Abstract

The soil conditions within the array of the Mad Dog suction anchor locations vary significantly. One anchor Cluster is located in a slump unit where both soft and stiff clay as well as sand layers are encountered. Due to the highly variable soil conditions five different anchor dimensions were used. The paper presents relevant installation considerations and field performance results. Predicted and observed embedment pressures, tilt and twist as the anchor penetrated are presented.

Introduction

The 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 11 polyester lines distributed in three Clusters with suction anchors at the end to provide holding capacity.

Suctions anchors were selected as the preferred option due to

  1. high 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 can be accommodated by using different anchor diameters and depth to diameter ratios and

  3. proven technology with more than 500 suction anchors installed worldwide at more than 50 locations per 2003 (Andersen et al (2005).

Soil investigation

The soil investigation was planned and performed following an integrated approach. Geophysical and geological analysis were performed prior to the main soil investigation and concluded that there was a high degree of confidence of homogeneous sediments at all anchors locations in Clusters 1 and 3. Therefore only one boring and one (two) CPTs were performed at each of these Clusters. The conclusion from reviewing geophysical data at Cluster 2 was that there was low confidence of homogeneous sediments at each anchor location and therefore the geotechnical field program consisted of one boring and one CPT at each anchor location (in total 4 borings and 4 CPTs were performed at Cluster 2), see Field Architecture shown on Figure 1 below.

Figure 1. Mad Dog field architecture with the 3 anchor clusters (available in fullpaper)

Penetration analysis

The penetration analysis includes calculation of

  • skirt penetration resistance

  • underpressure needed to reach target depth

  • allowable underpressure limited to avoid large soil Heave

  • soil heave inside the anchor

The penetration resistance, Qtot, is calculated as the sum of wall friction, Qside, and skirt tip bearing capacity, Qtip:
  • Mathematical equation (Available in full paper)

where
  • Aside = total side area

  • Atip = total tip area

  • ?= remolding factor, assumed equal to 1/St,

where St is the sensitivity
  • z = penetration depth

  • suDSS = characteristic direct simple shear strength

  • su,tipAV = characteristic average shear strength at skirt tip level

This content is only available via PDF.
You can access this article if you purchase or spend a download.