Keynote Address: Cyclic Loading of Offshore Piles: Potential Effects And Practical Design

Jardine, Richard (Imperial College) | Puech, Alain (Fugro Offshore Geotechnics) | Andersen, Knut H. (Norwegian Geotechnical Institute)

OnePetro 

ABSTRACT

This paper reviews some key issues regarding the cyclic loading response of offshore piled foundations. Starting with axial loading it considers: the cyclic loading that can be expected; the fundamental responses of piles driven in clays and sands; frameworks for understanding axial cyclic response and specifying cyclic soil testing; and approaches for practical application in design. The review then moves to consider pile responses to moment and lateral loading, distinguishing between flexible and relatively rigid piles and anchors. A range of possible design approaches is considered and it is argued that current routine practice needs to be reconsidered. Practical methods now exist to address the potentially highly significant effects on axial capacity of piles that experience high ratios of cyclic to average loads. New research and calculation procedures are emerging that offer significant improvements in a broad spread of topics.



1. Introduction

Interest in the behaviour of piles under cyclic loading grew in the 1980s to meet challenges posed by inherently fail-unsafe Tension Leg Platforms (with the first TLP being installed at Hutton in 1984) and heavily loaded deeper water fixed platforms, such as the Cognac jacket set in 320m water. Briaud and Felio (1986) assembled for API a database intended to resemble fine marine sediments covering the cyclic behaviour of clays in: laboratory tests, cyclic model experiments and axially cyclic field pile tests. They considered 16 studies on piles with diameters greater than 150mm, most of which were strain-gauged to measure axial load distributions. Local shaft friction, pore pressure and radial stress measurements were attempted in some cases, although these parameters are notoriously hard to sense reliably. The response of piles driven in sands was not addressed. The piles were submitted to significant numbers of load cycles (typically 100 to 1,000) with frequencies generally around 0.1 Hz.