The paper presents the salient aspects of the offshore geotechnical field work performed to characterize the top 150ft (46m) of sediments for the design of the Mad Dog suction anchors. The geotechnical investigation included 6 borings and 7 CPTs. Detailed findings from the borings and CPTs, performed are included. The work was performed under severe loop current (2.5+ knots currents) which required implementation of innovative drilling techniques. Work difficulty was also compounded by the presence of the Sigsbee Escarpment and the difficult subsurface conditions which consisted of soft clays under laid by stiff slump units. The slump units were found to contain sand and silt layers, adding to the complexity of the data interpretation.
The site investigation confirmed how difficult it is to perform geotechnical work in harsh environment. Shear strength data was very erratic showing that thick slump units can consist of many sub-units of different thicknesses and properties. Properly characterizing slump units and complex surficial sediments require in-depth planning. The required number of borings and CPTs can be much larger than at conventional sites.
In March and April of 2002 a geotechnical investigation was performed at the proposed Mad Dog spar location in Block 782, Green Canyon Area. The purpose of the geotechnical investigation was to provide engineering data for the design of the 11 suction caissons that would be used to moor the Mad Dog spar. Data collected during 3 previous geotechnical investigations, including piston cores and deep soil borings, are described by Al-Khafaji et al (2003).
The proposed mooring and anchor pattern for the spar is shown in Fig. 1. The geological setting of the anchors has been described by Berger et al (2006) and is briefly summarized in this paper. Two of the anchor clusters, Cluster 1 and 3, lie just above the Sigsbee Escarpment on the Lower Continental Slope, in water depths of approximately 4400 and 4500ft (1341 and 1372m), respectively. The remaining anchor cluster, Cluster 2, to the southeast of the spar location, lies on a bench on the face of the Sigsbee Escarpment in a water depth of approximately 5600ft (1707m).
Geophysical data sets including conventional and highresolution 3D seismic, high-resolution 2D lines and subbottom profile and side-scan imagery collected from an AUV were used in planning the geotechnical investigation. For Cluster 2 the seismic data showed slump units in the area. This was confirmed by an earlier geotechnical investigation (Al-Khafaji et al, 2003). Because of complex nature of the slump units it was determined that a separate investigation of soil conditions would be required at each anchor location in Cluster 2 (Berger et al 2006).
The geotechnical work was performed with the M/V ‘Uncle John’. Upon completion of the offshore campaign 7 Cone Penetration Tests (CPTs) and 6 soil borings had been performed.
