Dynamic Characteristics of Floating Breakwaters
Doctoral thesis, 1996
The dynamic behaviour of interconnected floating breakwaters has been studied theoretically and experimentally. For this type of shore-protection system, problems that must be dealt with include wave loads on the breakwaters caused by the interaction between the bodies and the waves, motions of the moored breakwaters caused by the waves, changed wave fields in front of and behind the breakwaters, and forces on the mooring cables and in interconnections between breakwater units.
These problems have been solved in two major steps. The first step consists in solving the hydrodynamic forces in the frequency domain by means of linearized diffraction theory. The boundary value problem involved in obtaining the velocity potential on or around the bodies is solved numerically by a panel method. The second step deals with the dynamic response problem. The equation of motion is extended to include the interaction between the bodies conveyed by mechanical couplings and the interaction between a body and its mooring cables. Some significant nonlinear effects are also taken into account in the dynamic modelling, such as wave drift force, viscous damping, and drag force on the mooring cables. Owing to the facts that restoring forces imposed by the mooring cables are highly non-linear even for a small motion, and that the non-linear stiffness for a complex cable arrangement cannot be linearized by any simple method, the Newton-Raphson method is applied with an iteration technique for the resulting non-linear systems of equations.
Based on the theories presented, a computer program FB (floating bodies) was developed for estimating the wave loads. An extension of the FB program has been made in conjunction with a well-developed dynamic mooring analysis program, MODEX. The program package should be of practical use for predicting the wave loads, system response, wave field near the objects, and the dynamic performance of the cables.
Experiments on box-type floating breakwaters were carried out in a model basin with waves of different heights and periods. Results from model tests are presented for motions, cable tensions and wave attenuation for regular beam and oblique waves. The 3-D motion tests were performed using a camera and video processor-based measurement system, MacReflex. Data aquisition for waves and forces was carried out by another system, MP100. Results indicate that the dynamic model should prove to be a useful tool in the design of floating breakwaters and offshore structures.