By Sam Phipps in Edinburgh
Gravity based structures (GBSs) with offshore storage capabilities – of which there are about 40 in the North Sea, including Norway – have been going out of fashion for more than a decade.
Originally favoured over conventional piled steel jacket structures as they could by floated out to installation sites with topsides already in place, they have largely been superseded by pipeline infrastructure – and new technology including subsea engineering and flexible risers.
Scenarios: 'Do nothing'
The task for operators now is to remove them safely, or seek derogation under OSPAR 98/3. That legislation bans disposal of offshore structures at sea but allows for possible exception on a case-by-case basis if they weigh more than 10,000 tonnes, or if removal could cause significant environmental damage.
This is a genuine GBS risk as uprooting is liable to cause several potential difficulties, ranging from ruptured seals to unbalanced buoyancy and sudden uncontrolled release from the seabed, even if the structure was designed with future dismantling in mind.
“We will seek derogation to leave them in place, as will most operators,” Austin Hand, Shell UK’s project director of Brent decommissioning, told Decomworld. He was referring to the three concrete GBSs in that field.
However, Shell is tackling another complex challenge at Brent: how to sample the contents of the giant storage cells before their removal or treatment.
The Brent Delta GBS has 16 of these cells at its base, each about 60 metres high and 20 metres in diameter. One third of each cell’s capacity comprises gravel ballast sealed by a concrete slab but the remainder left storage for up to a million barrels of oil in total.
Oil from the reservoir was originally stored in these cells temporarily before being exported. From the late 1990s, when gas became the main energy tapped in the field, the oil that was still produced contained a much higher proportion of co-produced water. This was separated in the storage cells, and the oil exported to Sullom Voe terminal in Shetland.
The upshot is that each cell contains a substance – part fluid, part sediment – whose precise makeup is not yet known. But it will certainly contain NORM (naturally occurring radioactive materials), requiring onshore treatment.
This must be sampled and treated or disposed of, in a way that satisfies safety, technical, environmental and cost criteria.
Shell was hoping to have completed this sampling by the end of 2011, but now expects to complete it in the first quarter of 2012, Hand said.
Weighing the options
At one time remotely operated vehicles (ROVs) were considered for this task. These were to travel along the existing narrow pipe work into the cell.
But this idea has since been shelved in favour of drilling a hole in the roof of the cell, an exacting job that requires an effective seal between the drilling equipment and the concrete dome. Otherwise seawater risks leaking into the storage cell and possibly flooding the GBS leg.
“We will put a baseplate on top of the cell, and a small drilling stack on, then we will drill through safely with isolation valves on. Then we will deploy a sonar tool to tell us the depth of the sediment, which is very important in terms of long-term storage considerations. Then we will take a sample, recover that and bring it to our labs for assessment,” Hand said.
It could be May 2012 before a sample is ready for examination, as Shell wants to pick a time when a vessel can moor alongside the GBS for 30 consecutive days in decent conditions to give a higher chance of success.
But the delay to original plans for this sampling operation will not have any impact on the overall decommissioning programme for submission to DECC, Hand said.
“It’s been disappointing that we had to shift it to next year but it’s not a schedule driver at all,” he said. “At some point we have to declare what we’re going to do with the contents but they are in the storage cells as part of the GBS and totally sealed.
“We’re removing the topsides first, which means the GBS has nothing done to it in the near term. So it’s definitely not on the critical path in any shape or form.”
In fact, Shell says it has a pretty clear notion of the cells’ contents through years of producing into Sullom Voe, separators and Brent Spa and so on. Modelling also feeds into the picture, and the sampling will merely “calibrate” this.
But stakeholders must be appeased, and that is part of the GBS decommissioning process.
“The public will say: when you’ve got a bucket of that stuff we’ll believe you,” Hand said.
To respond to this article, please write to the Editor: Rikki Stancich