Cone Penetrometer-Based Spudcan Penetration Prediction in Uncemented Carbonate Sand
Authors: Bienen Britta , Pucker Tim , Henke Sascha 
Published: April 2012

Installation of offshore platforms in carbonate soils is challenging, asevidenced by numerous incidents and accidents. The installation process ofmobile jack-up drilling rigs is no exception. The prevalence of these soils insome oil and gas producing regions, such as offshore Australia, necessitatesthorough understanding of the foundation behaviour as well as predictionmethods capable of capturing the salient features. This contribution thereforedevelops a method to predict footing penetration in uncemented carbonate sand. The method directly correlates the cone penetrometer tip resistance with thefooting load-penetration, i.e. without the requirement of selection of afriction angle. The development is based on centrifuge model experiments aswell as large deformation finite element analyses incorporating an advancedconstitutive model for the soil. The direct use of in situ site investigationdata increases the confidence in load-penetration predictions for rig moves inthese challenging soil conditions, while at the same time reducing the timerequired to obtain a prediction.

Introduction

Jack-up rig installation in carbonate soils is challenging, with Erbrich (2005)recounting details of two example cases in the Bass Straight, Australia. Despite the complex soil behaviour, accurate prediction of the spudcaninstallation response is required.

The prediction of load-penetration curves prior to a jack-up rig move istypically performed as a series of bearing capacity predictions at discretedepths. The use of the bearing capacity approach requires knowledge of theoperative friction angle. Unfortunately, the approach is rather sensitive, witha small uncertainty in the friction angle translating to a relatively largeuncertainty in the predicted bearing capacity. Further, based on experience ofoffshore jack-up installations, the SNAME (2008) guidelines recommend thefriction angle determined in the laboratory to be artificially reduced toaccount for " scale effects" (SNAME 2008).

The above applies to silica sands, where the general shear failure mechanism isobserved and penetration typically remains shallow. Carbonate soils areacknowledged to pose further challenges, with predictions expected to be lessaccurate than in silica sand (SNAME 2008, ISO 2011). This is due tocharacteristics such as crushablibity of particles, high in-situ void ratios, and high compressibility compared to siliceous sand, resulting from differencesin mineral composition and particle shape. Despite high friction anglesmeasured in laboratory tests, the penetration resistance in uncementedcarbonate sand is typically low, resulting in relatively large footingpenetrations. SNAME (2008) therefore recommends a larger reduction of thefriction angle as compared to silica sand, to artificially account for thedifference in response.

However, footing penetration in uncemented carbonate sand has been shown to begoverned by the punching shear mechanism (Finnie and Randolph 1994a, Yamamotoet al. 2009), rendering the bearing capacity approach inappropriate as it isbased on the general shear failure mechanism. This is reflected in ISO (2011), which proceed to recommend an alternative approach based on the concept of a‘bearing modulus’ (Randolph et al. 1993, Finnie and Randolph 1994a). Yamamotoet al. (2008, 2009) pursued the concept of a bearing modulus further for theprediction of the bearing response of shallow footings on uncemented carbonatesand.