Task-related wave groups for seakeeping tests or simulation of design storm waves

The chaotic wave field of a natural seaway can be decomposed into an infini te number of independent harmonic waves, and its spectrum follows from the associated wave amplitudes and frequencies. If superimposed with random pha se we register the well-known irregular sea, which is characterized by its significant wave height and zero-up-crossing period. As rare events very hi gh waves are observed accidentally. Since RAOs of wave/structure interactio ns are independent of the random phase shift between superimposing componen t waves this parameter can be selected arbitrarily to compose an optimum an d short-duration transient wave train which allows the precise determinatio n of all response amplitude operators within the relevant spectral range. A pplications of the wave group technique are presented for: (i) standard sea keeping tests of stationary or moving (self-propelled) marine structures; ( ii) simulation of design storm waves far the investigation of coastal and o ffshore structures. The paper illustrates the generation of task-related wa ve packets, the determination of the associated acceleration, velocity and pressure fields, as well as the related energy flux. Based on the dispersio n relation the propagation behavior is exactly predictable. Consequently, t he kinematics and dynamics of the wave field can be determined at any posit ion and time. If the converging wave group approaches its concentration poi nt the associated particle motions are analyzed by a nonlinear procedure us ing coupled Lagrangian expansion equations. The efficiency and the limitati ons of the transient wave technique are demonstrated by presenting typical test examples. These include the determination of the RAOs of stationary of fshore structures and towed or self-propelled ships as well as the investig ation of coastal structures in 100-year waves. As the entire process is det erministic, the action/reaction chains can be evaluated in detail. The pape r demonstrates that the wave group technique is a reliable and efficient to ol for all standard investigations related to wave/structure interactions, and opens a new area for the analysis of transient processes in the sea, e. g. dynamic stability of floating vessels or design wave impacts on coastal or offshore structures. (C) 1999 Elsevier Science Ltd. All rights reserved.