My research work has been in the areas of probability, risk analysis and mechanics. I am active as a consultant in all of these areas.
1.0 Probabilistic Methods and Risk Analysis
I have worked on the idealization of problems using decision theory so as to model both the probabilistic aspects, as well as the consequences, including aversion to risk. In this way it is possible to obtain rational and cost-effective solutions to engineering problems. This has involved the conceptual modelling of new problems and the development of methods of solution. Problems considered range from the movement of hazardous goods, to structural safety, to modelling of human error, to loads on offshore structures, to choice of structural systems. Some work in the latter areas is summarized in section 4.0.
The main focus of my work has been creep and viscoelastic theory, although work has been carried out using elastic methods as well as plasticity theory, and in the area of fracture. In viscoelastic theory, research has been carried out on concretes, and more recently on the mechanics of ice. This has focussed mainly on the failure of ice in compression. In addition to fracture, there is considerable modification to the microstructure, resulting in profound changes in the mechanical response. This is summarized in the following (section 3.0).
3.0 Specific Recent Application: Ice Failure in Compression
Research into ice mechanics at Memorial University of Newfoundland has focused on understanding the deformation of ice during impact with offshore structures. The objective of this research is to develop models for ice-structure interaction that will aid in the design of offshore structures and vessels that are subject to impacts with ice. The research program has consisted of an interlinked program of field, laboratory and analytical work. The load acting on structures during an ice impact event is transmitted through localised, short-lived zones of intense high pressure at the interaction interface. This has been shown in measurements during ship ramming trials. Medium scale indentation tests (two series, 1989 and 1990) on Hobson's Choice Ice Island were part of the present program. They have shown peak pressures reaching as high as 70 MPa locally. The high stresses result in the formation of a damaged layer of ice in the contact zone.
High pressure zones have been identified as a source of the crushed ice that has been observed to be extruded from the ice-structure interface in the field during full scale and indentation tests. Spalling is also present during an interaction, related to the formation of high pressure zones. Spalls create stress concentrations as the ice fractures and is quickly removed from the interaction area; high pressure zones cause cracks to propagate that can go on to reach free surfaces and cause spalls. The zones only exist for short periods of time and are randomly distributed in both space and time, appearing and disappearing within the interaction zone as the ice is crushed and deformed. A large pressure gradient at the edge of each zone results in an area of high shear damage near the outer boundary. Oscillations in the transmitted load that have been observed in ice structure interactions and in field testing are also associated with the high pressure zones and the associated damaged layer. Several high pressure zones together form the global load on the structure.
The present program includes tests to determine the deformation and failure of granular ice under triaxial confinement. Laboratory test conditions have covered a range of confinements from low pressures present at the periphery of the high pressure zone, to high pressures found at the centre of a high pressure zone.
Dramatic changes to the microstructure occur in ice under stress. These result in changes in the mechanical properties and behaviour of the material, in particular much enhanced creep. Microcracking will occur particularly in regions of high shear stress with low confinement. At higher confinement levels, cracking is suppressed and softening is associated with the occurrence of dynamic recrystallisation. Reduction in grain size has been associated with both of these mechanisms (See Figures at right of virgin ice, above, and recrystallized ice, below). Pressure melting has also been shown to be a source of microstructural change. Since pressures of 70 MPa have been observed under field condition, the effects of stress concentrations, for instance at grain boundaries, in addition to these pressures may be sufficient during ice-structure interaction to initiate this mechanism at grain boundaries.
With regard to the analytical part of the program, the results of the triaxial test program have been analyzed using damage mechanics. The resulting constitutive model has been implemented in the finite element program ABAQUS. The ice failure process has been simulated, and compressive failure in the crushed layer has been successfully modelled. The failure in the layer is associated with shear softening that is pressure-dependent, with softening associated with microfracturing initially near the edges of the high-pressure zone, and subsequently softening associated with pressure melting near the centre. When the two link together, dramatic failure of the layer occurs.
Off the east coast of Canada, the presence of icebergs, combined with other harsh environmental conditions, provides a challenge in design. Both fixed and floating production systems have been studied. This requires analysis of probabilities of encounter with icebergs, taking into account avoidance in the case of floating systems. The sea state is taken into account by considering a matrix of significant wave height versus iceberg length, with associated probabilities. The motions of the iceberg and the ability to detect in various sea states are taken into account. The calculation of global ice loads is based on an analysis of available full scale data, including that obtained from ship rams.
Various approaches using probabilistic methods have been implemented. These include analysis of the high-pressure zones of the preceding section as statistical entities, as shown in the figure. This has given promising results, with good agreement with local pressure algorithms developed separately from measurements on ship hulls. These results have been used in practical design methodology for local and semi-local pressure.
In the context of arctic shipping, all available data from full scale ship trials on local and global loads have been analyzed, and incorporated into a risk-based method for obtaining design loads and local pressures for arctic shipping. This was used to conduct a review of the Revisions to the Canadian Arctic Shipping Pollution Prevention Regulations (ASPPR), and also to update the provisions for maximum bow force.
Similar methodology was used in studies conducted to obtain ice loads on the 13 km Confederation Bridge, linking Prince Edward Island and New Brunswick.