Axial vibration of deep-water drilling risers under lifting conditions

Abstract

During the installation of deep-water drilling risers, the axial vibration caused by the heaving of drilling ships affects the safety and reliability of the operation. Conventional vibration analysis techniques consider the damping generated by the drag force to be uniformly distributed along with the risers, resulting in not real solutions. In this paper, an axial dynamics model of the drilling risers under the top excitation conditions is established, where the riser system is regarded as the assembly of an elastic rod and a block mass. Meanwhile, this model replaces the uniform damping with nonlinear distributed damping and considers the nonlinear concentrated damping of the block mass, which can be solved by the variable separation method and the iterative method. Consequently, the stress and displacement in the riser given by the present model are larger than that predicted by the model considering uniform damping or no damping. In addition, the sensitivity analysis of some parameters like the damping coefficient, the excitation amplitude and frequency, the lifting water depth, and the quality of the block mass was carried out. It shows that the stress in the top is the largest when the riser is lowered regardless of its self-weight, and it decreases almost linearly with the increase of water depth; the axial displacement increases with the increase of the block mass and the lowering water depth but reduces with the increase of the dimensionless damping coefficient. Moreover, excessive excitation frequency will increase the equivalent damping, resulting in greater stress.