I am doing some research for a paper on gas treatment and would like to know an estimate of the cost of a planned shutdown in a gas treatment plants. I don't need precise number but some "rule of thumb" estimates would be very helpful. 
Taking an average size plant of around 150 MMSCF/Day that has de-sulpherization, de-humidification, mercury removal and condensate removal. 
1/How often does a planned shutdown occur?
2/On average how long is the plant out of action including shutdown down and start-up?
3/What are the normal activities performed at a shutdown?
4/How many man days are involved including: 
  a/ planning,  
  b/ hazops, 
  c/ scope of work,  
  d/ method statements, 
  e/ risk assessments, 
  f/ permits
  g/ anything i forgot?
  h/ performance of tasks for shutdown maintenance and startup