Friday, 4 April 2014

The Essential Tension in Plant Design and The Modified Scotty Principle

Things are moving on with the design projects we have on the books, and we are close to pilot plant construction in both cases. With novel designs like these I use pilot plants as both prototypes, and a source of detailed, plant-specific data to allow the final design to proceed.

There is an tension between external design consultants and product managers in client organisations, explored in this paper, but even when designer and product manager are within the same organisation, there will be a tension. Designers are usually risk-averse, and management are usually risk-tolerant (I'd give you a link to a TCE article on this a while back, but they hide all of their articles behind a paywall)

In essence, management usually want to get a product to market as soon as possible, and with the absolute minimum possible margins of safety, and highest possible profit margin. Designers don't want to design things which don't work, they have a good idea of the limits of their analytical techniques, and they know that all design relies on approximation and heuristics.

When non-designers look at our calculations and drawings, it all looks very mathematical, very sharp-edged, but these are precise calculations of approximate values, and those straight lines on the drawing might be different in ten thousand ways.

The use of computer modelling and simulation programmes as design tools can cause many to be lulled into thinking that such slick-looking output must carry some weight. It can seem as if engineering has become such a straightforward application of scientific principles and mathematical modelling that computers can do it for us. It isn't, and the IChemE's guidance on use of computers has not been reversed.

I hear that management are now asking engineers to justify adding any margins of safety to designs done substantially using modelling programmes, but that engineers are then being asked to carry out de-bottlenecking of plants which have been designed in this way.

So there will always be a tension between engineers and management. The Scotty Principle addresses this tension with respect to timescales:

Scotty Principle(n.) The defacto gold star standard for delivering products and/or services within a projected timeframe. Derived from the original Star Trek series wherein Lt. Cmdr. Montgomery 'Scotty' Scott consistently made the seemingly impossible happen just in time to save the crew of the Enterprise from disaster.

The premise is simple:

1) Calculate average required time for completion of given task.

2) Depending on importance of task, add 25-50% additional time to original estimate.

3) Report and commit to inflated time estimate with superiors, clients, etc.

4) Under optimal conditions the task is completed closer to the original time estimate vs. the inflated delivery time expected by those waiting.

(Urban Dictionary)

Margins of safety are not just about gaining a Scotty-style reputation as a miracle worker, however. When things are uncertain, designers should err on the side of caution, "underpromise and overdeliver". Occasionally you will lose out to those who are willing to gamble, but winning in that way makes them lucky, not good. Let's not trust to luck.

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