Rigid Finite Element Method In Modelling The Dynamics Of Risers And Considering Large Deflections

Abstract The dynamic analysis of risers often requires the consideration of large deflections. Due to large deflections in slender elements, the distance between the upper and lower ends of the vertical riser is smaller than that resulting from linear theory. Accounting for flexibility means that the length of the riser has to be measured along its curvature. One of the discretisation methods, which includes an analysis of large deflections, is the rigid finite element method (RFEM). This paper presents a numerically effective planar formulation of the method, taking the deflections caused by bending and longitudinal flexibilities into account. The elaborated models are applied in calculations of the static deflections and ‘free frequencies’ of risers. The latter are used for the validation of the models. Investigations are concerned with how a riser’s parameters, the sea current and internal flows all influence the static deflection and frequencies of free vibrations. Analysis of the dynamics of a riser is also carried out. As part of these calculations, it is shown that large horizontal displacements of the lower end of the riser cause simultaneous and significant vertical displacements. This phenomenon is particularly visible when the possibility of rotation of the upper end is eliminated. The riser vibrations, forced by the displacement of its upper end (base), are also simulated. It is shown that, during horizontal displacements of the upper end, especially for longer risers, the effect of large nonlinear deflections is more than 10 m. The results presented in this paper also indicate how the length of the riser, its filling with the medium, the mass attached to the lower end, or the speed of the current, affect the vertical displacements of the lower end of the riser. These effects can be observed when applying the rigid finite element method for discretisation of the risers.