Elasto-mechanical simulation of a drill string in well-bore
A subsea drill string is often several kilometers long and subjected to compressive loads while confined in a closed wellbore. This leads to string buckling; first a sinusodial shape occur, then at increased compression, a sudden transition to a helix take place. The two transitions is expected to occur at the following compressive loads [ref 1]:
EI is pipe stiffness, ω is the buoyed linear weight of the pipe, Inc is the wellbore inclination, and r is the radial clearance between the pipe and wellbore. λ varies from 2.83 to 5.65.
The FEDEM model of the string does not only reproduce these effects, but it can also reproduce the dynamic loads on the rotating string, dynamic friction, longitudinal resistance, etc. These variables can be used for predicting with greater accuracy the the capacity of the string, the fatigue, and the attainable drilling length.
The model is using a pipe-in-pipe contact formulation with friction. Assembling the model was done with a specialized application written in C# using the FEDEM COM API. The particular work was carried out on behalf of Statoil, where the specific task was to validate the effect of new equipment attached to the string.
Dawson, R., and Paslay, P.R., "Drillpipe Buckling in Inclined Holes," JPT, October 1984, pp. 1734-38.