Physical and Numerical Modelling of a Jack-up Rig Subjected to Random Seas

Byron Byrne, Mark Cassidy and Prof. Guy Houlsby

During the past several years much work has been performed developing state of the art models for structural analysis, foundation response, wave loading and dynamics of offshore structures. This research is vital particularly when determining the reliability of mobile drilling units such as Jack-Up rigs, which are often exposed to severe working conditions outside their intended design. Typically the foundations for jack-ups are modelled as pinned footings (or at best linear springs), though this is clearly a conservative approach when undertaking structural reliability studies. Experimental work at Oxford University has been involved in developing state of the art foundation models to reduce this level of conservatism. The foundation response is interpreted through a work hardening plasticity framework where the amount of plastic/elastic deformation can be defined. These plasticity models, as well as advanced wave theories such as "NewWave", have been incorporated into the numerical models that are used for research at Oxford. A novel physical modelling technique will be used to evaluate the usefulness of the foundation theories. The foundation will be modelled physically on the lab floor whilst the rest of the structure will be modelled in the computer. It will be necessary to incorporate a feedback loop between the computer model and the physical model. This study would lead to:

  • A validation of the use of work hardening plasticity models as macro models in a numerical analyses package.
  • Comparisons to be drawn between the current drained loading models and the realistic transient loading cases.
  • The introduction of realistic random extreme loading to the model testing of offshore foundations, which is particularly important when dealing with the transient problem.