Ice-breaking platform legs
PA-B and Lun-A platforms’ giant concrete legs — each wider than 20 metres and some 56 metres tall — were built in Vostochny port in Russia in December 2004 and towed to the site on a barge pulled by three tugboats. They are extra thick to help withstand earthquakes and their rounded shape helps ice floes slide around them.
Sometimes sheets of ice edge up platform legs, peel backwards and crash back down on the frozen sea. The pressure of the ice against the four legs can amount to 30,000 tonnes.
A structure used to protect bridges and public buildings from earthquakes was installed in offshore platforms for the first time. Two of Sakhalin-2’s platforms are connected to their concrete legs by sliding joints. If an earthquake strikes, the topsides can move independently from the legs in a pendulum motion, preventing damage.
The design can withstand an earthquake measuring 8.0 on the Richter scale — greater than the strongest earthquake expected to occur around the platforms. Such an earthquake may occur on Sakhalin Island perhaps once in 3,000 years. A similar design protects San Francisco’s Oakland Bay Bridge and Istanbul’s Ataturk Airport.
Installation world record
In 2007, two specially built T-shaped barges, each the size of two football fields, towed the upper parts, or topsides, of the PA-B and Lunskoye-A platforms 3,000 kilometres from the Samsung Heavy Industries shipyard in South Korea where they were built.
The barges sailed into position between already-installed platform legs before being gradually lowered, allowing the topsides to settle on the legs. The PA-B’s 28,000-tonne topside makes it almost as heavy as three Eiffel Towers — it set a world record in 2007 for the heaviest topside ever installed this way. The fully assembled platform is as tall as a 30-storey building.
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