On August 23, 1991, the concrete base of the Sleipner A oil platform collapsed in the Gandsfjord near Stavanger, Norway, due to a critical error in engineering calculations. The base was a massive gravity structure, part of the Condeep family of platforms, made up of 24 interconnected cells covering 16,000 square meters. The collapse occurred during a routine controlled ballasting operation before the platform’s bridge was attached. The incident was caused by a 47 percent underestimation of shear forces in a computer model using the NASTRAN software, combined with a defect in the reinforcing anchoring system. These errors led to certain concrete walls being too thin to support the actual loads, resulting in the structure’s failure. The collapse happened at 5:49 am with a loud explosion, followed by two smaller bangs and a major leak. The control room recorded nearly 1,000 tons of water entering the drilling well, but the ballast pumps were unable to handle the volume. The entire base sank in just 18 minutes to a depth of about 200 meters. The event was recorded as a seismic event measuring 3.0 on the Richter scale, and the cells imploded as they sank under pressure. The wreckage left behind only a pile of debris at the bottom of the fjord. An investigation by Statoil traced the problem to a specific triangular junction in the structure known as a tricellule, where several cylindrical cells meet. The error was found in an inaccurate finite element model of the tricellule, which underestimated shear forces by 47 percent. A corrected analysis after the accident predicted a rupture at 62 meters depth, which closely matched the actual event at 65 meters. This highlighted the risks of relying on numerical simulations without proper verification. The incident resulted in an economic loss of about $700 million, with the destroyed structure itself valued at 1.8 billion Norwegian kroner. A new base was constructed in 19 months, and the platform began gas production in August 1993, two years after the accident. Sleipner A is now a well-known case study in offshore engineering, taught at institutions like the University of Minnesota. Since 1993, the platform has operated without major incidents and has been used for geological carbon dioxide storage since 1996.