The Foundation of the Frigg CDPl Gravity Platform: A Case Study
Authors: Lacasse S. , Goulois A. , Robberstad L. , Andersen E. , Boisard P. 
Published: October 2008

ABSTRACT
The geotechnical history of the gravity base structure CDPl on the Frigg Field in the North Sea is retraced in this paper. Erosion problems were encountered early in the platform life. Remedial measures were taken. The structural and foundation monitoring results, complemented with the results of in situ testing and sampling, provided the information necessary to verify that the structure was operating under safe conditions.

INTRODUCTION
Installed in September 1975 on the Frigg field in the British sector of the North Sea, the CDPl platform was decommissioned in 1990. The paper describes the geotechnical problems encountered, the monitoring results and the geotechnical reevaluation analyses during the platform life. The paper shows how the monitoring and site investigation results were used in re-evaluation analyses. In particular, the pore pressures in the sand at the platform base during wave loading are discussed.

CDP 1 PLATFORM

Structure and instrumentation
The CDPl platform is installed in 96 m water. The structure is a Howard-Doris pre-stressed concrete structure, with 107 m high circular lobed Jarlan wall resting on a ring base. Inside the wall, a central shaft extends 30 m above sea level to the steel deck. The structure is shown in Fig. 1. The structure was designed to be supported by an annular raft 101 m outer diameter and 54.4 m inner diameter. The permanent and environmental loads are transmitted to the foundation through diaphragm walls. The base central part is recessed 0.50 m and the outer ring is equipped with 60-mm corrugated steel plates. There are no foundation skirts. The predominant storms on the structure are oriented approximately in a north-south direction. An instrumentation program was initiated to monitor wave height, structural behavior, and geotechnical response. Figure 1 shows the instrumentation on the platform. When installed in 1975, the platform was equipped with a wave radar, four accelerometers, and a slab piezometer. Deep piezometers, depth settlement bullets, and horizontal displacement sensors were installed in 1978. To monitor crack elongation on the diaphragm walls, elongation sensors were installed in 1983. In 1985, five piezometers and three hydrodynamic sensors were installed on the ring poltion of the base. The piezometers were placed in the sand just below the base, and the hydrodynamic pressure cells just above the base at the location of three of the piezometers. Underwater surveys of the platform base and optical leveling were done regularly.

Soil profile and geotechnical characteristics
The soil profile at the CDPl location consists of alternating layers of sand, silt, and overconsolidated clay. The upper soil strata constitute an in-filled depression running approximately north-south, and layer thickness varies under the platform. The simplified average stratification and soil characteristics shown in Fig. 1 were adopted after a careful study of all available laboratory and in situ tests. In particular the laminated clay was very variable. Statistics were used to account for the spatial variability of the clay, which was subdivided into a "weaker" and a "stronger" layer to reflect the presence of sand and silt in the "stronger" layer. The static undrained shear strength of the clay for a horizontal failure was 90 kPa in the "weaker" clay and 140 kPa in the "stronger" clay.