A numerical model for the transient analysis of offshore foundations under cyclic loading

A comprehensive numerical model for the analysis of offshore foundations under a general transient loading is presented here. The theoretical basis of the model lies on the Swansea formulation of Biot’s equations of dynamic poroelasticity combined with a constitutive model that reproduces key aspects of cyclic soil behaviour in the frame of the theory of generalised plasticity. On the practical side, the adoption of appropriate finite element formulations may prevent the appearance of spurious numerical instabilities of the pore pressure field. In this respect, the use of a coupled enhanced-strain element is here proposed. On the other hand, the practicality of the presented model depends ultimately on its computational efficiency. Some practical recommendations concerning the solution strategies, the matrix storage/handling procedures and the parallel multi-processor computation are here provided. Finally, the performance of the model with a benchmark study case and its practical application to analyse the soil–structure interaction of an offshore monopile under a realistic transient storm loading are discussed.     

Rate measurements and detection of gas microseepage to the atmosphere from an enhanced oil recovery/sequestration project,Rangely, Colorado,USA

One of the proposals for large-scale sequestration of fossil fuel-derived CO₂ is deep geologic disposal in depleted oil/gas reservoirs or deep aquifers. Previously published scenarios for this inadequately proven technology have either ignored or dismissed the possibility of vertical migration of gases caused by overpressure. Overpressuring of a reservoir or aquifer will be necessary in order to have acceptable rates for dispersal of injected CO₂. This research describes methodology and the results of measurement of microseepage of CO₂ and CH₄ at a large-scale CO₂-enhanced oil recovery (EOR) operation at Rangely, Colorado, USA. Shallow and deep soil gas concentrations, and direct transport of CO₂ and CH₄ into the atmosphere were measured. The interpretation of the measurements was complemented by both stable and radiogenic isotopic measurements of C. The results have demonstrated an estimated microseepage to the atmosphere of approximately 400 metric tonnes of CH₄/a from the 78 km² area of the Rangely field. Preliminary estimates of deep-sourced CO₂ losses are <3800 tonnes/a, based on stable isotope measurements of soil gases. Several holes up to 10 m deep were drilled on, and off the field for nested gas sampling of composition and stable C isotopic ratios for CO₂ and CH₄. Carbon-14 measurements on CO₂ from these holes indicate that deep-sourced CO₂ microseepage losses were approximately 170 tonnes/a.