You can use paths:

and just do Edit > Stroke path
in Line mode

For a slightly more realistic look, you can use a brush.
Create an image with
- A crescent on transparent background (about the size of your wider pipes or larger, here 50x50px)
- Filled with a bi-linear gradient between a color and a slightly darker version of that color
- You can make a circle selection, fill it, and then delete a half circle, or make a crescent selection (select circle and then subtract half-circle)


Then use Edit > Stroke path
in Paint tool mode
and select the Paintbrush and Emulate paint dynamics
.

You can save your brush image as a plain Gimp brush for later use if interested, and it will be added to the brushes list.
For the record there is also a script that can draw a bilinear gradient along a path a gives a cleaner result:

Ask in comments if interested.
To use the plugin:
- Create a path...
- Set the current gradient to something light to dark, for instance, I created the gradient ❶ using the
FG to BG (RGB)
gradient, and then setting the FG ❷ and BG colors ❸.

- Call the script form the Paths list, right click you path and
Decorate > Gradient long path
towards the bottom

Then mostly set the width in the dialog:
