This is not necessarily true, and all structures must be removed (though the timeframe is flexible). For some platforms you're absolutely correct; those with g

This is not necessarily true, and all structures must be removed (though the timeframe is flexible). For some platforms you're absolutely correct; those with gravity-base foundations whose foundations were used for hydrocarbon storage are a technical and environmental nightmare. Unfortunately it's damned if you do, damned it you don't; removing them completely risks catastrophic spillage, leaving it in place also risks damage in decades to come. It was regretfully not well-considered in the wild west days of offshore drilling when many of these concrete-base platforms were installed. Many (most, but I haven't checked the numbers) other platforms are supported by steel towers, and these did not use their foundations for any sort of chemical storage. By law (in the North Sea at least) when these are removed they must be fully removed. Challenging, but not a considerable environmental risk.