I believe under international marine law, an offshore rig is an installation when exploring and only is considered a ship when moving from one location. Piracy laws only apply when the vessel is acting as a ship. However, I believe that force can be applied in international waters when it is "reasonable" to believe that actions may be taken by another vessel that threatens the safety or security of the structure or crew. I suspect that it is possible that the actions of Greenpeace could be considered in that league, as in other previous protests, they have broached the security of similar structures.

Below is a section from a oil rig security site I found- but lost the site to post as a source:

""Civil protest
Civil protest also poses a security threat to offshore oil and gas installations. Interferences with offshore operations can be caused by non-violent environmental activists, indigenous activists, labor activists, striking workers, and anti-government protesters. There have been at least ten security incidents where operations of offshore oil and gas installations were affected by the actions of protesters and activists. Greenpeace activists have caused interferences with operations of offshore installations on several occasions including an attempt to board an oil rig about 170 nautical miles off the coast of Massachusetts in the United States on 25 July 1981, the unauthorized boarding and occupation of Shell's Brent Spar floating offshore oil storage facility in the North Sea on 30 April 1995, an unauthorized boarding of the Stena Don offshore drilling rig off the coast of Greenland on 31 August 2010, interference with operations of the Stena Carron drillship in the waters off the Shetland Islands north-east of mainland Britain in the UK on 21 September 2010, and the unauthorized boarding of Leiv Eiriksson offshore drilling rig in Turkish waters. There have been other offshore security incidents caused by civil protest such as an unauthorized boarding of Parabe offshore oil production platform by protesters in Nigeria on 25 May 1998, and the seizure of offshore installations by striking oil workers off the coast of Nigeria in April 2003.""


Did Greenpeace not notice that Russia invaded another country a few years ago (Georgia) - which means thay haven't tended to "dither" when annoyed or "taken on".