RISERS NONLINEAR DYNAMICS INCLUDING THE UNILATERAL CONTACT WITH THE SEABED

Abstract

Risers are structures used for oil exploitation, being modeled as long beams, subjected to their own weight, external and internal pressure loads, sea current and sea wave loads. There is also a nonlinear boundary condition in certain riser configurations, such as catenary and lazy-wave, which consists of a unilateral contact between the riser and the seabed. To analyze these configurations one needs the statics and dynamics, possibly with large displacements and large rotations magnitudes, depending on the type of study and conditions considered. In this context, a nonlinear kinematically-exact beam formulation becomes an attractive choice for modeling a riser, in order to capture the geometric nonlinearities present. Furthermore, the representation of riser-seabed contact interaction may be done carefully. In this project a dynamic modeling of risers is proposed, with the usage of nonlinear beam models, with no theoretical magnitude limitation for displacements and rotations, and also a treatment of the nonlinear contact condition between the riser and the seabed using the Mortar Method, with the usage of Lagrange multipliers. Specifically for the contact interaction, the structure will be described using splines or the Moving Least Squares Method. The development of this new contact modeling technique will lead to interesting results of riser-seabed dynamic interaction. Furthermore, the model may be used for studies of practical dynamic problems, such as the risers' instability with a dynamic focus, loop formation scenarios and for predicting the curvature radiuses in post-buckling configurations, in dynamic compression situations of riser region close to the touchdown point.