The paper describes a practical method of solving the beam-column problem of a conductor with casings and production tubing between the drill floor and the sea floor. With a recognized formula for the interaction between axial force and bending moment as the basis, it is explained how to determine the various quantities to be introduced in the formula, with emphasis on the axial force and the conductor systems resistance to same. Different from a method proposed earlier by other authors [1), no additional term is introduced in the interaction formula. Instead, a correction is made on the global resistance on the basis of the true local stresses.
The buckling calculation of a conductor involves certain problems which are not involved in the calculation of an ordinary beam-column. There are problems both with regard to the load and with regard to the resistance, even if all--relevant physical circumstances are known. In the case of an ordinary beam-column there is usually no doubt about what is the load and what is the resistance when the relevant physical circumstances are known. The conductor with casings and tubing consists of several tubes, one inside another. This fact in itself is hardly the reason for the trouble, because built-up structural elements are frequently used, and do not usually create any special problems. However, combined with the fact that internal and external pressures acting on the wall of a tube affect its stability, a problem occurs regarding the corresponding influence on the conductor system (the conductor with casings and tubing) when various pressures are acting inside and outside the various tubes. The problem is related not only to which load that produces the buckling tendency, but also to the buckling resistance of the conductor system.