Laboratory Testing and Detailed Geotechnical Design of the Mad Dog Anchors

  • Schroeder K
  • Andersen K
  • Tjok K
  • 13

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Abstract

AbstractThe Mad Dog Spar is moored by suction anchors arranged inthree Clusters. Two anchor Clusters lie on top of the Sigsbee Escarpment while one anchor Cluster lies in a slump unit below the Escarpment. The soil conditions at the 4 pile locations in the slump deposit vary significantly and are very different from the soil conditions on top of the Escarpment. This dictated the need to make five different designs. This paper describes how the undrained shear strength selected for design was determined by combining the various soil conditions detected by CPT with the results from advanced and index laboratory testing. The design procedures used to design the anchors are explained in this paper. Also presented in the paper are the governing load cases together with results from holding capacity calculations using an efficient 3 D FE program.IntroductionThe Mad Dog prospect is located in the Green Canyon Area of the Gulf of Mexico. The water depth at the site varies from about 1370 m (4500ft) on top of the Escarpment to about 1680 m (5500 ft) below. The field architecture consists of a truss Spar platform moored by a taut leg system consisting of 11 polyester lines distributed in three Clusters, with suction anchors providing holding capacity.Suctions anchors were selected as the preferred option because of (1) their high degree of positioning accuracy, which is important in a slump deposit where soil conditions may change significantly within short distances, (2) robustness to varying/uncertain soil conditions, accommodated by using different anchor diameters/depth to diameter ratios and (3) they are based on proven technology with more than 500 suction anchors installed worldwide at more than 50 locations as of 2003 (Andersen et al 2005).Soil investigationThe soil investigation was planned and performed following an integrated approach. Reference is made to the accompanying papers (Berger et al 2006, Liedtke et al 2006 and Jeanjean et al 2006) for details about the considerations made. The practical consequence was that one boring and one or two CPTs were performed at each of the two anchor Clusters on top of the Sigsbee Escarpment, while one boring and one CPT were performed at each anchor location in the slump area at Cluster 2 (in total 4 borings and 4 CPTs were performed at Cluster 2), see Field Architecture shown on Figure 1.Figure 1 Field architecture at Mad Dog field (available in fullpaper)General soil conditions and seabed topographyThe soil conditions at the anchor locations in Clusters 1 and 3 are relatively uniform and dominated by homogeneous, soft to stiff plastic clay, with essentially linearly increasing shear strength with depth.

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Authors

  • K Schroeder

  • K Andersen

  • K Tjok

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