Site Investigation for Magnus

ABSTRACT In April 1982, BP's Magnus structure was installed about 150 km northeast of the Shetland Islands. The most northerly, deepest water platform in the North Sea, the steel tower is supported on groups of 2 m diameter piles that were driven, in good accordance with predictions, to an average penetration of 85 m in strong cohesive soils. The paper describes investigations performed at the platform site, and documents soil characteristics for conventional and state of the art pile analyses. Reference is made to several innovative techniques first used at the Magnus site that have since been incorporated into the larger North Sea investigations. Information is given about the geological history of the site. Test results are presented on soil strength and stiffness, including critical state soil mechanics parameters, on residual pore pressures after sampling, and on the effect of sample size on strength characteristics. INTRODUCTION Magnus Field is located in 186 m of water in Block 211/12 of the UK Sector of the North Sea, as shown on Fig. 1. The field is being developed with a four legged tower that was installed during 1982. The tower is supported on groups of nine 2134 mm diameter open end pipe piles driven to an average penetration of about 85 m below the seafloor. Pile driving was as predicted by soil data analyses. Site investigation at Magnus began in 1975 and continued until 1981. This paper presents the results of these investigations. The foundation design and installation are described by Rigden and Semple1 PLANNING Reconnaissance Preliminary site investigations at Magnus were performed in 1975 and 1976 before a decision had been reached on the location and type of possible platforms. A single boring to 26 m penetration was drilled about 4 km south of the final platform site in 1975. This reconnaissance boring provided an early indication of relatively strong cohesive soils likely to be suitable for founding a large platform. A preliminary geophysical survey consisting of 550 km of bathymetry, side scan sonar and deep towed multi electrode sparker profiling was performed in 1976, covering an area of some 200 sq km. Site Specific By 1977 it was known that a single platform would be used, and its position was fixed. The geophysical and geotechnical investigations then undertaken are summarisedin Tables 1 and 2. As no decision had been taken on platform type, the main geotechnical investigation of a 250 m square site in May 1977 provided for either a major piled or gravity base platform. At this time BP were considering using a large diameter vibrocorer for mini piles for a template at Buchan Field, and subsequently did so successfully. The vibrocorer was used at Magnus to obtain large diameter samples of the near surface soils and to provide BP with experience of its use. Site Detail By 1979 a large piled structure was being designed, and concern was expressed over its initial unpiled stability due to the possible variation in soil strength in the upper few metres.