I did some checking the River-Class were built with a traditional steel hull to a design based on a commercial offshore support vessel. The class was designed to be operated as deep sea team sweepers, to combat the threat posed to submarines by Soviet deep-water buoyant moored mines codenamed "Cluster Bay".
http://en.wikipedia.org/wiki/River_class_minesweeper
Here is picture from wikipedia of the HMS Orwell in the Bay of Biscay en route to Gibraltar, 1990
