Pile Instrumentation and Monitoring During Pile Driving Offshore Northwest Borneo
Authors: Wright Nigel , Tamboezer Albert J. , Vindle David , van Hooydonk William R. 
Published: October 2008

Premature refusal of large tubular piles experienced at platforms offshore Brunei and East Malaysia has caused concern since remedial action can be very costly. An attempt was made to establish the cause of the refusal and the nature of the resistance to driving by instrument piles during installation.

Two case histories are described, one from the Champion Field, offshore Brunei and the other from the Barton Field, offshore Sabah, both off N.W. Borneo (Figure 1). Instrumentation for the two tests was similar and is described.

In the former field problems have arisen due to "set-up" in heavily over consolidated clay layers. A typical case history was reviewed and used to predict the behavior of 42-inch diameter piles to be installed for an eight-pile living quarters platform. During installation the piles were instrumented, thus hammer performance, Soil Resistance during Driving (SRD) and set-up could be more accurately quantified to optimize the planning of future installations. Static load tests on similar piles are to be performed to verify the acceptance of piles based on driving behavior.

The Barton jacket was the first to be installed in that particular field. The site investigation indicated significant hard rock layers. During previous pile driving in the area, piles had been extensively damaged on encountering rock. To avoid this problem the 42-inch diameter piles were instrumented to enable the presence of rock to be detected as early as possible and to monitor stress levels within the pile with a view to minimizing damage.

The measurements showed that driving could be continued despite blow counts in 4 excess of the conventional refusal criterion over certain periods of driving. Examination of the data during driving indicated that these high blow counts were not caused by high pile tip resistance and that stresses were well below acceptable levels. With this information, driving was continued and target penetrations were achieved without the need to resort to costly drilling out and use of insert piles.

Introduction

Piled offshore platforms have been installed off the coasts of Sarawak, Brunei and Sabah since the early 1960?s and now total over 200. Despite 20 years of pile driving experience in the area, problems still occur due to premature refusal, low blow counts at final penetration or pile damage due to rock layers. Remedial action is costly.

The soil conditions in the area vary widely. In some parts they consist of soft normally consolidated clays with intermittent dense sand layers. Elsewhere they comprise heavily over consolidated clays interspersed with very dense sands. Sandstone bands occur at depths as shallow as 20 meters.

In an attempt to investigate the cause of the problems and quantify pile driving parameters and soil resistances, two sites were chosen for a programmed of instrumentation of the piles during installation.

Dynamic Pile Testing

The resistance that a pile meets during driving can be computed from records of total force and velocity at the pile head level resulting from each hammer blow. The total force is derived from strain measurements. The velocity can be found