ABSTRACT

Closely spaced conductor piles are sometimes deviated away from the group to reduce the risk of interaction, using an inclined driving shoe.

The theory developed by Poskitt1 for modelling the deviation of bent conductors or piles during driving has been extended to conductor piles with an inclined driving shoe. Variables include the axial tip and side resistance of the pile, the lateral soil resistance, and the length and inclination of the driving shoe. Parametric results are presented to illustrate the sensitivity of the results to input parameters. A comparison between the theory and the actual deviation of a conductor pile with a driving shoe confirms the reasonable agreement between theory and practice.

INTRODUCTION

The conductor pipes of offshore wellhead platforms are often installed closed together. In order to reduce the risk of interaction, the peripheral conductor piles can be deviated away from the group. One method that can be used is to provide an inclined driving shoe, thus creating a tendency for the pile to deviate in a particular direction. By adjusting the length and inclination of the driving shoe, the lateral deviation of a conductor pile can be controlled.

In some cases it is important to know the path of the conductor pile. As deviations may not be small, conventional beam-column theory is not applicable and a more general theory must be used. Poskitt1 has developed a method to compute the path of a bent pile during driving. From this theory it is possible to develop a method applicable to conductor piles with inclined driving shoe. The parameters acting on the deviation of the pile are the axial tip and side resistance of the pile, the lateral soil resistance, and the length and inclination of the driving shoe. In order to understand the influence of these parameters, a sensitivity analysis was performed. Then, the developed method was for a real case, where the deviation was measured, to check the relevance of the model.

Poskitt Method

Poskitt has described an incremental method to determine the path of a non-straight pile during driving. This method is based on finite differences and is applied to a pile with constant initial curvature. The path of a pile is function of the tip resistance, the shaft friction and the normal pressure. The forces acting on the pile are shown in Figure 1. Figure 1a corresponds to the forces acting on a perfectly straight pile if the soil is homogeneous. In practice, the piles are never perfectly straight, so lateral and moment forces appear at the tip of the pile deviating the pile away from the vertical (Figure 1b).

Fundamental equations

From Kirchoff's equations, Poskitt shows that a small curved element of pile of length, ds, is governed by the following equations (see Section 6 for notation definitions):

(Formula available in full paper)

Figure 1: Pile forces -

• perfectly straight pile and

• pile with initial curvature (available in full paper)

Figure 2: Incremental path (available in full paper)

This content is only available via PDF.