Non-linearities and Coupling Effects on Floating Breakwaters Eigenvalues

摘要

We have seen, the consequences of non-linearities and coupling effects on floating breakwaters eigenvalues and take a prominent part in the design of floating breakwaters anchored on piles. For some exposed sites, the greatest problem is no longer set in terms of extreme loads but in terms of stiffness and of natural periods. So, we can draw four concrete conclusions from this analysis: For floating breakwaters linked on piles with linear stiffness, limitation of motions and anchorage loads is assured by setting all piles bending eigen-periods smallest than half of modal period of sea state spectrum and typically at most of the order of 1.0 to 1.5 s. RAO plotted on Fig. 1 shows that this usual approach is inadequate as early as anchorage stiffness is nonlinear. If equivalent linear damping verify alpha~2 > alpha~2 (1 - alpha~2)~2 > |mu|, or better 4alpha~4 > 4alpha~4 (1 - 2alpha~2) > |mu|, RAO is broken in two parts and low frequency amplification concerns only periods greater than 5 or 10 times eigenperiods. We must try to obtain a good enough linearity of anchorage stiffness, in order to eliminate the three risks of eigenperiod sliding, superharmonic resonances and combined resonances. This objective can induce us to increase the flexural rigidity of the pile. If the depth of water or the thickness of soft soil is too important, it can be more convenient to fill up piles with concrete than to increase wrongly the steel thickness. We must try to obtain a sufficient viscous damping of hull, may be alpha > 0.15 or 0.20, in order to restrict the motions amplitudes of floating breakwater, in particular for the sway, roll and yaw degrees of freedom. It must be convenient to eliminate bilge radius and maybe to have recourse to bilge keels. When the modal period or the significant height of the sea state is too large, typically greatest than respectively 4.0 s and 1.0 m, it must be convenient to renounce to use classical breakwater pontoons and to adopt any special feature as, for example, a floating semi submersible wave attenuator (ASB) [7].