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

Abstract

The study discusses the problem of determining vertical displacements of a riser’s ends, which, despite its horizontal displacements induced by waves, mitigate stresses. A spatial model of riser dynamics is presented that considers the geometric nonlinearity due to large deflections. The Rigid Finite Element Method was used for riser discretisation. Analyses are reported that enabled riser’s vibration frequency determination according to the positions of its upper and lower ends. Then a dynamic optimisation task was formulated and solved. It consists of the selection of riser’s vertical displacements that provide bending moments’ stabilisation at its selected points, tension forces, despite the defined horizontal movements of the riser’s upper end induced by waves. The calculations were performed for variable amplitudes of riser end’s horizontal movements.