Abstract

Driving piles into carbonate strata, particularly weak rock (calcarenite) andcemented sands is traditionally thought to carry high risk in terms ofpremature refusal, exceedance of fatigue criteria or pile tip buckling. Although soil resistance to driving (SRD) algorithms have been developed forcarbonate deposits they are based on limited experience (primarily ArabianGulf) and there is a need to expand this database to include installationexperience elsewhere in carbonate conditions such as the Australian north westshelf where significant oil and gas developments are planned.

Much of the uncertainty surrounding the prediction of pile driving in carbonatestrata can be attributed to the difficulty in establishing a reliable andcontinuous soil resistance profile which is primarily due to i) the difficultyobtaining suitable samples of rocks such as calcarenite for compressivestrength testing, and ii) the equipment limitations for pushing conepenetrometers into weak rock.

It is standard industry practice to base hammer selection, fatigue damageanalyses and driveability assessments on upper bound plugged conditions which, for weak rocks such as calcarenite, gives significantly higher resistances thanfor the unplugged case and this can lead to the conclusion that driving iseither completely unfeasible or not possible beyond a certain depth.

This study presents data from a pile installation in the Timor Sea to supportmodifications that may be made to existing SRD algorithms to improvereliability of the predictions, at least for this particular site. Back-analyses of the installation data show that unplugged conditions are farmore likely to prevail when driving a large diameter pile which suggests thatautomatically assuming plugged conditions is over-conservative. Possiblecorrelations between cone tip resistance and unconfined compressive strength(UCS) of calcarenite are made by fitting SRD algorithms that require UCS as aninput to those that are based on cone tip data.

Introduction

Offshore pile fabrication and installation is expensive and anyover-conservatism in the design or in driveability predictions can result inproject costs being significantly higher than they might otherwise be. On theother hand, unplanned problems such as premature refusal or pile tip bucklingduring installation are potentially even more costly which is why a cautiousapproach is normally adopted. Where an over-conservative approach leads to piledriving appearing to be marginal, or indeed unfeasible, an alternative andpotentially very expensive solution such as grouted piles or contingencydrilling equipment may have to be adopted. A more balanced approach based onthe most likely driving behaviour would result in optimised project costs.

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