Shell has a Brent dilemma again; how to decommission the concrete structures left behind when the topsides are removed. (Last time was when the Brent Spar became a major issue):
Leaving the concrete structures, with the oil and chemicals in place, sounds to me like a logical solution. To re-float them would be very difficult, if not impossible after 40 years in place. Cleaning and disposing of the contaminants in place would also be difficult. Sealed off these structures could last “forever”.
If they were somehow re-floated, what do you do with them? To break them up would be an equally difficult, if not impossible task, even if done in a deep Norwegian fjord, slowly reducing their draft until the bottom slabs and skirts could be put into a dirt dock for final cutting. (Reversing the building sequence)
To recycle the concrete and reinforcement steel would be difficult and costly.