Troll A – giant move from fjord to field

If there is one occasion when you are going to check the weather forecast with particular care, it is when you are responsible for moving a structure almost 500 metres tall and costing billions of kroner out to the open sea. Everything was ready on 10 May 1995 for a historic towing job. Troll A would be leaving its sheltered life in the Yrkes Fjord.

By Ole Jone Eide, Norwegian Petroleum Museum

- Troll A is towed out to the field. Photo: Jan Ove Moen/Equinor

This platform comprised two main components – the concrete gravity base structure (GBS) and the topsides it supported. Construction of the GBS was launched by Norwegian Contractors in the summer of 1991 at Hinnavågen in Stavanger. Aker Stord further north was responsible for the topsides. Both units were towed to Vats on the Yrkes Fjord north of Stavanger, where the GBS was completed and mated with the topsides.

According to the original plans, 10 tugs would start moving the platform out to the field on 2 May 1995. But the GBS for Sleipner A had sunk in the Gands Fjord off Stavanger only a few years earlier. That boosted fears of an accident to Troll A while it was inshore, and meant such thorough precautions were taken to safeguard the structure that preparations for the tow fell somewhat behind schedule. The meter ran fast, and this delay is estimated to have cost NOK 2 million per day.[REMOVE]Fotnote: Bøe, Arnt Even, “Forsinket slep”, 26 April 1995, Stavanger Aftenblad: 10.

The tow finally began on the afternoon of 10 May. Eight of the tugs pulled the structure forward, while the other two helped to steer it from behind.[REMOVE]Fotnote: Jupskås, Stein Halvor and Risholm, Toril, “Temmet Troll på ferd ut fjorden”, 11 May 1995, Stavanger Aftenblad: 5. A little over 245 metres of the platform’s 472-metre height were visible, with 225 metres below sea level. A speed of one-two knots (about 1.8-3.6 kilometres per hour)[REMOVE]Fotnote: Storhaug, Eldrid Espedal, “Hektisk gigantslep en uke etter planen”, 10 May 1995, Stavanger Aftenblad: 8. was maintained through the main Bokna Fjord.

Eighty journalists from nine countries witnessed Troll A’s progress towards the open sea.[REMOVE]Fotnote: Bøe, Arnt Even, “Troll går nordover: Rogalending og europeer”, 12 May 1995, Stavanger Aftenblad: 6. By 15 May, the tow was 20 hours ahead of schedule and work could begin on ballasting the platform down by just over 50 metres, putting 278 metres below the waves.[REMOVE]Fotnote: Stavanger Aftenblad, 15 May 1995, “Troll-slep foran skjema”: 39.

The weather deteriorated on the following day, with winds of 40 knots and waves five metres high, and further progress was postponed. More tugs were positioned against the wind. Wave heights and wind strength had to be no more than two metres and 20 knots respectively before continuing towards the final goal on the field.[REMOVE]Fotnote: Stavanger Aftenblad, 16 May 1995, “Troll ligger værfast”: 25.

On 17 May, Troll A was lowered to the seabed in 303 metres of water. A few days later, the platform had been descended to its final position with the skirts below its base penetrating 36 metres into the soil. No further mooring was required.[REMOVE]Fotnote: Stavanger Aftenblad, 19 May 1995, “Trollet setter seg”: 27.

A lengthy process of readying the platform now remained before it could begin delivering gas to continental Europe.[REMOVE]Fotnote: Troll A is supplied with electricity from shore. The gas it produces travels by pipeline to the process plant at Kollsnes north-west of Bergen.