someone wrote:
"However, to support your position, I know of no offshore operation flown under Part 91, they are all Part 135, I believe."
In the U.S., Chevron USA operates under part 91 when flying their own people (although they do have a 135 certificate so they can use their aircraft for charters). There are other, smaller oil companies who own their own ships and operate them offshore under part 91 (day and night!), as well as some pipeline companies who do the same.