NUMERICAL EXPERIMENTS IN RINGING OF OFFSHORE SYSTEMS UNDER VISCOUS LOADS
Kurtis R. Gurley, University of Notre Dame, Notre Dame, IN
Ahsan Kareen, University of Notre Dame, Notre Dame, IN
ABSTRACT
A phenomenon which has recently received much attention in offshore engineering is the ringing response of structures. This high frequency transient type response has been observed in nature, particularly in tension leg platforms (TLPs). Given the implications of this behavior on the fatigue life of tendons, it is important that it be considered for response analysis. Significant progress has been made in recent years in identifying the nonlinear mechanisms that induce ringing in complex offshore structural systems. This introductory study-uses a simple model to numerically demonstrates several of the more salient features that are commonly cited in current literature, and shows that viscous loads may result in inducing ringing type response of members under certain conditions. Ringing response in pitch due to viscous loading is simulated on a column piercing the surface, and the significant contributing force mechanisms are identified. System characteristics are altered to ameliorate the performance of these systems.
A MULTIPOLE ACCELERATED DESINGULARIZED METHOD FOR COMPUTING NONLINEAR WAVE FORCES ON BODIES
Stephen M. Scorpio and Robert F. Beck, Department of Naval Architecture and Marine Engineering, University of Michigan, Ann Arbor, Michigan
ABSTRACT
Nonlinear wave forces on offshore structures are investigated. The fluid motion is computed using an Euler-Lagrange time domain approach. Nonlinear free surface boundary conditions are stepped forward in time using an accurate and stable integration technique. The field equation with mixed boundary conditions that result at each time step are solved at N nodes using a desingularized boundary integral method with multipole acceleration. Multipole accelerated solutions require o(N) computational effort and computer storage while conventional solvers require o(N2) effort and storage for an iterative solution and o(N3) effort for direct inversion of the influence matrix. These methods are applied to the three dimensional problem of wave diffraction by a vertical cylinder.
