Many thanks for the background - very interesting.
The only one I've seen in anger so far has bene the line pipe spec on a Shell project. The latest Shell line pipe DEP basically has two lines on additional items from the IOGP spec, but pages on the DNV F101 spec for the pipe material part.
So at least for Shell then they don't add anything more. Now other adding further things to the IOGP spec makes it lose the purpose of doing it IMHO but each specialist / SME will have their onw particualr little hobby horse they want added which isn't either in the base spec or the IOGP one.
The aim as said would appear to be to give vendors a single additional spec to meet rather than one for Shell, one for BP, one for Exxon, one for ....
Time will tell if they succeed or not and how much more these extra items will cost compared to a minimalist / cut down spec based on the ISO / API standard.
I must admit I thought that the latest API5L pipe spec also tried to do the same by giving you so many additional optional items (annexes) in the spec to add to the "base" API 5L document.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
Precisely - a lot of the supplementary standards are just stipulating the specific options to apply, or not to apply. Having been involved in the production of the first ISO 3183, the only way that we could move forward in a way that satisfied the majority, and reach a vote, was to leave in every option under the sun to keep everyone happy. Shambles.