Sleipner A offshore platform

The Sleipner A offshore platform was a North Sea, Condeep-type oil platform, built for Statoil in Norway by the company Norwegian Contractors. It is known for its catastrophic failure on August 23, 1991, due to a design flaw, that resulted from an error in the finite element analysis program NASTRAN. [Arnold DN. The sinking of the Sleipner A offshore platform. URL: [http://www.ima.umn.edu/~arnold/disasters/sleipner.html http://www.ima.umn.edu/~arnold/disasters/sleipner.html] . Accessed on: June 27, 2008.] [cite journal |author=Selby RG, Vecchio FJ, Collins MP |title=The Failure of an Offshore Platform |journal=Concrete International |volume=19 |issue=8 |pages=28-35 |year=1997 |pmid= |doi= |url=http://www.concrete.org/PUBS/JOURNALS/AbstractDetails.asp?SearchID=461713&date=anytime&allwords=ff&aftermonth=1&searchmonth=8&searchday=1&publication=Concrete+International&ID=56]

Design

The hull was a gravity base made up of support pilings and concrete ballast chambers from which three or four shafts rise and upon which the deck sits. Once fully ballasted, the hull was to sit on the sea floor. There were 24 chambers, of which four formed the 'legs' supporting the facility on top in the case of the Sleipner A oil rig.

Collapse

The hull was towed into Gandsfjord where it was to be lowered in the water in a controlled ballasting operation at a rate of 1 m per 20 minutes. This was necessary for the fitment of the deck platform to the hull. As the hull was lowered to the 99 m mark,Fact|date=June 2008 rumbling noises were heard followed by the sound of water pouring into the unit. A cell wall had failed and a serious crack had developed, and sea water poured in at a rate that was too great for the deballasting pumps to deal with. Within a few minutes the hull began sinking at a rate of 1 m per minute. As the structure sank deeper into the 220 m fjord, the buoyancy chambers imploded and the rubble struck the floor of the fjord creating a Richter magnitude scale 3 earthquake.

Investigation

The post-accident investigation by SINTEF in Norway discovered that the root cause of the failure resulted from inaccurate finite element calculations in the design of the structure. Stresses on the ballast chambers were underestimated by 47% and some concrete walls were designed too thin to resist foreseeable hydrostatic pressure when submerged. As the pressure increased, the walls failed and cracked, allowing sea water to enter the tank at an uncontrolled rate, eventually sinking the hull.

The hull was redesigned and the Sleipner A Platform was successfully completed in June 1993.

References

External links

* [http://www.ima.umn.edu/~arnold/disasters/sleipner.html Description of failure]
* [http://home.versatel.nl/the_sims/rig/sleipnera.htm Oil rig disaster web site]