Journal of Waterway, Port, Coastal, and Ocean Engineering

We consider the problem of estimating in real time the profile of ocean currents on towed arrays from positioning and tension measurements. This ill-posed inverse problem is addressed in the context of Bayesian statistics using maximum a posteriori estimation. Drawing the analogies to the generalized Tikhonov formulation, we implement an efficient algorithm that optimizes regularization and, subsequently, derive confidence intervals on the estimated currents. We then address the problem of reconstructing the profile of the angle of attack on the basis of the estimated ocean velocities and propose a stochastic simulation algorithm to evaluate it. To demonstrate the performance of our methods, some results from simulated experiments are presented.

This paper addresses coupled wave and wind-induced motions of spar-type 5-MW wind turbines in harsh and operational environmental conditions. Global dynamic motion responses have been analyzed by aero-hydro-servo-elastic time-domain simulations. The aerodynamics is based on an advanced blade element momentum theory. Panel method and Morison formula accounting for the instantaneous position of the structure are applied for hydrodynamics. Hydrodynamic drag and considering geometrical updating introduce nonlinearities. Hydrodynamic nonlinearities were found to cause excitation of the natural frequencies in the low frequency range more than in the wave frequency range. Extrapolation methods are applied to estimate the maximum responses. A previous study showed that the uncertainty of such an extrapolation for the present concept is less than 2%. In this study it is found that the mean values of the dynamic responses are primarily wind induced and the standard deviations of the responses are primarily wave induced. However, the standard deviation of the nacelle surge motion under operational conditions is primarily wind induced. The maximum of the responses under operational and survival conditions are wind induced and wave induced, respectively. For the present turbine, a power of 5 MW can be achieved at a mean wind speed greater than $15\mathrm{m}/\mathrm{s}$ , whereas the rated wind speed for the original land-based wind turbine is approximately $11\mathrm{m}/\mathrm{s}$ . It is found that the wind turbulence does not affect the dynamic motion and structural responses significantly. However, turbulence affects power production. The ratio of the bending moment and the shear force in the tower-spar interface under a 100-year environmental condition to the corresponding responses associated with the rated wind speed can be as high as 1.5 and 2.3, respectively. It is therefore important to consider survival environmental conditions in the ultimate limit state checks of floating turbines, whereas operational conditions are normally most critical for land-based wind turbines. Moreover, it is shown that under survival conditions, the normalized maximum responses, i.e.,  $(\mathrm{\text{maximum mean}})/(\mathrm{\text{standard deviation}})$ , corresponding to an upcrossing rate of 0.0001 based on five 1-h analyses varies between 4 and 6, whereas for operational cases the normalized maximum varies between 2 and 4.

A set of numerical model experiments has been conducted to simulate the circulation driven by oscillatory forcing over a theoretical continental slope configuration used previously in laboratory experiments. The test case considered was the numerical simulation of the flow over a model of a submarine canyon, and the numerical model used in the analysis was a coastal ocean model version based on an adaptation of the finite-calculus–finite-element method (FIC-FEM) approach implemented in the commercial package Tdyn. Two cases were analyzed involving changes in fluid density. Structured and unstructured finite-element spatial discretizations were generated for the same study domain to compare the resulting velocity field with outputs from the laboratory experiments and to assess which mesh provided a better representation of the complex geometry of the channel model and the water circulation process. The comparison between the laboratory results from the reference article and the output of the numerical model showed good agreement in the structure and magnitude of the phase-averaged and residual velocity fields.

Floating fish cages provide the main production utilities for salmon farming. However, despite their pivotal role in production safety as well as in protection of the environment, much room exists for further development in relation to verified structural design procedures and computerized tools for structural analysis. In this paper, a statistical analysis of stress range histories is presented and compared with results from rainflow counting. The study is based on a structure being applied by the Norwegian fish farming industry today. The floater is made of steel cylinders configured as a square. The fatigue design analysis is based on time domain analysis of the structure in irregular waves. The statistical properties of the stress range histories are investigated using Weibull plots, histograms, damage plots, and, finally, estimation of short- and long-term distributions and evaluation of its applicability in relation to simplified methods. The Rayleigh distribution and the two-parameter Weibull distribution are applied. A visual inspection of the short-term Weibull plots indicates a quite good fit, and the shape parameter is typically between 0.6 and 0.8. However, a closer inspection of the histograms shows that the process seems to be a combination of a high-frequency/low stress range level process with a Weibull shape parameter $h<1$ and a low-frequency/high stress range level process with a Weibull shape parameter $h>1$ .

Prediction of wave overtopping is a key task in the design and safety assessment of coastal structures. In this study, M5′ model tree as a new soft-computing approach was used to develop a model for the prediction of the wave overtopping rate at rubble mound breakwaters. The main advantages of model trees are that they are easier to deploy and more importantly, they produce understandable formulas. Selected data from the CLASH database were used for training of the model, and the conventional governing parameters were used as the input parameters. The obtained results were also compared with those of previous models. The accuracy of the model was evaluated by statistical measures, and it was shown that the developed model is more accurate than previous models. Furthermore, the model was validated with the prototype overtopping measurements in three sites. Results indicated that the developed model outperforms the previous models in predicting the full scale overtopping rates as well.

Coastal regions are continually plagued by high water levels induced by river flooding or hurricane-induced storm surge. As with any protective structure, it is essential to understand potential problematic locations that could result in structural failure and devastating loss. Common coastal protective systems are composed of floodwalls and levees, for each of which practiced methodologies have been used to estimate their performance under design conditions. Methodologies concerning spatial variability are limited, however, and transitions where earthen levees merge with floodwalls are considered areas vulnerable to erosion and possible breaching. Physical modeling of a levee transition is undertaken in a three-dimensional wave basin to evaluate this hypothesis, and the detailed results of this assessment are presented in this paper. From the physical model testing, analysis of the data reveals that overtopping rates tend to be larger immediately near the transition than away from it. The run-up values and floodwall wave heights tend to show potential problematic areas and mimic the variation of overtopping along the levee transition. Under the design conditions tested, extreme overtopping conditions and associated water level values indicate that for the structure to sustain the hydraulic conditions, it must be well armored. It is shown that the variation of the still water level plays the largest role in the magnitude of the measured values, and increasing the peak wave period and wave heights also yields greater overtopping and water levels at the structure. This study highlights the need to understand specific spatial variability along coastal protective systems, and provides a better understanding of the mechanisms affecting overtopping for the specific structure tested.

The drive to develop marine renewable energy has made resource assessments a matter of pressing need and ongoing research. In the case of tidal in-stream power generation, a central objective of such assessments is to estimate the maximum power that may be extracted from tidal motions based on observable properties of the flow in the natural undisturbed state. To this end, analytical models have been developed for simple geometries, such as a channel connecting the open ocean to an inner basin. A key assumption of these models is that the along-channel volume flux is nondivergent. This requires the channel length to be small in comparison to the tidal wavelength and also that $\mu \ll 1$ , where $\mu$ is the ratio of the surface area of the channel to that of the basin. A practical consequence of assuming nondivergent flow is that the extractable power is then independent of position along the channel. In the present study, a linear one-dimensional model is developed to study the power potential of a uniform tidal channel that is spanned by a turbine fence and that links a bay to the open ocean. The assumption of nondivergent flow is relaxed, permitting an exploration of the sensitivity of the extractable power to nondimensional parameters measuring the relative channel length and area. Depending on parameter values, the results show that assuming nondivergent flow may lead to an underestimation of the available power. For long waves, the maximum extractable power is shown to scale as $(1+\mu )$ , thus providing a simple correction to account for the effects of finite channel area. It is shown that the power potential may vary appreciably with the position along the channel. However, for long waves the optimal location of the turbine fence is always at the mouth where the magnitude of the along-channel flow is greatest.

Field measurements of ship-generated waves were performed in the Cagliari Harbor where tugboats are significant wave generators that can adversely affect small-sized moored vessels. A method is proposed in which the major characteristics of the ship-generated waves in areas where wind waves and swell occur can be assessed and presented. The requirements and limitations of the method are also discussed. The results of the method application to field data were compared to an extensively used empirical equation from the literature for the prediction of the maximum wave height generated by tug passages. Although this comparison was affected by the uncertainties in the formation and propagation of ship-generated waves, it confirmed the practical validity of the proposed method.