Finite Element Method for Critical Top Tension Analysis of Neutrally Buoyant Riser

Abstract

This paper aims to present the critical top tension of a neutrally buoyant riser in deep water. Variational formulation was developed based on the principles of virtual work-energy, which involve the strain energy due to bending and axial deformation, and the works done by current force and internal fluid flow. The finite element method was used to obtain the numerical solution, which was then verified by the shooting method. The critical top tension can be found by considering the lowest point on the stiffnessdisplacement curve between the top tension and total arc-length; the critical top tension is the minimum tension that is required to maintain the equilibrium state of the riser. If the top tension is lower than the critical top tension, the equilibrium would not be satisfied. If the top tension is greater than the critical top tension, there are two possible equilibrium configurations. The configuration associated with a smaller displacement is the stable configuration, while the others with a larger displacement is the unstable configuration. For the case of unstable equilibrium, the riser may be unable to maintain the equilibrium state if a small disturbance induces the riser motion. The effects of riser’s span lengths, horizontal offsets, current forces and internal flow velocities on the critical top tensions of the risers are highlighted.