For nearly 40 years, these massive concrete cells were used to store oil before it was exported to shore. When the platform ceased production, some oily sediment remained trapped at the base of the cells. The exact volume and composition of that sediment had not, however, been substantiated.
Duncan Manning is Business Opportunity Manager for Brent Decommissioning: “We had a good idea of what was left in the Brent Delta storage cells, thanks to our historical records and data modelling. But in order to check our assumptions about the quantity and composition, we wanted to obtain real samples.
“This desire was shared by our stakeholders who, during engagement sessions, told us they would have more confidence in our decommissioning recommendations if they were based on real samples.”
Accessing the cells and obtaining these samples however, presented a huge technical challenge. Duncan said: “This was not a simple matter of taking off the lids and sucking out some sediment. For a start the cells are located 180 kilometres offshore and the cell tops are 80 metres below the surface of the sea. Their original internal access points are old and complex and their concrete walls are almost a metre thick. And there are only a few weeks in the year when the weather is stable enough to attempt this kind of operation.”
Perseverance and meticulous planning
Work started on the sampling project in 2008. After numerous engagements with industry suppliers, it became clear that sampling operations of the kind we would need to deliver had never been successfully conducted before.
Over the next few years, thousands of man hours were devoted to the sampling task and multiple concepts examined and tested. Two ideas progressed beyond the drawing board to become offshore projects in 2008 and 2012, but eventually both had to be abandoned due to feasibility challenges.
Thoughts also turned to companies outside the oil and gas sector. John Gillies, Brent Decommissioning Execution Manager, was in charge of the sampling project: “One of the companies we worked with was the space agency NASA who helped us with a sonar mapping project to measure the volume of sediment. Their expertise in miniaturisation and navigation enabled the first successful access into one of the storage cells. But in the end, the waxy material near the top of the cell prevented the sonar from generating the image necessary for measurement.”
In 2014 a fourth attempt to obtain samples was made offshore. Crucially this concept was based on creating a new subsea access point in the top of the cell, and on integrating five different technologies which had been tested extensively on shore.
Three storage cells were chosen for sampling that could be accessed using the topside crane.
Working at 80 metres below the surface of the sea, divers first installed base plates on each of the three cells and bolted them onto the concrete surface. Phase two was carried out from the Brent Delta topside using the platform cranes, supported by a platform-based, remotely-operated underwater vehicle (ROV). A drilling tool was attached to the base plate so it could core into the 0.9m thick concrete tank tops and extract the samples.
Finally in August 2014, during a period of calm weather, we were able to collect between one and three kilogrammes of sediment from each cell – as well as water samples. A 3D sonar device was also successfully launched in each to measure the sediment volume and its surface topography.
John said: “In the end perseverance, good engineering and meticulous planning paid off, but it was a long, costly and technically difficult journey. Integrating multiple technologies and managing teams from multiple contractors was a real challenge. The use of the platform-based ROV was a game-changer in terms of cost, saving us hundreds of thousands of pounds a day because we didn’t need a diving operations vessel. I’m proud to say the whole job was completed safely as well.”
In order to give assurance that the samples were genuine and collected in controlled conditions, the offshore operation was witnessed throughout by independent observers. Each sampling canister was temperature controlled, sealed and signed to ensure it could not be tampered with or compromised before onshore analysis.
Once back onshore in Aberdeen, a specialist independent laboratory carried out detailed chemical and physical analyses of the samples. The results are now being used in the various studies needed to compare different decommissioning options, such as the Environmental Impact Assessment.
Duncan Manning said: “Obtaining samples from the Brent Delta storage cells has been a monumental challenge. It’s taken years of work, millions of pounds and a variety of specially adapted and bespoke technologies. But the results from the sample analysis, together with data from the modelling, will be invaluable in helping us move forward. They will give us – and our stakeholders – a new degree of confidence that we know exactly what we are dealing with.”