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ABSTRACT This paper addresses some noteworthy findings relating to particular hydrodynamic and hull features leading to the capsizing of a box-shaped pontoon barge in a wave basin. Experimental evidence suggests that these features may substantially influence the capsizing behavior of a shallow deck barge. Most significant are the influences of viscous damping and green water on deck. Both factors substantially shift the resonance frequency and delay barge motions relative to wave excitations. A shallow barge constantly experiences changes in its wetted hull shape that drastically change its stability characteristics in heavy seas. Whereas these subtle changes in resonance frequency and phase lag may be irrelevant to the stability of a large conventional hull, these same changes may lead to the dramatic loss of a pontoon barge with limited freeboard. Test observations raise concerns about the legitimacy of a stability assessment procedure based on potential theory. A proper means capable of capturing the relevant stability features is a necessary step in setting loadline criteria for pontoon barge. A watertight pontoon barge with reasonable reserve stability rarely capsizes. Yet, mishaps were demonstrated in a model basin for conditions of vessel metacentric height close to zero. INTRODUCTION Pontoon platforms, which originated primarily for inland uses, have been assuming more important roles offshore in recent years. The attributes of modular construction and large payload capacity make them a low-cost candidate for contingent operations at sea. Recent advances in technology, which allow on-site assembly of a floating structure in rough waters (Huang, 1997) further enhance their attractiveness for offshore applications. Primary uses include working platforms and lighterage as shown in Figure 1. (figure 1 shown in paper) Although these floating assets exhibit certain functional qualities resulting from the box shape, they also exhibit the weather sensitive motions typical of a large waterplane hull.
ABSTRACT A Reynolds-Averaged Navier-Stokes (RANS) numerical method has been employed in conjunction with a chimera domain decomposition approach for time-domain simulation of large amplitude ship roll motions. For the simulation of arbitrary roll motions, it is convenient to construct body-fitted numerical grids for the ship and ambient flow domain separately. The ship grid block is allowed to roll with respect to its center of rotation under either forced or free roll conditions. The roll moments are computed every time step by a direct integration of the hull surface pressure and shear stresses obtained from the chimera RANS method. The simulations for prescribed roll motions of a full-scale motor vessel clearly show that the bilge keels at the mid-ship produced large roll damping but generated very little waves. On the other hand, the ship skag acts like a wavemaker during the roll motion and produced large wakes in the stem region. Time-domain simulations were also performed for a freefloating pontoon barge in free decay motions and under large amplitude incident waves. The simulation results successfully predicted the roll resonance when the incident wave coincides with the free decay period of the barge. INTRODUCTION The roll motions of ships and barges to incident waves are one of the primary concems in naval architecture and ocean engineering. For small amplitude wave and body motions, linear or perturbation theories are often used in frequency domain to predict the responses. During the past decade, several numerical and experimental studies have been conducted for roll motions of floating bodies. Time-domain fully nonlinear simulations of parametric roll motions were studied by, among others, Cointe et al. (1990) and Tanizawa and Naito (1997, 1998). However, most of the numerical studies were using potential flow methods with the artificial damping terms added to dynamic and kinematic free surface boundary conditions.