ABSTRACT

The largest offshore soil penetration test ever undertaken was planned and carried out in 1985 in the Gullfaks Field, North Sea. In more than 200 m water depth a large scale test structure was successfully penetrated 22 m into the seabed to give important information for the design of the Gullfaks C fixed concrete platform to be installed in 1989. The test structure was made up of two steel cylinders, height 23 m and diameter 6.5 m, attached to each other through a concrete panel with crossectional area 2.4 m × 0.4 m. Penetration of the concrete panel into the clay, clayey sand and medium dense sand layers was achieved by the submerged weight and suction inside the two steel cylinders. 13 types of instruments, and a total of 70 sensors recorded continuously during penetration and gave specific geotechnical information like soil friction, earth pressure and pore pressure at 6 elevations and tip resistance of the concrete panel. Two tests were carried out and included variation in penetration rate, check for set-up effects, cyclic tests and water injection at tip of concrete panel. The paper presents the instrumentation program, field performance and results from the offshore large scale penetration test.

INTRODUCTION

The CONDEEP Gullfaks C platform, ordered by Statoil in 1986 with scheduled installation in 1989, will be the largest and heaviest offshore concrete structure ever installed. Designed for 220 m water depth it has a displacement of more than 1.4 million tonnes which is twice the size of previously installed platforms. This huge fixed platform which represents an investment of 14 billion NOK ($ 1.8 billion) is however, offered one of the most complex and poor sites in the North Sea to date, Fig. 1. At the planned location on the slope of the Norwegian Trench, the upper 45 m of soil consists of normally consolidated clay and relatively loose clayey and silty sands.

The platform will have 1400 running metres of 22 m high concrete skirts penetrated into the seabed. These skirts will consist of circular cells with diameter 24 m and wall thickness 0.4 m.

During the foundation design it became evident that the penetration of these 22 m high platform skirts, which where required for stability reasons, could become difficult. Predicted skirt penetration resistance exceeded the available driving force from the structural submerged weight and ballast water, and it was realized that underbase suction most probably would be required to achieve the required driving force. Even with this additional force, the platform had a rather low margin against penetration refusal.

Facing this fact, the Statoil geotechnical staff and Gullfaks C project team members decided to perform a large scale field test at the Gullfaks C site by penetrating a segment of the skirt wall into the seabed. This was done to improve the confidence in the predicted soil response and to clarify uncertain aspects with respect to penetrating the 22 m high concrete skirts.

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