes a dessication from the face exposed to 0%, 50% or 90% relative humidity, and thatthere is a diffusion controlled water ingress from the face exposed to 100% relative humidity. Both ofthese processes are very slow and reach a steady state resulting from an equilibrium between dessicationon 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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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 isconcerned, the studied concrete (N Kossa high performance concrete) performs well, assuring no leakageduring 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-groundLNG tank.

A good resistance to cold spot

The storage of LNG imposes temperatures of about —170¡C. For this range of temperature, a specificcontainment 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 — andtanks covered with the Technigaz membrane, including corrugated stainless steel membrane andinsulation panels. These panels maintain the temperature of the internal face of the concrete shell at about0 / -5 ¡C. This temperature is really acceptable for the concrete material, and does not represent asignificant loading.

Membrane leakage is a very unlikely scenario. However, what can be the consequences in that case? LNGis directly in contact with the concrete slab and causes a thermal shock. Such a loading may last severaldays before emptying the tank and decommissioning it. As the concrete acts as a secondary barrier, inaddition to the Technigaz membrane, the concrete slab must withstand this thermal shock withoutsignificant damage. The demonstration of keeping this integrity has been demonstrated through a finiteelement 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 beingmodeled (see model in figure 10). LNG leakage is represented through nodal heat of 5 m x 2 m extensionon the upper face of the concrete slab in the corner of the tank. The choice has been made to model thebottom 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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10˚m along the longitudinal axis of the hull. The model has been performed with COSMOS/M softwareusing volume elements.

Figure 10: model for analysis of cold spot

- Environmental conditions have been taken into account by modeling forced convection betweenseawater and concrete,- Thermal properties of concrete and insulation depending on temperature have been considered.- Thermal loads are insulation temperature (-170¡C) and cold spot (-200 W.m-_).

A steady state thermal analysis has been performed (see figure 11 for results). The temperatures field hasbeen used to perform a structural analysis. Thermal stresses have been added to stresses resulting from thestructural analysis already performed. Verifications of concrete sections have been realized by comparingfinal strains to ultimate strain limits of concrete and reinforcement steels.

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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 cablesare added in the transverse bulkheads or by adding corner protection (LNG tight facing below the steelmembrane). Then the hull is able to resist the effects of the local cold spot until the LNG tank has beenemptied. In addition, concrete repair is unnecessary.

An excellent behavior in lower temperatures

Concrete is mainly composed of aggregates, cement, water, and some additive products. Duringhardening of the paste, water and cement react to precipitate and form a gel that guaranties the cohesionof the whole mixture. All the water content will not react with the cement, as it is necessary to add somewater 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 ofthe containment system, the water contained may freeze. Consequently ice formation may begin in thebiggest pores. This formation could implicate some water migration in the concrete. If the structureremains in water, as it is the case for offshore constructions, migration of water toward the cold faceimplicates 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 thesecrystals in the pores could induce an important over-pressure that creates micro cracking and the structureexpands 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, theseexplanations have been developed to justify the observed cryosuction for freezing / thawing tests adaptedto harsh winter conditions with temperatures about —25¡C. The purpose here was to test concrete, one facebeing submitted to very low temperatures corresponding to cryogenic storage, the other face being incontact with sea water at 25¡C.

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 (N’Kossa 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 stabilizedwithin a few hours. Temperatures in the concrete, cryogenic tank and seawater were monitored bythermocouples. Minimal temperature measured in the concrete was about —140¡C.

Figures 13 shows the general arrangement of the testing bench.

Gaseous LN2Liquid LN2at —196¡C

Cryostat (LN2)

Concrete

Insulation

Seawater 25¡

Tightness protection(triplex)

Diameter: 450 mmThickness: 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 highperformance concrete to resist this type of thermal shock, which may permit the conclusion that thecryosuction effect is not important in our case. The aim of the tests in progress is to measure the followingparameters 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 thedurability 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 ableto 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 oftemperature data

Testing benchLiquid nitrogentank (6 m3)

Sea water tank to maintainwater 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 forthe 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 realizationand 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 demonstrationof 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 forthe Java sea", by A.R Anderson (Concrete Technology Corp.) — OTC 2487, Dallas, 1976

2 "The N’Kossa concrete oil production barge" by C.Valenchon & R.Nagel (Bouygues Offshore),J.P.Viallon & H.Belbeoc’h (Bouygues), J.Rouillon (Elf Congo) — DOT 1995, Rio

3 "LNG storage: adaptability of the membrane containment system to existing and future conceptsalong 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 1993Conference, Paris, February 1993

5 Norwegian Standard NS 3473 E "Concrete structures, Design rules", 19986 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) andL.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 del’Atlantique, Fincantieri, FMC Europe, Gaz Transport et Technigaz, Bureau Veritas, RINA, IRCN areacknowledged for financial and technical support provided.