Find the cheapest pipe network with positive pressure

You must find the lowest cost network upgrade that ensures positive pressure for all possible demand cases. Some locations need new pipe. Some locations have existing pipe where you can (i) do nothing (ii) clean existing pipe or (iii) place an additional duplicate pipe. Pipe sizes given in millimetres. Larger pipes give better flow, but are more expensive. Of the existing pipes, some are more deteriorated. The two pipes from the reservoir are much longer (i.e. more expensive). Details of pipe roughnesses, lengths, elevations and pressure baselines omitted from plot.
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