The Northumbria was certainly not a great testament to British shipbuilding. I remember her at Fawley when I was Shift Super there in 81/82 - she had a metre of solidified crude in 2 wings ballast tanks, permanently in place, ensuring that those tanks could never be completely filled or emptied.
Of course, as Chris Allport rightly said, the stresses weren't completely understood at the time, but there has to be an element of penny-pinching when the 2 x Esso and 2 x Texaco were ordered - they needed scrapping relatively quickly, but another of the series, layed up after building as Tyne Pride but eventually renamed Thermidor, was a fine ship which lasted much longer than the Northumbria. I inspected this ship, and it was hard to believe it had come from the same yard as the Northumbria - suggesting to me that you get what you pay for.
I sailed on the Scotia (bit of a wreck - lots of holes in the stripping system for Neville Humble and me to fix); the Demetia (beautiful ship); and the Caledonia (too dreadful for words)!!