Abstract

Accurate spudcan penetration prediction is important for the safe and economic management of mobile offshore jack-ups. However, the current method for dealing with loose sand - by selection of a low friction angle in a general shear failure analysis - is unsatisfactory because the material also experiences significant compression under load. A new simple method is proposed that includes compression and allows spudcan penetration analysis to be based on actual measured site soil parameters. The method is explained, guidance given on site investigation requirements and illustrative case histories documented. The new method's predictions are a good match to field records.

1. Introduction

An important aspect of the safe management of jackup rigs is the provision of accurate leg penetration predictions, with a narrow uncertainty range. When predictions consistently match the eventual site outcomes, they become a resource that is consulted and relied upon by the rig move team. This means that discrepancies between prediction and actual leg penetrations will be taken seriously, and feedback sought from the geotechnical engineer on the appropriate response. While much attention is given to making accurate predictions at soft clay and layered soil sites, where leg length and punch-through are issues, less attention is given to sand sites. At a dense sand site this is understandable, as spudcan penetration tends to be small. However, in loose sand the same approach can grossly underestimate the penetration recorded in practice. This also has implications for the prediction at a punch-through site, where the crust is loose rather than dense sand. The most common technique used by geotechnical engineers to deal with loose sand is to select an artificially low friction angle (φ) that, when used in the conventional bearing capacity equations, matches the penetration results at previous sites.

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