One involves turning the 450ft tall structure on its side in deep water, moving it on to a barge, then bringing it to land - so-called horizontal dismantling. The other, vertical dismantling, involves towing the 14,500- tonne Spar to a sheltered, deep-water site, then gradually cutting off section after section while it is floating upright in the water. After each cut, the Spar would be raised to keep the top of what remained above the sea surface.
While it was building up its case for deep-sea disposal at below 7,000 feet, Shell argued that if it had to land the structure, then horizontal decommissioning would be best.
Once upended, the bouy would be towed to a sheltered site, such as Scapa Flow in Orkney, where it would then be lifted on to a barge before being taken to land and broken up.
The oil company ruled out vertical dismantling because it could find no sheltered water along the UK coastline with sufficient depth (about 350 feet). There are other deep water sites in Europe, but Shell UK judged it would not get the necessary permissions from any foreign government to dismantle the Spar and take out its toxic contaminants.
The likely offer of an achorage in a Norwegian fjord, where the battered and weakened Spar can be sheltered from storms, may change that. If the Spar can be towed back across the North Sea to Norway, then Shell may try to persuade the Norwegian authorities to allow at least some dismantling in the vertical position. The most hazardous material would probably then be ferried back to Britain for final disposal.
The costs of both options is put at about pounds 45m by Shell. Both will involve extensive repairs by divers before the dismantling begins. This is necessary to repair two of the six giant oil storage tanks, ruptured by a build-up of pressure 18 years ago.
The Spar was also overstressed at the end of its construction and has to be handled gingerly to avoid the dangers of it breaking up, releasing all contaminated contents into the sea.
Once onshore, the problems are far from over. The Spar contains mildly radioactive salts coating its tanks and pipework, asbestos in the accommodation deck and 100-tonnes of oily, silty sludge and much more sea water with high concentrations of toxic metals such as zinc, cadmium and copper. Disposing of any of these requires approval from government bodies.
The Spar could be towed into the oil-field fabrication yards at Nigg on the Cromarty Firth or Ardersier on the Moray Firth, both on Scotland's east coast where it could be dismantled. That worries some Scottish fishermen.Reuse content