Flexibility investigation of a marine riser system based on an accurate and efficient modelling and flexible multibody dynamics

The flexibility of marine riser systems in deepwater settings is studied using flexible multibody dynamics with geometrically exact beam for static, eigen and recoil analyses. The aim is to form an accurate and efficient modelling method and the corresponding solution strategies/algorithms for considering the spatial flexibility of the marine riser and movable constraints simultaneously, where the former is usually oversimplified in rigid multibody dynamics based works and the latter is inefficient in the finite element method based works. Therefore, a geometrically exact beam with the Euler's rotation vector is adopted for the marine riser, and its uniform formulation with rigid bodies is presented together with the preconditioning and solving strategies in static, eigen and recoil cases. Some other technical details for speeding up the algorithm are also described, such as parallelisation, matrix sparsity and variable step size control. To verify the present algorithm, a model with a depth of 1547 m based on a real platform is built, here considering the hydrostatic and hydrodynamic loads on riser, hydro-pneumatic tensioner and platform motion. The numerical results are compared with the commercial software Deepriser, and up to 93% coincidence is obtained in all cases with six times speedup is achieved. In addition, an extended model with more details of the real system obtained by considering independent modelling of auxiliary lines is simulated in the recoil analysis with the presented algorithm, and the results show that ignoring the stiffness of the auxiliary lines in the traditional method and commercial software may lead to incorrect conclusions regarding the tension safety of the marine riser.