Analysis of fixed and floating structures in random multi-directional waves

Offshore structures have traditionally been designed on the assumption of long-crested or uni-directional incident waves. Realistic sea states are however short-crested or multidirectional with a distribution of wave energy over both frequency and direction. The present thesis investigates the influence of the directional spreading of wave energy on the forces on fixed structures and motions of floating structures. The work is both theoretical and experimental, with the experiments carried out at the multi-directional wave basin of the Hydraulics Laboratory at the National Research Council in Ottawa. Different methods of estimating directional wave spectra are evaluated using numerically synthesized time series of the water surface elevation and horizontal orbital velocities at a single location, and the maximum entropy method is found to provide the best directional resolution. The maximum entropy method is developed further to estimate directional wave spectra from an array of wave probes. Expressions are developed for the spectral densities of the inline and transverse components of the force on a slender cylinder in random multi-directional waves, and for the probability distribution of the peaks of the corresponding resultant force. The former are based on a linearization of the Morison equation, while the latter is based on the assumption of a narrow-band spectrum. Experiments were carried out to measure the forces on a segmented vertical cylinder in random multi-directional waves. The theoretical expressions for the force spectral density and probability distribution match the measured data reasonably well. Reduction factors relating the forces in short-crested seas to the forces in long-crested seas are also presented. Experiments were also carried out with a moored floating barge in regular and random multi-directional waves. The experiments show an increase of the sway, roll, and yaw motions due to directional spreading, and a slight reduction of the pitch and first order surge motions. The second order, low frequency surge motions are however significantly reduced in multi-directional waves. Linear diffraction theory is used to predict the transfer functions for the first order surge, heave, and pitch motions of the barge. Reasonable agreement was obtained between the measured and predicted first order transfer functions. A procedure to compute the spectral density of the second order drift forces in multi-directional waves based on the concept of a bi-frequency, bi-directional quadratic transfer function is also presented.Applied Science, Faculty ofCivil Engineering, Department ofGraduat

Wave loads and motions of long structures in directional seas

The effects of wave directionality on the loads and motions of long structures is investigated in this thesis. A numerical method based on Green's theorem is developed to compute the exciting forces and hydrodynamic coefficients due to the interaction of a regular oblique wave train with an infinitely long, semi-immersed floating cylinder of arbitrary shape. Comparisons are made with previous results obtained using other solution techniques. The results obtained from the solution of the oblique wave diffraction problem are used to determine the transfer functions and response amplitude operators for a structure of finite length and hence the loads and amplitudes of motion of the structure in short-crested seas. The wave loads and body motions in short-crested seas are compared to corresponding results for long-crested seas. This is expressed as a directionally averaged, frequency dependent reduction factor for the wave loads and a response ratio for the body motions. Numerical results are presented for the force reduction factor and response ratio of a long floating box subject to a directional wave spectrum with a cosine power type energy spreading function. Applications of the results of the present procedure include such long structures as floating bridges and breakwaters.Applied Science, Faculty ofCivil Engineering, Department ofGraduat

Response of a Compliant Cylinder to Irregular Waves

NRC publication: Ye

Wave Transformation Over Reefs: Evaluation of One-Dimensional Numerical Models

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Seaworthiness through intelligent trajectory control

Navigation of autonomous surface vessels, less than 20 m in length, in large sea states is difficult and often precludes successful completion of the assigned mission or, in the worst case, survivability. Operation in high seas requires sensing of the local wave environment and determining a vessel trajectory that maximizes survivability based on knowledge of the vessel response functions and prediction of the incident wave field forward in time. To achieve this objective, new technologies are being developed and tested in full scale at the Marine Autonomy Research Site (MARS), located in central Lake Superior and operated by Michigan Technological University. In this initial set of experiments, a skilled human operator was used as a surrogate for an envisioned wave-adaptive autonomous control system. The test vehicle is a fully instrumented personal water-craft, operated by a U.S. Coast Guard-trained surf-boat operator in moderate sea states with Froude number (Fr) = 1.0 through a course consisting of up-wave, cross-wave, and down-wave legs. Results dramatically document that the wave-dodging maneuvers employed are designed to minimize vessel pitch (preserve propulsor and rudder control) while allowing increased vessel roll. Comparisons of the straight line with wave-dodging circuits during constant sea state conditions show that vehicle roll is at times twice greater in wave-dodging runs while vehicle pitch averages half to one third of that in the straight-line course. These data suggest that optimum paths do exist through steep, evolving incident wave fields and these optimum paths can produce significant improvements in vessel survivability

Peregrine's system revisited

43 pages, 91 references, 15 figures, 2 tables. Other author's papers can be downloaded at http://www.denys-dutykh.com/International audiencePeregrine's system revisited arXiv.org / halIn 1967 D. H. Peregrine proposed a Boussinesq-type model for long waves in shallow waters of varying depth. This prominent paper turned a new leaf in coastal hydrodynamics along with contributions by F. Serre, A. E. Green & P. M. Naghdi and many others since then. Several modern Boussinesq-type systems stem from these pioneering works. In the present work we revise the long wave model traditionally referred to as the Peregrine system. Namely, we propose a modification of the governing equations which is asymptotically similar to the initial model for weakly nonlinear waves, while preserving an additional symmetry of the complete water wave problem. This modification procedure is called the invariantization. We show that the improved system has well conditioned dispersive terms in the swash zone, hence allowing for efficient and stable run-up computations