Shell submitted plans in 2017 to leave the giant legs of three of its four Brent platforms and some other infrastructure in the North Sea.
Concerns have been raised, with formal objections by Germany and the Netherlands, around the contents of 64 oil storage cells – concrete cases within those legs – being eventually released into the environment.
Shell’s preferred option is to leave the cells in place, with the cost of removing them for onshore treatment “disproportionate to any environmental legacy benefit”.
It added that the outer structure of the cells are expected to last “hundreds of years”.
The oil itself is not free floating but bound to sediment comprised of sand, grit and water, which Shell said is encased within one metre thick reinforced concrete structures which will take “centuries” to degrade.
Shell said, as the structures do eventually degrade, the oil will be exposed to the water gradually and broken down by natural organic process.
Brent, which started production in 1976, is the only UK field with these concrete structures, Shell said.
However, an independent report published last week which had been commissioned by the Dutch government stated the “timing and rate” of the contents being released into the environment is “highly uncertain”.
It said there is uncertainty around the future state of local ecosystems around the Brent field, “which could be more vulnerable”, in which case any release may have larger impacts than what Shell assesses.
In terms of the cells’ contents, Shell has said there was a “higher hydrocarbon content than expected” within the water of one of the cells it tested, but subsequent tests of other water samples were below its expectations.
Therefore it is “continuing to investigate”.
The oil major added there are “no significant amounts of non-biodegradable compounds” but there are traces of naturally occurring radioactive materials and heavy metals within the cells.
However, in June the German government registered its formal objection to Shell’s plans after an Aberdeen-based consultancy – Scientia et Sagacitas – produced a report highlighting “major issues” with Shell’s assessments.
Part of the report stated there was “inadequate poor-quality data” relating to the contents of the storage cells.
It also described Shell’s assessment process as “flawed” with a bias to minimal decommissioning work.
Shell has long argued that its plans are safe and “environmentally sound”, being the result of more than 300 scientific and technical studies.
A special meeting of Ospar, the intergovernmental agreement of several European countries to protect the marine environment, takes place on Friday to discuss the plans.
The decision to allow infrastructure to remain in place lies with the UK’s department for Business, Energy and Industrial Strategy (BEIS), but they need to consult with Ospar to do so.
Professor John Paterson, of Aberdeen University’s law school, has previously said this allows other countries to require “intense discussions” through Ospar, but the net effect is they cannot veto what the UK ultimately decides to do.