Research On Generation Of Freak Wave And Its Effect On Truss SPAR Motions

This thesis focuses on freak wave generation and Truss SPAR motion due to freak waves,main contributions as follows.1.Freak waves are generated in physical wave tank and some significant conclutions are summarized on generation results of freak waves.The wave-making signal is generated by spational-temporal focusing of wave energy,in order to keep the whole original wave-making signal and its executability,the original signal is processed by pre-interpolation and extention.A series of tests of freak waves are performed in physical wave tank,measured results from freak wave tests are compared and influences from different parameters are summarized on generation results,which is a practical reference for researches related to freak waves.2.Modified Finite Difference Interpolation scheme was developed and studied results indicate considerable improvement on accuracy and efficiency of gradient estimation in nonlinear wave field.The basis function of Finite Difiference Interpolation scheme is definited as hyperbolic or exponential feature,Investigations on calculation of gradient in Stockes 5th order wave,the 2nd order wave field and tested waves demonstrate this method has considerably higher accuracy,shorter computing time than the original Finite Difference Interpolation scheme and Linear moving least square method,besides this scheme is more independent to mesh layer,which could improve simulated results of strongly nonlinear waves.3.An estimation formulation was obtained for how to choose time and space parameters of freak wave and research results indicate that after high-frequency wave energy coefficient reaches a steady value,freak waves will emerge.A nonlinear numerical wave tank for simulating freak wave was built by QALE_FEM-solver and modified finite difference interpolation scheme,critical time parameters corresponding to different space parameters were calculated by wave amplitude critia,as a result a formula on selection of time and space parameters was concluded by mathematical fitting.Three wave energy coefficients,i.e.low-frequency coefficient CEL,inputting coefficient CEC and high-frequency coefficient CEH,those coefficient results of different freak waves display freak wave will generate after the high-frequency coefficient reaching a steady value.The formulation above could be a preference on selection of time and space parameters in its physical generation and numerical simulation;in addition the high-frequency coefficient CEH can be as an indicator of freak wave prediction.4.Nonlinear hydrodynamic loads on Truss SPAR were investigated in random waves,the results revealed those nonlinear hydrodymanic loads mainly affected on low-frequency motion and mooring force of Truss SPAR,and occasionally their influences was significant.A new estimation scheme of mooring loads was developed for taut mooring line and appied to Truss SPAR,hydrodynamic force on Truss SPAR was calculated by the slender formula,and Truss SPAR motion was predicted by solving nonlinear equations of rigid body motion.Several hydrodynamic terms negelected in slender formula were investigated,studied results indicate low-frequency motions of Truss SPAR may significantly larger than its real motions if without including those nonlinear hydrodynamic loads,thus it is necessary to consider those nonlinear loads in prediction of Truss SPAR motion,in order to improve solution accuracy.5.Truss SPAR motions in freak waves were measured,and comparison results demonstrate motion amplitudes are relied on wave energy distribution of freak wave around natural frequencies of motions.According to spational-temporal focusing of wave energy freak waves were gengerated in physical wave tank,and the floatation of Truss SPAR was adjusted as mirror imaging principle.A optical capturing equipment was used to record motions of Truss SPAR,from comparisons among results of freak waves with different frequencies and amplitude distribution of wave components,it is found that if the more wave enengy is focused around natural frequency of motion,the larger corresponding amplitude of motion is performed and vice versa.Those results are helpful to understand motion characteristics of Truss SPAR in freak waves,which is meaningful guaidance for designing Truss SPARs under freak wave states.6.A numerical method was developed to calculate motions of Truss SPAR in freak waves,and motion coupling was investigated.The conclusion is the coupling effect among motions is nonlinear process and acts significantly.A simulation method is developed by combination of QALE_FEM-solver,MSDFI scheme and slender formula.It is applied to investigate coupling among motions of Truss SPAR in freak waves,results display if allowing or restricting some motions,and other motions may not change or vary largely.Those results indicate some motion could be reduced by motion coupling effect,which could make Truss SPAR performs its task normally under some conditions.