Tow-out of Statfjord B
When the final chain was cut at 03.00, eight tugs with a combined bollard pull of 107 000 horsepower were waiting to tow the structure for 422 kilometres down the fjord and out into the open sea. Five of these vessels were attached at the forward end to pull, with three aft to maintain control.
The wind, which had been the only uncertain factor, stayed light until the tow was out into the North Sea. A southerly gale then blew up, but its only effect was to push the platform’s speed well above the planned level.
Operator Mobil had allocated responsibility for Statfjord B to Norwegian Contractors before the tow started, and the latter had contingency plans for all possible and impossible emergencies. It hired Norwegian survey company Bloms Oppmåling to investigate bottom conditions in a kilometre-wide band along the whole route. All obstacles were mapped and the seabed cleared at the destination.
All this had been done well in advance, but everything was rechecked just before the tow was due to start. The critical sections were the inshore waters and the last stage of the voyage from the deep Norwegian Trench to shallower waters. Over the final 10-15 kilometres, clearance between the platform base and the seabed was a mere 15 metres.
Most of the voyage over the open sea passed along the Trench, where the water depth was a great help. The platform was towed semi-submerged, with its base 130 metres below sea level. While in the Trench, it could be submerged even further if wind or waves made that necessary.
The tow was under the command of Captain Ronald Seim from NC, and involved a crew of 53 people on the platform. These included personnel from Mobil and NC as well as catering staff and various consultants.
Vital functions such as electricity, water, safety installations and communication had to be kept operating around the clock. Meteorologists and navigation specialists were also represented.
No problems arose during the operation. When the platform reached the open sea, the three aft tugs dropped the towing lines. However, two of these and other standby ships accompanied the structure all the way to its destination in case stabilisation was required.
The work of ballasting the gravity base structure (GBS) began during the final stage of the tow, and the colossus was only a few metres above the seabed when it reached the field. After adopting a star formation, the tugs manoeuvred the platform into place and kept it stable until its skirts had penetrated the sea bottom.
At 22.00 on 6 August, the tow was completed – one day ahead of schedule. The voyage had lasted 138 hours. Ballasting for further penetration continued for three days and was completed on 9 August.
That date was a red-letter day for both Statfjord platforms. The B structure was securely in position, and Statfjord A set a production record of 236 714 barrels per day.
