What was the North Sea Empire?

During the Viking Age, Norsemen settled throughout England and became rulers, forming the North Sea Empire, a personal union of Denmark, England, and Norway, for about 30 years, starting in 1013 CE under the reign of Danish King Sweyn Forkbeard. This empire was unique in medieval European history because it was a thalassocracy, a maritime empire, where the vast North Sea was the only way of connecting these three kingdoms.