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

Abstract

The chaotic wave field of a natural seaway can be decomposed into an infinite number of independent harmonic waves, and its spectrum follows from the associated wave amplitudes and frequencies. If superimposed with random phase we register the well-known irregular sea, which is characterized by its significant wave height and zero-up-crossing period. As rare events very high waves are observed accidentally. Since RAOs of wave/structure interactions are independent of the random phase shift between superimposing component waves this parameter can be selected arbitrarily to compose an optimum and short-duration transient wave train which allows the precise determination of all response amplitude operators within the relevant spectral range. Applications of the wave group technique are presented for: (i) standard seakeeping tests of stationary or moving (self-propelled) marine structures; (ii) simulation of design storm waves for the investigation of coastal and offshore structures. The paper illustrates the generation of task-related wave packets, the determination of the associated acceleration, velocity and pressure fields, as well as the related energy flux. Based on the dispersion relation the propagation behavior is exactly predictable. Consequently, the kinematics and dynamics of the wave field can be determined at any position and time. If the converging wave group approaches its concentration point the associated particle motions are analyzed by a nonlinear procedure using coupled Lagrangian expansion equations. The efficiency and the limitations of the transient wave technique are demonstrated by presenting typical test examples. These include the determination of the RAOs of stationary offshore structures and towed or self-propelled ships as well as the investigation of coastal structures in 100-year waves. As the entire process is deterministic, the action/reaction chains can be evaluated in detail. The paper demonstrates that the wave group technique is a reliable and efficient tool 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.