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The use of concrete in deep-sea offshore energy projects is becoming more common as explorers are forced into harsher ocean environments.   BY ALAN TITCHALL

Reinforced concrete has been used for shoreline marine structures such as lighthouses and docks for centuries. Success with concrete ships since the 1920s, and floating bridges since the 1950s, paved the way for lightweight concrete as a common material for floating structures.

During the construction of the North Sea oil project in the early 1970s concrete was used for the first time as a construction material for offshore platforms. These structures have now survived four decades in a harsh ocean sea environment, points out Dr Tor Ole Olsen, of Norwegian company Dr.techn.Olav Olsen, and who was a keynote speaker at the 2009 Concrete Conference.

The North Sea now features 15 concrete platforms installed in the Norwegian sector, including the floaters Heidrun TLP and Troll B Semi, and another 12 concrete platforms installed in the UK sector, including the storage/foundation tank for Harding, he says.

In addition there are concrete platforms installed in the Baltic Sea, Brazil, Indonesia (concrete barge), Gulf of Mexico, Canada, Germany, USA, Holland, Congo and Australia. The most recent concrete platforms have been installed in Denmark (South Arne), the Philippines (Malampaya), Sakhalin and the Adriatic.

“The concrete substructure can be predictably and robustly designed to resist extreme loading conditions such as significant wave and wind loads, high water pressure, seismic actions, impact loading and ice abrasion,” Tor says, which makes the material ideal for oil and gas extraction in hard environments such as the Arctic.

“Some 25 percent of the world’s undiscovered reserves of oil and gas are believed to be located in the Arctic. It seems reasonable therefore to expect Atlantic Canada, North Sea, Barents Sea, Caspian Sea and Sakhalin to be the prime markets for future concrete production facilities,” he says.

Challenges associated with the Arctic include high ice loads and abrasion and limited installation windows. “In more general terms the challenges for future projects in the areas mentioned above are associated with extreme loadings such as earthquake, iceberg impact, ice abrasion, and the safe removal of the entire installation upon completion of the field production.”

The use of concrete in for offshore petroleum construction was pioneered by a constellation of the contractors Selmer and Høyer-Ellefsen – two construction companies with extensive experience building concrete structures on land. In 1973 the two companies signed a contract with Mobil to construct the Beryl A ‘condeep’ (concrete deepwater structure), with the help of Tor’s father, Olav Olsen, an expert on thin shell structures and the founder of Norwegian engineering company Dr.techn.Olav.

The condeeps are built onshore in temporary dry docks and towed out to their ocean sites to be completed – foundations (piling, grouting) and topside installation. The degree of inshore completion obviously influences the cost and safety of the field development, says Tor. While being towed, some over distances of several hundred kilometres, these structures are subjected to a considerable hydrostatic pressure difference before being ballasted on to the seabed, he adds.

Probably the most impressive condeep is the Troll A platform off the Norwegian coast. With its 22,000 tonne topside sticking some 150 metres above the sea level during its tow out, the Troll A platform was installed at 303 metres depth in the North Sea in 1995. Parts of this structure were subjected to a water pressure of 3.5 MPa during construction, says Tor.

The typical offshore concrete structure is still gravity based (GBS), which means that it sits on the seabed by its own weight. If on soft soil, the structure may be equipped with skirt piles.

“One of the more elegant condeeps is Shell’s Draugen platform which was installed in 1993. Draugen is equipped with nine metre long concrete skirts beneath the base.”

Most condeeps have a tower configuration designed to minimise wave action and some are equipped with perforated Jarlan walls to reduce the wave force.  

Concrete GBSs may be re-floated and removed. Alternatively, they can be decommissioned and stripped of all mechanical plant and equipment and left fully or partly in place as a well-marked reef.

Floating structures

Another category of offshore concrete structure consists of semi-submersible floating hulls or large barges capable of carrying considerable payloads and accommodate storage of LNG or other products.

“Then there are smaller platforms such as concrete islands, mono-towers founded on small caissons and hybrid platforms consisting of a concrete base with a steel truss tower,” says Tor.

Floating concrete platforms for offshore oil and gas production first appeared in 1976 with the Arco barge. Since then the construction of the Heidrun TLP and the Troll Oil Semi, both installed in the North Sea during 1995, opened the way for inexpensive concrete floating structures that are also well suited for floating LNG plants, he says.

Permanently floating offshore concrete vessels related to the petroleum industry are now installed in the Java Sea, in the North Sea and on the Congo coast in West Africa.

Concrete future

“The conclusion drawn from service performance of these earlier offshore concrete structures is that they have proved excellent behaviour and require significantly lower expenditure for inspection, maintenance and repair than steel structures,” says Tor.

A number of international codes now cover all aspects of design and construction, he adds.

The inshore construction of concrete offshore structures also provides good conditions for quality construction.

The increasing importance of local content in the oil and gas development projects is favouring concrete as the building material in countries with limited number of offshore steel yards, says Tor.

“Every country has a concrete construction industry. 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.”

Typical fabrication cost for offshore concrete structures in the past range from US$1500-$3500 per cubic metre of concrete, inclusive mechanical equipment and installation, says Tor.

“Experience has shown that offshore concrete structures currently in use are virtually maintenance-free. Many platforms have passed their intended design lives, many have got their functional duties enlarged, and many have shown a remarkable strength towards abnormal events.”  

-- Edited by admin on Monday 31st of October 2011 02:22:02 PM

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