Forensic Engineering Case Studies

We were instructed to investigate the cause of the fire. The starting point of the project was to interview all personnel involved and to sift through and examine the physical remains of the wax plant furnace to ascertain the cause. The investigation demonstrated that the fire resulted from the failure of the support lugs of an air inlet cone in a centrifugal forced draft fan.

The cone and the lugs were manufactured from pure aluminium. The impeller of the fan had 16 vanes and 1500 rev/min. Therefore, the fundamental aerodynamic forcing frequency on the support lugs was 400Hz. This induced 34.56 million stress reversals per day. The fan failed approximately 10 months after commissioning, therefore, the lugs will have experienced 10 billion cycles from commissioning to the time of failure.

We were not able to calculate the dynamic stress to which the support lugs were subjected in order to determine the probable number of cycles to that such a material could have sustained within its S-N curve to predict the fatigue life. However, it is known that pure non-ferrous alloys including pure aluminium do not exhibit a fatigue limit or a lower stress level at which it may operate for an infinite number of stress reversal cycles. Therefore, there is a finite number of stress reversals that will always induce a fatigue failure.

In support of this view we found the 10 lugs which had been supporting the air inlet cone. All of them were broken into two parts with one half welded to the air inlet cone and the other half bolted into the casing of the fan. Two of them exhibited catastrophic tensile failures but eight exhibited the classic characteristics of fatigue failure on the failed surfaces. This was determined by microscopic metallographic analysis.

The Client took our findings to the Fan Contractor who denied liability, rejecting the findings in our report. However, by coincidence, a colleague in the hydrocarbon processing industry happened to meet Dr Stoneman some two weeks after the matter had become an impasse. By chance the two discussed what they had done since they last met. The colleague who was working on a new furnace project with a number of forced draft fans manufactured by the same company, which had not yet been commissioned. The colleague told Dr Stoneman of a peculiar development that the fan manufacturer had come to site one day and without explanation had dismantled all the fans to replace the air inlet cones.

The dispute between the Client and Contractor was resolved shortly after with the contractor accepting total liability for the cause of the fire!!

Case History #2 Roller Coaster Disaster

Whilst returning from a business trip to Germany Dr Stoneman sat by a lady who was a Barrister defending an action on behalf of an Insurance company, holding liability for the consequences of the failure of a roller coaster. A number of people were killed in the disastrous failure.

Whilst describing the technical issues it was felt that the technical arguments being provided to the Barrister were not valid. We were engaged to review the issues and advise accordingly. There were two issues related to financial liability. The first was the fundamental physics of the cause of the mechanical failure and secondly, at what part of the design and commissioning processes did the error occur.

The first issue required a judgement of the failure process and extensive technical analysis to demonstrate that the mechanical component that failed was inadequately designed to fulfil its function. That was the relatively easy part.

The second issue was more important to the Insurer client as they were responsible for faults in the Commissioning but a second Insurer was responsible for any fault in the Design. We argued and successfully demonstrated that the fault was one of fundamental design and so the liability of our Client was relieved.

Case History #3 Oil Industry Cherry Picker

A workman was injured when the tension chain that supported the boom of his mobile elevating work platform otherwise known as a Cherry Picker failed. The chain catastrophically failed and allowed the boom supporting his work cage to retract without support falling a distance of more than 40 feet.

The workman suffered substantial physical injuries and financial damages from his inability to work in his well-paid employment within his trade in the Oil Industry. He sued inter alia the employer, the Cherry Picker hire company and the Cherry Picker manufacturer. All denied liability and retaliated by endeavouring to attribute blame on the other parties to the action.

It was claimed that the injured Claimant was accused of contributory negligence by not using the Cherry Picker properly, causing the chain to overload and fail.

The Hire Company was accused by all sides of not having maintained the machine adequately, allowing shot blasting grit to accumulate such the chain was overloaded by fouling from the shot blasting grit.

The employer was accused by all sides of not providing adequate instruction to the Claimant on the use of the Cherry Picker.

Finally, the manufacturer was accused of providing a machine that was not fit for purpose by their not making provision for the design to prevent fouling of the tension by shot blast grit.

All sides vigorously denied their various accusers, especially the manufacturers. However, we judged that the design of the boom assembly was deficient in that any foreign material that entered the chain tube of the boom could not fail to accumulate in and around the chain, with the inevitable failure at some time. On investigation we found that new versions of the Cherry Picker incorporated a sealing system to exclude foreign debris from the chain. We requested disclosure of the design records of the new Cherry Pickers that the company had manufactured since the failure, to establish what the reasons were for the known design revisions. Disclosure of the documents was resisted and the case was settled by the manufacturer making a "without liability" settlement out of Court.

Case History #4 Marine Mooring Winch

An operator on a marine jetty for an Oil Company was severely injured when a steel hawser on a pneumatic winch, broke and lashed his body.

The winch was used to haul marine mooring lines that secure Very Large Crude Carriers (VLCC's) to deep water jetties. The operation involved one man controlling the air winch whilst the second man stood at the edge of the jetty signalling to the winch man when to apply load. He signalled for the winch to be started but soon after the mooring line had cleared the rope running launch it became entangled with the structure of the jetty. The second man called to the winch operator to stop but the winch operator did not hear him and the winch was so powerful that it loaded the steel hawser to beyond it's breaking point. The elastic energy in the hawser was such that it sprang back and lashed the man's legs and arms as he tried to protect himself.

It was later discovered that the winch man had been distracted at the critical moment and was not watching his partner on the edge of the jetty for a signal. It was also discovered that the winch man a hearing deficiency and so was only able to receive visual signals, which he could not do because he was distracted, or very loud audible signals.

These matters contributed to the damage sustained by the victim but had the hawser not failed then no damage could have occurred. We investigated the mechanical arrangement of the winch and hawser and found that the ultimate strength of the hawser was approximately 1 tonne. However, the tensile capacity of the winch was 2 tonne. Therefore, the situation existed that if the hawser was ever constrained from being hauled in, without an adequate means of controlling the winch, it was inevitable that the hawser would be loaded to failure.

On further investigation it was discovered that the manufacturer and installer of the rope retrieval system had consciously accepted the situation winch following complaints by the operators when the system was first installed that the original 2 tonne hawser was too stiff and too heavy to handle. The 2 tonne hawser was therefore replaced with a 1 tonne hawser without any protection being provided that would prevent overloading by the winch. The situation had been operated without incident for five years but the coincidence of the design weakness and the hearing impaired operator produced a dangerous and ultimately injurious situation.

Case History #5 Navigation Buoy

A new generation of navigation buoy was deployed at sea, with life expectancy of 20 years. After approximately 20 weeks they started to fail by physical disintegration under the dynamic loads imposed by wave motion.

The buoys housed solar panels for powering warning lights and  a horn, via two large  industrial rechargeable lead-acid battery packs. The batteries stored energy to deliver power to the warning lights and horn at night or when solar radiation was insufficient.

We examined the failed buoys and found that the structural capacity of the support and encapsulating body were wholly inadequate for the hostile environment that they would experience. Even a simple estimate of the loads imposed by orthogonal wave loading, from modest wave heights, caused stress levels that were likely to induce immediate failure.

The problem was made worse by fact that the restraint system provided for the batteries had little strength.  When the buoy was impacted by a wave, the relative inertia of the structure of the buoy and the battery pack was such that the batteries exerted loads that caused their restraints to fail.

This in turn allowed the batteries to be thrown around inside the supposedly weatherproof enclosure of the buoy that housed the control gear for the solar panels and warning systems. The batteries first destroyed the electrical equipment, rendering the warning function of the buoys inoperable and then continued to cause further damage to the structure, in some cases puncturing the enclosure allowing seawater to enter.

We judged that the fundamental design was defective and so the client claimed damages against the designers and manufacturers. The issue was settled without going to litigation.

Case History #6 Flutter Instability in Heat Treatment Furnace

A new heat treatment furnace for steel sheet functioned by blasting the steel sheet with jets of air from 25000 nozzles to cool the sheet at a pre-determined rate. The system failed because the air blast caused the sheet to oscillate violently such that it came into contact with the nozzles. The nozzles were variously damaged or dislodged and the steel sheet product was damaged beyond recovery. The cost of lost production was estimated at £1 Million per day.

It was thought that the flutter phenomenon was due to an aerodynamic instability within the system. We were asked to measure the aerodynamic characteristics of a mock up to determine if we could reproduce the instability. Using specialist anemometry and flow visualisation methods, we determined that the aero-elastic properties of a tensioned flat steel sheet positioned between two sets of elastic jets, were susceptible to a flutter type phenomenon.

In collaboration with colleagues, we went further to investigate whether the flutter phenomenon was able to be reduced or at least controlled by the physical characteristics by the disposition of the cooling jets in relation to the tensioned flat steel sheet. In the event, we demonstrated that if the angle of the jet impinging on the surface of the steel sheet was taken from orthogonality, to an off centre-angle of approximately six degrees, then the flutter-energy feedback loop was destroyed and the oscillatory motion of the steel sheet ceased.

Therefore, in seeking the underlying cause of the problem, we also produced a potential practical solution.