Theoretical And Applicational Study On The Nonlinear Hydroelasticity Of Ships With Forward Speed

When a ship is moving in the large amplitude waves, the motions and wave loads shows evident nonlinear character; In addition, the loads of the traveling ship and the structural deflections and damage actually are a dynamic process. The traditional analysis method which deals with the hydrodynamic and structural problems respectively has certain disadvantages. While it has great engineering practical value to establish the nonlinear hydroelastic analysis method for ships moving in large amplitude waves, taking the perspective of the fluid structure interaction. This method which can be used to study the safety performance of the traveling ships in rough sea is also rational and original theoretically.In the past two decades, the theory and corresponding numerical analysis methods of hydroelasticity of ships have been developed from two dimensional to three dimensional, from linear to nonlinear. The hydroelasticity theory has been extensively applied to a wide range of fluid structure interaction problems in the field of naval architecture and ocean engineering, such as the motions, loads and the structural responses assessment of the large scale ships, high performance multi-hull ships and Very Large Floating Structures (VLFS). The theory of hydroelasticity is now a new rising subject with great importance and potential of further development and engineering application.The present research works are developed in the field of three dimensional hydroelasticity of ships. Firstly the available three dimensional linear hydroelastic analysis methods(Wu, 1984; Du, 1996) of ships are inherited and are extened to accounting for the influence of the forward speed and the steady wave flow effects to the hydroelastic responses of ships numerically more rigorously; Based on the second order nonlinear hydroelasticity (Wu, Maeda & Kinoshita, 1997) of floating body, including the forward speed and steady wave flow effectrs, taking into account the nonlinear hydrodynamic actions induced by the the large rigid body motions and variation of the instantaneous wetted surface area, the three dimensional nonlinear hydroelastic analysis methods for ships moving in large amplitude waves are established. The following works have been achieved in the present thesis:(1) A brief review on the development history of the hydroelasticity theory of ships and the corresponding numerical methods has been given. Based on the theoretical foundations of Wu, Maeda & Kinoshita (1997), the generalized second-order nonlinear hydroelastic theory for ships with forward speed in rough seas is deduced, only considering the contribution of the first order velocity potentials responses to the second order hydrodynamic forces. The formulations for each second order hydrodynamic coefficients in the general three dimensional nonlinear hydroelastic motion equations are presented.(2) The state of the art of the analysis methods dealing with the steady flow fields in seakeeping theory has been summarized. A simple desingularized virtual panel method is proposed which can be used to solve the higher order derivatives of the non-uniform steady wave flow velocity potential. A moving submerged ellipsoid is selected as an example and the computation results are compared with the analytical solutions, showing that this method is useful and effective, particularly for smoothed surfaces without sharp corners. In addition, applying the boundary integral formula derived by Kim (2005), the higher order derivatives of the velocity potential when the ship is moving steadily can also be solved numerically by using the Havelock translating source Green function and the first order partial derivatives.(3) Including the steady wave flow effects, the three-dimensional hydroelastic analysis program of ships with forward speed is developed. The moving submerged ellipsoid and a water-piecing elastic half-ellipsoid like ship and a SWATH ship is utilized as the numerical models to study the influences of the uniform flow, double body flow and steady wave flow model to the hydroelastic responses.(4) Taking into account the non-uniform steady wave flow surrounding the elastic ship hull, the nonlinear hydrodynamic actions induced by the rigid body rotations and the variation of the instantaneous wetted surface area are deduced, the three dimensional second order nonlinear hydroelastic analysis methods of ships moving in large amplitude waves are established. The differences of the predicted linear and nonlinear responses (deflections, stresses) are illustrated by the numerical examples of a SWATH ship traveling with forward speed in irregular waves.(5) Based on the previously linear and nonlinear hydroelastic analysis method, the wave loads of the SWATH ship in waves with and without forward speed are predicted and compared with the model tests data abailable.The present work provide the possibility of predicting both the linear and nonlinear hydroelastic responses of traveling ships or floating structures in large amplitude waves, mathematically more close to the theoretical formulae presented by Bishop et al (1986) and Wu, Maeda & Kinoshita (1997), and numerically more rigorously accounting for the forward speed effect than the previous work (Du, 1996; Chen, 2001). The main points of originality in this dissertation are as follows:(1) Based on the available steady flow analysis method in the seakeeping theory, a desingularized virtual panel method is proposed which can be used to solve the higher order derivatives of the non-uniform steady wave flow velocity potential. The numerical example of a submerged moving ellipsoid shows that the computation results correlates well with the analytical results.(2) The steady wave flow effects is firstly included in the three dimensional hydroelastic analysis of ships, a more rational three dimensional hydroelastic analysis of ships with forward speed is developed (THAFTS). In addition the translating and pulsating source Green function is utilized to accounting for the forward speed effects more rigorously in the hydroelastic responses.(3) Aaccounting for the steady wave flow effects, the second order nonlinear hydroelastic analysis methods are established firstly, which can be used to predict the steady wave flow, motins and the dynamic wave loads and stresses of arbitrary shape floating structures with forward speed in large amplitude waves. A numerical example (Tian & Wu, 2006a) is given for the first time to illustrate the three dimensional nonlinear hydroelastic responses of ships with forward speed in random waves. (4) The 3D nonlinear hydroelasticity theory is firstly applied in a real SWATH ship, and the wave resistance, motions, wave loads, deflections and stresses are systematically analyzed. The conclusions drawn here may have some reference value to the practical design and application of the high performance catamaran and multi-hulled ships.Finally, the nonlinear hydroelastic analysis method developed in the present thesis can also be applied to the conventional ships. It has significant practical value in develping the ship structure fatigue analysis method based on the direct loads calculation (DLA) for the classification societies.