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A CGBS is generally a very large and extremely heavy reinforced concrete structure which is placed on the seabed. It withstands the extreme environmental forces by virtue of its own weight and inherent strength.
CGBS platforms are among the largest and most impressive structures ever built and moved by man. To date the Troll platform is installed in the deepest water (303 m), and the Hibernia platform is the heaviest weighing 1.2 million tons on land. The designs of CGBSs vary considerably, and their weights range from 3000 to 1.2 million tons with corresponding topsides weighing between 650 and 52,000 tons.
A typical CGBS has a concrete base (often called a caisson) with one or more shafts to support the topside platform. When empty, voids within the base known as cells (along with the hollow shafts) provide buoyancy during the latter stages of construction, tow-out, and installation. When a CGBS is in operation, the cells are flooded with seawater or act as storage and in some cases separation facilities for crude oil (https://www.iogp.org/bookstore/product/decommissioning-of-offshore-concrete-gravity-based-structures-cgbs-in-the-ospar-maritime-area/).
Basic Characteristics of Concrete Platform
The dimension and weight of typical concrete gravity platforms (Sharp 1979)
150,000 barrels/day oil
Caisson base area
Weight of substructure in air
Total height of platform
Concrete platforms can be used for drilling, oil extraction, oil storage, and so on. The water depth that concrete platforms can work well is within 200 m and the best from 100 to 150 m.
Concrete platform has its own advantages and disadvantages.
Save steel and economical. The total cost of constructing a concrete gravity platform is lower than that of the steel jacket platform.
The installation workload at the sea site is small.
Anti-seawater corrosion, low maintenance costs, and long service life.
Require good seabed conditions.
The structural analysis and manufacturing process are complicated.
Reuse is difficult.
History of Concrete Platform
At the end of 1969, industrial oil was discovered in the Ekofisk field of the North Sea. However, due to the harsh sea conditions in the North Sea and the surging price of steel at that time, it is difficult to adopt the traditional steel jacket platform. Then, the Norwegians built the first concrete gravity platform. In the design, the Norwegians used their experience of building concrete breakwaters and offshore gravity lighthouses, as well as the abundance of concrete in Norway. This concrete gravity platform was designed in 1970 and started to be built in 1971 and was finished in June 1973. This concrete platform with 92 m diameter, about 100 m high, has an oil storage capacity of 158,900 m3. The successful construction of this platform provided valuable experience for the future use of concrete gravity platforms in the North Sea and other parts of the world.
Concrete platforms can be used for drilling, oil extraction, and oil storage and transportation. It has the advantages of saving steel, anti-seawater corrosion, low maintenance costs, and long service life. So 17 concrete platforms were built in the North Sea and off the coast of Brazil in just 5 years from 1973 to 1978. The maximum depth of these platforms’ operating depths is 150 m and the minimum depth 15 m.
However, some shortcomings of the concrete platform appeared gradually, and in the early stages of development, some problems emerged. For example, the concrete gravity platform requires good seabed condition during working and deepwater depth during construction. These problems made it difficult to promote the concrete gravity platform to other waters in the world. At the same time, due to the improved techniques of steel jacket platforms, steel jacket platforms became popular again. This reduced the demand for concrete platforms.
From medium water depth to shallow and deepwater depth.
Safer and more reliable concrete gravity platform appeared.
Structural Components of Concrete Platform
Concrete platform contains a cellular base, columns, and deck structure. Figure 1 also shows the structures of a concrete platform. The cellular base is the foundation of the entire building. In order to resist the huge storm force, the platform requires a large base structure. To prevent the base from sliding on the surface of the sea floor, skirt plates could be inserted into the seabed around the base. Large base structure can be used to store crude oil, which provides the concrete platform the advantages of oil storage.
The deck provides a workplace for production. The deck is fitted with various production and living facilities.
The column is connected between the base and the deck, to support the deck.
Construction and Installation Process of Concrete Platform
Gravity platform construction process is complicated, compared to those of other types of offshore platforms. Basically, there are two construction methods for concrete platform. One is that the building site can be located at the low-lying water area that is drained. The other is that construct the base on the land to a certain extent and then launched into the water.
In the dry dock, construct the base to a predetermined height and pour water into the dry dock.
Tug the base together with lifting equipment to the deepwater storm-sheltered area, and moor it firmly.
Continue building the upper part of the base and the legs until the concrete base and concrete legs are constructed completely.
Ballast the base, and the platform goes down. Then the prefabricated platform deck is transported by barges right up to the legs. Drain the base, and the legs float up a little until the legs support the deck at the right positions.
Connect the legs and the deck firmly together, forming a concrete gravity platform.
The Strength of Concrete Platform Main Structures
The strength of a concrete platform is highly important, and it is a basic problem needed to be considered when engineers design a concrete platform. However, sometimes it was still neglected and an accidental example is provided here.
On August 23, 1991, the loss of the Sleipner A concrete gravity platform caused huge economic losses (Collins et al. 1997). The concrete base structure of the Sleipner A platform was lowered into the Gandsfjord for deck-mating. When deck-mating, a Condeep structure was about 20 m deeper than it is during operation. The Condeep structure experienced the critical hydrostatic pressure. This high pressure caused the cell wall to break, allowing water to rush into the drill shaft. Later, the structure sank. As the structure went deeper into the fjord, the buoyancy cells imploded. Finally, this structure of $180 million dollars was totally lost. Only the pile of rubble at the bottom of the fjord remained.
This accident gave a big lesson on the structural design for concrete platform. The design of concrete gravity platforms should consider the buckling or crushing of the cellular base bulkhead. When the base is also used as a storage tank, temperature stress due to internal and external temperature differences should be considered. In addition, when the concrete gravity platform works in the area with high seismicity, the seismic performance of the platform has to be considered. The water-structure interaction and soil-structure interaction have much importance in the analysis of offshore concrete gravity platform. Besides, the inelastic behavior of the hollow concrete members deserves attention (Whittaker 1987).
Furthermore, because concrete platforms mostly work in the North Sea, where ice may be expected in the winter, the ice-concrete interactive effect has also to be taken into account if the concrete gravity platform works in the ice zone. Level ice on offshore concrete platform is the main concern for general concrete abrasion, and the other type of ice, for example, iceberg and freshwater ice, should not be forgotten (Tijsen 2015). Concrete abrasion mainly occurs below water level, because level ice adheres to the concrete above water level.
- Collins MR, Vecchio FJ, Selby RG, Gupta PR (1997) The failure of an offshore platform. Concr Int Detroit 19:28–36Google Scholar
- Tijsen J (2015) Experimental study on the development of abrasion at offshore concrete structures in ice conditions. Delft University of Technology, DelftGoogle Scholar
- Whittaker D (1987) Seismic performance of offshore concrete gravity platformsGoogle Scholar