Uninterrupted periodic nudge alarm app
- watch only app

Once you set timer, it chimes with intervals of set time. It works even on the background. You can check out the set time on the complications.
You can set number of repeats, the process of chimer times on the complications. when there is no number of repeats set, chimer goes infinitely.

p.s. In case you don't like Chimer app opens on wrist raise, go to watch's settings -> General -> return to clock -> Chimer -> select customize and select always on "while not in session" then it won't pop up on wrist raise. It's unable to be turned off by default by Apple.

p.s. 2 Since this is an Apple watch only app, you can't install it on your iPhone's App store app. Please install this app through App store app on your apple watch. This is how Apple set up for its every watch stand-alone app