Moscow steps up maritime presence in North Sea after UK threats to seize shadow fleet oil tankers Moscow steps up maritime presence in North Sea after UK threats to seize shadow fleet oil tankers Britain’s Royal Navy tracked and followed a Russian frigate every day last month as it sailed from the Atlantic to the North Sea, as Moscow steps up its maritime presence after UK threats to seize shadow fleet oil tankers . The Russian navy’s Admiral Grigorovich escorted six Russia-linked vessels