This study is aimed at the development of a rigorous finite element technique for the analysis of rigid and flexible deep water marine risers. The finite element developed is based on an isoparametric formulation of a three nodded cylindrical beam element. The formulation takes into account large displacements and rotations, axial-torsion-bending coupling, nonlinear hydrodynamic loads evaluated on the current configuration of the riser, effect of buoyancy due to both external and internal fluid pressure. Furthermore, an axialhybrid element and axial-torsional hybrid element were derived from the conventional one to over come the ill-conditioning of the stiffness matrix due to the difference between the axial-bending stiffnesses and the torsion-bending stiffnesses. The axial force and torsional moment are linearly interpolated along the element.
For this study a general overview of wave theories and hydrodynamic forces on slender bodies is presented. The Green-Lagrange strain tensor, the second Piola-Kirchhoff stress tensor and the rigidity tensor are presented in a curvilinear system of coordinates.
The research includes the description and derivation of both conventional and axial-hybrid elements. Static analysis examples are presented to illustrate the performance of the axial-hybrid element. The equations of motion for the dynamic analysis of risers are derived. A comparative study of a deep water marine riser with the general purpose computer program ABAQUS for the comparative study is based on the simplified Cosserat beam theory of Dupuis in stretch, shear, bending and twist strains are used, whereas the elements derived in this study are based on a solid-element formulation approach in which the components of the Green-Lagrange strain tensor are used. Also included is a three dimensional dynamic analysis and a parametric study of different factors affecting the response of the deep water mariner riser. The formulation of the torsional-axial hybrid element for which three dimensional static and dynamic examples are shown.
Related Publications: Boubenider, R., Tassoulas, J.L. and Roesset, J.M., “A Hybrid Axial-Torsional Finite Element for Flexible Risers,” Proceedings, Second International Offshore and Polar Engineering Conference, ISOPE, San Francisco, California, June 14-19, 1992.
Tassoulas, J.L., “Finite Element Analysis of Marine Risers,” Proceedings, Workshop on Marine Riser Mechanics, National Science Foundation, The University of Michigan, Ann Arbor, September 29-October 1, 1992.
Tassoulas, J.L., Boubenider, R. and Roesset, J.M., “A Hybrid Axial-Torsional Finite Element for Flexible Risers,” International Journal of Offshore and Polar Engineering, Vol. 3, No. 4, pp. 299-305, December, 1993.