Compensation of top horizontal displacements of a riser

Abstract

The nonlinear dynamic problem of catenary risers is solved by means of the rigid finite element method. The method enables us to model slender systems such as lines, cables and risers undergoing large base motions. The formulation allows elimination of large values of stiffness coefficients: shear, longitudinal and torsional (dependent on permissibility of the system analyzed). The analysis presented in the paper is concerned with the dynamics of a riser initially bent and undergoing heave excitation. Comparison of the results with those obtained using the finite difference and finite element methods shows good compatibility and indicates the correctness of the models obtained by means of the rigid finite element method. Numerical effectiveness of the method enables it to be applied in solving the dynamic optimization problem. The problem described is the compensation of the horizontal vibrations of the vessel or a platform by vertical displacements of the upper end of the riser. Bending moments which arise during the motion of a platform or a vessel can seriously influence and in some cases damage the structure. Thus the aim of the optimization is to ensure that the maximum bending moment at a given point does not exceed its static counterpart. The results of numerical simulations show that the systems enabling heave compensation can be used to minimize the difference between the bending moment caused by displacements of the platform or a vessel and the static moment.

Application of the finite segment method to stabilisation of the force in a riser connection with a wellhead

“…For modelling such structures as risers, the finite element method is used most often [2], which is reflected in many software packages like ANSYS, Abaqus or Riflex. However, many other methods are also used: finite difference method [3], lumped-mass method [4][5][6], finite segment method [7][8][9][10], or the similar rigid finite element method [11][12][13][14][15][16]. Most of the methods mentioned use Morison equations in order to describe the influence of the sea.…”

Application of dynamic optimisation to stabilise bending moments and top tension forces in risers

“…This may be important if some areas along the riser length need to be protected against overloads. A similar problem with regard to planar was considered in the study [5]. To formulate a riser dynamics model in this study, a modification was applied of the Rigid Finite Element Method (RFEM) presented in [6,7] for planar systems, and in [8] for spatial systems.…”