Australian Research Council Centre of Excellence Geotechnical Science and Engineering

Mr Xiaojun Li

Research Interests

  • Uplift behaviour of offshore foundations
  • Wave-soil-structure interaction
  • Flow and scour of sediment

Supervisors

  • Professor Christophe Gaudin
  • Winthrop Professor Mark Cassidy
  • Dr Yinghui Tian

Research Topic

Study on the pore pressure mechanism in clays and its application in the uplift of offshore foundations

Research Abstract

Offshore structures are subjected to environment loadings, which are by nature cyclic and may introduce tensile components. Typical examples include various skirted or embedded foundations (e.g., caissons and anchors) used in deep water, which are subjected to tensile forces from the buoyancy of upper floating structures, and pipelines which may experience uplift loadings during storm events. In some particular cases, subsea structures and their underlying foundations (e.g. mudmat foundations) need to be extracted from the seabed for decommissioning of projects, maintenance of aging infrastructures, saving costs by reusing in other drilling fields or abiding the environment regulations.

The aim of present study is to understand the mechanism of suction generation and investigate the uplift behaviour of offshore foundations in clays. A conceptual framework is being proposed to interpret the suction development during extracting foundations under different loading rates. 1g and centrifuge tests will be carried out to verify the proposed model. The soil failure mechanisms during uplift and the effects of preloading on uplift will also be investigated by using centrifuge tests. Meanwhile, complementary numerical analysis will be undertaken to validate the centrifuge test results and conduct more comprehensive parametric study.

Why my research is important

In low permeable clays, the application of tensile loadings result in the generation of negative pore pressures at the foundation inverts. These further result in suction forces that augment the uplift loadings. No methodology currently exists to predict the occurrence and magnitude of these suction forces. As a consequence, the suction forces are not considered in situation where they could increase uplift resistance, leading to significant conservatism or overestimation in design of offshore foundations. During subsea structure recovery, the poor estimation of the suction forces often causes difficulty to achieve extraction. The development of mitigation measures requires a reasonable assessment of the suction forces, as a function of the soil parameters and foundation geometry.

Therefore, the proposed study aims at revealing the fundamental science behind the generation of suction forces in fine soils and investigating the uplift behaviour of offshore foundations over a range of soil drainage conditions by changing loading rates and drainage length. The outcomes from this study are expected to provide a better understanding of the suction development phenomenon and to develop robust methods to predict the uplift resistance of offshore structures and potential mitigation measures to facilitate their retrieval.

Funding

  • Scholarship for International Research Fees (SIRF)
  • The Lloyd’s Register Education Trust (LRET) PhD Top Up Scholarship
  • Australian Research Council Linkage Grant and Keppel Offshore and Marine, Singapore