Z - Offshore Structures Overview (Concrete Structures)

5/28/2018 Z - Offshore Structures Overview (Concrete Structures)

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XIV National Conference on Structural Engineering,

Acapulco 2004

Offshore Structures A new challenge

How can the experience from the marine concrete industry be utilized

Knut Sandvik, Rolf Eie and Jan-Diederik Advocaat, of Aker Kvaerner Engineering & Technology AS

Arnstein Godejord, Kre O.Hreid, Kolbjrn Hyland and Tor Ole Olsen, of Dr.techn.Olav Olsen a.s

Norway


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XIV National Conference on Structural Engineering, Acapulco 2004Offshore Structures A new challenge

TABLE OF CONTENT

Summary

1 Marine Concrete Structures ............................................................................................ 41.1 General .................................................................................................................................... 41.2 Floating Concrete Sea-structures............................................................................................. 41.3 Performance in the marine environment ................................................................................. 61.4 Considerations for new field development.............................................................................. 81.5 Design aspects ......................................................................................................................... 8

3 The Norwegian Experience and Know-how................................................................. 133.1 The structures ........................................................................................................................ 133.2 Research and development .................................................................................................... 16

3.3 Decommissioning of offshore concrete platforms................................................................. 17

4 Project Execution Typical ...........................................................................................18

5 Ongoing Projects .............................................................................................................265.1 The Sakhalin II Project.......................................................................................................... 265.2 The Adriatic LNG Terminal Project...................................................................................... 29

6 Novel Concepts ................................................................................................................ 326.1 Floating LNG Terminals ....................................................................................................... 326.2 Floating airport or navy base................................................................................................. 33

6.3 MPU Heavy Lifter................................................................................................................. 346.4 MPU Semo ............................................................................................................................ 356.5 Other novel concepts............................................................................................................. 36

7 Project Execution in Mexico .......................................................................................... 397.1 Engineering and Design ........................................................................................................ 39

7.1.1 Conceptual design ......................................................................................................... 397.1.2 Detail design.................................................................................................................. 397.1.3 Rules and Regulations proposed for concrete projects in Mexico ................................ 40

7.2 Fabrication Site...................................................................................................................... 41

7.3 Options for fabrication .......................................................................................................... 427.4 Construction and Methods..................................................................................................... 437.5 Material Qualities .................................................................................................................. 43

7.5.1 Concrete......................................................................................................................... 447.5.2 Ordinary reinforcement ................................................................................................. 447.5.3 Prestressed reinforcement.............................................................................................. 45

7.6 Cathodic Protection ............................................................................................................... 45


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7.7 Steel/Concrete Connection Methods..................................................................................... 467.7.1 Riser support.................................................................................................................. 467.7.2 Mooring brackets / Towing brackets / Fairleads ........................................................... 46

7.7.3 Embedment plates ......................................................................................................... 47References ............................................................................................................................... 48


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Summary

The concrete construction industry is widely spread and every country has its own.

Concrete structures have been used in the marine environment for a very long time. Examples arebridges, docks and lighthouses. Particularly in war times, when steel is scarce, concrete has also beenused for barges and ships. The long history of marine concrete structures is interesting and representsvaluable experience.

Concrete structures have proven especially well suited to develop offshore oil and gas fields.

More than 40 major offshore concrete structures make a good job at supporting the processingfacilities of hydrocarbon plants offshore. They are constructed over the past 30 years, and performwell in all the different environments from the arctic to tropical waters, and from sandy stiff seabed to

very soft clays.

A number of the platforms are permanently floating, and they also show good and efficient behaviour.

How may all this experience be utilized to further develop the offshore oil and gas industry?

The authors of the present paper, being representatives of the Norwegian offshore concrete industry,have experience from all levels of design and construction of small and large offshore concretestructures. Examples are described in this paper, including the important elements of projectexecution, the experiences from design and construction, the durability of offshore concrete structuresand the associated required maintenance, as well as the issues of removal and recycling of suchstructures.

Ongoing projects (Sakhalin II in Russia and Adriatic LNG Terminal in the Adriatic Ocean) are brieflypresented.

Recent trends and some novel concepts for further development are discussed.

The paper concludes with some thoughts on project execution in Mexico, including engineering,construction and construction methods, materials and labour.

It is the hope of the authors that the paper will represent a fairly comprehensive description of offshoreconcrete structures, or at least a place to start in the search for improved oil and gas field development.


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1 Marine Concrete Structures

1.1 General

Fig. 1-1 shows the still floating caissons for a roll on/roll off facility in Port dAutonome Abidjan,built by Selmer Skanska.

Figure 1-1. Floating caissons

The picture shows that having access to water and an innovative concrete construction industry maybe a good, and possibly sufficient, starting point for building offshore structures.

Some situations may call for more complicated structures, and deep sheltered waters may be arequirement for the construction. Many of the offshore structures described in this paper are fromNorway which has deep fjords, protected from the ocean.

Although many of the examples in this paper describe complex offshore concrete structures, it isimportant to recognize the value of simplicity. Ingenuity and standard means of construction will bringthe best results.

1.2 Floating Concrete Sea-structures

The history of floating concrete sea structures goes back to the 19th century. In 1848 Lambot for the

first time used reinforced concrete to build a boat. During World War I, 14 concrete ships were builtdue to the steel shortage - including the 130 m long U.S.S. Selma. At that time reinforced concrete hadalready been used in shipbuilding (small ships) in the Scandinavian countries.

World War II concrete ships saw widespread wartime service in battle zones. Twenty-four of theseships were large sea-going vessels and 80 were sea-going barges of large size. The cargo capacitiesranged from 3.200 to 140.250 tons. Ref. 1, by Morgan, gives a good description of the earlydevelopment of the concrete hull.


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A number of notable pontoon bridges have been built of concrete. Ref. 2 gives an overview of the longtraditions within this area. The first floating concrete bridge was built across Lake Washington in1940.

In the late 1950s, a number of pre-stressed concrete ocean-going barges were constructed in the

Philippines (additionally 19 barges from 1964 to 1966), and concrete lighthouses were constructed ascaissons in the 1960s. Concrete lighthouses are installed in the Irish Sea, in Eastern Canada and in theGulf of Bothnia. Many pontoons, barges and other crafts have been successfully built in the formerUSSR, Australia, New Zealand and the UK.

Over the years, starting back in mid 1920s, some 70 temporary floating immersed concrete tunnelshave been built in the following countries: USA, Canada, Argentina, Cuba, UK, Denmark, Sweden,Holland, Belgium, Germany, France, Hong Kong, Taiwan (Republic of China), Japan and Australia.

During the 1970s concrete gained recognition as a well-suited material for construction of offshoreplatforms for the exploration of oil in the North Sea. Permanently floating offshore vessels related tothe petroleum industry are now installed in the Java Sea, in the North Sea and outside the coast ofCongo in West Africa.

From 1950 to 1982 it was registered that approximately 1.130 concrete hulls had been built. Most ofthem are small with overall length less than 50 m. Among the bigger ones, two groups of sizes aredominant, - approximately 250 hulls with length ranging from 58 to 67 m, and 40 hulls with a lengthof 110 m.

Concrete hulls and barges - examples from practice

The ARCO barge (ref. 3)The Ardjuna Sakti is a floating pre-stressed concrete LPG storage facility with overall dimensions140.5 x 41.5 x 17.2 m (length x beam x depth). Fully loaded, the vessel displaces 66.000 tons. TheARCO barge was built and completely outfitted in Tacoma (Washington) and towed 16.000 km(10.000 miles) across the Pacific Ocean to the Java Sea in 1976, where it is permanently moored.

Concrete barge C-Boat 500.The prototype barge, of 37 m length, 9 m beam and 3.1 m depth and of 500 dwt loading capacity wasbuilt in Japan in 1982.

Heidrun TLP (ref. 4)Conocos Heidrun platform is the worlds first TLP with a concrete hull and the largest permanentlyfloating concrete structure ever with a concrete volume of 67.000 m3. The topside related weight is89.000 tons (net 65.000 t topside) and the displacement 285.000 tons. The platform was installed onlocation in the North Sea in 1995, at a water depth of 345 m.

Troll Oil Semi (ref. 5)Norsk Hydros Troll Oil FPS platform is the worlds first concrete catenary anchored floater. TheTroll Oil semi submersible hull has a concrete volume of 46.000 m3and supports a topside weight of32.500 tons. The displacement is 190.000 tons. The platform was installed on location in the NorthSea in 1995, at a water depth of 335 m.

Nkossa barge(ref. 6 & 7)Elf Congos Nkossa barge is the worlds largest pre-stressed concrete barge. The floating productionvessel of which the dimensions are 220 x 46 x 16 m was built in Marseille, France, and towed 4500nautical miles to the west coast of Congo in West Africa where it was permanently anchored in 170 m


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water depth in 1996. The total displacement fully loaded is 107.000 tons, and the concrete volume ofthe barge is 27.000 m3. The hull supports six topside modules with a total weight of 33.000 tons.

1.3 Performance in the marine environment

Considering the wide use of concrete for marine applications there is surprisingly little documentationto be found on in-service performance. The apparent cause for this is that provided satisfactory designand execution, concrete is an optimal material for harbour, coastal and offshore construction as itcombines durability, strength and economy. This fact is supported by studies of floating concretedocks back in the 1970s, showing dramatic savings, requiring less than 10% the maintenance ofsimilar all-steel docks, ref. 1.

Other structures also utilize the water-tightness properties of concrete; storage tanks, nuclearcontainment structures and submarine tunnels.

Sare and Yee, ref. 8, report negligible repair and maintenance costs for the 19 pre-stressed concretebarges constructed in the Philippines during 1964-66 for Lusteveco, with no need for dry-docking.After many years in service, average annual maintenance cost of the concrete barges are found to beabout 1/3 compared to steel barges.

The fabrication cost of Yees barges showed a saving of 16 percent compared to that of steel. In theperiod 1974 to 1975, the total downtime per floating barge per year for maintenance work was sixdays for the concrete structures. The similar steel barges had an average downtime of 24 days.

The Refiner I barge, checked by Bureau Veritas for issuing necessary certificates for the towedvoyage, was designed for 4.2m wave height. It is worth noting that the vessel in fact endured a stormin the Bay of Biscay during which time the conditions were undoubtedly more severe than thosecontemplated in the calculations (the pontoon drifted in winds of force 10-11 and angles of roll and

pitch of 14 and 10

respectively were observed). The unit behaved perfectly well through thisunexpectedly severe environment. It seems to be general consensus that concrete vessels and barges

have proved to have good seagoing qualities, to be safe and strong, and suffer much less fromvibration than steel ships - to the crews satisfaction.

The 1970s and 1980s saw the spectacular development of offshore bottom fixed concrete structures,installed in up to 300 meters (1000 ft) of water depth in the midst of one of the worlds stormiestoceans, the North Sea. It is remarkable how well these structures have performed in the hostile marineenvironment, successfully withstanding the extreme loads from waves approaching 30 meters inheight as well as the dynamic cyclic forces. Experience has shown that the offshore concrete structurescurrently in use are virtually maintenance-free. It is generally recognized that the first concreteplatforms in the North Sea were over-inspected and that the need for extensive instrumentation ofplatforms of common types should be reconsidered.

A comprehensive list of references to information pertaining to the performance of North Sea concretestructures is presented in ref. 9 and 19. No significant sign of material deterioration, corrosion ofreinforcement or other material-related deficiencies have been observed. Falling objects or rammingships mainly cause observed damages. Platforms designed for 20 years operation have now passed theend of their prescribed design life. Inspections and investigations confirm that their lifetime in generalcan be extended.

Various codes give well-established rules for assessing fire resistance. Two hydrocarbon fires insideNorth Sea concrete platform shafts in the late seventies are reported. The consequence was a surface


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scaling about 10-20 mm deep over a height of 5-10 m. This marginal impact is attributed to the large heatcapacity and low thermal conductivity of concrete. No repair was found necessary - clearly demonstratingthe excellent fire resistance of concrete.

Concrete is normally considered to be one of the best fire proofing materials available, a factor of

unquestionable importance for an offshore oil or gas platform/storage. There are many instances, bothashore and afloat, of fire causing no more than local to no damage to concrete structures. As an exampleconstituting the most impressive testimonial that could possible be called for, Derrington (ref. 11) reportshow two concrete barges survived the Bikini Atoll nuclear bomb tests in good shape when their cargo offuel oil was set alight - moored only 100 yards from the test centre.

Wartime brought the additional hazards of bombs and mines. Morgan ref. 12 reports that in 1944 a 1000-tonne German concrete barge hit a mine, which exploded under the stern - the vessel was able to reachshore by being repaired while afloat with underwater concreting. Lusteveco, operator of Yees concretebarges, was quite pleased with their performance and say one of the most endearing aspects of pre-stressed concrete hulls is their ease of repair. The barges serviced the Vietnam War area for a periodof nine years and a number of barges were rocketed or damaged by plastic bombs. The damage wasusually confined to severely cracking of concrete within a limited area of 1m x 2m on the surface of

the hull - a damage consequence limitation credited to the rigid pre-stressed concrete hull. In March1973 one of Lustevecos 2000 dwt dry cargo barges, L-1960, hit a mine at the starboard side transitingthe Mekong River fully loaded with rice intended for Phnom Penh. After some temporary repairs, thebarge was towed safely to the Philippines for permanent repairs. The cost of this 10 days repair jobwas US$ 4381.

Pre-stressed concrete was chosen as the hull material for the ARCO barge because of itsseaworthiness, competitive cost, fire resistance, durability and speed of construction. After almosttwenty years of continuous service, various tests were carried out for the concrete barge. Due to itsexcellent condition, ARCO has given its barge an indefinite lifespan - a solid proof of the excellentperformance of concrete in a marine environment as well as its good fatigue resistance.

There are also examples of premature failures for concrete structures in coastal areas (e.g. bridge piersand quay structures) - suggesting that the marine environment is demanding and imposes specialrequirements on materials and workmanship. It is in this context important to distinguish between theonshore and offshore concrete industry. The problems experienced in coastal areas for the onshoreconcrete industry is caused by, for example ref. 13: improper cover, misplaced reinforcement,improper handling, placing of concrete or poor quality of concrete (e.g., seawater contaminatedaggregates, improper concrete mix proportions).

In Norway, the experiences with coastal bridges have shown that the principal causes of failure are thesame as reported above. In general, however, marine structures built by the onshore concrete industryhave suffered very limited from degradation - refer for example list of surveys presented in FIPs stateof the art report on inspection, maintenance and repair of concrete sea structures, ref. 14.

In 1999 a large Norwegian research program, ref. 10, investigated the durability of concrete structures,covering bridges, industrial structures, quays and offshore platforms. Main emphasis was set onchloride penetration and reinforcement corrosion. Six offshore concrete structures were investigated:

Statfjord A (16 years in operation at time of inspection)

Gullfaks A (7)

Gullfaks C (4)

Oseberg A (8/9)

Troll B (2)


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Ekofisk Tank (17/22)

Calculations based on the chloride profiles showed that the investigated offshore platforms were inexcellent condition and that there would be no risk for corrosion within their expected lifetime. Two ofthem would theoretically not reach chloride concentrations representing risk for corrosion at the

position of the reinforcement bars, until having serviced for more than 200 years.

The good performance of the offshore concrete structures is attributed mainly to high quality concrete(i.e. high strength / low permeability concrete in situ), proper design including sufficient cover toreinforcement, good workmanship & construction techniques and thorough quality assurance.

1.4 Considerations for new field development

The general conclusion drawn from service performance of the offshore concrete structures is that theyhave proved excellent behaviour and require significantly lower expenditure for inspection,maintenance and repair than steel structures. Experience has shown that offshore concrete structurescurrently in use are virtually maintenance-free.

Over the years wide experience has been gained in the field of post-tensioned concrete offshorestructures. The successful construction of the Heidrun TLP, the Troll Oil Semi and the Nkossa bargehas opened for interesting potentials to the offshore industry as to the suitability and economics ofconcrete floating structures. Recent studies also conclude that floating concrete structures are wellsuited for floating LNG plants.

The increasing importance of local content in the development projects is favouring concrete as thebuilding material in countries with limited number of offshore steel yards. Concrete structures can bebuilt in greenfield areas with very little infrastructure. The majority of the workforce does not needspecial education and can be recruited locally. Hence, choosing concrete may significantly increase

the local content of a project.

1.5 Design aspects

Design Life and Reuse:The offshore concrete structures installed to date have been designed for 25-70 years. The Troll GBS was designed for a 70-year life and the Heidrun TLP is designed for 50 years.There is not a significant additional cost related to extension of design life from for instance 30 yearsto 50 years or 70 years. One reason is the fact that reinforced and pre-stressed concrete is not sensitiveto fatigue. With the extensive design life possible for a concrete platform there is obviously a verygood possibility for reuse. The investigations carried out on durability and conditions of existingconcrete platforms clearly prove a great potential for reuse.

Stiffness: Concrete structures generally have large stiffness. The result is less flexibility and lessdeformation applied onto outfitting steel etc.

Robustness: Under maximum credible accidents, such as major leakage, collision or fire, a properlydesigned and constructed pre-stressed concrete vessel has better inherent safety than a comparable steelvessel. This is one of the conclusions from a technical feasibility and safety study of a 297 m (974 ft) longstorage/processing vessel carrying LPG in free-standing tanks performed by Gerwick et. al., ref. 15. Herea pre-stressed concrete vessel was designed and compared with an existing steel vessel designedaccording to the ABS requirements. It was also found that the concrete hull, being stiffer, developed


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significantly lower dynamic amplification and had a lower risk of failure than the steel vessel. Theconcrete hull was found to have adequate safety to justify its use for vessels in hazardous cargo servicewithout limitation as to length.

Impact Resistance: The concrete material has excellent resistance to impact loads. This has been

proven through history, and the result is that concrete is widely used in military installations, shelters,in buildings which need to be failsafe and which are regarded as exposed to terror attacks etc. Theconcrete hull of a concrete floater will typically be designed for impact loads from any possibledropped object. Still there will be a design requirement to design for accidental filling of anycompartment adjacent to sea, or adjacent to piping, which is connected to sea.

Fire Resistance:As mentioned above concrete is normally considered to be one of the best fire proofingmaterials available. Two fires inside shafts of North Sea concrete structures have been reported, anddamages have been too small to decide any repair work to be done. The combination of excellent fire- andimpact-resistance is of course very important for units producing hydrocarbons.

Maintenance Free:Appropriately designed and constructed concrete hulls in the marine environmentare almost free of maintenance. Regulatory inspection of the concrete structure is mainly limited to

visual inspection and entails no significant cost. Recently constructed concrete platforms have beendesigned for an operational life of 50-70 years. The maintenance/OPEX aspect was decisive in ElfCongos decision to chose a concrete barge on the Nkossa project, ref. 6 and 7, as the concrete hulloffered significant savings in expected maintenance cost. The barge will fulfil its functions on sitewithout interruptions for 30 years and there is expected virtually no maintenance.

To quote Campbell, American Bureau of Shipping Surveyor ref. 16: The history of concrete formarine construction is very favourable. There is little doubt that a well designed, well built, concretestructure will have a longer service life than a comparable steel structure.

Motion Characteristics: The motion characteristics of a concrete hull are typically better than for asteel floater designed for the same purpose. This conclusion can be drawn based on reports from shipcaptains (World War II ships and Yees barges), several studies and recently confirmed by bothanalyses and model testing for very large FPSOs (BP Atlantic Frontier Stage 2 / Schiehallion, hulllength 280 m). The generally somewhat larger mass and draught, result in improved motioncharacteristics. For the very harsh environmental conditions West of Shetland (the SchiehallionFPSO), the mooring size and cost was reported nearly identical for steel and concrete hulls (ballastedcondition governing). The general picture, however, is that the mooring costs for concretevessels/semis are approximately 10 per cent more expensive than for a steel hull - illustrating that themooring costs must be included in cost comparison steel versus concrete.

Material properties; strength and weight: It is obvious that the weight of the platform is ofimportance. A vessel must carry its own weight plus a payload. For the concrete platform the payloadis the topside and equipment, as well as any ballast required for hydrostatic and/or geotechnicalstability.

In ref. 17 Jan Moksnes presents some of the results of Norwegian research on concrete over the past20 years. The benefit of this research has been and is significant for the concrete construction industry.

In brief relevant design aspects of importance for the type of structures under consideration are:

o High stiffness, providing a stable foundation for tanks and other attachements

o Good resistance to environmental loading


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o Excellent behavior at low temperatures

o Favorable in ice-infested waters

o Robust with respect to accidental loading such as ship impact, dropped objects or terroristattacks

o Good resistance to oil and gas process hazards

o Functional and safety features common to a land based plant.

o Good resistance to cold spot incidents

o Enhanced material properties with decreasing temperature

o Excellent fatigue resistance

o Good durability, and basically maintenance-free

o Standard offshore concrete quality applied

o No need for skilled labor for the bulk of the construction work, enabling local execution

o Good resistance to seismic loading

o

May be decommissioned and removed, possibly reused

Important also is the cost of the structure. Key qualitative parameters for cost of concrete structuresare summarised below:

o Low complexity structures are both faster and more cost effective to construct.

o Close integration of engineering and construction as well as main operations

o Local availability of labour and materials

o Design basis requirements (waves, soils, functional requirements)

o Generally cost effective to complete as much as possible in the graving dock. Unnecessarystops in slip forming should be avoided

o Preparation of the graving dock may add cost to the project. However, it may proveeconomically sensible to extend and/or deepen the dock to increase the completion grade.

o Construction schedule. Construction time is related to simplicity and concrete volume. ForLNG terminals, assembling the tanks from pre-cast elements may decrease the length of theschedule.

For small structures the initial construction may be performed on barge(s). After float off from thebarge the remaining construction is performed while afloat. However, in most cases a dry-dock hasbeen used for the initial and also sometimes the complete construction phase.

All concepts developed are designed to be removed from the offshore location in a controlled way.

Followin g the removal the structure may be reused or deconstructed and recycled.


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2 Offshore Concrete Structures for the Oil and Gas Industry

Since the Ekofisk Tank was installed in 1973, 41 major offshore concrete structures have been built,see ref. 18.

Fig.2-1 shows the tow of Beryl A in 1975, from its construction site in a sheltered Norwegian fjord, onits way to the harsh environment of the North Sea, fig. 2-2.

Figures 2-1 and 2-2 illustrate some of the design criteria for offshore structures, and at the same timeindicate why concrete may be the best choice of construction material.

Many offshore locations are calm and friendly, but not all. Fig. 1-3 shows an example of an oceanwhere it is not straightforward to build. The platforms may then be prefabricated elsewhere, installedand possibly completed with respect to the foundation (piling, ballasting, grouting) and topsideinstallation. The degree of inshore completion influences the cost and safety of the field development.

The typical offshore concrete structure is of the caisson type, often termed Concrete Gravity Structure,CGS. The caisson provides buoyancy in the construction and towing phases, and acts as a foundationstructure in the operation phase. The caisson may also provide storage volume for oil or other liquids.

This multiple usage of the structure may prove very economic, particularly when storage is required.

Steel structures may of course also be built to provide buoyancy and storage, but buoyancy at largewater depth is complicated and expensive for steel structures as they are exposed to significantpressure loading.

Figure 2-1. Beryl A during tow to installation site, in 1973


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Figure 2-2. The environment of the sea.

The offshore concrete structures for the oil and gas industry are located at various and very differentparts of the world. There are structures in ice-infested waters, in seismic zones and in very harshmarine environments, but also in relatively calm areas. Some are located at large water depth, others inshallow areas. The foundation conditions vary from very stiff sand to very soft clays, and some of thestructures float permanently.

Some of the structures have storage facilities, and all have a hydrocarbon processing plant facility ofsome kind.

Such various conditions for the offshore concrete structure call for different designs.

As stated previously the offshore concrete structures behave very well, and perform their task of

supporting the oil and gas processing facility. The oil companies are frequently evaluating extensionof operational life, and modifications to enable further facilities as the offshore field may containadditional hydrocarbons.


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3 The Norwegian Experience and Know-how

3.1 The structures

The inshore construction of concrete offshore structures provides good conditions for qualityconstruction. The construction site of Aker Kvrner at Hinna, near Stavanger in Norway, was likelythe best and most professional and effective construction site in the world. The construction of 17large offshore concrete structures, created a large amount of expertise, see Fig. 3-1. Multidisciplinarygroups of specialist companies participated in design of these platforms and their various units andoutfitting. Some of the companies contributing considerably were Aker Kvaerner with their designs oftopside, mechanical outfitting and marine operations, Multiconsult, Aas-Jakobsen, SWECO Grnerand Dr.techn.Olav Olsen designed the concrete substructures, both concept and detail designs, and

NGI designed the foundation.

Figure 3-1. Concrete Structures Constructed at Hinna, Norway

The structures built by Aker Kvaerner represents, in terms of concrete volume, approximately half ofthe total volume of the offshore concrete structures of the world.


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Many of the concrete platforms built by Aker Kvaerner are rewarded, in Norway and internationally.In 1976 and 1995 the Condeep platform was awarded the Norwegian Betongtavlen, in 1990 and 1998FIPs prize for outstanding structures. In 2000 the readers of Teknisk Ukeblad (a Norwegianengineering magazine) elected Troll A the engineering achievement of the century.

Olav Olsen was the first Norwegian recipient of the prestigious FIP medal and the Gustave MagnelGolden Medal, mainly due to the pioneering work of designing offshore concrete structures.

Of the more impressive structures is the Troll A platform, shown in Fig. 3-2. The Troll A platform, agas wellhead and processing platform, was complete with skirt piles and topside when towed out andinstalled in the North Sea, see fig. 3-3.

Figure 3-2. The Troll A Condeep, with other structures.

The lower part of the Troll A structure was subjected to a water pressure of 350 m (1150 ft) duringconstruction, and carried the topside weight of 22 000 t 150 m (490 ft) above the sea level during towfrom construction site to the offshore installation site.


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Figure 3-3. The Troll A Condeep during tow-out.

The soil condition at the site of the Troll A platform is very soft, popularly termed yoghurt. For thisreason the base of the platform (more than 16000 square meters in area) is equipped with 36 m longskirts to enable the safe foundation of the structure. The 100-year design mud line moment is 100 000MNm.

Most of the existing CGS s are resting on dense sand with short steel skirts penetrating the sand forprotection against scour. For soft soils, deeper skirts are required. This skirt pile principle wasdeveloped for Gullfaks C in the mid 1980s. The soil conditions with very soft clay, required skirtspenetrating down to stiffer layers of soil. Gullfaks C has skirts penetrating 22 meters into the soil.Troll A has skirts as mentioned, so does Draugen (fig. 3-4), with concrete skirts penetrating 9 meters

into the soil. The GBS platforms Gullfaks C, Draugen and Troll A are all located in the North Sea.


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Fig. 3-4. The Draugen CGS-MONO

In recent years the principle has been used also for TLP foundations (Snorre, Heidrun), jackets

(Europipe 16/11, Sleipner T) and suction anchors for floaters as an alternative to ordinary piles.

Provided feasible soil conditions, skirt foundations have many advantages both economically andtechnically compared to other solutions:

o Lower material and fabrication costo Reduction of foundation areao No piling or need for heavy subsea hammerso High position accuracyo Short installation timeo Reduced need for solid ballast

3.2 Research and development

Many of the offshore concrete structures of the world are located in moderate waterdepths. As is seenin fig.3-1 most of the Norwegian built platforms are located in medium to large waterdepths, with thefoundation structures for the Heidrun tension leg platform being deepest, at 345 m of waterdepth.

For installation by bouyancy it is very important to use high strength concrete. For this reason aconsiderable amount of research has been performed in Norway, as described by Moksnes in ref. 17.


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This research has been very successful, and the results benefit not only the oil and gas industry, but theentire concrete industry.

One inherent benefit is that high strength concrete is also very durable in the marine environment.

In the general sense, pushing the limits for the application of offshore concrete structures required avast amount of development in many areas, such as construction techniques (slipforming etc.), marineoperations, analyses for environmental loads and structural response, soil testing and instrumentation,etc.

3.3 Decommissioning of offshore concrete platforms

Even though the structures may be fit for many years, international regulations will put constraints onthe use of the oceans. Particularly important here is the OSPAR (OSlo PARis) Convention. In July1998 it was decided that all platforms in the North Sea should be removed after completing theirduties. An exemption was made for concrete platforms, because of the believed complexity of theoperation.

Extensive work has been performed on the subject, particularly JIP work that has proprietary rights.

The international fdration internationale du bton, fib, initialised work on the subject, in their TaskGroup 3.2 Recycling of Offshore Concrete Structures.

The conclusions were:

It is feasible to remove the offshore concrete structures.

Due respect is required; we shall be humble to the task.

Removing the entire installation is most likely the safest and most cost efficient way toremove the topside.

The Task Group 3.2 realises the political and economic aspects of the issue. Several joint industry projects have addressed the subject, but their conclusions are not, as of

now, publicly available.

The OSPAR Convention requires that the topside of the concrete platforms must be removed.

The work of TG 3.2 is described in Ref. 18 and 20.

The OSPAR applies to the North East Atlantic Ocean, including the North Sea. Other parts of theworld will have different rules to relate to, but the essence is likely to be similar.


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4 Project Execution Typical

Typically the execution of offshore concrete structures consists of several main project phases. Someprojects may constitute all the below phases whereas others may just include a few of these. Aftercompletion of front end engineering design and decision to develop the project and award ofcontract(s) the projects may go through the following main phases:

- Detail design of dry dock and construction site- Detail design of concrete structure prior to concrete structure construction start- Dry dock and construction site development- Construction of lower part of concrete structure inside the dry dock- Float out of dry dock and mooring at inshore wet construction- Construction of upper part of the concrete structure at wet construction site

- Installation of topsides facilities and/or other type of outfitting- Tow to installation site, positioning and installation at location

The main typical construction phases are shown in the below figure.

Construction in dry dock (Draugen) Float out from dry dock (Troll)

Construction at wet site (Draugen) Installation of topsides (Draugen , deck mating)


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Tow to offshore installation site (Draugen) Installation at offshore location (Troll)

Figure 4-1: Typical main construction phases

Detail design of dry dock and construction site

During this phase detail design of the dry dock and the construction site facilities are performed. Thegeotechnical design of the dry dock should be given special attention, as this is the key to ensure awater tight and safe dry dock. The floor in the dry dock is normally located 10 to 15 meters below sealevel and hence the dock is exposed to this differential pressure. This phase is normally on critical pathof the project execution schedule.

Detail design of concrete structure prior to concrete structure construction start

Prior to start construction a global design of the entire structure is performed to define all wall

thicknesses and prestressing layouts etc. as well as overall quantities and layout of reinforcement.This phase also includes an overall design of all the mechanical systems that should go inside theconcrete structure. Further local design and detailing of rebar arrangements and the mechanicalsystems for the lower parts (the first casting sequences) has to be completed before start of anyconstruction work. If the project includes development of a construction site this design work isnormally conducted concurrent to the completion of the construction site and dependant on theduration of this work, this phase may be on critical path of the execution schedule. If the project doesnot include construction site development this phase will be on critical path.

The remaining part of the concrete and mechanical systems detail design will be performed with anoverlap to the construction work. This part of detail design is not normally on the critical path of theexecution schedule.

Dry dock and construction site developmentDuring this phase the dock will be excavated or digged out. Further all the facilities needed to supportthe concrete structure construction work will be established. This phase will normally be on criticalpath of the execution.

In some cases it is possible to use an existing dry dock and only minor adjustments and mobilisationactivities will be required.


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Construction of lower part of concrete structure inside the dry dock (See Figure 4-1)

When the dock is established construction of the lower sections of the concrete structure cancommence. Normally as much as possible would be built in the dry dock as the access and logistics isbetter that at a wet construction outside the dock or any other nearby location. However, in some cases

it may be more economic to complete more of the structure at the wet site rather than to establish adeeper dock. Normally the concrete construction work will be on the critical path for most ofexecution schedule following start concrete structure construction.

Float out of dry dock and mooring at inshore wet construction (See Figure 4-1)

Following completion of the construction work inside the dry dock the dock will be flooded withwater to the same level as the external sea. Then the dock berm or gate will be removed and tugs/winches attached to move the structure out of dock and tow the structure to a wet construction site orin some cases directly to the installation location. To get the structure afloat internal water ballast willbe removed and the lift off carefully monitored. To ensure a sufficient under base clearance and acontrolled lift off a comprehensive weight control system is used.

Construction of upper part of the concrete structure at wet construction site (See Figure 4-1)

When moored at the wet construction site the construction work will continue to complete thestructure. For structures to be installed in deeper waters, say 50 metres or more, a wet constructionsite would be required. During this phase a floating rig will be established to support the constructionwork, this normally consists of a number of storage and other type barges. The construction crew willnormally be shuttled from shore during this phase. A well organised and planned logistics is of utmostimportance to ensure an efficient construction during this phase.

Installation of topsides facilities and/or other type of outfitting (See Figure 4-1)

Upon completion of the concrete structure the topsides facilities (deck) may be installed at the wetsite. If installed at the wet site this is normally performed as a float over (deck mating) operation. Thedeck will arrive to the wet site supported on one or two barges. To transfer the deck onto the concretestructure the structure is ballasted down with just a few meters remaining above sea level. Then thedeck is floated over the structure, which will be gradually de-ballasted to transfer the weight of thedeck from the barges onto the concrete structure.

Alternatively the topsides facilities can be installed at the offshore site. This may either be performedas a high deck float over or by lifting of modules.

Tow to installation site, positioning and installation at location (See Figure 4-1)

Following completion of the concrete structure end possibly installation of the topsides facilities thestructure will be towed to the offshore installation location where it will be positioned and installed.The towing operation is normally conducted by utilising 3-5 ocean going tug boats and average towspeed is in most cases around 3 knots. At the offshore location the tugs will be reconfigured to somekind of a star formation to control the positioning of the structure. When in position the structure isballasted down to the seabed by sluzing in of water.

The in place foundation stability will either be ensured by sufficient weight and friction or bypenetrating the lower parts of the structure (skirts) into the soil skirt piling.

Project execution methodology and strategy

The complexity of this type of projects requires a clear execution strategy and a well developed andsystematic methodology. Aker Kvaerner and the Norwegian know-how cluster have over the yearsdeveloped a systematic and efficient way to execute this type of projects.


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The main elements of the project execution strategy are:

- Develop and maintain one single integrated project team.- Provide management focus and emphasis on the final result through Improvement Leadership.

- Empower and align team members through team buildings.- Organise the project in distinct phases to continuously control schedule and divide the product

into manageable parts with clear objectives and delegated responsibility for the economicalresult.

- Focus on improved standard of HSE and quality in work from day 1.- Systematic technology transfer and training of local personnel.

Each of these bullet points are further outlined below.

Develop and maintain one single integrated project team.The project shall develop a competent organisation supported by effective systems and specialists forkey performance areas such as Cost, Schedule, Quality and Safety.

The organisation will be structured for project execution through effective control and co-ordination ofthe main process from Engineering, Procurement and Construction through to Marine Operations.

The organisation should be staffed by highly qualified personnel, expatriates and locals based on theprinciple of best man on the job. The integrated project management team should be responsible forand have the experience to manage the total EPCI process, see Figure 4-2 below.

Figure 4-2: Integrated Project Management

An overall project risk assessment shall form the basis for prevention based programs to ensure thatproject milestones and key targets are met.

Identification and management of the project schedule critical path will hold the key to a timelydelivery of the project. Extensive work will therefore be performed to identify the critical path andreview measures to reduce its length and consequential effects.

Integrated

Project

Management

Civil

Construction

Engineering

Finance

Acco unt ing

Facilities

Maintenance

Materials

Logistics

Project

Control

Procurement

Mechanic

Installation


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Improvement LeadershipIn general, an Improvement Leadership process should be established. This process shouldfocus continuous improvement effort on all key result areas such as cost, schedule, HSE,

quality and regulatory compliance.

Early in the project a plan for Improvement Leadership shall be established to support the key resultareas given by the project objectives. This plan shall contain the Management Improvement Policy,the goals and the approach for each key result area as well as how the achievement of the goals will bemeasured. The Improvement Leadership is a continuous process that will support prevention-basedprograms such as risk assessment, technology transfer, value engineering, constructability reviews,potential problem analysis and quality improvement.

These leadership and management principles are currently applied with great success for the ongoingSakhalin II project under construction in Far East Russia.

Team Building and Alignment to Project GoalsTeam building sessions shall be arranged to develop the project organisation into a single integratedteam focused on the project processes and goals and to establish a common understanding andacceptance of cultural differences, organisational goals, responsibilities and good workingrelationships. Team strength means empowered team members promoting personal quality and havingthe confidence to openly discuss problems.

Local ContentThe Construction of a offshore concrete structure will provide many jobs to the local society. Inaddition to direct employment, a project will give many indirect benefits to people and businesses.Individuals will develop new skills improving their job prospects. For business, this type of work willtranslate into new hiring and additional training of staff, enhancing new skill levels, capabilities andcompetitiveness of many companies. This will enable them to attract additional oil and non-oil related

work on a local, national and international scale.

Such projects should lead to a major transfer of technology to the local society.

Based on previous experience from Norway, Newfoundland and Southeast Asia the value of the localcontent will typically be in the range of 50 to 90 % of the total project value

The local content will normally be related to staff and labour man-hours expenditures, local materialsupply, rented services and equipment and comes in the following categories:

Personnel resources- Management.- Supervision.

- Labour force.

Materials- Civil.- Mechanical.- Materials for dock and site construction.

Equipment for site development and operation- Camp.


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- Transport.- Stores.- Offices.- Security.

Labour for such projects will be recruited from the local area and supplemented with handpickedskilled labour. An extensive training and induction program will be put in place to prepare the labourand local supervision for the task and to secure a proper transfer of technology.

Local supervisors will be employed to directly supervise the local workforce under the control ofspecialist expatriate supervision.

A number of specialist personnel with extensive construction experience will be included in themanagement and supervision of the project. An important role for the experts is the transfer ofknowledge to the local supervising staff and to the work force. Training sessions and mock-ups ofsome parts of the structures will be used in this context. The learning curve is reasonably short if aproper information and training system is established.

In general, therefore, it is not necessary to bring in especially skilled labourer for the constructionwork. It will, however be required to use a limited number of hands-on foremen or lead hands in thestart up phases of new construction operations, for example for rebar installation in different parts,slipforming operations, pre-stressing and other special work operations.

All commodity bulk materials such as Cement, Fly Ash, Concrete Aggregates and Reinforcing Steel,Post -Tensioning tendons and equipment, Wood and Plywood in addition to structural steel and pipingrelated to mechanical outfitting should to the extent possible be procured locally.

Materials for concrete production and the properties for fresh and hardened concrete shall be qualifiedin due time before start of construction. The qualification is a relatively comprehensive exercise thatcould be combined with mock-ups for training purposes.

All supplies are to be sourced on a competitive bidding basis.

Materials for temporary facilities including the construction camp, site office, workshops and storesand associated services will be procured locally. Specialised materials and equipment will be tenderedinternationally if not available locally.

Execution Methodology

Aker Kvaerner has together with partners over several years developed a compressive projectexecution model that is focused on the quality of the end delivery to the clients. To achieve thisattention has to be given to the following items:

Predictability! More effective work processes

Building commercial awareness Effective communication and utilisation Enhanced risk management & control Zero mindset HSE philosophy Common approach provides the basis for continuous improvement

The below figure gives a high level overview of the execution model. The core of the model is tocontrol quality of information at all levels. Further stringent gates are introduced to check out that


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sufficient quality of the information is reached to move forward into the next phase or sub-phase of theproject. A high degree of automatisation of this process is obtained by using state of the art IT/IStools.

Figure 4-3: Project execution model, phases and levels.

STRATEGIC

EXECUTION

CONTROL

SPECIFIC WORKPROCESSES,

SYSTEMS AND PROCEDURES

Corporate

Business Areas

Project


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The philosophy of the model is to ensure end product quality for all parties. This is best obtained byfocusing on the system definition and maintaining a system focus all the way through to finalcompletion of the product. This philosophy is illustrated on the below figure.

Figure 4-4: Project execution philosophy.

Each of the phases as shown in Figure 5-3 is further divided into sub-phases and detailed work processfor each discipline is developed. The end product is divided into a large number of quality objects onwhich the completion (or quality level) is constantly monitored by detailed checklists.

Utilisation of this systematic and comprehensive model has proven to give predictable and reliableproject execution.

System Design Completion OperationProcurement

Fabrication

Construction

ContractAward

Take-over

Client requirements:

- operation

- maintenance

- HSE

Client specs & rules Client experience

SYSTEM FOCUS

Internal knowledge &

lessons learned


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5 Ongoing Projects

Offshore concrete structures has to be considered niche products that are especially well suited forsome special applications such as:

Structures for harsh environments such as ice and iceberg infested waters Structures for LNG facilities due to the good behaviour of concrete when subjected to cryogenic

temperatures Structures that require local construction

Over the last 10-year period there has typically been 1-2 project ongoing at any time. The laststructure completed was the Malampaya project that was installed in 2001.

Presently there are 2 construction projects ongoing constituting a total of 5 concrete structures. An

additional 2-4 projects are in the front-end engineering phase and most of them may hopefully beapproved by the end of next year, some of these are outlined in the next section.

The two projects currently under construction is the Sakhalin II project for Sakhalin EnergyInvestment Company (SEIC, with Shell being the lead party) and the Adriatic LNG Terminal (ALT)project with ExxonMobil as the lead party. Both these projects are executed using the Norwegianknow-how cluster with Aker Kvaerner being in a lead role.

5.1 The Sakhalin II Project

SEIC is currently developing phase II of the Sakhalin II block outside the Sakhalin island in far eastRussia. The development comprises eight main items; The PA-B Platform, PA-A Platform, an

Onshore Processing Facility, the Lunskoye Platform, an Infrastructure Upgrade Project, OnshorePipelines, an LNG Plant and an Oil Export Terminal. Concrete structures will be utilised for the PA-BPlatform and Lunskoye Platform as also shown below.

Figure 5-1: The Sakhalin II Phase 2 development. Concrete structures for the PA-B and Lunskoye platforms


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Construction work is currently ongoing at the construction site in Vostochny Port about 4 hours carride from Vladivostok. The construction site has been developed and construction of the two concretestructures is well underway and the project is on schedule. The picture in Figure 5-2 below shows thesite prior to site development (April 2003).

Figure 5-2: Concrete structures construction site in Vostochny Port prior to start site development

The pictures in Figure 5-3 below shows the status in April 2004 (one year after start site development)

and September 2004. As can be seen the lower parts of the two structures are completed andpreparations are ongoing to start construction of the concrete towers (shafts).

Status April 2004: Dock completed and construction of both structures started.

Construction Site Location


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Status Sept. 2004: Lower parts of both structures completed. Preparations for construction ongoing

Figure 5-3: Sakhalin II construction work

The below Figure 5-4 identifies the locations where the main portions of the work are performed. Ascan be seen the majority of the work is performed locally and in Russia. However, design, intialprocurement and work preparation is performed outside Russia (Norway and Finland).

Figure5-4: Main work locations

Offshore Field

LUN-A and PA-B

Tow

18 days

CGBS WorkSite

Vostochny Port

CGBS / Mechanical Outfitting Detail engineering

Work preparation/Procurement

Mechanical Prefabrication

Mechanical Prefabrication


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Major achievements on the project so far includes:

One lost time injury approaching 4.000.000 hours since lost time injury

The project is on schedule More than 90% local labour at site More than 95% (weight) of material purchased in RussiaTechnology transfer program is working well and the need for expatriates is gradually reduced

5.2 The Adriatic LNG Terminal Project

ExxonMobil, Qatar Petroleum and Edison Gas is currently developing a facility to receive, store andregasify LNG in the Adriatic Sea about 15 km off the coast of Italy. The gas will be piped to shoreand sold in the Italian market. The LNG is shipped from Qatar through the Sues channel as shown onthe below figure.

Figure 5-5: LNG shipping rout and terminal location

The facility will be developed based on a bottom fixed concrete structure with internal LNG storagetanks and the regasification plant on the top deck, see Figure 5-6 below. Aker Kvaerner is executingthe project under a FEED & EPCI contract.

LNG

Terminal

Location

LNG Plant

LNGTransport

Rout


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Figure 5-6: The Adriatic LNG Terminal general configuration

The FEED has been completed and detail design has commenced. The concrete structure as well asinstallation of the LNG storage tanks and the regasification plant will take place in Algeciras, Spain.Deepening of the dock is completed and establishment of the facilities to support the constructionwork is due to start.

Figure 5-7: The Algeciras construction site modification work ongoing

Work is performed at several locations as shown on the map in Figure 5-8 below.

Footprint Concrete Structure


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Figure 5-8: Main work locations

Following the work in at the construction site the completed structure will be towed to the installationlocation in the Adriatic Sea where it will be installed and made ready for receiving LNG. The LNGcarriers will be moored directly to the side of the structure. The mooring and berthing facilities are

displayed in Figure 5-6 above.

Client HQ

Project ManagementTopside Engineering

LNG Tanks Engineering

Project Management

Technical Co-ordination

GBS Engineering

GBS Construction and

Terminal Assembly

Pipeline Engineering

Topside modules - Europe,

Korea etc.Tank fabrication - Europe or

Japan

Installation, Commissioning

and Start-up


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6 Novel Concepts

Over the last couple of years we have seen a growing interest for offshore concrete structures. This hasmainly been driven by the oil companies desire to start developing oil and gas fields in more harshenvironmental regions such as outside the Sakhalin island (Russia), in the Barents (Russia) and atGrand Banks outside Newfoundland (Canada). Further it has also been driven by the need to developmore LNG export and import terminals, especially we see a number of terminals being considered forthe North American market. In addition there is a growing desire to increase the local content forthese types of developments. Concrete structures are well suited to meet all of these three demands.

Aker Kvaerner and our partners are constantly monitoring the market to be in front with regards todeveloping the concepts and technology required to respond to the market demand. Three areas areconsidered to be of special interest:

Further developments outside the Sakhalin island, where we may see a need for more that 15 newplatforms over the next 10-15 years. If so it should be possible to develop a long-term sustainablebusiness in the region.

On Grand Banks outside Newfoundland it is very likely that we will see another large oil fieldbeing developed on the basis of using a large ice berg resistant concrete structure.

LNG terminals may represent the most promising market for this type of structures over the next5-10 year period. It is likely that we will see a number of these terminals being developed forNorth America, Mediterranean region and also for far east countries such as Japan and Korea. Justnow we are working on a number of possible prospects where the two most advanced are:

The Port Pelican terminal (ChevronTexaco) in the Gulf of Mexico, and

The Baja California terminal in Mexico.Both these terminals are currently in the front end engineering design phase.

In addition to the near term project opportunities briefly outlined above we are constantly striving todevelop new and novel solution targeted for the somewhat more distant future. Some examples aregiven below.

6.1 Floating LNG Terminals

As the gas fields to be developed is situated at deeper and deeper waters the industry is looking for

solutions to liquify the gas (make LNG) at the field rather than build long and expensive pipelines toshore. Together with some key technology partners we are trying to develop solutions for this. Themain challenges are:

LNG transfer in the open sea sufficient regularity Sloshing of LNG inside the tanks for partly filled tanks Vessel motions and in impact on large rotating equipment on topsides Significant topside facilities large weights


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An example of such a concept is shown in Figure 6-1 below:

Figure 6-1: Floating LNG export terminal with concrete hull

6.2 Floating airport or navy base

A few years ago we worked with US Navy to develop a concept for a relocatable US Navy base. Theconcept is based on four large self-propelled semisubmersible hulls that could be connect together tocreate a complete navy base. The idea is that is will be much more economic and faster way tomobilise large military forces. This would also reduce the need for regional presence. The concept isshown in the below Figure 6-2. The same basic principle may be applied for regular airports.

Figure 6-2: Mobile Offshore Navy Base.


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6.3 MPU Heavy Lifter

As mentioned previously there are requirements to remove offshore steel platforms (OSPAR

Convention). For the North Sea alone this represents a market value of some 10 billion $. This marketpotential initiated the development of a robust, inexpensive Heavy Lifter that utilises the simpleprinciple of Archimedes in order to lift straight up.

One solution is a concrete U-shaped semi-submersible Heavy Lifter, designed based on the principlesSimple, Safe, Robust and Cost-effective. Dr.techn.Olav Olsen developed the design; the ownershipis by MPU Enterprise (ref. 21, 22 and 23.)

Figure 6-3. The MPU Heavy Lifter for removing and installing offshore structures.

Figure 6-4. The MPU Heavy Lifter in the tank-test.


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6.4 MPU Semo

The experience from the hydrodynamic tests of the Heavy Lifter in fig. 6-4 gave strengthened

confidence in the MPU SEMO, shown in fig. 6-5. The SEMO is a floating mono-hull structure, builtof concrete. The incentive of the design was the complexity of the moored ship solution, currently apopular option for offshore field development. Such ships are swivelling around a turret that is mooredto the seabed. The design philosophy of the SEMO is that it may be round, and not ship-shaped, as it isnot going anywhere. It may also be considered an enlarged turret, without a complicating bodyattached to it.

Local content will be more important in the future, and the MPU-SEMO creates interestingopportunities with regard to local fabrication and assembly. For many countries it is important to buildnew industry and to further develop the economy. The fabrication of an MPU-SEMO can givesignificant amount of work locally, which could give such a concept a political advantage compared tosolutions built in SE Asia or Europe.

Figure 6-5. The MPU SEMO.


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6.5 Other novel concepts

Figure 6-6. Fixed /floating LNG storage

Figure 6-7. GBS Nnwah-Bilah LNG Project

Figure 6-8.

A proposal for an offshore terminal, containing

storage for oil, condensate and gas.


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Figure 6-9. Submerged Floating Tunnel

A solution for future infrastructure,by the

Norwegian Submerged Floating Tunnel Company(ref.25)

Figure 6-10. LNG Terminals

Figure 6-11. Suction anchors

Built for the Snorre TLP.


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Figure 6-12.Urban development on floating

concrete structures, developed by Marfloat

Figure 6-13. Urban development. High rise on

prefabricated stranded cellar/parking structure.

Yogamid developed by Finn Sandml


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7 Project Execution in Mexico

ChevronTexaco is currently developing a solution for an offshore LNG receiving terminal to beinstalled inside the Coronado island outside Rosarito in Mexico, Figure 9-1 below shows the overallconfiguration of the proposed terminal. Aker Kvaerner and our partners are supportingChevronTexaco in this effort and one of our tasks is to assist in assessing project execution in Mexico.

Figure 7-1: The proposed Baja California terminal configuration

7.1 Engineering and Design

7.1.1 Conceptual design

The success of conceptual design requires an overall understanding of the key elements that isgoverning for the offshore concrete structure. Most of these elements are described in this paper, such

as functional requirements and close relation with the construction methods.

In this respect conceptual design of offshore concrete structures is similar to any other conceptualdesign of structures. The main difference is the required understanding of the water, both as an actingload and as a medium for floating.

7.1.2 Detail design

A typical construction project may require somewhere in the range of 1000 drawings, for the concretestructure only. These drawings do not only describe the concrete geometry and reinforcement, but all


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details of connections, anchors, pipes, openings etc. Such projects are multidisciplinary, and it is aconsiderable task to organize and manage them.

For the structural design specialized tools have been developed for analyses and design. These toolsare generally commercially available.

7.1.3 Rules and Regulations proposed for concrete projects in Mexico

There are several recognized international rules and regulations pertaining to the design and executionof offshore concrete structures. The overall common requirement is that the structure shall bedesigned, executed, transported and installed in such a way that:

The reliability level of the installed platform meets the intended reliability level.

All functional and structural requirements are met.

The present draft ISO/CD 19903 Petroleum and natural gas industries - Offshore structures - Fixedconcrete structures,lists all those areas of design that are particular to offshore concrete structures,and acknowledges that design may be performed according to national standards provided it is

supplemented with additional rules for all those areas not properly covered by the national standard. Itthen in a note states that the Norwegian Standard NS 3473, ref. 24, is recognized to meet all thoserequirements relevant for the design of offshore concrete structures. This Norwegian Standard isavailable in the English language.

To the best of our knowledge, there should not be any special difficulties with respect to rules andregulations for a concrete offshore structure constructed in Mexico. National regulations and standardsapplicable in the place of use of the structure can be different from those given in internationalstandards, such as the ISO suit of standards. In such cases it must be ensured that the requirements ofsafety and durability are met. This applies to all phases of planning, design, execution, transportation,installation and possible removal.

In Norway the following main class notation is used for ship-shaped floating structures complyingwith Det Norske Veritas (DNV) class requirements and the Norwegian rules and regulations as issuedby the Norwegian Petroleum Directorate:

+1A1 Oil Production and Storage Vessel (N)

The analyses and design will then follow the extended calculation procedures for hull structures -additional class notation CSA-2.

The wave loading experienced by a permanently moored barge is quite different compared to that of asailing merchant ship (may in general vary some 25 to 35% above that of the response calculated fromDNV rules for merchant ships). This calls for a separate hydrodynamic analysis, independently of thefact that concrete barges are unusual and cannot easily be fitted into existing standard steel

categories.

The analyses and design of a concrete hull will therefore follow the traditional approach for offshoreconcrete floaters, as demonstrated and approved in detail engineering of the ARCO barge, HeidrunTLP, Troll Oil Semi and the Nkossa barge:

Hydrostatic analyses (drafts, internal/external water levels, still water moments andshear forces, afloat stability - intact and damage)

Hydrodynamic analyses (global responses and hydrodynamic wave pressures)


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Mooring analyses and design

Structural response analyses (finite element analyses)

Structural design verification (code checking according to Norwegian standardNS3473 or DNV concrete design rules harmonized with the rules in NS3473)

We are not aware of any offshore concrete structure that has been built to other national standards thanthe Norwegian NS 3473, without the need of extensive supplements. Areas normally not adequatelycovered for offshore structures are such as; fatigue, tightness, design provisions for shell typemembers, design for durability and cracking etc. Elf Congos Nkossa barge has been designed toBureau Veritas shipbuilding specifications, using the NS3473 for the concrete design verification,where it is registered as a certified hull. The Canadian Hibernia platform and the Australian WestTuna, Bream B and Wandoo platforms have all been designed according to NS 3473.

The detail design of the Sakhalin GBS is carried out in accordance with the approach set out by DetNorske Veritas Rules for Classification of Fixed Offshore Installations (DNV Rules). The referencestandard for concrete design is British Standard BS8110, but according to what stated above, specificinterpretations and additions have been necessary in order to make this standard applicable foroffshore structures. For this purpose the Norwegian standard NS 3473 is used as supplement. BS8110is most likely used because the initial conceptual phases were performed by a UK consultancy.

The reference standard to be used should be agreed at an early stage in the project, as the choice ofstandard might strongly influence the platform geometry and dimensions, while standards not intendedfor offshore use might be unnecessarily conservative on certain aspects relevant to offshore conditions.

The reference standard shall give the design parameters required for the type of concrete, e.g. normalweight or lightweight concrete, and strength class used. For high strength concretes and lightweightconcrete, the effect of reduced ductility shall be considered. This in particular applies to thestress/strain diagram in compression, and the design parameter used for the tensile strength incalculation of bond strength, and transverse shear resistance.

7.2 Fabrication Site

A general concrete substructure construction facility is described previously. It should have reasonablesupporting infrastructure facilities and labour resources, to support the scale of construction required.Detailed evaluation of specific sites would allow cost and land availability issues to be determined.


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Figure 7-2. A typical construction dock for a GBS facility.

By way of example, the green-field construction facility prepared for the Malampaya concrete gravitysubstructure is illustrated in Figure 7-2. The construction facility was located in a remote part of SubicBay in the Philippines.

7.3 Options for fabrication

A medium size concrete platform built in concrete will require a fabrication dock with an area of some140 x 140 m, and a water depth of 10-12 m. The concrete hull can be completed in the dock if asufficient water depth is available, or it can be completed in a floating condition at a place outside thedry dock, with sufficient water depth.The cost of a fabrication dock will depend on the soil, size, depth etc. In Australia and in thePhilippines a gravel dock was prepared for the Wandoo and the Malampaya platforms at the cost ofUS$ 8 mill.


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There is also a possibility that the concrete can be built on flat land and skidded into the sea. Thelower hull may be built to a level when it can float, and the remaining of the hull will be built whilefloating.

7.4 Construction and Methods

The small concrete platform is ideal for slip forming of the vertical walls. Slip forming is a veryeffective fabrication method for high concrete structures. Even the lower hull with an assumed heightof some 10-12 m will most likely be slip formed. There are advanced slip-forming systems allowingfor changes in slip forming geometry over the height of the structure. I.e. the upper hull can be builtwith a sloping outer and/or inner wall. In Figure 7-3 below one slip forming system is presented.

The mechanical outfitting will be placed during construction, in sequence with other constructionactivities. All steel will be attached to the concrete at preinstalled embedment plates. Such plates areplaced during the slip forming, and after slip forming steel pipes etc will be welded onto embeddedsteel plates. Penetrations through walls or slabs will also be placed during cast/slip forming.

Figure 7-3. Interform slipforming system

7.5 Material Qualities

Any concrete platform and all appurtenances, piping and fitting shall be fabricated from materialssuitable for the service and life of the facilities. The concrete contractor shall develop a concrete mixthat satisfies strength, durability requirements in their operating environment and constructionmethodology. Steel elements shall be selected to resist the factors of design stresses, fatigue, corrosionand brittle fracture. Additional factors shall be considered with respect to hot and cold working andweld ability including resistance to lamellar tearing.


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The concrete mix designs shall be in accordance with the concrete specification given by the civilcontractor. As a minimum, the concrete specification shall cover the following items: mechanical andchemical strength, mix design, batching, workability, durability, testing, reinforcement corrosion,warm weather concreting and quality control.

For small concrete platforms there are 2 types of concrete, which can be used; normal density (ND)and light weight aggregate (LWA) concrete. The LWA-concrete has about 20% less density than theND-concrete, which is an advantage for floaters. However, small concrete platforms are partly weightstable platforms and needs weight in the base. Hence it may be beneficial to use ND-concrete in thelower part and LWA in the upper part. The advantage must be compared to the possible disadvantageof dealing with 2 different materials at the site and in engineering. In a feasibility study the design ofthe concrete hull is recommended to be based on Normal Density (ND) concrete grade C60 andlightweight aggregate concrete (LWA) concrete grade LC55, both as defined by NS3473. Theseconcrete qualities are well proven in many countries, and are regarded as fairly straightforward toproduce. For ordinary and pre-stressed reinforcement the grades KT500TE (NS 3570) and St1570/1770 (EURONORM) are recommended.

7.5.1 Concrete

Concrete grade : ND C60 LC55Structural material strength fcn(MPa) : 36.4 33.6Design compressive strength, ULS fcd(MPa) : 29.1 26.9Nominal structural tensile strength ftn(MPa) : 2.375 2.25Design tensile strength, ULS ftd(MPa) : 1.90 1.8

Modulus of ElasticityPlain concrete, ULS Ecn(MPa) : 29 399Plain concrete, SLS Eck (MPa) : 30 534 23 226Plain concrete, FLS (0.8Eck) Eck (MPa) : 24 427 18 581Static, short-term loads (incl. reinf.) Ec(MPa) : 35 000 25 000Dynamic Ecdyn(MPa) : 40 000

Poisson's Ratio, Coefficient of Thermal ExpansionPoisson's ratio = 0.20

The coefficient of thermal expansion T = 1010-6oC-1

Density ND C60 LWA C60

Plain concrete: 24.0 kN/m3(2.45 t/m

3) 19.3 kN/m

3(1.95 t/m

3)

Reinf. concrete:150kg/m3

300kg/m3

500kg/m3

25.0 kN/m3(2.55 t/m3)

26.0 kN/m3

(2.65 t/m3

)27.5 kN/m3(2.80 t/m3)

20.4 kN/m3(2.08 t/m3)

21.1kN/m3

(2.15 t/m3

)22.6kN/m3(2.30 t/m3)

7.5.2 Ordinary reinforcement

Design Strength, Grade K500TEYield stress :fy= 500 MPaDesign strength, ULS :fs= 435 MPa

The yield strain is y= 2.5 x 10-3


46/50


45

Nominal diameters are 8, 10, 12, 16, 20, 25 and 32 mm.Modulus of Elasticity

Es= 2.0105Mpa

7.5.3 Prestressed reinforcementGeneralThe pre-stressing steel to be used is Grade 270 according to ASTM A 416-85.Strand PropertiesThe properties of pre-stressing strands are tabulated below.

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