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The wash water with chemicals and scale was being stored aboard the Oseberg C platform and was inadvertently introduced to the oil flow in May 2011.
Wash water is defined as hazardous waste and Statoil admits it has not been handled in accordance with the regulations by the platform.
The wash water was sent to through the oil export pipeline to the Sture terminal in Hordaland, which did not have permission to receive or handle such waste.
After the incident, a number of measures were implemented to prevent a recurrence.
New procedures for reporting the content of fluid sent to the terminal through the oil export system have also been introduced.
"This was a highly regrettable incident that should not have occurred," Oseberg operations head Terje Gunnar Hauge said.
"We fortunately have no indications that this has resulted in local pollution at Sture."
