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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, Kåre O.Hæreid, Kolbjørn Høyland and Tor Ole Olsen, of Dr.techn.Olav Olsen a.s Norway

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



Marine Concrete Structures ............................................................................................ 4

1.1 1.2 1.3 1.4 1.5

General .................................................................................................................................... 4 Floating Concrete Sea-structures............................................................................................. 4 Performance in the marine environment ................................................................................. 6 Considerations for new field development.............................................................................. 8 Design aspects ......................................................................................................................... 8 The structures ........................................................................................................................ 13 Research and development .................................................................................................... 16 Decommissioning of offshore concrete platforms................................................................. 17


The Norwegian Experience and Know-how ................................................................. 13

3.1 3.2 3.3

4 5 6

Project Execution ­ Typical ........................................................................................... 18 Ongoing Projects............................................................................................................. 26

5.1 5.2 6.1 6.2 6.3 6.4 6.5

The Sakhalin II Project.......................................................................................................... 26 The Adriatic LNG Terminal Project...................................................................................... 29 Floating LNG Terminals ....................................................................................................... 32 Floating airport or navy base................................................................................................. 33 MPU Heavy Lifter................................................................................................................. 34 MPU Semo ............................................................................................................................ 35 Other novel concepts ............................................................................................................. 36 Engineering and Design ........................................................................................................ 39 Conceptual design ......................................................................................................... 39 Detail design.................................................................................................................. 39 Rules and Regulations proposed for concrete projects in Mexico ................................ 40

Novel Concepts ................................................................................................................ 32


Project Execution in Mexico .......................................................................................... 39


7.1.1 7.1.2 7.1.3

7.2 7.3 7.4 7.5

Fabrication Site...................................................................................................................... 41 Options for fabrication .......................................................................................................... 42 Construction and Methods..................................................................................................... 43 Material Qualities .................................................................................................................. 43 Concrete......................................................................................................................... 44 Ordinary reinforcement ................................................................................................. 44 Prestressed reinforcement.............................................................................................. 45

7.5.1 7.5.2 7.5.3


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

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


Steel/Concrete Connection Methods ..................................................................................... 46 Riser support.................................................................................................................. 46 Mooring brackets / Towing brackets / Fairleads ........................................................... 46 Embedment plates ......................................................................................................... 47

7.7.1 7.7.2 7.7.3

References ............................................................................................................................... 48

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


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 are bridges, docks and lighthouses. Particularly in war times, when steel is scarce, concrete has also been used for barges and ships. The long history of marine concrete structures is interesting and represents valuable 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 processing facilities of hydrocarbon plants offshore. They are constructed over the past 30 years, and perform well 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 concrete structures. Examples are described in this paper, including the important elements of project execution, the experiences from design and construction, the durability of offshore concrete structures and the associated required maintenance, as well as the issues of removal and recycling of such structures. Ongoing projects (Sakhalin II in Russia and Adriatic LNG Terminal in the Adriatic Ocean) are briefly presented. 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 offshore concrete structures, or at least a place to start in the search for improved oil and gas field development.


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



Marine Concrete Structures


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

Figure 1-1. Floating caissons

The picture shows that having access to water and an innovative concrete construction industry may be 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 a requirement for the construction. Many of the offshore structures described in this paper are from Norway which has deep fjords, protected from the ocean. Although many of the examples in this paper describe complex offshore concrete structures, it is important to recognize the value of simplicity. Ingenuity and standard means of construction will bring the best results.


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 built due to the steel shortage - including the 130 m long U.S.S. Selma. At that time reinforced concrete had already 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 these ships were large sea-going vessels and 80 were sea-going barges of large size. The cargo capacities ranged from 3.200 to 140.250 tons. Ref. 1, by Morgan, gives a good description of the early development of the concrete hull.


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

A number of notable pontoon bridges have been built of concrete. Ref. 2 gives an overview of the long traditions within this area. The first floating concrete bridge was built across Lake Washington in 1940. In the late 1950's, 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 as caissons in the 1960's. Concrete lighthouses are installed in the Irish Sea, in Eastern Canada and in the Gulf of Bothnia. Many pontoons, barges and other crafts have been successfully built in the former USSR, Australia, New Zealand and the UK. Over the years, starting back in mid 1920's, some 70 temporary floating immersed concrete tunnels have 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 1970's concrete gained recognition as a well-suited material for construction of offshore platforms for the exploration of oil in the North Sea. Permanently floating offshore vessels related to the petroleum industry are now installed in the Java Sea, in the North Sea and outside the coast of Congo in West Africa. From 1950 to 1982 it was registered that approximately 1.130 concrete hulls had been built. Most of them are small with overall length less than 50 m. Among the bigger ones, two groups of sizes are dominant, - approximately 250 hulls with length ranging from 58 to 67 m, and 40 hulls with a length of 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 dimensions 140.5 x 41.5 x 17.2 m (length x beam x depth). Fully loaded, the vessel displaces 66.000 tons. The ARCO 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 was built in Japan in 1982. Heidrun TLP (ref. 4) Conoco's Heidrun platform is the world's first TLP with a concrete hull and the largest permanently floating concrete structure ever with a concrete volume of 67.000 m3. The topside related weight is 89.000 tons (net 65.000 t topside) and the displacement 285.000 tons. The platform was installed on location in the North Sea in 1995, at a water depth of 345 m. Troll Oil Semi (ref. 5) Norsk Hydro's Troll Oil FPS platform is the world's first concrete catenary anchored floater. The Troll Oil semi submersible hull has a concrete volume of 46.000 m3 and supports a topside weight of 32.500 tons. The displacement is 190.000 tons. The platform was installed on location in the North Sea in 1995, at a water depth of 335 m. Nkossa barge (ref. 6 & 7) Elf Congo's Nkossa barge is the world's largest pre-stressed concrete barge. The floating production vessel of which the dimensions are 220 x 46 x 16 m was built in Marseille, France, and towed 4500 nautical miles to the west coast of Congo in West Africa where it was permanently anchored in 170 m


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

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


Performance in the marine environment

Considering the wide use of concrete for marine applications there is surprisingly little documentation to be found on in-service performance. The apparent cause for this is that provided satisfactory design and execution, concrete is an optimal material for harbour, coastal and offshore construction as it combines durability, strength and economy. This fact is supported by studies of floating concrete docks back in the 1970's, showing dramatic savings, requiring less than 10% the maintenance of similar all-steel docks, ref. 1. Other structures also utilize the water-tightness properties of concrete; storage tanks, nuclear containment structures and submarine tunnels. Sare and Yee, ref. 8, report negligible repair and maintenance costs for the 19 pre-stressed concrete barges 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 be about 1/3 compared to steel barges. The fabrication cost of Yee's barges showed a saving of 16 percent compared to that of steel. In the period 1974 to 1975, the total downtime per floating barge per year for maintenance work was six days 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 towed voyage, was designed for 4.2m wave height. It is worth noting that the vessel in fact endured a storm in the Bay of Biscay during which time the conditions were undoubtedly more severe than those contemplated 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 this unexpectedly 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 from vibration than steel ships - to the crew's satisfaction. The 1970's and 1980's 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 world's stormiest oceans, the North Sea. It is remarkable how well these structures have performed in the hostile marine environment, successfully withstanding the extreme loads from waves approaching 30 meters in height as well as the dynamic cyclic forces. Experience has shown that the offshore concrete structures currently in use are virtually maintenance-free. It is generally recognized that the first concrete platforms in the North Sea were over-inspected and that the need for extensive instrumentation of platforms of common types should be reconsidered. A comprehensive list of references to information pertaining to the performance of North Sea concrete structures is presented in ref. 9 and 19. No significant sign of material deterioration, corrosion of reinforcement or other material-related deficiencies have been observed. Falling objects or ramming ships mainly cause observed damages. Platforms designed for 20 years operation have now passed the end of their prescribed design life. Inspections and investigations confirm that their lifetime in general can be extended. Various codes give well-established rules for assessing fire resistance. Two hydrocarbon fires inside North Sea concrete platform shafts in the late seventies are reported. The consequence was a surface


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

scaling about 10-20 mm deep over a height of 5-10 m. This marginal impact is attributed to the large heat capacity and low thermal conductivity of concrete. No repair was found necessary - clearly demonstrating the 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, both ashore and afloat, of fire causing no more than local to no damage to concrete structures. As an example constituting the most impressive testimonial that could possible be called for, Derrington (ref. 11) reports how two concrete barges survived the Bikini Atoll nuclear bomb tests in good shape when their cargo of fuel 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 1000tonne German concrete barge hit a mine, which exploded under the stern - the vessel was able to reach shore by being repaired while afloat with underwater concreting. Lusteveco, operator of Yee's concrete barges, was quite pleased with their performance and say "one of the most endearing aspects of prestressed concrete hulls is their ease of repair". The barges serviced the Vietnam War area for a period of nine years and a number of barges were rocketed or damaged by plastic bombs. The damage was usually 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 March 1973 one of Lusteveco's 2000 dwt dry cargo barges, L-1960, hit a mine at the starboard side transiting the Mekong River fully loaded with rice intended for Phnom Penh. After some temporary repairs, the barge was towed safely to the Philippines for permanent repairs. The cost of this 10 days repair job was US$ 4381. Pre-stressed concrete was chosen as the hull material for the ARCO barge because of its seaworthiness, competitive cost, fire resistance, durability and speed of construction. After almost twenty years of continuous service, various tests were carried out for the concrete barge. Due to its excellent condition, ARCO has given its barge an "indefinite" lifespan - a solid proof of the excellent performance 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 piers and quay structures) - suggesting that the marine environment is demanding and imposes special requirements on materials and workmanship. It is in this context important to distinguish between the onshore and offshore concrete industry. The problems experienced in coastal areas for the onshore concrete 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 contaminated aggregates, improper concrete mix proportions). In Norway, the experiences with coastal bridges have shown that the principal causes of failure are the same as reported above. In general, however, marine structures built by the onshore concrete industry have suffered very limited from degradation - refer for example list of surveys presented in FIP's state of 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 on chloride 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)


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


Ekofisk Tank (17/22)

Calculations based on the chloride profiles showed that the investigated offshore platforms were in excellent condition and that there would be no risk for corrosion within their expected lifetime. Two of them 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 to reinforcement, good workmanship & construction techniques and thorough quality assurance.


Considerations for new field development

The general conclusion drawn from service performance of the offshore concrete structures is that they have proved excellent behaviour and require significantly lower expenditure for inspection, maintenance and repair than steel structures. Experience has shown that offshore concrete structures currently in use are virtually maintenance-free. Over the years wide experience has been gained in the field of post-tensioned concrete offshore structures. The successful construction of the Heidrun TLP, the Troll Oil Semi and the Nkossa barge has opened for interesting potentials to the offshore industry as to the suitability and economics of concrete floating structures. Recent studies also conclude that floating concrete structures are well suited for floating LNG plants. The increasing importance of local content in the development projects is favouring concrete as the building material in countries with limited number of offshore steel yards. Concrete structures can be built in greenfield areas with very little infrastructure. The majority of the workforce does not need special education and can be recruited locally. Hence, choosing concrete may significantly increase the local content of a project.


Design aspects

Design Life and Reuse: The offshore concrete structures installed to date have been designed for 2570 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 years to 50 years or 70 years. One reason is the fact that reinforced and pre-stressed concrete is not sensitive to fatigue. With the extensive design life possible for a concrete platform there is obviously a very good possibility for reuse. The investigations carried out on durability and conditions of existing concrete platforms clearly prove a great potential for reuse. Stiffness: Concrete structures generally have large stiffness. The result is less flexibility and less deformation applied onto outfitting steel etc. Robustness: Under maximum credible accidents, such as major leakage, collision or fire, a properly designed and constructed pre-stressed concrete vessel has better inherent safety than a comparable steel vessel. This is one of the conclusions from a technical feasibility and safety study of a 297 m (974 ft) long storage/processing vessel carrying LPG in free-standing tanks performed by Gerwick et. al., ref. 15. Here a pre-stressed concrete vessel was designed and compared with an existing steel vessel designed according to the ABS requirements. It was also found that the concrete hull, being stiffer, developed


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

significantly lower dynamic amplification and had a lower risk of failure than the steel vessel. The concrete hull was found to have adequate safety to justify its use for vessels in hazardous cargo service without 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. The concrete hull of a concrete floater will typically be designed for impact loads from any possible dropped object. Still there will be a design requirement to design for accidental filling of any compartment 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 proofing materials available. Two fires inside shafts of North Sea concrete structures have been reported, and damages have been too small to decide any repair work to be done. The combination of excellent fire- and impact-resistance is of course very important for units producing hydrocarbons. Maintenance Free: Appropriately designed and constructed concrete hulls in the marine environment are 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 been designed for an operational life of 50-70 years. The maintenance/OPEX aspect was decisive in Elf Congo's decision to chose a concrete barge on the Nkossa project, ref. 6 and 7, as the concrete hull offered significant savings in expected maintenance cost. The barge will fulfil its functions on site without 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 for marine construction is very favourable. There is little doubt that a well designed, well built, concrete structure 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 a steel floater designed for the same purpose. This conclusion can be drawn based on reports from ship captains (World War II ships and Yee's barges), several studies and recently confirmed by both analyses and model testing for very large FPSO's (BP Atlantic Frontier Stage 2 / Schiehallion, hull length 280 m). The generally somewhat larger mass and draught, result in improved motion characteristics. For the very harsh environmental conditions West of Shetland (the Schiehallion FPSO), the mooring size and cost was reported nearly identical for steel and concrete hulls (ballasted condition governing). The general picture, however, is that the mooring costs for concrete vessels/semi's are approximately 10 per cent more expensive than for a steel hull - illustrating that the mooring 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 of importance. A vessel must carry its own weight plus a payload. For the concrete platform the payload is the topside and equipment, as well as any ballast required for hydrostatic and/or geotechnical stability. In ref. 17 Jan Moksnes presents some of the results of Norwegian research on concrete over the past 20 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 o High stiffness, providing a stable foundation for tanks and other attachements Good resistance to environmental loading


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

o o o o o o o o o o o o o

Excellent behavior at low temperatures Favorable in ice-infested waters Robust with respect to accidental loading such as ship impact, dropped objects or terrorist attacks Good resistance to oil and gas process hazards Functional and safety features common to a land based plant. Good resistance to cold spot incidents Enhanced material properties with decreasing temperature Excellent fatigue resistance Good durability, and basically maintenance-free Standard offshore concrete quality applied No need for skilled labor for the bulk of the construction work, enabling local execution Good resistance to seismic loading May be decommissioned and removed, possibly reused

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

o o o o o o o Low complexity structures are both faster and more cost effective to construct. Close integration of engineering and construction as well as main operations Local availability of labour and materials Design basis requirements (waves, soils, functional requirements) Generally cost effective to complete as much as possible in the graving dock. Unnecessary stops in slip forming should be avoided Preparation of the graving dock may add cost to the project. However, it may prove economically sensible to extend and/or deepen the dock to increase the completion grade. Construction schedule. Construction time is related to simplicity and concrete volume. For LNG terminals, assembling the tanks from pre-cast elements may decrease the length of the schedule.

For small structures the initial construction may be performed on barge(s). After float off from the barge the remaining construction is performed while afloat. However, in most cases a dry-dock has been 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.


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


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, on its 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 time indicate 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 ocean where it is not straightforward to build. The platforms may then be prefabricated elsewhere, installed and possibly completed with respect to the foundation (piling, ballasting, grouting) and topside installation. 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 foundation structure 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 large water depth is complicated and expensive for steel structures as they are exposed to significant pressure loading.

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


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

Figure 2-2. The environment of the sea.

The offshore concrete structures for the oil and gas industry are located at various and very different parts of the world. There are structures in ice-infested waters, in seismic zones and in very harsh marine environments, but also in relatively calm areas. Some are located at large water depth, others in shallow areas. The foundation conditions vary from very stiff sand to very soft clays, and some of the structures float permanently. Some of the structures have storage facilities, and all have a hydrocarbon processing plant facility of some 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 extension of operational life, and modifications to enable further facilities as the offshore field may contain additional hydrocarbons.


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



The Norwegian Experience and Know-how

The structures

The inshore construction of concrete offshore structures provides good conditions for quality construction. The construction site of Aker Kværner at Hinna, near Stavanger in Norway, was likely the best and most professional and effective construction site in the world. The construction of 17 large offshore concrete structures, created a large amount of expertise, see Fig. 3-1. Multidisciplinary groups of specialist companies participated in design of these platforms and their various units and outfitting. Some of the companies contributing considerably were Aker Kvaerner with their designs of topside, mechanical outfitting and marine operations, Multiconsult, Aas-Jakobsen, SWECO Grøner and 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 of the total volume of the offshore concrete structures of the world.


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

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 1998 FIP's prize for outstanding structures. In 2000 the readers of Teknisk Ukeblad (a Norwegian engineering 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 Magnel Golden 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, a gas wellhead and processing platform, was complete with skirt piles and topside when towed out and installed 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) during construction, and carried the topside weight of 22 000 t 150 m (490 ft) above the sea level during tow from construction site to the offshore installation site.


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

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 this reason the base of the platform (more than 16000 square meters in area) is equipped with 36 m long skirts to enable the safe foundation of the structure. The 100-year design mud line moment is 100 000 MNm. Most of the existing CGS 's are resting on dense sand with short steel skirts penetrating the sand for protection against scour. For soft soils, deeper skirts are required. This skirt pile principle was developed for Gullfaks C in the mid 1980s. The soil conditions with very soft clay, required skirts penetrating 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.


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

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 and technically compared to other solutions: o Lower material and fabrication cost o Reduction of foundation area o No piling or need for heavy subsea hammers o High position accuracy o Short installation time o Reduced need for solid ballast


Research and development

Many of the offshore concrete structures of the world are located in moderate waterdepths. As is seen in fig.3-1 most of the Norwegian built platforms are located in medium to large waterdepths, with the foundation 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 a considerable amount of research has been performed in Norway, as described by Moksnes in ref. 17.


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

This research has been very successful, and the results benefit not only the oil and gas industry, but the entire 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 a vast amount of development in many areas, such as construction techniques (slipforming etc.), marine operations, analyses for environmental loads and structural response, soil testing and instrumentation, etc.


Decommissioning of offshore concrete platforms

Even though the structures may be fit for many years, international regulations will put constraints on the use of the oceans. Particularly important here is the OSPAR (OSlo PARis) Convention. In July 1998 it was decided that all platforms in the North Sea should be removed after completing their duties. An exemption was made for concrete platforms, because of the believed complexity of the operation. Extensive work has been performed on the subject, particularly JIP work that has proprietary rights. The international fédération internationale du béton, fib, initialised work on the subject, in their Task Group 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 to remove 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 the world will have different rules to relate to, but the essence is likely to be similar.


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


Project Execution ­ Typical

Typically the execution of offshore concrete structures consists of several main project phases. Some projects may constitute all the below phases whereas others may just include a few of these. After completion of front end engineering design and decision to develop the project and award of contract(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)


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

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. The geotechnical design of the dry dock should be given special attention, as this is the key to ensure a water tight and safe dry dock. The floor in the dry dock is normally located 10 to 15 meters below sea level and hence the dock is exposed to this differential pressure. This phase is normally on critical path of 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 the concrete structure. Further local design and detailing of rebar arrangements and the mechanical systems for the lower parts (the first casting sequences) has to be completed before start of any construction work. If the project includes development of a construction site this design work is normally conducted concurrent to the completion of the construction site and dependant on the duration of this work, this phase may be on critical path of the execution schedule. If the project does not 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 an overlap to the construction work. This part of detail design is not normally on the critical path of the execution schedule. Dry dock and construction site development During this phase the dock will be excavated or digged out. Further all the facilities needed to support the concrete structure construction work will be established. This phase will normally be on critical path of the execution. In some cases it is possible to use an existing dry dock and only minor adjustments and mobilisation activities will be required.


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

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 can commence. Normally as much as possible would be built in the dry dock as the access and logistics is better 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 a deeper dock. Normally the concrete construction work will be on the critical path for most of execution 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 with water 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 or in some cases directly to the installation location. To get the structure afloat internal water ballast will be removed and the lift off carefully monitored. To ensure a sufficient under base clearance and a controlled 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 the structure. For structures to be installed in deeper waters, say 50 metres or more, a wet construction site would be required. During this phase a floating rig will be established to support the construction work, this normally consists of a number of storage and other type barges. The construction crew will normally be shuttled from shore during this phase. A well organised and planned logistics is of utmost importance 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 wet site. If installed at the wet site this is normally performed as a float over (deck mating) operation. The deck will arrive to the wet site supported on one or two barges. To transfer the deck onto the concrete structure the structure is ballasted down with just a few meters remaining above sea level. Then the deck is floated over the structure, which will be gradually de-ballasted to transfer the weight of the deck from the barges onto the concrete structure. Alternatively the topsides facilities can be installed at the offshore site. This may either be performed as 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 the structure 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 tow speed is in most cases around 3 knots. At the offshore location the tugs will be reconfigured to some kind of a star formation to control the positioning of the structure. When in position the structure is ballasted down to the seabed by sluzing in of water. The in place foundation stability will either be ensured by sufficient weight and friction or by penetrating 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 and systematic methodology. Aker Kvaerner and the "Norwegian know-how cluster" have over the years developed a systematic and efficient way to execute this type of projects.


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

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 economical result. 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 for key performance areas such as Cost, Schedule, Quality and Safety. The organisation will be structured for project execution through effective control and co-ordination of the 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 the principle of "best man on the job". The integrated project management team should be responsible for and have the experience to manage the total EPCI process, see Figure 4-2 below.

Civil Construction Engineering & D i Project Control

Integrated Finance & Accounting Project Management Materials & Logistics


Facilities & Maintenance Mechanic l Installation

Figure 4-2: Integrated Project Management

An overall project risk assessment shall form the basis for prevention based programs to ensure that project milestones and key targets are met. Identification and management of the project schedule critical path will hold the key to a timely delivery of the project. Extensive work will therefore be performed to identify the critical path and review measures to reduce its length and consequential effects.


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

Improvement Leadership In general, an Improvement Leadership process should be established. This process should focus 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 result areas 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 be measured. The Improvement Leadership is a continuous process that will support prevention-based programs 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 ongoing Sakhalin II project under construction in Far East Russia. Team Building and Alignment to Project Goals Team building sessions shall be arranged to develop the project organisation into a single integrated team focused on the project processes and goals and to establish a common understanding and acceptance of cultural differences, organisational goals, responsibilities and good working relationships. Team strength means empowered team members promoting personal quality and having the confidence to openly discuss problems. Local Content The Construction of a offshore concrete structure will provide many jobs to the local society. In addition 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 will translate into new hiring and additional training of staff, enhancing new skill levels, capabilities and competitiveness 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 local content 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 material supply, 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.


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


Transport. Stores. Offices. Security.

Labour for such projects will be recruited from the local area and supplemented with handpicked skilled labour. An extensive training and induction program will be put in place to prepare the labour and 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 of specialist expatriate supervision. A number of specialist personnel with extensive construction experience will be included in the management and supervision of the project. An important role for the experts is the transfer of knowledge to the local supervising staff and to the work force. Training sessions and mock-ups of some parts of the structures will be used in this context. The learning curve is reasonably short if a proper information and training system is established. In general, therefore, it is not necessary to bring in especially skilled labourer for the construction work. It will, however be required to use a limited number of hands-on foremen or lead hands in the start 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 piping related 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 qualified in due time before start of construction. The qualification is a relatively comprehensive exercise that could 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 stores and associated services will be procured locally. Specialised materials and equipment will be tendered internationally if not available locally. Execution Methodology Aker Kvaerner has together with partners over several years developed a compressive project execution model that is focused on the quality of the end delivery to the clients. To achieve this attention 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 to control quality of information at all levels. Further stringent gates are introduced to check out that


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

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




Business Areas Project



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


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

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

Contract Award


Procurement Fabrication Construction

System Design




Internal knowledge & lessons learned

Client specs & rules

Client requirements: - operation - maintenance - HSE

Client experience

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 process for each discipline is developed. The end product is divided into a large number of quality objects on which the completion (or quality level) is constantly monitored by detailed checklists. Utilisation of this systematic and comprehensive model has proven to give predictable and reliable project execution.


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


Ongoing Projects

Offshore concrete structures has to be considered niche products that are especially well suited for some 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 last structure 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 be approved 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 Energy Investment 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 "Norwegian know-how cluster" with Aker Kvaerner being in a lead role.


The Sakhalin II Project

SEIC is currently developing phase II of the Sakhalin II block outside the Sakhalin island in far east Russia. The development comprises eight main items; The PA-B Platform, PA-A Platform, an Onshore Processing Facility, the Lunskoye Platform, an Infrastructure Upgrade Project, Onshore Pipelines, an LNG Plant and an Oil Export Terminal. Concrete structures will be utilised for the PA-B Platform 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


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

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

Construction Site Location

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 and preparations are ongoing to start construction of the concrete towers (shafts).

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


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

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. As can be seen the majority of the work is performed locally and in Russia. However, design, intial procurement and work preparation is performed outside Russia (Norway and Finland).

CGBS / Mechanical Outfitting Detail engineering Work preparation/Procurement

Offshore Field LUN-A and PA-B

Tow 18 days

Mechanical Prefabrication

Mechanical Prefabrication

CGBS WorkSite Vostochny Port

Figure5-4: Main work locations


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

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 Russia Technology transfer program is working well and the need for expatriates is gradually reduced


The Adriatic LNG Terminal Project

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

LNG Terminal Location

LNG Plant

LNG Transport Rout

Figure 5-5: LNG shipping rout and terminal location

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


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

Figure 5-6: The Adriatic LNG Terminal ­ general configuration

The FEED has been completed and detail design has commenced. The concrete structure as well as installation 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 construction work is due to start.

Footprint ­ Concrete Structure

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.


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

LNG Tanks Engineering Project Management Technical Co-ordination GBS Engineering

Installation, Commissioning and Start-up

GBS Construction and Terminal Assembly Client HQ Project Management Topside Engineering

Pipeline Engineering

Topside modules - Europe, Korea etc. Tank fabrication - Europe or Japan

Figure 5-8: Main work locations

Following the work in at the construction site the completed structure will be towed to the installation location in the Adriatic Sea where it will be installed and made ready for receiving LNG. The LNG carriers will be moored directly to the side of the structure. The mooring and berthing facilities are displayed in Figure 5-6 above.


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


Novel Concepts

Over the last couple of years we have seen a growing interest for offshore concrete structures. This has mainly been driven by the oil companies desire to start developing oil and gas fields in more harsh environmental regions such as outside the Sakhalin island (Russia), in the Barents (Russia) and at Grand Banks outside Newfoundland (Canada). Further it has also been driven by the need to develop more LNG export and import terminals, especially we see a number of terminals being considered for the North American market. In addition there is a growing desire to increase the local content for these 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 to developing the concepts and technology required to respond to the market demand. Three areas are considered to be of special interest: Further developments outside the Sakhalin island, where we may see a need for more that 15 new platforms over the next 10-15 years. If so it should be possible to develop a long-term sustainable business in the region. On Grand Banks outside Newfoundland it is very likely that we will see another large oil field being 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 next 5-10 year period. It is likely that we will see a number of these terminals being developed for North America, Mediterranean region and also for far east countries such as Japan and Korea. Just now 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 to develop new and novel solution targeted for the somewhat more distant future. Some examples are given below.


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 to shore. Together with some key technology partners we are trying to develop solutions for this. The main 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


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

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

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


Floating airport or navy base

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

Figure 6-2: Mobile Offshore Navy Base.


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


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 market potential initiated the development of a robust, inexpensive Heavy Lifter that utilises the simple principle of Archimedes in order to lift straight up. One solution is a concrete U-shaped semi-submersible Heavy Lifter, designed based on the principles "Simple, Safe, Robust and Cost-effective". Dr.techn.Olav Olsen developed the design; the ownership is 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.


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


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, built of concrete. The incentive of the design was the complexity of the moored ship solution, currently a popular option for offshore field development. Such ships are swivelling around a turret that is moored to the seabed. The design philosophy of the SEMO is that it may be round, and not ship-shaped, as it is not going anywhere. It may also be considered an enlarged turret, without a complicating body attached to it. Local content will be more important in the future, and the MPU-SEMO creates interesting opportunities with regard to local fabrication and assembly. For many countries it is important to build new industry and to further develop the economy. The fabrication of an MPU-SEMO can give significant amount of work locally, which could give such a concept a political advantage compared to solutions built in SE Asia or Europe.

Figure 6-5. The MPU SEMO.


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


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.


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

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.


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

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 Sandmæl


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


Project Execution in Mexico

ChevronTexaco is currently developing a solution for an offshore LNG receiving terminal to be installed inside the Coronado island outside Rosarito in Mexico, Figure 9-1 below shows the overall configuration of the proposed terminal. Aker Kvaerner and our partners are supporting ChevronTexaco 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



Engineering and Design

Conceptual design

The success of conceptual design requires an overall understanding of the key elements that is governing 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 conceptual design of structures. The main difference is the required understanding of the water, both as an acting load and as a medium for floating.


Detail design

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


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

details of connections, anchors, pipes, openings etc. Such projects are multidisciplinary, and it is a considerable task to organize and manage them. For the structural design specialized tools have been developed for analyses and design. These tools are generally commercially available.


Rules and Regulations proposed for concrete projects in Mexico

There are several recognized international rules and regulations pertaining to the design and execution of offshore concrete structures. The overall common requirement is that the structure shall be designed, 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 - Fixed concrete 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. It then in a note states that the Norwegian Standard NS 3473, ref. 24, is recognized to meet all those requirements relevant for the design of offshore concrete structures. This Norwegian Standard is available in the English language. To the best of our knowledge, there should not be any special difficulties with respect to rules and regulations for a concrete offshore structure constructed in Mexico. National regulations and standards applicable in the place of use of the structure can be different from those given in international standards, such as the ISO suit of standards. In such cases it must be ensured that the requirements of safety 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 complying with Det Norske Veritas (DNV) class requirements and the Norwegian rules and regulations as issued by 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 a sailing merchant ship (may in general vary some 25 to 35% above that of the response calculated from DNV rules for merchant ships). This calls for a separate hydrodynamic analysis, independently of the fact 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 offshore concrete floaters, as demonstrated and approved in detail engineering of the ARCO barge, Heidrun TLP, Troll Oil Semi and the Nkossa barge: · · Hydrostatic analyses (drafts, internal/external water levels, still water moments and shear forces, afloat stability - intact and damage) Hydrodynamic analyses (global responses and hydrodynamic wave pressures)


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

· · ·

Mooring analyses and design Structural response analyses (finite element analyses) Structural design verification (code checking according to Norwegian standard NS3473 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 than the Norwegian NS 3473, without the need of extensive supplements. Areas normally not adequately covered for offshore structures are such as; fatigue, tightness, design provisions for shell type members, design for durability and cracking etc. Elf Congo's Nkossa barge has been designed to Bureau 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 West Tuna, 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 Det Norske Veritas Rules for Classification of Fixed Offshore Installations (DNV Rules). The reference standard for concrete design is British Standard BS8110, but according to what stated above, specific interpretations and additions have been necessary in order to make this standard applicable for offshore structures. For this purpose the Norwegian standard NS 3473 is used as supplement. BS8110 is 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 of standard might strongly influence the platform geometry and dimensions, while standards not intended for 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. normal weight or lightweight concrete, and strength class used. For high strength concretes and lightweight concrete, the effect of reduced ductility shall be considered. This in particular applies to the stress/strain diagram in compression, and the design parameter used for the tensile strength in calculation of bond strength, and transverse shear resistance.


Fabrication Site

A general concrete substructure construction facility is described previously. It should have reasonable supporting 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.


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

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 gravity substructure is illustrated in Figure 7-2. The construction facility was located in a remote part of Subic Bay in the Philippines.


Options for fabrication

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


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

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


Construction and Methods

The small concrete platform is ideal for slip forming of the vertical walls. Slip forming is a very effective fabrication method for high concrete structures. Even the lower hull with an assumed height of some 10-12 m will most likely be slip formed. There are advanced slip-forming systems allowing for changes in slip forming geometry over the height of the structure. I.e. the upper hull can be built with 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 construction activities. All steel will be attached to the concrete at preinstalled embedment plates. Such plates are placed during the slip forming, and after slip forming steel pipes etc will be welded onto embedded steel plates. Penetrations through walls or slabs will also be placed during cast/slip forming.

Figure 7-3. Interform slipforming system


Material Qualities

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


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

The concrete mix designs shall be in accordance with the concrete specification given by the civil contractor. As a minimum, the concrete specification shall cover the following items: mechanical and chemical 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 the ND-concrete, which is an advantage for floaters. However, small concrete platforms are partly weight stable platforms and needs weight in the base. Hence it may be beneficial to use ND-concrete in the lower part and LWA in the upper part. The advantage must be compared to the possible disadvantage of dealing with 2 different materials at the site and in engineering. In a feasibility study the design of the concrete hull is recommended to be based on Normal Density (ND) concrete grade C60 and lightweight aggregate concrete (LWA) concrete grade LC55, both as defined by NS3473. These concrete qualities are well proven in many countries, and are regarded as fairly straightforward to produce. For ordinary and pre-stressed reinforcement the grades KT500TE (NS 3570) and St 1570/1770 (EURONORM) are recommended.



fcn (MPa) fcd (MPa) ftn (MPa) ftd (MPa) Ecn (MPa) Eck (MPa) Eck (MPa) Ec (MPa) Ecdyn (MPa) : : : : : : : : : : ND C60 36.4 29.1 2.375 1.90 29 399 30 534 24 427 35 000 40 000 LC55 33.6 26.9 2.25 1.8

Concrete grade Structural material strength Design compressive strength, ULS Nominal structural tensile strength Design tensile strength, ULS Modulus of Elasticity Plain concrete, ULS Plain concrete, SLS Plain concrete, FLS (0.8Eck) Static, short-term loads (incl. reinf.) Dynamic

23 226 18 581 25 000

Poisson's Ratio, Coefficient of Thermal Expansion Poisson's ratio The coefficient of thermal expansion T

= 0.20 = 10×10-6 oC-1

Density Plain concrete: Reinf. concrete: 150kg/m3 300kg/m3 500kg/m3

ND C60 24.0 kN/m (2.45 t/m ) 25.0 kN/m3 (2.55 t/m3) 26.0 kN/m3 (2.65 t/m3) 27.5 kN/m3 (2.80 t/m3)

3 3

LWA C60 19.3 kN/m3 (1.95 t/m3) 20.4 kN/m3 (2.08 t/m3) 21.1kN/m3 (2.15 t/m3) 22.6kN/m3 (2.30 t/m3)


Ordinary reinforcement

:fy = 500 MPa :fs = 435 MPa

Design Strength, Grade K500TE Yield stress Design strength, ULS The yield strain is y = 2.5 x 10-3


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

Nominal diameters are 8, 10, 12, 16, 20, 25 and 32 mm. Modulus of Elasticity Es = 2.0105 Mpa


Prestressed reinforcement

General The pre-stressing steel to be used is Grade 270 according to ASTM A 416-85. Strand Properties The properties of pre-stressing strands are tabulated below. Strand type Nominal diameter Nominal area Nominal mass Yield strength Young's Modulus 15 mm (0.6") 15.2 140 1.10 1670 1.95105

Tendon units: 6-7, 6-12, 6-19, 6-20, 6-27 and 6-37 may be used. For this study a pre-stressing force of 150 KN/strand will be used.


Cathodic Protection

In a concrete structure there is a considerable amount of reinforcement steel. For a concrete offshore structure the reinforcement steel is covered by minimum 50 mm of concrete to protect against seawater penetrating into the reinforcement and subsequent corrosion. In addition to a significant concrete cover the reinforcement steel is connected to a cathodic protection system together with all other steel structures attached to the concrete structure. The reinforcement steel is acting as wires between the different anodes, usually made of aluminium. The steel outfitting will, in addition to being connected to anodes, be surface protected according to regulations and design life requirements. The aluminium anodes are mounted onto steel rods, which are welded to embedment plates. Such anodes can be spotted on the upper part of the Snorre TLP anchors in Figure 7-4 below.


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

Figure 7-4. Snorre TLP anchors with anodes mounted in upper part

The design of the corrosion protection system is usually based on the following assumptions: · there is no electrical insulation between topside structural steel and mechanical outfitting (MMO and CMO), risers, moorings etc., attached to the hull · all steel in flooded tanks, cells/shafts is electrically continuous with the reinforcement via the embedment plates


Steel/Concrete Connection Methods

Steel/concrete connections are generally based on the principle of embedding steel into the concrete behind reinforcement steel layers so that the embedded item becomes a part of the structure. In order to reach behind the layers of reinforcement typically dowels with heads, steel rods with shear keys or similar can be used. Examples of steel/concrete connections are connection between topside and hull, connection between concrete lower hull and steel upper hull, attachment of risers, fairleads, towing and mooring brackets, piping support, support of external and internal steel decks etc.


Riser support

The riser support will be attached to the concrete hull via embedment plates described below. The size and capacity of the embedment plates will be selected to resist the loads from the riser system. In case of steel catenary risers the flexi-joints may be attached to the upper edge of the lower hull. This will give a good load-bearing capacity.


Mooring brackets / Towing brackets / Fairleads

Pre-stressing tendons are used to attach the steel plate to the concrete. In addition the rear steel surface may be fitted with shear keys etc.


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


Embedment plates

Figure 7-5. The plate is connected to the concrete via dowels with forged head. Behind each plate there will be an additional amount of ordinary reinforcement. Typically there are many types of standard plates designed covering the full spectrum of loads. Some embedment plates have shear profiles embedded into the concrete, which make them able to resist large shear forces. Towing and mooring brackets are also a type of embedment plates usually connected to the concrete with post tensioning cables instead of, or in addition to, dowels.

Figure 7-5. Typical embedment plate


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


[1] Morgan, R. G. Development of the concrete hull. "Concrete Afloat", Proceedings of the conference on concrete ships and floating structures organized by The Concrete Society in association with the Royal Institution of Naval Architects and held in London on 3 and 4 March, 1977. Gloyd, C. S. Concrete Floating Bridges. Concrete International, May 1988. Anderson, A. R. Design and Construction of a 375.000 bbl Prestressed Concrete Floating LPG Storage Facility for the JAVA Sea. Offshore Technology Conference, OTC 2487, 1976. Sannum, H. Heidrun, The First Concrete TLP. The Future Development of the North Sea and Atlantic Frontier Regions. OCS, Aberdeen 25 and 26 January 1995. Ruud, M. The Troll Olje Development Project. Vision Eureka, New Technology for Concrete Structures Offshore. Lillehammer 13 & 16 June 1994 Valenchon, Nagel, Viallon, Belbeoc'h, Rouillon: The NKOSSA concrete oil production barge. OMAE 1995 - Copenhagen - 14th International conference - June 18-22 1995. Valenchon, Nagel, Viallon, Belbeoc'h, Rouillon: The NKOSSA concrete oil production barge. Paper presented at DOT, 30 Oct. / 1 st Nov. 1995, Rio de Janeiro, Brazil. Sare and Yee Operational experience with pre-stressed concrete barges "Concrete Afloat", Proceedings of the conference on concrete ships and floating structures organized by The Concrete Society in association with the Royal Institution of Naval Architects and held in London on 3 and 4 March, 1977. Fjeld (NC), Hall (Phillips), Hoff (Mobil), Michel (Doris), Robberstad (Elf), Vegge (Norw. Petrol. Directorate), Warland (Statoil): The North Sea concrete platforms - 20 years of experience, OTC 1994, Houston

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Bech, S., Carlsen, J.E.: "Durability of High-Strength Offshore Concrete Structures".

Proceedings - 5th. International Symposium on Utilisation of High Strength/High Performance Concrete. Sandefjord, Norway, June 1999.


Derrington, J. A. Prestressed concrete platforms for process plants. Proceedings of the conference on concrete ships and floating structures organized by The Concrete Society in association with the Royal Institution of Naval Architects and held in London on 3 and 4 March, 1977. Morgan, R. G. History of and Experience with Concrete Ships. Proceedings of the conference on concrete ships and floating structures, Sept. 15-19, 1975 / Berkeley, California, Ben C. Gerwick jr. Editor. Nanni, A. and Lista, W.L. Concrete Cracking in Coastal Areas: Problems and Solutions. Concrete International, Dec. 1988 FIP (Federation Internationale de la Precontrainte) state of the art report: The inspection, maintenance and repair of concrete sea structures, August 1982


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


Gerwick, Mansour, Price & Thayamballi. Feasibility and Comparative Studies for the Use of Prestressed Concrete in Large Storage/Processing Vessels. The society of naval architects and marine engineer. Paper presented at the annual meeting November 16-18, 1978, New York. Campbell, R. Classification of Concrete Vessels. Proceedings of the conference on concrete ships and floating structures, Sept. 15-19, 1975 / Berkeley, California, Ben C. Gerwick jr. Editor.



Moksnes, J.:"20 years of R&D into HPC ­ has it been a Profitable Investment?" 6th International Symposium on Utilization of High Strength/High Performance Concrete, Leipzig 2002. Olsen T.O.:"Recycling of offshore concrete structures" Structural Concrete, 2, No.3, September 2001

Fjeld, Hall, Hoff, Michel, Robberstad, Vegge, Warland (1994) The North platforms - 20 years of experience. OTC, Houston Sea concrete

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K.Høyland, T.O.Olsen.: "Recycling of Offshore Concrete Structures". International Workshop, Consec `98

Maage M., Olsen T.O.: "LETTKON - A major joint Norwegian research programme on light weight aggregate concrete". Proceedings - Second International Symposium on Structural Lightweight Aggregate Concrete. Kristiansand, Norway, June 2000. Olsen T. O.: "Heavy Duty Floating Unit for the Offshore Industry". Proceedings - Second International Symposium on Structural Lightweight Aggregate Concrete. Kristiansand, Norway, June 2000. NS 3473 Norwegian Standard: Concrete structures - Design rules, 5th edition, 1998.


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