The watchdog accused BP of poor maintenance and "serious breaches of regulations". The criticism is another blot on the company's safety record and is the second such censure in the last two years.
Norway's Petroleum Safety Authority said BP's maintenance systems were deficient, and demanded a review of the company's management of the platform at the Ula field "with a view to assessing whether it is adequate for identifying and managing risk". The UK company has also been asked to explain why its maintenance was so inadequate that a "substantial" leak occurred last September.
An estimated 125 barrels, or 20 cubic metres, of oil flowed out of the leak, along with 1,600kg of gas, from the installation on the Ula field at the southern end of the Norwegian continental shelf.
Production was halted for 67 days as a result of the leak, from a field that is estimated to hold nearly 100m barrels of recoverable oil.
While no one was injured, "the incident had the potential to become a major accident, with the risk that a number of lives might have been lost and substantial material damage caused," according to the PSA's report.
The incident followed a fire in July 2011 at a platform on the Valhall field, also in the North Sea, which forced a shutdown of more than two months and was also investigated by Norway's PSA. No one was seriously injured in that incident either, but the regulator warned that the fire could easily have spread and caused fatalities.
Both incidents were an embarrassment to BP, which has been under fire for its safety breaches, notably the Deepwater Horizon explosion in 2010, which killed 11 workers and led to the worst offshore spill in US history, and the Texas City refinery disaster in 2005, in which 15 workers were killed.
The PSA's investigation into the Ula leak, the results of which were made public on Monday, identified "a number of serious breaches of the regulations, related in part to BP's management system for activities" in Norway's North Sea oil and gas fields. The PSA issued BP with an order to review its systems.
After the 2011 fire on the Valhall installation, BP was also served with similar orders to review and assess its "maintenance management on ageing installations and ensuring that maintenance programmes and the execution of such work were tailored to the age and condition of the installations and equipment".
But even after those reviews, from its investigation of the Ula incident, the PSA has concluded that "deficiencies still exist in the maintenance system".
The PSA said an order of the type issued to BP was "a powerful preventive measure and legally binding on the recipient".
BP said: "[We have] received the PSA's investigation report following the hydrocarbon leakage on Ula last year. The findings closely match BP's own investigation and work has already started to address the issues raised."
The oil and gas leak at Ula was caused by "fracturing of the bolts holding together a valve in a separator outlet", the PSA found. Seepage from the valve exposed the bolts to water with a high content of chlorides at a temperature of about 120C, resulting in stress corrosion cracking, which weakened the bolts to the extent that in the end they fractured.
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