The Petroleum Safety Authority Norway (PSA) requires all facilities on the Norwegian shelf to be able to withstand a hundred year storm - that is a wind force that statistically only occurs once in a hundred year period (up to 40 meters a second - 40 m/s).
All facilities built after 1984 must also withstand a ten thousand year storm without the main safety functions breaking down and without personnel being injured or significant pollution taking place. The wind force which statistically will occur once in a ten thousand year period can reach 50 m/s.
Less wind - bigger waves
Tropical hurricanes form when there is a big difference between the air temperature and the sea temperature, and when the air is very much warmer than the sea. "Katrina" had a speed of 78 m/s - far more than the ten thousand year value for the Norwegian shelf. The hurricane held 56 m/s when it hit the New Orleans/Mississippi area, according to the National Oceanic & Atmospheric Administration (NOAA).
- Our low pressures are normally formed when cold air from Greenland reaches open, ice-free water. These low pressures do not reach the same kind of wind force. However, they may create bigger waves than those made by the tropical hurricanes, says Principal Engineer Arne Kvitrud in the PSA.
The size of the waves depends on the wind force as well as on how long the wind has been blowing in the same direction, and the length of the stretch where the wind can move freely.
- Wind from the north towards the south, which is across the longest stretch, will create the largest waves in the North Sea, Kvitrud explains. - Britain shields the North Sea from the biggest waves brought by the westerly winds, he adds. Arctic low pressures
In our part of the world, arctic low pressures are the phenomena that most closely resemble tropical storms. They form when cold air which has been lying above the arctic sea ice moves out to the open sea. However, these low pressures cover a smaller area than tropical storms like "Katrina".
- Normally the centre of a tropical storm will move faster than the centre of a storm on the Norwegian shelf. The storm centre that moves most slowly will create the biggest waves, Kvitrud adds. For a hundred year storm the waves have been calculated to be largest at Haltenbanken outside Mid Norway - a little over 30 meters. In the Barents Sea and on the Ekofisk field the wind might whip up waves of up to 25 meters.
According to figures from the PSA's US sister organisation Minerals Management Service (MMS), 41 facilities and five mobile drilling facilities in the Gulf of Mexico were totally destroyed by "Katrina". Another 15 installations have received extensive damage. "Katrina" is not the first hurricane to wipe out petroleum installations in the USA. Just last year seven facilities were totally destroyed by the storm Ivan. Several more were damaged. Likewise, "Andrew" caused the total destruction of 29 facilities in 1992, and in 2002 the hurricane Lilly caused severe damage on six production facilities and four mobile drilling facilities. Different regulations
The manning situation and the safety regulations are other elements which differ between the Norwegian shelf and the Gulf of Mexico. Most of the roughly 4000 production facilities in the US are unmanned during a storm. - On our own shelf most of the facilities have people on board during a storm, says Technical Supervisor Helge Ivar Vestre in the PSA.
- Whilst all the facilities on the Norwegian shelf must be designed for a hundred year storm, only permanently placed facilities in the Gulf of Mexico are subject to the same requirement, Vestre says.
- The anchoring on the floating exploration rigs in the Gulf must withstand wind forces that statistically will occur once every five years, says Vestre. A five year storm is roughly equivalent to 80 per cent of the force of a hundred year storm. Measures prior to a storm
The great majority of permanent facilities will have the same clearance between the waves and the deck throughout its lifetime. On several field in the southern part of the North Sea, however, the seabed is subsiding. This means that the facilities gradually come to stand deeper in the sea. The fields Ekofisk, Valhall and Eldfisk are subject to subsidence and are therefore vulnerable to waves in connection with a storm.
The Ekofisk Centre has the greatest seabed subsidence, last year measured to 8.4 meters. Valhall has sunk just over six meters, but it has the greatest subsidence rate - 0.25 meter a year. Thus the probability of waves reaching the deck will increase as long as the subsidence continues.
- A number of measures will therefore be implemented if a storm is forecast for these facilities. The personnel will be evacuated, equipment will be moved higher up and production will be shut down, Vestre concludes.
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