If you're on the east coast of England, in the North Sea (English coastal shipping mission), the requirement is 50,000 tons... so you've more than met your requirement. Go back to base (either Kiel or Wilhelmshaven... whichever one is displaying the little submarine icon on the map) and click the anchor symbol that pops up in your screen upon entering the harbor. Choose "end patrol." That should help you out... hopefully!! Good hunting, Herr Kaluen!!
