An Elusive Utopia: Systems that Work

Transcription

1 An Elusive Utopia: Systems that Work Alice E. Fischer October, 2018 Elusive Utopia... 1/14

2 Systems Fail Two American DIsasters Poison Gas in Bhopal Elusive Utopia... 2/14

3 Systems Fail Systems Fail All kinds of systems fail: Bridges fall down Buildings collapse Dams break Nuclear reactors overheat Aircraft crash Spaceships explode... and computer systems malfunction. Elusive Utopia... 3/14

4 Systems Fail Why Systems Fail These disasters are diverse, but a small number of causes, in various combinations, account for all of them. Inadequate or incorrect statement of requirements. Defective design based on an inappropriate model Complexity and concurrency. Lack of redundancy. Defective parts and/or inappropriate parts. Safety equipment is turned off. Poor instructions; operator error. Change happens. Political, corporate, or financial pressure. Red tape and constraints on the bidding process. Elusive Utopia... 4/14

5 Two American DIsasters The Dome Collapse The collapse of the roof of the new auditorium. Long Island University, Post College, C.W. Post Dome Auditorium collapsed seven years after completion. Saturday, January 21, 1978 between 2:00 and 3:00 AM The center of the dome caved in under mounds of snow and ice. The $2 million auditorium, which had received several architectural awards, was completely destroyed. Nobody was hurt. Causes? The theory did not fit the application. Elusive Utopia... 5/14

6 Two American DIsasters It looked like a giant cracked eggshell Causes? The theory did not fit the application. This dome was a triodetic, reticulated structure, invented in the 1940 s and used in the United States since The load-bearing capacity was calculated according to a membrane-theory model for thin-shell domes. The membrane theory (load equally distributed on all parts) does not apply to a reticulated structure (load concentrated on the joints). The dome had been under-designed based on an inapplicable model that assumed a uniform load on all parts. A snow load of only one fourth required by the code, distributed over one third of the dome s surface, was bound to cause a collapse. The snow and wind load was concentrated on one side. Elusive Utopia... 6/14

7 Two American DIsasters The Aerial Walkway Collapse Atrium of Hyatt Regency Hotel, Kansas City, Missouri July 17, 1981, with 1600 people in the atrium, many dancing. Suspended walkways on the second and fourth floors were crowded with people, stomping to the music. They suddenly and simultaneously dropped onto the people at the bar, below. People screamed, glass flew, pipes were broken, water sprayed. 114 died, 200 were injured, many severely. The engineering firm was judged to be at fault. Causes? Change. Money. Elusive Utopia... 7/14

8 Two American DIsasters Plunge at the Hyatt The original design called for very long rods, from ceiling to second floor, to support the suspended walkways. The contractor suggested (and the engineer accepted) a change to two-part suspension to simplify construction. The new design had one rod from ceiling to 4th floor, another from 4th to 2nd floor. The lower rod was connected to the same beam as the upper rod, a few inches away. This kind of jointed rod has less load-bearing capacity than a single rod. Even the original design did not meet the city s building code, but it might have been adequate. The modified design certainly was not. There was no redundancy. When the first hanger failed, the rest followed immediately. Elusive Utopia... 8/14

9 Poison Gas in Bhopal Death in Bhopal Chemical plant in Bhopal vents tons of poison gas over city. Bhopal, India, factory of Union Carbide, India, Ltd. December 3, For two hours, a 100-ft tall factory smokestack spewed out a deadly cloud of methyl isocyanate. This gas attacks and destroys mucous membranes. The railway station manager lived long enough to warn other trains not to come into the city. Thousands of people fell dead in the streets, many more died slowly and painfully. Altogether, 18,000 to 20,000 deaths is a conservative estimate. Causes? Change, financial pressure, disabled safety systems, poor operator information. Elusive Utopia... 9/14

10 Poison Gas in Bhopal Death in Bhopal: Background A chemical plant had been built in heavily populated Bhopal. Originally, it produced pesticides by a relatively safe process. In 1978, the plant was converted to using a cheaper process that used methyl isocyanate (MIC). The MIC was stored in two large nitrogen-pressured tanks. Defective batches were stored in a third tank, prior to reprocessing or being destroyed. The local government objected to this change because of the potential danger. The state and national governments, however, supported the new plant and process. A corporate safety survey in 1982 warned of slipshod maintenance. Local managers assured headquarters that all problems had been corrected. Elusive Utopia... 10/14

11 Poison Gas in Bhopal Death in Bhopal: Safety Systems Originally, four safety systems were supposed to guard against disaster: A vent gas scrubber, which could destroy defective MIC gas. A cooling system for the storage tanks. A big torch to burn off escaping MIC vapor. A water spray for firefighting. The business lost money and started to practice deferred maintenance. The cooling system stopped working because workers siphoned off the refrigerant for use in other equipment. In October, 1984, the production reactor was turned off, and workers turned off the scrubber tower. This saved money, but the storage tanks still held 62 tons of MIC. Then they turned off the torch to replace a pipe. Elusive Utopia... 11/14

12 Poison Gas in Bhopal Death in Bhopal: Getting worse In November, the storage tanks started having trouble maintaining nitrogen pressure. Alkaline water from the scrubber began to leak into the storage tanks system and react with vapors to form a gunk that stuck to pipe walls. Workers repaired the nitrogen system on one tank, so they would be able to pump out the MIC. They left the second tank for later repair. On December 2, workers set out to clean part of the system. There was no maintenance supervisor, and the process used skipped an essential safety step. Drains clogged and the pool of wash water rose. Nothing came out of the bottom. Elusive Utopia... 12/14

13 Poison Gas in Bhopal Death in Bhopal: The Tragedy A worker noticed the problem but was told by a supervisor brought from another factory told him to keep going. The water rose 20 feet, to a pressure-relief system, then went down a pipe toward the other tank. A valve in that pipe was supposed to be closed. it wasn t. A hundred gallons of wash water flowed into the tank at about 10:00 p.m., starting a heat-producing reaction. By midnight, workers knew that MIC was leaking, somewhere. They could feel it. The operator noted the pressure was out of range and inspected the tank. He heard gas escaping and tried to bring the scrubber online. It didn t respond. No effective safety systems were left. The gas escaped. Elusive Utopia... 13/14

14 Poison Gas in Bhopal Bibliography James R. Chiles. Inviting Disaster, Lessons from the Edge of Technology Harper Business, N. Leveson and C. Turner An Investigation of the Therac-25 Accidents IEEE Computer volume 27 #7, July M. Levy and M. Salvadori Why Buildings Fall Down W.W.Norton & Company Elusive Utopia... 14/14