| More and more observations on the sea show that freak wave exists widely(from offshore to Ocean)and has sudden and concentrated energy.Its universality,non-small probability,suddenness and extraordinary destructiveness bring obvious harm to the safety of marine engineering structures and navigating ships.According to the existing design standards,H1%with different return periods are usually used as the design wave height.Obviously,the design wave height in the existing specifications is far less than the freak wave height,which almost ignores the existence of freak wave.This prompted the engineering and academic circles to carry out extensive research on freak wave.In this paper,the physical model test is used to investigate the dynamic response of the floaters,i.e.semi-submersible square cylinder and submerged horizontal cylinder,which are typical and have engineering background.Extensive experiments have been conducted to investigate the effect of freak wave on the dynamic responses of two typical floaters and its variation with freak wave parameter ?1,relative wave height,relative period.The results are compared with those under irregular waves without freak wave in the same spectrum.The main conclusions are as follows:Semi-submersible square cylinder:(1)Freak waves can induce low-frequency oscillation of mooring system,which makes the low-frequency response superimposed with the low-frequency oscillation caused by the second-order drift force.(2)With the occurrence of freak wave,the time that affects the dynamic response is not limited to the period of freak wave,which can last for dozens of wave periods,and increases with the freak wave parameter ?1.(3)The time domain and frequency domain responses of the mooring floating body have significant dynamic amplification effects under freak wave.In time domain,the amplification effect is mainly reflected in the maximum value of dynamic response.In frequency domain,the amplification effect is mainly reflected in the low frequency response of surge and mooring tension.(4)Compared with the direct time domain description,the time-frequency spectrum and generalized energy spectrum can capture the dynamic response characteristics of the mooring system under freak wave more sensitively and intuitively,and can indirectly quantitatively describe the duration of the freak wave effect on the dynamic response of the mooring system.(5)The influence time of freak wave on dynamic response increases significantly with the increase of freak wave parameter ?1.In the range of ?1=2.0?2.8,the influence time of freak wave on surge is in the range of 8?45 TP and the influence time on mooring tension is in the range of 10?30TP but the influence time on heave and pitch is limited to the "short period"(2?5TP).(6)There is a significant transient effect on the dynamic response under freak wave,which is significantly correlated with ?1.The transient effects of surge and mooring tension are higher than those of heave and pitch.Two dimensional submerged floating tunnel:(1)For the constraint along the submerged floating tunnel,the double inclined cable mode is better than the single inclined cable+vertical cable mode.(2)The natural frequency of the floating tunnel is easy to coincide with the wave frequency.There is an "appropriate" submergence depth:with a certain relative wave height,when the suspension tunnel is set to the "appropriate" submergence depth,the dynamic response of the system is always stable with the relative period.Under the condition of Hs/do=0.2,the"appropriate" relative submergence depth is do/LP?0.15;The "appropriate" submergence depth can also be judged by the KC number at the center of the floating tunnel:under the same condition,the KC number corresponding to the "appropriate" relative submergence depth is KC?0.8.(3)Under the same submergence depth,the freak wave will make the horizontal acceleration of the floating tunnel greater than that of the irregular wave with the same spectrum.Under the experimental conditions of Hs/do=0.1 and ?1=2.0,the maximum horizontal acceleration under freak wave is about 1.8 times that of irregular wave with the same spectrum.(4)There is dynamic amplification effect in two-dimensional floating tunnel under freak wave,which is significantly related to the freak wave height.(5)The influence time of freak wave on the dynamic response of submerged floating tunnel is much shorter than that of the semi-submersible floating body,and the increase is not significant with ?1.With ?1=2.0?2.6,the influence time of freak wave on the dynamic response of two dimensional submerged floating tunnel is 2?5TP.(6)The main parameters of time-frequency spectrum and generalized energy spectrum of the dynamic response are significantly larger than those of irregular wave with the same spectrum and increase nonlinearly with ?1.(7)The effect of freak wave on the submerged floating tunnel,especially when the freak wave parameter ?1?2.5(close to "shock"),which shows that the time-frequency spectrum of surge,heave,pitch and mooring tension appears significant energy concentration phenomenon,and the maximum value of time-frequency spectrum is significantly greater than that of irregular wave with the same spectrum.The degree of energy concentration increases with the increase of ?1.At the same time,some high frequency energy appears in the time-frequency spectrum of each motion response. |
PDF Full Text Request