The time element in Operational Research (2)

In part 1 of this item, I reflected on problems where there are repetitive decisions at comparatively short time intervals; replacing vehicles is something which happens quite frequently for fleet managers, and production lines need to have regular maintenance.
What happens, though, where there are much longer time gaps between the need to do something?  O.R. can help suggest the frequency of action, but I suspect that the problem is implementation, rather than of the O.R. approach.
At one time, I lectured on the methodology of O.R. to undergraduates, and on conflicting objectives.  The example that I used was one that I had picked up from Devon County Council.  How often should a farmer trim the hedges between their fields and minor roads?  The hedges grow steadily all year round.  You can't cut the hedges at the time when birds are nesting (constraint).  Each cut costs money, so the farmer doesn't want to cut too often.  Equally, if you cut very rarely, then the hedge gets out of control, and needs more attention than simply cutting with a mechanical tool.  There are environmental reasons for not cutting every year.  So there are objectives of cost, aesthetics, environment to consider.  So the decision comes down to a decision of annual cuts, every second year cuts, every third/fourth year cuts.  And it doesn't really matter whether you do it precisely at intervals of 365 days.  The county council's advice is every third year, with some hedges on busier roads being trimmed annually.
That was a decision that could be implemented reasonably easily.
Not so easy to implement was a problem which affects many councils, organisations and individuals.  How often should you give a fresh treatment of wood preservative to items of exterior woodwork?   In my visits to several churches around the county, I found that hardwood gates and some other woodwork were drying out through lack of treatment.  As we have walked on some popular footpaths, Tina and I have noted that seats and benches beside the path are prone to decay through lack of attention.  From our experience with the old gate of our garden, and the wooden furniture on the patio, we know that these items need treatment from time to time -- but because they are in the vicinity of the house, it is easy to monitor them and decide when to do the work, usually every two to four years.  We also have some wooden trellis, supporting climbing plants, which we have decided is not worth treating again, because its cost is small compared with the time it will take to move the plants away from the wall.  Maybe the owners of the gates, seats, benches have decided the same?  And if they have decided to give treatment how does the policy get implemented?  Somebody has to inspect the items, and then arrange the repair and treatment into a worker's schedule.  Because this is a "one-off" item, it disrupts that person's schedule.  So, as it is hard to plan the inspection and treatment, the job doesn't get done, because of the implementation problem.  It requires record keeping, and a job description which lists the whole set of items that need maintenance.
And what prompted my thoughts about this topic at the moment?  A matter of industrial archaeology!  I have been reading about the medieval tin-miners on Dartmoor.  Apparently, it was the practice to smelt the ore in stone buildings on the moor, with stone chimneys and thatched roofs.  These days, it is suggested that thatched roofs be rethatched every 20 years or so.  Owners of thatched houses budget for this.  But on the moor, the weather was so hostile, and the thatching of the buildings was "rough and ready".  So the thatch was burnt when it had worn out, by which time the building needed repairs to the walls as well.  This was a rather drastic form of maintenance, and would seldom be adopted these days.  However, there was a further purpose in the burning of the thatch, as it allowed tin ore to be retrieved from the thatch.  Hence the maintenance cycle was:
Build - about twenty years of decay - burn thatch and rebuild - about twenty years of decay - burn thatch and rebuild -etc
Now the interesting aspect of this is how people learnt the process of reclaiming the ore from the burnt thatch.  It could only happen a few times in the lifetime of any miner or mine manager.  Perhaps rival managers were prepared to share their expertise?
And is this the longest maintenance cycle around?


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