Observed Behavior of the Ekofisk Oil Storage-Tank Foundation
Authors: Clausen C.J.F. , DiBiagio E. , Andersen K.H. , Duncan J.M. 
Published: December 2007

In June 1973 an oil storage tank was installed successfully in the Ekofisk field in the North Sea. In Nov 1973 the structure was subjected to several storms that resulted in loads close to the 100-year design values. Observations of settlements and pore-water pressures show that the tank has behaved satisfactorily.

Introduction

The Ekofisk oil field was the first North Sea oil discovery to be produced commercially. The discovery, made by the Phillips Norway Group in 1969, proved to be a major oil reservoir. This success initiated efforts by others in the North Sea, and the search has successfully resulted in several major discoveries. In the hostile North Sea environment, the exploration and production of oil have offered great engineering challenges. One challenge has been the design and construction of the Ekofisk storage tank, 93 m (305 ft) in diameter and 90 m (295 ft) high, which rests on the sea floor in 70 m (230 ft) of water in mid-North Sea.

The Ekofisk tank is the first of a number of concrete gravity structures to be placed in the North Sea. Since its placement, 12 other such structures have been ordered. These platforms will be used for drilling, production, and oil storage. Placed in water depths up production, and oil storage. Placed in water depths up to 150 m (492 ft), some of them are likely to experience sea conditions worse than those experienced by the Ekofick tank.

Artificial Island

When production began at the Ekofisk field, oil was loaded onto tankers from buoys. With rough sea states, such loading is not possible; therefore, the Phillips Norway Group decided to construct a 160,000-cu m (5.7-MMcf) storage tank to maintain production in bad weather. The tank probably will not be required for oil storage after pipelines from the Ekofisk field are completed to Teeside in the United Kingdom and to Emden, West Germany, in 1975. However, the tank carries two decks with a total area of 11,000 sq m (118,000 sq ft), thus making an artificial island in the middle of the North Sea with valuable space for processing and pumping equipment. processing and pumping equipment. Design and Construction

Designed by the French company C. G. Doris, the Ekofisk tank is shaped like a square with rounded corners and covers an area of 7,360 sq m (79,200 sq ft) (Fig. 1). The crude oil is stored in nine interconnected compartments in the center of the tank; these compartments are surrounded by a perforated breakwater wall that reduces the wave forces on the structure. Beneath the tank, 40-cm-high (16 in.) concrete skirts or ribs extend around the periphery and across the base, as indicated in Fig. 1. The base is covered by corrugated plates.

The Ekofisk tank was built near Stavanger, Norway, by contractors F. Selmer and Hoyer-Ellefsen. The tank base, 6 m (20 ft) high, was constructed in a dewatered area. After completion of the base, the dry dock was flooded and the base was floated to the nearby deep-water construction site. After completion, the tank was towed to the Ekofisk field with a draught of 62 m (203 ft). On June 30, 1973, it was placed on bottom by flooding the inner compartments. A photograph of the tank after installation is shown in Fig. 2. photograph of the tank after installation is shown in Fig. 2. JPT

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