The photograph shown is of a tank cleaning vessel. There were at least 3 ex armed trawlers/minesweepers converted to that roll, the other two being Switha and Graemsay. They were run by the Port Auxiliary Service/ Royal Maritime Auxiliary Service. (MoD) Succeeded by purpose built vessels.
I remember her well at Chatham between 1974 to 1977, she was a tank leaner and run on the recovered oil for her boilers. She used to shudder up and the basins, wether this was because of slugs in her fuel or the t/c gear I never found out, her last months before disposal was at Portsmouth when Chatham closed.