What do you mean by "low"

Low compared to the max / design flow rate or 100 bbls/day?

Low velocity in a pipeline can lead to laminar flow, water drop out and inefficient use of resources.

You're going to need to expand quite a lot to get some more useful advice.

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at the end life of oil well the production rate decreased and the transported treated crude oil also decreased , for example may be pipeline with 300 km length designed to transfer 200,000 bbl/day of oil but what will be happen if this production dropped to 30,000 bbl/day instead of 200,000 bbl/day

A few things can happen.

Your velocity goes down and your pump costs and pressure required go down considerably
However you can get water drop out or if you're in a hilly location you can operate in slack flow mode.
Waxing up can also become an issue as temperature falls a lot faster than before.

Sometimes it falls to such a level that you can operate on the mainline booster pumps alone...

You build up storage upstream and operate in a batch operation at a higher flowrate for as long as possible then shut down to build up stocks.

You find some more oil

You tap into the line and transport someone elses oil

You connect to someone elses nearby facility and shut the pipeline down

Lots of possibilities.

Was there any specific you're thinking of?

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Also: If you get a response it's polite to respond to it.

i think in build up storage upstream and operate in a batch operation but the problem will raise at start up and cooling down of oil in the pipeline
also i think in mix the oil with hot water before transportation to overcome the barfine deposition issue and high pour point of crude , but the problem will be the treating and disposal of this water after that