A New Method for the Design of Laterally Loaded Anchor Piles in Soft Rock
Authors: Erbrich C.T. 
Published: October 2008

This paper will discuss a new design method (termed the CHIPPER method) that has been specially developed to assess the performance of laterally loaded anchor piles in soft rock (both carbonate and non-carbonate). The theoretical basis for the model will be presented along with the results of full 3D FE analyses that were used to calibrate a number of the key input parameters. A program of centrifuge model tests was also undertaken to verify the model, and the results from these will also be described. Finally, application of the CHIPPER model to the detailed design of anchor piles for two moorings will be outlined (Bayu Undan FSO in the Timor Sea and Legendre CALM buoy mooring on the North West Shelf).

Introduction

This paper presents a new design model for assessing the behaviour of laterally loaded anchor piles in soft rocks. Such materials are commonly found in offshore environments around the world, including Australia, South Africa and the Middle East.

The key features of the CHIPPER model will be described, which includes an algorithm to model the development of a fractured ('chipped') zone near the surface of the rock, an algorithm to model compressibility beyond the materials yield pressure and an algorithm to explicitly model the effect of cyclic loading. The form of the 'p-y' curves that are used in the CHIPPER model have been developed based on the results from full 3D FE analyses of typical anchor piles using a nonlinear constitutive model accounting for different levels of compressibility and shear strength. These results will be presented. Results from centrifuge model tests of anchor piles under monotonic and cyclic loading will also be discussed and compared with predictions made using the CHIPPER model.

The new model is believed to have a better theoretical basis than previous models and is backed up by model test results. It has been found that the new model gives a more optimistic load displacement response than earlier models enabling smaller piles to be adopted confidently. In regions such as offshore Australia, where often only small construction spreads are available, the ability to adopt smaller piles can have substantial economic benefits.

Background to Method

The lateral pile analysis problem under consideration is presented schematically on Fig. 1. The new method is a p-y curve based approach and has been implemented into a proprietary spreadsheet program. Various input parameters that define the shape of the p-y curves have been established from 2D and 3D finite element (FE) analysis, plasticity analyses and centrifuge model tests.

The model used is essentially a 'cohesive - compressible' model. The ultimate capacity is solely determined from the cohesive strength and takes no account of frictional components. This is fundamentally conservative for drained conditions but reasonable for undrained conditions. Weakly cemented but high void ratio carbonate rocks generally exhibit a soft compressive response under drained conditions and this is also accounted for when appropriate.

Breakout of the upper rock material will occur progressively, with 'chipping' of near surface material leading to a highly brittle stress-strain response.