Production is still suspended after the leak, which occured Sept. 12. No people were injured, but the PSA "considers the incident to have had a substantial potential," it said in a statement.
"We have launched an internal investigation of the incident," said BP spokesman Olav Fjellsaa, who confirmed that both oil and gas had leaked from the Ula platform, but said it was too early to estimate how much.
The leak arose in the separator module on Ula's production platform, the PSA said. The platform was automatically shut down, and all personnel was evacuated to Ula's drilling platform. The Ula field consists of three conventional steel platforms for production, drilling and quarters, linked by bridges.
The Ula field produced 317,500 barrels of oil in July, or about 10,200 barrels per day, according to the Norwegian Petroleum Directorate.
Ula is an oil field in the southern part of the North Sea, at a sea depth of 70 meters. The main reservoir is 3,345 meters below the seabed.
The field's peak production was 52.4 million barrels in 1993, or 143,600 barrels per day on average. In 2011, production had fallen to 4.8 million barrels, or 13,200 barrels per day on average.
The oil from the field is transported by pipeline via Ekofisk to Teesside.
Operator BP Norge AS has an 80% stake in the Ula field, and partner Dong E&P Norge AS has a 20% stake."