Risers Model Tests: Scaling Methodology and Dynamic Similarity Felipe Rateiro 1 Celso P. Pesce 2 Rodolfo T. Gonçalves 1 Guilherme R. Franzini 2 André Luís Condino Fujarra 1,2 Rafael Salles 2 Pedro Mendes 3 1 TPN – Numerical Offshore Tank, Escola Politécnica – University of São Paulo São Paulo, SP, Brazil 2 LIFE&MO – Fluid Structure Interaction and Offshore Mechanics Laboratory, Escola Politécnica – University of São Paulo São Paulo, SP, Brazil 3 CENPES/PDEP/TDUT – Petrobras Rio de Janeiro, RJ, Brazil ABSTRACT This works addresses the problem of how to scale catenary riser model tests, properly considering dynamic similarity. A review on the most important dynamic characteristics and parameters is presented and a relevant group of representative nondimensional numbers is analyzed. Not only FPU induced global dynamics is taken into account as well as VIV and other important phenomena of localized nature, such as those that are typical of the touch-down zone. The general rationale is based on analytical and asymptotic dynamic solutions, previously constructed by means of standard perturbation techniques and asymptotic methods. Such a rationale gave rise to a new concept for small scale riser model design and construction. A riser model made of a silicone tube filled with stainless steel microspheres showed to best fit the intended dynamic similarity, focusing on experiments in two distinct laboratories: a wave ocean basin and a towing tank. A series of experimental tests, specially designed to assess the main dynamic characteristics and structural parameters of the riser model, was carried out. The experimental methodology and a summary of results are shown. Finally, a critical analysis, comparing static and dynamic numerical simulations, both in full and model scales, is also presented. KEY WORDS: Riser model tests; dynamic similarity; small-scale model; numerical analyses. INTRODUCTION Offshore production risers are very slender structures, conveying oil and gas from the well head to the floating processing unity. Such structures are excited at the top, through motions imposed by the floating system and all along the span length, by hydrodynamic loads. The mechanical problem is inherently nonlinear. The main sources of nonlinearities are of two types: (i) geometrical and (ii) associated to hydrodynamic forces due to the action of ocean currents; see, e.g, Pesce and Martins, 2005. The geometrical nonlinearities are related to two main aspects: (a) time varying boundary conditions, of contact type, along the touchdown zone on the sea bottom; (b) large displacements in the static equilibrium configuration. Due to the very large slenderness of the structure, in a common free-hanging configuration, bending stiffness effects are usually restricted to small regions close to the extremities, where high curvature variations are expected to occur, causing large cycling stresses. Dynamically, as far as high modes of vibrations are concerned, bending stiffness plays its role. On the other hand, viscous drag and inertial hydrodynamic forces are essentially nonlinear and strongly dependent on the kinematic state of the structures. Moreover, vortex-induced vibrations caused by ocean currents are always present. Aside from this, vortex self-induced vibrations, caused by motions imposed at top may also occur; see, e.g., Le Cunff et al, 2005, Fernandes et al, 2008, 2011. The scenario is, therefore, rather complex, still demanding research efforts. Usually, riser dynamics is treated through numerical or analytical formulations, either in time or frequency domain. The dynamic problem is commonly formulated around the static equilibrium configuration, through perturbation techniques (Triantafyllou, 1984, Aranha et al, 1997, Pesce et al, 1999, Chatjigeorgiou, 2008a,b) making sure the nonlinear contact problem at the touch down zone is treated consistently(Pesce et al 2006), sometimes considering, as well, soil- structure interaction modeling (Leira et al, 2004, Zhang and Nakhaee, 2010). Recently, exact kinematic finite element formulations and nonlinear dynamics techniques have been also applied in order to help further understanding some puzzling nonlinear interactions and internal resonances that are prone to occur; see, e.g., Sanches et al, 2007, Mazzilli and Sanches, 2009, Srinil, 2010. 439 Proceedings of the Twenty-second (2012) International Offshore and Polar Engineering Conference Rhodes, Greece, June 17–22, 2012 Copyright © 2012 by the International Society of Offshore and Polar Engineers (ISOPE) ISBN 978-1-880653-94–4 (Set); ISSN 1098-6189 (Set) www.isope.org