Seabed subsidence

by the Norwegian Petroleum Museum

“I was away from the complex for several years, on the outlying platforms,” Steinar Undheim later told Ragna Ervik in an interview.

— Mounting modules on Ekofisk 2/4 T. Photo: ConocoPhillips/Norwegian Petroleum Museum

“When I returned, I noticed that the boat landing which [originally] stood four-five metres above sea level had disappeared. And the number of rings visible up the tank [breakwater] had fallen since I last worked there.” [REMOVE]Fotnote: Mechanic Steinar Undheim on Ekofisk 2/4 Q interviewed by Ragna Ervik, MSc student at the University of Bergen, 13 April 2003.

A subsiding field

In the late autumn of 1984, the platforms on Ekofisk were observed to be closer to the sea surface than when they were installed. A series of measurements was made to determine the rate of this subsidence.

It became clear that the seabed was subsiding in some place by 40-50 centimetres per year. By the spring of 1985, the platforms were 2.5 metres deeper than they had been at the start. The biggest subsidence was at the Ekofisk Complex.

Unlike many other Norwegian North Sea fields, Ekofisk has a chalk reservoir. Great uncertainty prevailed from an early stage how the field would behave during production.

Large quantities of oil had never previously been produced from chalk reservoirs with the abnormally high pressure found in Ekofisk.

The question was whether reducing this pressure would mean the rocks could no longer withstand the pressure from overlying strata.

Subsidence had been mentioned as a possibility as early as the application for trial production in 1970. Three kilometres of overburden were expected to stay firm, but they proved too unconsolidated to prevent the seabed from subsiding.

Such movements over reservoirs producing oil or gas are a known phenomenon. As hydrocarbons are extracted, pressure declines and the rock structure of the reservoir must carry a larger proportion of the overlying weight.

Chalk is a soft material and, when the pressure declines, it becomes compressed. The 3 000 metres of rock above the reservoir proved too weak to prevent this, and the compression propagated up to the seabed.

The question was whether this could be tolerated, with the 100-year wave posing the main challenge. This is theoretical estimate of the height North Sea waves could reach in given circumstances, under given weather conditions. It was determined that the Ekofisk installations would no longer be able to withstand such a surge.