Yield design upper bound approach to uplift bearing capacity of offshore strip anchors

Resumo

The offshore oil and gas exploration activities require several subsea infrastructures to be installed
over the ocean floor, this equipment being commonly supported by shallow foundations namely anchor plates or
mudmats. By the environmental conditions, live loads imposed during the structure lifetime, and the requirement
for extraction processes, the design practice for anchor plates strongly relies on the evaluation of the vertical
uplift capacity of the foundation system under undrained settings, being subjected to suction forces acting below
the anchor interface which compose a rather complex subject in geomechanics, usually not addressed by the
classical foundation theories. In this context, this paper aims to evaluate the uplift bearing capacity of a shallow
foundation embedded in a purely cohesive material based on the theoretical framework of limit analysis theory
and related kinematic approach. The latter allows for the formulation of rigorous upper bound estimates of the
uplift bearing capacity by implementation and analysis of appropriate failure mechanisms that involve the anchor,
the surrounding soil, and their mechanical interaction along with the interface. It is shown that the optimal uplift
estimate bearing capacity amounts to minimizing a non-convex functional with respect to the parameters defining
the geometry of considered class of failure mechanisms. The considered minimization procedure is handled by
analyzing the stationary condition of the associated variational form, thus leading to the analytical determination
of the optimal failure mechanism. For application purposes, the strength capacity of the soil is modeled through a
Tresca-like yield condition with a tension cut-off, whereas the soil/anchor interface is modeled by a tensile stress
threshold. The predictions obtained from the approach are compared to available finite element solutions derived
in a particular situation with a perfectly smooth soil/anchor interface. An extensive parametric study is then carried
out to assess the effects of relevant problem parameters on the uplift bearing capacity, such as the threshold stress
controlling the breakaway condition at the interface and the strength parameters of the soil.