Up to 25.7 tonnes of diesel oil spilled into the water after additional tests were not carried out on a pressure valve on the Judy Riser Platform in December 2013.
An engineer working on the ConocoPhillips-operated platform had noticed that the wrong valve was possibly fitted to a diesel drain line because it did not operate properly while tests were being carried out in November.
The valve supplier suggested turning the valve 180 degrees as a temporary fix because it would take 18 weeks for the correct part to arrive offshore.
However, the work permit generated for the task did not specify the requirement to test the valve after it was rotated to make sure it was working as intended.
The valve should have stayed closed until facing a high level of pressure but instead remained open resulting in diesel being pumped continuously through the valve and into the hazardous open drains tank.
Rig staff were dealing with an unplanned shutdown of the platform when the incident happened on the rig.
The oil giant pled guilty to releasing oil into the sea from the new Judy Riser Platform contrary to Offshore Petroleum Activities regulations at Aberdeen Sheriff Court yesterday.
Fiscal depute Richard Brown said: “When the volume of fluid reached a high level in the hazardous open drains tank this activated an alarm in the control room.
“The first alarm in the hazardous open drains tank initiated at 23:40 on 16th December 2013.
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“Control room operators who were dealing with the shutdown acknowledged the alarms but did not take any follow up action as they believed that the drains system had not yet been commissioned and therefore was not their responsibility.
“The diesel then overflowed from that tank to the hazardous drains caisson. The caisson then began to fill with diesel eventually overflowing resulting in diesel being released into the North Sea.
“Had the alarms been responded to, a pump in the drains tank or the caisson could have been activated manually and the fluid would have been redirected back into the system.”
The court heard that an oil sheen was spotted on the surface of the sea spanning approximately 50 metres wide and 100 metres long at 9am the following morning.
An investigation was launched and managers later identified the source of the release being the wrong pressure valve on the diesel drain line.
Every oil or chemical spill needs to be reported to the Business, Energy and Industrial Strategy Department of the UK Government.
Officials later confirmed that there would be no significant detrimental impact to the environment because the diesel would over time disperse into the North Sea.
Mr Brown told the court: “On 22nd December 2013, the control room staff were issued with a work instruction. This confirmed that control of the JRP (Judy Riser Platform) hazardous drains system was now the responsibility of the control room and not the commissioning team.
“Prior to this, the control room staff could not have been expected to have taken action in respect of the alarms.”
Morag McLaughlin said she had taken into account the fact that ConocoPhillips had pled guilty at the earliest opportunity and had never before been convicted of a crime.
She fined the company £7,000.