|23 August 1991|
|Norwegian Continental Shelf|
The Sleipner A Platform was Condeep-type platform (abbreviated from concrete deep water structure), built for Statoil in Norway by the company Norwegian Contractors. The Condeep-type platform consists of two units, the hull and the deck. The hull is 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 sits on the sea floor. In the case of the Sleipner A, there were 24 chambers, of which four formed the 'legs' supporting the facility on top.
In August 1991, prior to the mating of the hull and the deck unit, the hull was towed into Gandsfjord where it was to be lowered in the water in a controlled ballasting operation at a rate of 1m per 20 minutes. As the hull was lowered to the 99m mark, 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 1m per minute. As the structure sank deeper into the 220m fjord, the buoyancy chambers imploded and the rubble struck the floor of the fjord creating a 3.0 magnitude record in a local seismograph station.
The post-accident investigation by SINTEF discovered that the root cause of the failure resulted from inaccurate finite element approximation during calculations in the design of the structure. Essentially, stresses on the ballast chambers were underestimated by 47% and some concrete walls were designed too thin. Upon reaching a given pressure, these walls failed and cracked allowing sea water to enter the unit at an uncontrolled rate, eventually sinking the base unit.
The base structure was redesigned and the Sleipner A Platform was successfully completed in June 1993.