EnQuest has revealed plans to temporarily leave a pair of 1,200-tonne oil storage tanks from its Thistle platform at the bottom of the North Sea after structural concerns forced their removal.
Underwater checks showed the redundant tanks had to be removed “as soon as possible” to maintain the platform’s structural integrity, according to approved decommissioning plans.
Both tanks are 91 metres in height, far greater than Nelson’s Column at 51 metres, and more than 100 workers were evacuated in October as a result of the checks, with the platform since being kept in shut down.
The government has now greenlit EnQuest’s removal plan, which includes leaving the structures on the seabed within 500 metres of the platform, with the aim of bringing them back to shore “sometime in the future”.
The operator has not given a specific date for their recovery, but the most likely window for “onshore waste management” is the two-year period between Q3 of 2021 and Q2 of 2023.
Meanwhile, their physical removal from the platform is due to take place in the second or third quarter of this year.
EnQuest said the tanks will be “flushed” but there is a possibility of some of the contents, such as seawater, residual oil and minor quantities of sediment being released on the seabed.
However, the environmental assessment said any impact from the plans to local sea life would be “minor”.
EnQuest had already been seeking to remove the storage tanks prior to the platform evacuation but the structural checks have forced the timeline to be moved forward.
Decommissioning analyst Simon Axon described the tanks’ position as “precarious” in emails as part of plans to remove the structures.
The plan had been for their removal in September this year, and EnQuest had initially hoped to bring the work forward to January, pending weather conditions, after subsea checks revealed the extent of the problem.
Mr Axxon said: “For structural reasons the tanks are in a precarious condition and so they need to be removed as soon as possible.
“Notwithstanding weather conditions the intention is that will be removed early January 2020.”
Last week Italian firm Saipem revealed its Saipem 7000 crane vessel had been contracted out for decommissioning work for EnQuest’s Thistle installation.
The Thistle platform lies around 125 miles north-east of Shetland in the northern North Sea and EnQuest hopes to restore production once the tank removal is completed.