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Ekofisk is an oil field in the Norwegian sector of the North Sea. Discovered in 1969, it remains one of the most important oil fields in the North Sea. Production began in 1971 after the construction of a series of off-shore platforms by Phillips Petroleum. Oil production is planned to continue until at least 2050.
Subsidence
In the mid-1980s the Ekofisk field as a whole and the platforms in particular were found to be suffering from an unexpected degree of subsidence. Detailed geological investigation showed that it was the result of delayed compactional diagenesis of the Chalk Formation reservoir rocks. As hydrocarbons were produced and replaced with water, the Chalk began to re-dissolve at microscopic high-pressure inter-grain contacts and redeposit in a more compact, lower porosity configuration. Total movements of several metres was recorded, and it was calculated that the total subsidence would almost be 6 metres at the end of the concession of Phillips Petroleum. The Norwegian government pressed Phillips to take action. A French consultant was ordered to find a solution. As 5 of 7 rigs were interconnected, most of the rigs had to be jacked with some 6 metres at the same time. The solution was that the steel tubular legs of the rigs would be cut. Subsequently large flanges were welded to these legs and when all flanges were welded, in one operation five rigs would be lifted simultaneously approximately 6 metres and pipes with flanges would be mounted in between. After bolting all flanges the rigs would be safe again.
The lifting was done with a huge number of hydraulic cylinders; also during the welding of the flanges to the legs, these hydraulic cylinders took over the load on the legs for the welding period. A couple of days before this great jack-up, the hotel platform was lifted, as this platform was less interconnected than the other platforms. The total lifting capacity of all these cylinders was approximately 80 000 tons and was published in the Guinness book of records as being the largest jack-up ever.
External links
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