I blogged yesterday about the recent Russian seizure of a Greenpeace vessel, which had been planning to stage a protest against a Russian oil rig in the Arctic. According to Russian authorities, Greenpeace activists may have been committing an act of piracy. As I wrote yesterday, whether any actions against an oil rig constitute piracy turns on whether the oil rig is a ship (because Article 101 of the United Nations Convention on the Law of the Sea requires the presence of two ships, an aggressor and a victim vessel). My initial assessment was that in the Russian case, the oil rig was not a ship and Greenpeace activists could not have been committing piracy under international law. I was then kindly reminded by Diane Marie Amann that oil rigs could in fact be ships. After doing a bit of research on this, here is what I found out. Numerous mobile oil rigs exist, such as jack-up rigs and other floating production systems, These systems are essentially mobile platforms, which can be moved either vertically or horizontally across the ocean. In addition, drillships are maritime vessels which have been fitted with a drilling apparatus. Drillships are vessels and had another vessel attacked a drillship, such an act could constitute piracy provided that other requirements of Article 101 were satisfied (that the act took place on the high seas and for private ends). It is questionable whether other types of mobile oil rigs could be considered ships. UNCLOS does not define the term “ship” or “vessel” and I am not aware of any case law on this subject. Comments on this would be much appreciated!