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Cao Y.,MARINTEK United States Inc. | Beck R.F.,University of Michigan
Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE

Desingularized boundary integral equation methods (DBIEM) and their applications in water wave dynamics and body motion dynamics over the past 30 years are reviewed. In solving the potential flow problem for wave dynamics, unlike conventional boundary integral methods, a DBIEM separates the integration surface and the control (collocation) surface, resulting in a BIE with non-singular kernels. The desingularization allows simpler and faster numerical evaluation of the boundary integrals, and thus a fast numerical solution. The paper reviews the fundamental aspects and advantages of DBIEMs. Examples of applications of DBIEMs in wave dynamics and body motion dynamics are given and the outlook of future DBIEMs development is discussed. © 2015 by ASME. Source

Cao Y.,MARINTEK United States Inc. | Graczyk M.,Norwegian Marine Technology Research Institute | Pakozdi C.,Norwegian Marine Technology Research Institute | Lu H.,George Mason University | And 2 more authors.
Proceedings of the International Offshore and Polar Engineering Conference

Motion of fluid in a moving liquid tank can be very complicated and violent. It remains a great challenge to accurately predict the flow and the hydrodynamic load on the tank which affects the motion of the vessel carrying the tank. Although significant progress has been made in the experimental techniques and advanced nonlinear computational methods (CFD), they are still very costly and time consuming to perform and it is not feasible to use these methods to study the liquid sloshing effects at the early stage of the vessel design. At the moment, a more practical way to include the liquid sloshing effects on the vessel motion is use of the potential flow model. Although the potential flow model cannot handle the violet free surface motion with wave breaking, it still can be used effectively in many applications. For instance, an effective way to achieve a good vessel motion performance is to avoid the vessel natural frequencies being close to the dominant frequencies of the environment conditions. This may be achieved with a linear wave model for the wave motion in the tanks. Some numerical methods based on the potential flow model and associated computer programs have been developed. These programs compute the sloshing load coefficients (in terms of added mass and hydrostatic stiffness) and the vessel wave frequency motion responses in the frequency domain. They give reasonably good predictions of the natural frequencies. In many situations, however, the computations of the motions of a floating system (with a vessel, mooring, risers, dynamic positioning system, and other devices⋯) need to be carried out in the time domain. The frequency-dependent added mass and hydrostatic stiffness due to the liquid sloshing must be transferred to the sloshing loads in the timedomain. Or it may be more efficient to solve for the flow and calculate the sloshing load directly in the time domain. It is important to know the limits within which the potential flow models (linear and nonlinear) are valid. This paper presents an attempt to gain a better feeling about the range of the validity of the potential flow models by comparing the predictions from the potential flow models with those by other CFD simulations and experiment measurements of the liquid motion in an oscillating tank. © 2010 by The International Society of Offshore and Polar Engineers (ISOPE). Source

Cao Y.,MARINTEK United States Inc. | Zhang F.,MARINTEK United States Inc. | Liapis S.,Global Solutions U.S. Inc.
Proceedings of the International Offshore and Polar Engineering Conference

Sloshing in liquid tanks is a serious safety and operational concern. A computer tool that can be employed to predict the onset of sloshing in tanks and estimate the severity of the hydrodynamic pressures applied on the tank walls was developed in 2010 and reported in ISOPE 2011. The work reported here is a continuation of this effort with particular emphasis on speeding up the code so it is faster than real time. The code uses a potential flow model with linear boundary conditions. The liquid in the tank is assumed incompressible and inviscid, and the flow is assumed irrotational. The flow can then be described by a velocity potential Φ(t, x, y, z) which is governed by the Laplace equation. The flow problem can be formulated as an initial-boundary value problem. At each time instant, a boundary value problem for the velocity potential is solved using the Desingularized Rankine Singularity method. A time-stepping approach is used, in which the kinematic and dynamic boundary conditions on the liquid free surface are integrated in time to update the surface elevation and the velocity potential for the next time instant. A significant speedup is achieved through parallel computation and better memory management for faster accessing stored data. Two test cases were chosen to assess the performance of the code: A square-base tank and a prismatic tank. Using a standard desktop, the code runs faster than real time, which is a prerequisite for any predictive on-line sloshing tool. Copyright © 2013 by the International Society of Offshore and Polar Engineers (ISOPE). Source

Cao Y.,MARINTEK United States Inc. | Tahchiev G.,MARINTEK United States Inc.
Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE

The paper presents a theoretical study on an active hybrid decomposed mooring system for model testing of offshore platform in wave basin. The basic concept and the working principles are described. Important issues for achieving a correct simulation will be discussed. The feasibility of the approach is demonstrated based on numerical investigations. Plans for potential implementation in an ocean basin are also discussed. Copyright © 2013 by ASME. Source

Cao Y.,MARINTEK United States Inc. | Cao Y.,C Z Marine Technology LLC | Zhang F.,MARINTEK United States Inc. | Zhang F.,C Z Marine Technology LLC
Journal of Offshore Mechanics and Arctic Engineering

This paper presents a simple and fast method to include the effect of liquid tanks of a vessel in the prediction of the vessel motions. The effects are expressed in terms of modifications to the added mass and stiffness matrices of the vessels calculated with the liquids in the tanks assumed being rigid. The flows in the liquid tanks are solved using a panel method based on the desingularized boundary integral equations (DBIEs). The numerical results were validated by the experiments of a square tank partially filled. An application example for a vessel with two internal liquid tanks is demonstrated. © 2016 by ASME. Source

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