Stduy On Dynamic Response And Anti-collision Performance Of An Assembled Floating Anti-collision System

With the economic expansion of many major coastal cities around the world and the associated increase in the number of navigation vessels in the coastal waters,the risk of vessel collisions with coastal and offshore structures rise significantly.The collision accidents can lead to serious consequences with massive economic losses and even human casualties.Nowadays,the flexible floating anti-collision system(FFAS)is has been proven as an effective way to prevent the possible vessel collisions.There are many advantages to a typical FFAS—for example,it is easy to install,expand,relocate,and remove,and it is cost effective for deep water and poor foundations.However,it is not long since the concept of FFAS was put forward and thus its engineering application and relevant researches are still limited.Particularly,there is a noteworthy lack of the survival under harsh wave conditions,the mechanism of anticollision,the performance of the energy absorption and dissipation,which greatly restricts the development and application of FFAS.Therefore,to solve these afore mentioned key issues,extensive studies are carried out to investigate hydrodynamic responses of the FFAS under harsh wave conditions and the dynamic responses of the FFAS under vessel collisions.The study is expected to provide an important reference for the evaluation of the safety and anticollision performance,the structural design and practical application of the new-type FFAS.Firstly,model tests are conducted on the hydrodynamic response of the pontoon-type modular system under regular waves and irregular waves.The motions of the buoys,their relative motion,and the tensions of the connecting and mooring lines are acquired and analyzed synchronously.The emphasis is focused on the changing rules of the coupled characteristics of the modular system under various wave conditions.And the sensitivity of the dynamic responses of FFAS is discussed to the wave period,wave height and wave obliquity.The results showed that the resonance of the buoy in the sway,surge and roll directions have an important influence on the dynamic responses of the entire system.The tensions of the connecting lines have strongly nonlinear characteristics and are affected by several frequency components of multiple-wave frequency.Compared to the beam wave conditions,the relative motion between buoys is more prominent in oblique waves,resulting in considerable snap loads on the connecting lines.Then the dynamic responses of the FFAS under vessel collisions were investigated experimentally.The electromagnetic and optical measuring methods are combined to realize the synchronous measurements of the horizontal movements of each buoy,the 6-DOF motions of the vessel and collided buoy.Both the collisions from propelled ships in calm water and uncontrolled vessels driven by wave-current actions are considered carefully.The importance influence of the collision parameters(collision position,collision gesture of the vessel,initial collision velocity),and the wave-current parameters(relative wave period,relative current velocity)on the dynamic responses of the FFAS are discussed in detail.The varying force distribution along the buoy chain against various relevant factors is also analyzed.It is found that the buoy motion and tensions of the mooring and connecting lines of the FFAS increase with the increase of the collision velocity.The growing trend is approximately linear under the head-on vessel collisions,while it is logarithmic and exponential trends for the side-on collisions.The head-on collisions scenarios are more hazardous than the side-on collision cases.The mooring line tensions are greatest for the head-on collision in the near-center region of the buoy,while the maximum connecting line tensions happens for the head-on collision in the connecting lines.With respect to the collisions of the uncontrolled vessels driven by wavecurrent actions,the vessel would squeeze and beat the buoys frequently,and this leads to noteworthy dynamic responses of the FFAS and consequently the fatigue risks of the mooring and connecting lines.Moreover,it should be considered carefully that the snap loads occur frequently in the connecting lines and can be up to 1.5-2.0 times of the static forces.Finally,the anti-collision mechanism and anti-collision performance of the FFAS are estimated based on the energy method.Applying the kinematics equation,momentum theorem and the energy conservation equation,the transfer process of the impact energy of the vessel is theoretical analyzed based on the dynamic response results of the vessel collision tests.Subsequently,the anti-collision mechanism of the FFAS is fully revealed.The energy storage coefficient and the effective energy-absorption zone are defined,and the influences of various collision parameters and wave-current parameters on them are explored.The evaluation methods of the anti-collision performance of the FFAs are proposed and the suggestions for its design and application are also addressed.The results showed that the impact energy is mainly converted into the elastic potential energy of the mooring lines for the head-on collision cases.And the energy storage coefficient increases exponentially with the ship's collision velocity and this trend is most prominent for collision on edge area of the system.Considering the influence of energy absorption range,the anti-collision capacity of the FFAS under head-on collision is close to the energy storage limit of the single mooring line.However,as for the side-on collision cases,the energy storage coefficient always keeps around 20%,which strongely enhances the anti-collision capacity of the FFAS up to the energy storage of 5 times of the straight section of ships.