Effect of Jackup Spud Cans on Piles
Authors: Siciliano R.J. , Hamilton J.M. , Murff J.D. , Phillips R.J. 
Published: October 2008

This paper describes the determination of lateral soil displacement profiles surrounding a jackup rig spud can penetrating in soft clay. The displacement profiles were inferred from pile moment data obtained from centrifuge tests which modeled a spud can penetrating next to three instrumented piles. A beam column model that incorporates movable soil supports was employed to infer the soil displacements from the pile moment data. At distances as close as 0.5 spud can radii from the spud can edge, the soil displacements were found to be small (? 0.02 spud can radii). The inferred displacements were found to be consistent with physical measurements within the model.

INTRODUCTION

In soft clays, independent-leg jackup rig spud cans often penetrate to significant depths. This causes the displacement and remolding of large volumes of soil. Where jackups are employed in work overs of existing facilities or drilling new wells, this soil movement can place significant loads on in-place foundations.

The problem of determining the magnitude and extent of soil movement surrounding the spud can is difficult. The soil displacements must also be converted into loads on the piles adjacent to the spud can. Existing analytical and numerical methods for evaluating penetration of objects in a semi-infinite medium do not appear well suited for determining the soil displacements in this case. A jackup rigspud can may penetrate to significant depths, but such depths may not be large in comparison to the spud can diameter. Therefore, surface effects may need to be taken into account. Also, unlike penetrating piles where laterally displaced soil moves primarily outward, a spud can may have soil flow around and above it as it penetrates.

In order to address this problem, a series of centrifuge experiments have been initiated at the Geotechnical Centrifuge Center of the University of Cambridge. The objective of the tests is to simulate the penetration of a spud can adjacent to installed piles. Three piles instrumented with strain gauges along their length are used. The piles are spaced at increasing lateral distance away from the spud can edge. As the spud can penetrates, the moment distribution in each of the piles is recorded.

The following sections contain a description of the test program and present some of the results. A method of analysis that uses a beam-column model with movable soil supports is also described. Using this model and the measured moment data, the associated optimum lateral soil displacement profiles were determined for various levels of spud can penetration and increasing distance from the spud can edge. The optimum soil displacement profiles were used to construct a generalized four-parameter model of displacement as a function of the spud can penetration and distance from the spud can.

ANALYTICAL STUDIES

In association with the data interpretation efforts reported in this paper, several analytical studies were also conducted. Limit analysis procedures were used to examine potential failure mechanisms of the spud can penetrating the soil. It was concluded that extensive remolding of the soil is localized laterally to within a spud can radius.