Effect of structural and hydrodynamic nonlinearities on the dynamics and failure of marine risers.

Abstract

The problem of designing marine risers presents several challenges which range from modeling of structural and hydrodynamic nonlinearities, to convergence of nonlinear solution algorithms in regular and random seas, to failure analyses. The aim of this dissertation is to address some of these challenges and advance the state of the art by providing numerical solutions to some long standing problems. All of the aforementioned complexities appear once nonlinearities become important. This is the case not only in deep sea drilling and production but also in severe environmental conditions in moderate water depth. In these situations, nonlinearities in riser statics and dynamics are not weak. Since a general theoretical method for solution of nonlinear dynamics problems for continuous structures does not exist, it is important to develop robust numerical techniques that converge for various extreme conditions and at all water depths. Consequently, the most reliable approach that can be adopted to solve the general marine riser dynamics problem is numerical. In that respect innovative numerical algorithms that converge for strong nonlinearities are first developed; second, the importance of nonlinearities is assessed in order to conclude whether it is worth using time domain nonlinear simulation codes for riser design. The ultimate test for assessing the importance of nonlinearities is to evaluate their effect on riser failure. First excursion and fatigue failures are studied with the new algorithms that can provide solutions to the nonlinear dynamics problems even in the range of strong nonlinearities. To achieve these goals, the following computer codes to implement new solution algorithms have developed for static, dynamic, two or three dimensional, deterministic or random riser response: STARI-2D.V2, STARI-3D.V2, DYNARI-2D.V2, DYNARI-3D.V2, DYNARI-2D.L, DYNARI-2D.LR, DYNARI-2D.R and DYNARI-3D.R. It is shown that the difference in riser dynamics and failure due to structural and hydrodynamic nonlinearities is important and worth the additional computational effort required. Nonlinearities for very severe loading conditions may result in differences of about 54% in bending stress in a regular wave; 33% in extreme bending stress in random seas; and 96% in fatigue damage in random seas.