Cable sheath induced voltages vs. pipeline induced voltages

[Power0020]

A two point bonded sheath will have voltages to ground with maximum in the middle of the cable and almost zero towards the ends. circulating current will appear due to the closed loop.

Pipelines have a different story, the induced voltage shows a V-curve, with minimum stress to ground around the middle of the span and maximum towards ends!

I can't figure out the reason for these two different shapes, as it looks very similar mechanism of action with voltages building up due to induction.

any clue?

 

[7anoter4]

See EPRI EL-904/September 1978 Mutual Design Consideration for Overhead AC Transmission Lines and Gas Transmission Pipe.
Appendix C
Appendix G