It accounted for around ten per cent of the oil and gas production from the North Sea at the time.
At the peak production of Piper Alpha, it produced 3.86 million tonnes per year of crude (77.2 MBPD)
http://163.164.19.97/information/bb_updates/appendices/fields/piper.htm
http://www.eppo.go.th/ref/UNIT-OIL.html
In 1988, when Piper burned, the North Sea produced 3,495 MBPD. Piper was producing 2%, not 10% of the North Sea. When the article starts out that way, everything else is not believeable.