Combined Loading Of Offshore Foundations
E.H. Davis Lecture 2012
Prof Mark J. Cassidy
Mark was selected as the E.H. Davis Lecturer for 2012. This biennial lecture commemorates the work in geomechanics by Professor Edward Hughson Davis. The E.H. Davis Lecturer is selected by the National Committee of the Australian Geomechanics Society as having made a distinguished recent contribution to the theory and practice in geomechanics in Australia.
Predicting the bearing capacity of a foundation under combined vertical (V), horizontal (H) and moment (M) loads is a fundamental soil mechanics problem. Traditional solutions utilise ad-hoc adjustment factors to reduce the pure vertical bearing capacity in order to account for inclination, eccentricity, foundation shape and soil strength gradients. These procedures have their roots in the work on bearing capacity by Meyerhof (1953) and are typified by the procedures described by Brinch Hansen (1970) and Vesic (1973). This EH Davis Lecture reviews a different approach.
It is becoming increasingly popular, particularly in the offshore industry, to write the combined loading capacity of a foundation directly in terms of the combined loads being applied. This provides more accurate solutions and is more readily extended to different foundation geometries, such as caissons, spudcans, anchors, mudmats and pipelines.
Furthermore, by utilising concepts familiar to engineers from classical mechanics – elastic domain, yield surface, flow rule and hardening law – these combined loading surfaces have been incorporated within the framework of plasticity theory to develop models that predict the load-displacement behaviour of foundations. These models encapsulate the behaviour of the foundation and surrounding soil without the need to discretise the soil or formulate complex constitutive models. As they are written in a terminology consistent with structural finite element theory they also have the distinct advantage that they can be incorporated into the software that the majority of offshore engineers use.
A review of experimental and numerical methods to develop combined loading surfaces and encapsulated plasticity models are provided in this Lecture. Four example models applicable to the offshore oil and gas industry are used to illustrate how elasticity, hardening and finally non-associative flow can be built-into the approach in order to account for subtleties in foundation-soil behaviour. These examples span the applications of shallow foundations, suction embedded plate anchors, inverted conical spudcan footings of mobile jack-up platforms and shallowly embedded pipelines.
About the speaker
Prof Mark J. Cassidy Director, Centre for Offshore Foundations, University of Western Australia
Mark is an Australian Research Council Future Fellow, the Lloyd’s Register Foundation Chair of Offshore Foundations and Director of the Centre for Offshore Foundations at The University of Western Australia. He is also the Deputy Director of the recently established Australian Research Council Centre of Excellence for Geotechnical Science and Engineering. Mark graduated in Civil Engineering from the University of Queensland in 1994, and as a Rhodes Scholar, attained a doctorate in Engineering Science from the University of Oxford in 1999.
His research interests are in offshore geotechnics and engineering, predominantly developing wave-structure-soil interaction models for the analysis of oil and gas platforms, mobile drilling rigs, anchors and pipelines. Mark has published over 150 refereed journal and conference paper and jointly holds two international patents with Singaporean mobile jack-up builders Keppel Offshore and Marine. He is a fellow of the Australian Academy of Technological Sciences and Engineering and Engineers Australia.
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