The platform that could not miss its window

The Troll gas field sits approximately 80 kilometres northwest of Bergen, in the Norwegian North Sea. Its reservoir lies beneath 303 metres of water. No fixed steel jacket could reach the seabed there — the water was too deep, the geometry of a steel structure at that depth too unstable. The solution was concrete: a gravity base structure, anchored to the seabed by its own weight, its massive skirts penetrating the seafloor and holding it in place against the full force of the northern North Sea.

What was designed was not large by accident. It was large by necessity. The concrete shaft had to span 303 metres of open water and still extend above the surface to mate with a topsides. The structure that resulted — the Troll A GBS — stands 472 metres tall in total. Its dry weight is approximately 656,000 tonnes — the tallest structure ever moved by human hands. Some 245,000 cubic metres of concrete in the GBS, with cells at the base capable of ballasting the platform to its final draft.

The GBS was built at Hinnavågen and Vats, Norway. The topsides at Aker Stord. In January 1995, both units were mated in the deep-water fjord at Vats — the GBS partially submerged, the topsides floated in and set on top. What would make the tow was not two components. It was one integrated platform.

On 10 May 1995, ten tugs took hold of the complete Troll A platform and began the passage north. The structure rode with the majority of its height below the waterline — 245 metres visible above the surface. Speed when underway: approximately 1 knot. The complete passage — including the weather hold — would take seven days, within a carefully defined summer weather window: the narrow band of months when wave heights and wind allow a 656,000-tonne structure to be moved safely, positioned, and ballasted to the seafloor.

On 16 May, the convoy stopped. Winds reached 40 knots. Waves reached 5 metres. Ten tugs — all on charter, all on rate — held the platform in position. It could not be parked. It could not return to Vats. The window was not paused.

The window was not a schedule item. It was the contract.

This is not a towing problem. It is a commercial exposure that no weather clause can fully contain — because the charter clocks of ten vessels do not stop when the sea does not cooperate.

The platform arrived on location on 17 May 1995, approximately one week ahead of schedule. It was ballasted down — seawater admitted through controlled valves until the skirts penetrated the seabed and the structure settled, irreversibly, by approximately 19 May. First commercial gas followed on 1 October 1996. The Troll field holds approximately 40% of Norwegian Continental Shelf gas reserves.

The tow completed ahead of schedule because it started on time and was prepared adequately — and because the 16 May weather hold resolved in hours rather than days. But the programme did not know any of that on the morning of 16 May.

The decisions that determined whether the Troll A tow met its weather window were made onshore, months before departure — in construction programme reviews, in weather-window modelling, in the tow preparedness sign-offs. The people making those decisions were not the same people who would hold the platform in 5-metre seas with ten charter clocks running and the summer window somewhere ahead.