What is the longest pipe you have ever built?

Question: What is the longest pipe you have ever built?
- Keywords:
  built,
  longest,
  pipe
Asked by Ratman and Bobbin to Neil on 17 Mar 2015.
- Neil Taylor answered on 17 Mar 2015:
  
  Hi!
  
  Hmm, I can’t remember the longest one that I designed – I think it was around 25 km long. The longest pipe I have analysed is over 50 km long though!
  
  That’s a lot of pipe! 😀

Question: What is the longest pipe you have ever built?