Tuesday, August 14, 2007

A cautionary tale on the use of software

An occasional commenter to my blog wrote an entry on "Where is engineering going". One of the subtopics in that was the on use and abuse of computers. In particular the use of computers as a black box. I have a cautionary tale about using computers as a black box.

About ten years a nuclear facility, "the customer," was buying a deployment system from a vendor. The vendor was a well known name in nuclear operations. The deployment system was to insert a high pressure cleaning lance into a waste tank to break up sludge on the bottom the of the tank. Once the sludge was broken up it was to be removed.

The vendor had had a finite element analysis of the entire deployment system performed by an outside consultant. The consultant's analysis showed the deployment system design to be satisfactory.

Because of the seriousness of a failure the customer wanted an independent assessment by an outside third party of the deployment system provided by the vendor. The customer approached my employer to have the assessment performed and the job landed on my desk. I was not to redo the entire analysis but only to analyze the wrist segment of the deployment system which was deemed the most critical. The significance of the wrist was that its failure would result in either the deployment system being unretrievable from the waste tank or dropping the payload which could result in breaching the tank.

I recieved a 3-D CAD file of the wrist, some drawings of the overall mechanism, and a table of loads. The vendor, viewing this whole excersize as a waste of time, was not interested in cooperating further. The cooperation I recieved was only due to arm twisting on the part of the customer.

The mechanism was a complex 3-d mechanism, as opposed two a 2-d planar, mechanism. It took me a few days to decipher the drawings, figure out the static equilibrium equations for the wrist, and set up the finite element analysis model. I ran the model and .... portions wrist near the mechanism were shown to be at yield under static loading conditions with no factor of safety. I was quite dismayed. I automatically assumed I made a mistake.

I went back through my analysis, both the hand calculation to determine the loading on the part, and the finite element analysis - same result. I refined my model of the pin joints to more accurately calculate the stresses at the connection points. It made matters worse - the stresses were now shown to be slightly higher than yield.

I was quite aware of the consequences to the customer/vendor relationship once I reported my results. I had two engineers I worked with review what I had done and they agreed with my conclusions. At this point I was still convinced I had overlooked something. It simply did not seem that an experienced vendor could make this sort of design error. With a heavy heart I called the customer and reported my findings. The customer, of course, was also convinced I had made a mistake but requested that I fedex my calculations and analysis results. The next day the vendor called and after expressing some doubts about my abilities and my employer's reputation also requested a copy of my work.

I heard nothing more for almost a week. By this time I was convinced I was in the dog house when the customer called. They agreed with my results and, reluctantly, so did the vendor.

What had happened?

The vendor's consultant had inadvertently modelled the spherical and pin joints as transferring moment loads where in actuality they can only transfer forces. That error dramatically changed the stresser on the part. The factor of safety showed in the vendor's original analysis was non-existent.

What were the end results of this? The project lead engineer was fired. Although he did not do the analysis, he signed off approving it. The consultant was fired. The customer eventually backed out of the contract. Although this flaw was fixed. This and other technical difficulties led to the customer losing confidence in the vendor.

Ever since then I have been leery of automatically trusting results from computer analyses. This poses a real problem in today's ever shortening design cycles. The assumption is if the computer did it, it's correct. There is no patience for checking the results or for performing alternative manual calculations.

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Wednesday, August 08, 2007

Salesmen I have met - The Retro Encabulator

This reminds me of sales presentations I have been to. Once you pin the salesman down and start asking questions they have no idea what they have just said.

Saturday, August 04, 2007

Nothing is as quick as you think it should be

Today I replaced an outdoor light for a friend of my wife's. I figured it would take two hours or so. I took my tools and headed over to install the light. It turned out I needed some conduit, a junction box, some toggle bolts, and a few other tools. I headed out to Home Depot and back home to the tools I needed. I started installing the light. It turned out the toggle bolts were too short. I made another trip to Home Depot.

What was supposed to be a short job ended up taking a good part of Saturday.

A few weeks ago we completed design and the drawings for a piece of process equipment. We (the lead engineer, the designer, and myself) sat down on Thursday morning to do a final review of the drawing package (45 drawings, ~80 sheets) before releasing it to the customer. The plan was to meet quickly and incorporate those final changes.

This was not a design review. We already had a design review. We were simply reviewing the weld notes, quality assurance notes, and material callouts for completeness and consistency. As this was for a piece of nuclear grade equipment going into a nuclear facility these notes are not a triviality. Nonetheless, we expected to be done well before lunch.

We ate lunch in the conference room and finally finished our "short meeting" at five o'clock that afternoon. The customer had initially been told the drawings would be ready Friday morning. It took us two days to incorporate all the changes from that review into the drawing package.

Don't underestimate even the simplest of things.