Investigation On Dynamic Response Of Multiple Body System In Waves

Multiple floating body systems, such as oil/gas offloading in FPSO,grouped-columns structure and very large floating structure, are increasinglyutilized in ship and ocean engineering industry. Owing to the coupledstructural and hydrodynamic effects between modules constituting floatingbody system, its dynamic response is distinct from that of single floatingsystem.As to weakly-connected multiple floating body system with small sizeand several modules, multiple rigid body dynamics is universally applied,where wave loads is obtained by potential theory. In this paper, a numericalmodel composed of two ships with Wigley hull arranged side-by-side ischosen to investigate the second-order wave forces, which is the focus ofmooring design and dynamic position system operation. The influence ofsheltering effects and radiation phenomenon acting on second-order forces isstudied by comparing hydrodynamic interactions in the free-floating modeland fixed model. According to the pressure field of fluid domain as well aspeak periodic oscillation of response values versus distance between ships,near-trapping mode phenomenon is revealed in this system. Subsequently,general conclusion is given to minimum resonance frequency, gap betweenadjacent resonance distance and forward shift phenomena of resonancefrequency when near-trapping mode occurs. Newman approximation method is then approved satisfactory as to multiple floating systems by comparingstatistical values with results of full QTF method.A model composed of two rectangular boxes connecting by spring isthen proposed to investigate the influence of conectors on multiple floatingsystems. Its hydrodynamic performance is computed by adjusting relevanthydrostatic restoring matrix. It is found both the stiffness and position of thespring would significantly affect the hydrodynamic results. Resonancephenomenon occurs under the action of wave-frequency forces when stiffnessof the spring fits certain condition.As to strongly-connected multiple floating body system with enormoussize and many modules, hydroelasticity mechanics is commonly adopted,where wave loads take flexible deformation into consideration. Amultisegment beam model is proposed to investigate the dynamic response ofa very large floating structure in waves. The relationship between verticaldisplacement (including rigid part, flexible part and total value) and otherparameters, including wave length, wave frequency and connector’s rigidity,is then discussed. The importance of rigid motion and flexible displacementis illustrated as to different ocean environments; and the relationship betweenVLFS’s length and wave length cooresponding to maximal flexibledisplacement is found. Significant influence of the connector’s rigidity onvertical motion and cross-section bending moments is also dissucssed bycomparing the numerical results of different conncectors.