You can just Google "open marriage success rate". But here are a few that quoted the same study, but I've been unable to find the original research.

https://www.emotionalaffair.org/do-open-marriages-work/

https://www.psychologytoday.com/intl/blog/magnetic-partners/201307/the-slippery-slope-open-marriage

https://katiecouric.com/lifestyle/relationships/what-is-an-open-relationship/#:~:text=If%20you've%20considered%20expanding,an%208%20percent%20success%20rate.

It hasn't been studied very much, since it appears only maybe 1% of the population is living this way. In addition it is usually young people (20s and 30s) so it seems to fizzle out as people mature.