THE LESSONS LEARNED from failure analysis have resulted in improvements in the overall housekeeping and discipline in many industries. Disaster investigations have led to improved de-signs and use of better materials, with enhanced reliability and safety of operations. In some cases, the entire plant has been re-designed or the entire manufacturing process altered to ensure bet-ter safety standards. The design codes, standards, specifications, and regulations for various engineering applications have been suitably modified, after identifying the deficiencies. These modi-fications are very significant from the safety point of view. A few examples are presented to illustrate these aspects.
8.1 Process Modification
The Flixborough Disaster. A plant at Flixborough, U.K., manufacturing caprolactam was completely destroyed and reduced to ashes, and the surrounding community was demolished with many fatalities, following a powerful explosion of warlike dimen-sions inside the plant (Ref 1). The details of this disaster have been described in Chapter 2, “Common Causes of Failures.” The pro-cess used for making caprolactam before the disaster occurred was oxidation of cyclohexane, which is a highly inflammable and very hazardous material to handle. When copious amounts of hot cy-clohexane escaped to the atmosphere due to the bursting of a pipe, spontaneous explosion was but natural, and the damages to the plant and the community were inevitable.
It may be recalled that the court of inquiry concluded that when the leaking reactor No. 5 was replaced by a dog-leg pipe with bellows at the two ends, to bridge reactors 4 and 6, there was no proper design study, no proper consideration for the need for a support, no safety testing, no reference to the relevant standards, and no reference to the bellow manufacturer’s “Designer’s Guides.” The important outcome of the investigation was the re-alization that any modification in a plant should not be ad hoc, but be designed, constructed, tested, and maintained to the same stan-dards as the original plant.
Following the disaster investigation, at Flixborough, the process itself for making caprolactam was changed, based on hydrogena-tion of less-hazardous phenol instead of oxidahydrogena-tion of the highly inflammable cyclohexane.
8.2 Design Philosophy for Better Safety
In the accident at the Markham colliery, as described in Chapter 2, several miners died as a result of the cage carrying them crashing
into the bottom of the pit with great force (Ref 2). The shoes of the brake system were actuated by a nest of compressed springs.
The main member transmitting the force from the nest of springs through levers to the brake system was a single steel rod at the center of the nest of springs. This steel rod fractured due to a fatigue crack that was not detected earlier. It resulted in the failure of the brake system, and the miners’ cage could not be stopped from rapid descent and crash.
It was obvious that the steel rod at the center of the nest of springs was an important structural member of the brake system for winding and unwinding the cable of the shaft. Thus, the safety of the miners was vitally dependent on the integrity of this single steel rod, which is an example of a “single line component.”
After this accident, the design philosophy for safety was changed so as to eliminate such single line components in vital systems where overspeed and overwind protection is extremely important. Or, the system should be so designed that in the event of such a failure, the winding drum will be slowly brought to rest.
Condition monitoring should be incorporated to provide warning of failure.
8.3 Use of Cleaner Materials of Construction
In a passenger aircraft, there was an uncontained fracture of a compressor disc whose fragments hit the fuel line, causing an in-flight fire and resulting in a serious accident. All the occupants of the aircraft died and the aircraft was a total loss. Investigations revealed that the subject steel disc had been made by conventional air melting followed by casting, forging, and machining to shape.
Air-melted steel is not a clean steel and is known to contain del-eterious nonmetallic slag inclusions. If these inclusions are of sig-nificant size and are located in certain critical locations in the ro-tating disc, cracks can initiate from these inclusions and propagate to dangerous dimensions. Following this accident, the manufac-turers replaced the air-melted compressor discs with vacuum-melted discs, which are known to be cleaner.
8.4 Modification of Codes and Standards
Collapse of Ashland Oil Storage Tank. On Jan. 2, 1988, a 4 million gallon oil storage tank collapsed during the first filling after a reconstruction, producing a major environmental impact. The
DOI:10.1361/faes2005p059 www.asminternational.org
tank was first built in 1940, dismantled in 1986 leaving the old welds in place, and reconstructed in 1987, radiographed and leak tested. It was a case of brittle fracture in the base metal of the tank, alongside a weld. The investigation revealed that the fracture ini-tiated from a defect in the base metal. The defect existed prior to reconstruction. The metal did not have adequate toughness to ar-rest the propagation of the crack. The region around the defect had been embrittled by heating due to cutting and welding. The steel did not meet the API 650 Standard of the American Petroleum Institute.
A task force of API studied a number of tank failures built in the period 1897 to 1979 and found that most of the failures were after some modification or repair. The Standard 650 did not cover repair, reconstruction, or modification of oil storage tanks. Later, requirements to minimize fracture were added. Further investiga-tions and follow-up work resulted in the modification of their stan-dard for “Welded Steel Tanks for Oil Storage.” API 653 (1990) for “Tank Inspection and Repair” was introduced to include main-tenance, inspection, repair, alteration, and relocation and also to provide procedures to assess risk of brittle fracture (Ref 3).
8.5 Safety Regulations for Transmission Pipelines
Failures of Oil Transmission Pipelines. A pipeline for gaso-line, in Minnesota, constructed in 1957 with electric resistance welded (ERW) pipes, burst on July 8, 1986, without warning, caus-ing fatal injuries and damage to environment and property. The failure was due to a defect in the ERW seam weld and corrosion.
On Dec. 24, 1988, in Missouri, a pipeline carrying oil burst without warning and 800,000 gallons of crude oil spilled into the Missouri River, causing a major ecological problem. The pipeline was con-structed in 1949, using ERW pipes. Here again, the failure was at the ERW weld seam.
A number of agencies studied the problem of liquid pipeline failures. The findings were very interesting; ERW pipelines con-structed before 1970 had weld defects and low toughness in the seam where there was preferential corrosion. Hydrostatic testing of pipes and nondestructive testing of seam weld were not re-quired.
Subsequently, safety regulations by the Department of Trans-portation stipulated hydrostatic testing of all interstate pipelines constructed before 1971 and all intrastate pipelines constructed before 1985 to 125% of maximum operating pressure or reducing the maximum operating pressure to 80% of the original level (Ref 3).
Failures of Gas Transmission Pipelines. In another incident, in Kentucky, a seamless steel tube ruptured completely during fill-ing of compressed natural gas from a gas well after six months of service. The steel was A-372 class 5, quenched and tempered to a tensile strength of 1100 to 1170 MPa (160 to 170 ksi). The cause of the rupture was environmentally assisted cracking due to water and hydrogen sulfide in the gas (500 ppm).
Following this mishap, the regulations were modified to limit the maximum tensile strength to 870 MPa (126 ksi). Gas purity requirements were also enforced to limit the water and hydrogen
sulfide content of the gas, to a level of 0.5 lb per million cubic feet and 0.1 gm per 100 cubic feet, respectively (Ref 3).
8.6 Aviation Security and Explosives Sensors
Aircraft Accidents by Explosive Sabotage. Explosive sabo-tage of aircraft continues to be a serious threat to aviation throughout the world. After investigating the various aircraft ac-cidents caused by deliberate in-flight detonation of explosive de-vices, an increased awareness to air safety, aircraft security, and airport security has been felt by every airline. The types of ex-plosive devices deployed, the exex-plosive species employed, the methods of concealment, and the timing devices used by the sab-oteurs are now better understood. The catastrophies to the Boeing 747 aircraft of Air India off the west coast of Ireland and that of Pan American Airways at Lockerbie have proved that it has been possible for someone to place the explosive device inside the aircraft without being detected. These two accidents have led to a significant increase in the security measures adopted at the air-ports for baggage and cargo check-in.
Airlines stretched technology to its limits in the search for ex-plosive devices in passengers’ baggage (Ref 4). Large-scale ef-forts and funding continued for developing fool-proof instruments for detecting the presence of explosives in the baggage. One such machine that was developed and tested in a few major interna-tional airports was the thermal neutron analysis (TNA) machine in which the baggage is made to pass through a beam of thermal neutrons generated from radioactive californium-252. Plastic ex-plosives are denser than other organic materials and contain two to six times more nitrogen than other plastics. Nitrogen atoms absorb the thermal neutrons and emit gamma rays. Detection of more gamma rays is an indication of higher nitrogen density and hence the possible presence of plastic explosives. Thermal neu-tron analysis machines had certain drawbacks because of their weight and the possibility of false alarms.
In another type of instrument, x-rays of two different energies are made to pass through the material. Each material attenuates the two sets of x-rays differently. By comparing the attenuated beams, the effective atomic number of the material can be esti-mated. Using such a system, dense organic material such as ex-plosives can be distinguished from dense inorganic materials such as metals. Explosives can be distinguished from less-dense or-ganic materials such as ordinary plastics.
In yet another machine, which is a computerized tomography scanner, x-ray scans are taken around an object and the different views combined on a computer to produce a slice image in a second. Screening a bag takes 9 to 12 slices. For bags arousing suspicion, the machine projects two images taken from different directions for the operator to make a decision. If necessary, fur-ther scanning can be done focusing on particular areas in the baggage.
Techniques and instruments are also being developed for de-tecting the vapors given out by the explosives. However, there are limitations because plastic explosives are not very volatile.
These are some of the significant developments toward aircraft security against bomb threats.
The examples cited indicate the importance of continuous im-provement in safety and reliability standards of engineering
struc-tures, which has been possible through the lessons learned from systematic failure analysis and accident investigations.
REFERENCES
1. R.J. Parker et al., “The Flixborough Disaster—Report of the Court of Inquiry,” Her Majesty’s Stationery Office, London, U.K., 1975
2. J.W. Calder, “Accident at Markham Colliery,” Her Majesty’s Stationery Office, London, U.K., 1974
3. J.L. Gross, J.H. Smith, and R.N. Wright, Ashland Tank Col-lapse Investigation, J. Performance of Constructed Facilities, Vol 3 (No. 3), 1989, p 144
4. D. Clery, Can We Stop Another Lockerbie?, New Sci., 27 Feb 1993, p 21