Mitigating Early Pile Refusal to Meet Foundation Capacity Requirement for Offshore Platform

Abstract

The scope of this paper is to present the issues faced during pile installation for offshore platforms specifically due to early refusal and its implication to the required platform foundation capacities. The scope of the paper will include a discussion on pre-development activities, soil reports, drivability analysis, pile and conductor installation using jack-up rig (JUR) and advantages of drill and drive to mitigate early refusal. Soil parameters obtained from soil boring during pre-development activity are used by the design consultant as upon relied information in developing the foundation design. Pile termination depth is formulated based on required pile capacity than translate into number of piles and size for both operating and storm condition of the platform. Early refusal means that the pile has not reached the termination depth but has met the refusal criteria. This is normally stated in blowcounts per depth at a given measured depth. Meeting the pile refusal criteria does not mean the pile has met the required foundation capacity as per platform design but it only indicates the pile cannot be driven further using the same hammer size. A larger hammer size maybe required provided that the stresses induced during driving is lesser than the allowable stress. During engineering stage, pile driving pattern can be predicted by performing pile drivability analysis and any sign of pile refusal prior to target penetration depth is reported. The outcome can be different for upper bound and lower bound cases that takes into consideration both continuous and set-up case during driving due to equipment breakdown. Mitigation to avoid pile refusal should be deployed to meet pile target penetration depth in ensuring long-term integrity of the platform. If unavoidable due to limited piles and hammers selection, early refusal can be mitigated with planned drill and drive. Refusal can also occur if actual site condition differs from expected drivability analysis report. Internal skin friction contributed by the soil in the pile annulus is removed to reduce the resistance during pile driving. It is also critical to understand how JUR operates in terms of pile installation, handling of hammer and clean out activities to fully appreciate the complexities of drill and drive. Based on two recent JUR-installed light weight structure (LWS) projects which is similar in design at two different locations, this paper will outline the notable difference in soil boring data, drivability analysis, planned and unplanned hard driving and corresponding methods on drill and drive.