Driven piles are often used for supporting jackets and providing anchors for mooring patterns. In noncarbonate conditions, driveability assessments are routine and generally reliable. However, piles are now more frequently being installed in areas where carbonate seabed conditions prevail and the database of offshore installation experience in calcareous soil and rock is still relatively small. Offshore pile installation time is costly, and reliable prediction of driveability is essential towards selecting the correct piling hammer and predicting driving conditions including potential refusal. This paper considers a recent project on the North West Shelf of Australia where significant oil and gas field developments are planned. It highlights the challenges of installing driven piles in weakly cemented calcareous sands and calcarenite, and demonstrates how back-analysis of driving data can play an important role in predicting the field outcome, especially that related to risk of premature refusal and the requirement for contingency measures such as relief drilling equipment.

1. Introduction

An increasing number of offshore oil and gas fields are being exploited in areas of the world where calcareous seabed sediments (commonly referred to as ‘carbonate soils’) prevail, and the pace of development has been nowhere greater than for the North West Shelf and Bass Strait areas of Australia. Many of the planned developments for these areas will utilise driven piles for anchoring mooring systems and subsea structures, or for supporting platforms, but past experiences have shown that calcareous soils can pose problems for both the pile designers and the installer (e.g. King and Lodge, 1988). At one end of the scale, there are the problems associated with the extremely low shaft resistance mobilised on driven piles when compared to silica sand, with subsequent reduction in capacity such as that documented for the Rankin A Platform (Jewell and Khorshid, 1988).

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