It's possible oil and whaling ship, in the case of GP's at-sea protests against whaling, workers are put in a little danger. I've yet to read a case where they were - you'd be sure Ivo would've gleefully mentioned it. In fact, it's non-violent GP's activists who are in danger - a risk they, ordinary men and women who probably receive some training, willingly take as all activists do - from direct, offensive action by security - water cannons, physical manhandling, etc not counting the dangers of scaling rigs and ships from GP's vessels at sea. In short, the physical danger GP pose to oil, etc workers is minimal, less than working in a dangerous environment on a rig and at sea.
So, is this all that's left of your (and Ivo's) argument? If I were an oil rig worker I'd be annoyed by any disruption of my routine and which might add to the danger of the job (it's probably as safe as can be, though). But really there's little harm to operations, and an important environmental point is made.