Study On Deepwater Riser Vibration Characteristic

Drilling riser is the important equipment from sub-bottom wellhead to marine platform.As offshore drilling activities expand to deeper water areas, the force acting to the marineriser becomes more and more complicated. With the water depth increasing, thecharacteristic analysis of the marine riser becomes more important. Firstly, the wave theory,the current theory and the method on computing environmental loads on riser with MorisonEquation are shown separately. Secondly, the mathematical model of underwater riser isintroduced in this paper, the dynamic equation that governs the motions of riser in thehorizontal direction. Considering the characteristics of big deformation with small strainunder outside loads and the axial force influence for the marine riser, and using nonlineartheories, the calculated results match the real situation better. A static analysis is numericallysimulated by using a finite-difference method,the effect of top tension on the offshore riser isstudied. Then the governing equation is discredited by using the Hermite interpolationfunction and the dynamic responses obtained through the Newmark method, displacement andbending moments response curves of all nodes are plotted. The computation programs arewritten with Matlab and there are also some computation examples.The study shows these research results provide a basic foundation for practical design ofmarine risers. The section of the riser near the water middle has the biggest deformation andnear the water surface has the biggest bending load for deep-sea riser. So, special attentionshould be paid to the strength of the riser for the section where the biggest deformation andbending moment is. The riser has the same length and withstands marine of environmentloads. As the riser with tensioned increasing, the displacements and bending moments ofnodes of the riser are obviously decreasing. So axial loads of deep-sea riser has an obviousinfluence on static force and dynamic characteristics, they should be specially consideredwhen the analysis design and use of the riser are conducted. The top of the riser should keep enough tensioned in practice so that the riser is not in the compression situation. There is thesame marine of environment loads and tensioned on the riser. With the length of the riserincreasing, axial deformation has little influence on the displacement of nodes.