Collapse resistance of tubular strings under geotectonic load

The tubular string collapsing process is significantly different when an external load is applied through a solid formation or a fluid. A series of experiments were conducted to analyze the differences in tubular string stability with a fluid force and geotectonic loads. The laboratory tests showed that tubular strings started yielding significantly earlier if the load was applied through a formation rather than through a confining liquid. However, during additional loading after tubular string yielding, the deformation of the tubular string installed in a formation remained small since most of the external load was supported by the surrounding formation. Analytical solutions were developed to calculate the stress state and deformation before yielding and after yielding for gravel packed or cemented tubular strings under directional in-situ stresses. The API tubular string failure criteria were also modified so that they could be applied to more general stress state around a tubular string. Using the tubular string stress state and the generalized API pipe failure equations, a guideline was given for field design of tubular strings under geotectonic load based on the experimental observations.