Swell Partitions Research Articles

Improvements in Wave Age-Based Sea and Swell Separation for Offshore Industry Applications

Abstract For offshore wind turbines, an accurate description of the wave conditions is crucial for their design, construction, installation, maintenance, and operation. Typical sea states in the open ocean are often a combination of wind-sea and swell systems which cannot be fully described by a single set of parameters. In such cases the wave conditions are better defined by separating and quantifying the individual wave systems. In this paper, the wave age method has been revisited, and a modification has been proposed to add more functionality to better describe the wind/wave alignment of the wind-sea. A qualitative analysis was performed using offshore spectral energy data from a global wave model. The analysis was carried out for four different geographical locations: the Philippine Sea, the North Sea, the Atlantic Ocean, and the Pacific Ocean. The analysis compares the wind-sea and swell partitions separated by the standard and modified wave age methods. It shows an improved description of the wind-sea compared to the standard equation, especially in the case of high wind speed events and where the wind-sea partition is expected to dominate most of the spectral energy.

Wind-Sea and Swell Climate in the Black and Azov Seas, Based on 42-Year Spectral Wave Hindcast

Long-term wave climates of the Black and Azov Seas are widely assessed based on the bulk wave parameters of the mixed sea. However, studies based on spectral wave partitions are rarely documented. Assessment of the wave climate based on the individual wave systems provides a more reliable description of the sea states' characteristics and the variability in the long-term wave climate. For instance, the wind-sea climate may reflect a real change in the local climate. However, local change in swell characteristics may not necessarily reflect a change in the local climate, and it can locally influence the mixed sea climate variability. This study assessed the spatial distributions of the long-term averages and maxima of mixed seas, wind-sea, and swell partitions at annual and seasonal scales. The contributions of wind-sea and swell partitions to the mixed-sea were computed, and their spatial and temporal variability was also assessed for overall and extreme sea state conditions. Finally, the occurrence in main wave systems' directions and occurrence of crossing, opposing, and following wind-sea and swell interactions were computed. The wind-sea contribution to the mixed-sea exceeds 70 % in all Black Sea and >95 % in the Azov Sea. The swells are more dominant on the Northeastern and Northwestern Black Sea basins. Thus, the results show spatial and temporal variability in the wind-sea climate close to the mixed sea but largely different compared to the swell climate. The inter-annual trends in the swell partitions and wind-sea are inversed at some locations.

An analytical transient coal permeability model: Sorption non-equilibrium index-based swelling switch

Long-term permeability experiments have indicated that sorption-induced swelling can switch from internal to bulk depending on the evolutive sorption status. However, this sorption swelling switch mechanism has not been considered in current analytical permeability models. This study introduces a normalized sorption non-equilibrium index (SNEI) to characterize the sorption status, quantify the dynamical variations of matrix swelling accumulation and internal swelling partition, and formulate the sorption swelling switch model. The incorporation of this index into the extended total effective stress concept leads to an analytical transient coal permeability model. Model results show that the sorption swelling switch itself results in the permeability switch under stress-constrained conditions, while the confined bulk swelling suppresses the permeability recovery to the continuous reduction under displacement-constrained conditions. Model verifications show that current experimental observations correspond to the early stages of the transient process, and they could be extended to the whole process with these models. This study demonstrates the importance of the sorption swelling switch in determining permeability evolution using simple boundary conditions. It provides new insights into experimentally revealing the sorption swelling switch in the future, and underscores the requirement of a rigorous model for complex coupled processes in large-scale coal seams.

Validation and calibration of partitioned integral ocean wave parameters from co-polarized synthetic aperture radar data

The inversion of wave spectrum is the embodiment of the fine observation of ocean swells by synthetic aperture radar. Due to the nonlinear distortion from velocity bunching, the operational Sentinel-1 Wave Mode (WV) level-2 (L2) Ocean (OCN) wave products retrieved by quasi-linear inversion are still quite different from the real wave spectrum, requiring further calibration. Limited by the azimuth cut-off, the quasi-linear inversed wave spectrum mostly concentrates on swells with a wavelength longer than 150 m. 2D spectral partitioning separates different swell systems, denoted as partitions in the Sentinel-1 WV L2 OCN product. We compared and calibrated the Sentinel-1 WV L2 OCN products' swell partitions (More specifically partitioned integral ocean wave parameters: partitioned wave height, partitioned wavelength, and partitioned wave direction) with WAVEWATCH III (WW3) in different polarization modes (HH and VV polarization) and different incidence angles (∼23°-WV1 and ∼ 36°-WV2). The accuracy of swell partitions in the OCN products varies with peak wave direction relative to the Sentinel-1 flight direction, and it is difficult to calibrate the partitions derived from quasi-linear inversion unless deleting partitions severely distorted by nonlinearity and large-scale features. A machine learning method is proposed to calibrate the wave partitions. Spectral distances of collocated Sentinel-1 and WW3 partition pairs larger than two are considered “bad” partitions to be filtered. Inputting high-dimensional features (twenty-one features) derived from Sentinel-1 WV L2 OCN products into the extreme gradient boosting (XGBoost) model can effectively reduce the bias (root-mean-square error, RMSE) of significant wave height (SWH) of all the partitions from ∼0.10 m (∼0.70 m) to ∼0.00 m (∼0.46 m) and the bias (RMSE) of wavelengths of all the partitions from ∼20 m (∼35 m) to ∼0.20 m (∼26 m) compared with WW3 swell partitions. This method improves the precision of the wave partition and retains more partitions in the Sentinel-1 WV L2 OCN products than previous studies, especially for wave partitions with moderate wave height (< 4 m), potentially improving the understanding of swell dynamics globally.

Extreme Wave Statistics in Combined and Partitioned Windsea and Swell

We investigate how the extreme wave statistics of a combined windsea and swell appears to be different from the extreme wave statistics of the corresponding windsea and swell partitions. We consider the situation of following long-crested windsea and swell in laboratory experiments and in simulations using a high-order spectral method (HOSM). We also consider the situation of short-crested windsea and swell crossing at nearly right angle, corresponding to the sea state when the Prestige accident happened, in hindcast simulations combined with HOSM. For cases when the two wave systems do not interact much, the combined wave system appears to be more Gaussian than the corresponding partitioned wave systems, consistent with the central limit theorem. This result is found for kurtosis and exceedance probability of envelope and crest height.

Directional correction of modeled sea and swell wave heights using satellite altimeter data

This paper proposes a buoy independent directional calibration methodology that uses satellite altimeter data to correct the simultaneous wind-sea and all swell partitions available from wave hindcasts. The proposed technique was applied to a 20-year wave hindcast in the New Zealand region and showed promising results. The method could potentially be applied worldwide to fully assess systematic errors in wind-sea and swell partitions.

Validation and Parameter Sensitivity Tests for Reconstructing Swell Field Based on an Ensemble Kalman Filter.

The swell propagation model built on geometric optics is known to work well when simulating radiated swells from a far located storm. Based on this simple approximation, satellites have acquired plenty of large samples on basin-traversing swells induced by fierce storms situated in mid-latitudes. How to routinely reconstruct swell fields with these irregularly sampled observations from space via known swell propagation principle requires more examination. In this study, we apply 3-h interval pseudo SAR observations in the ensemble Kalman filter (EnKF) to reconstruct a swell field in ocean basin, and compare it with buoy swell partitions and polynomial regression results. As validated against in situ measurements, EnKF works well in terms of spatial–temporal consistency in far-field swell propagation scenarios. Using this framework, we further address the influence of EnKF parameters, and perform a sensitivity analysis to evaluate estimations made under different sets of parameters. Such analysis is of key interest with respect to future multiple-source routinely recorded swell field data. Satellite-derived swell data can serve as a valuable complementary dataset to in situ or wave re-analysis datasets.

Model development and analysis of the evolution of coal permeability under different boundary conditions

Coal permeability is an important parameter for coalbed methane (CBM) production and CO2-enhanced coalbed methane (ECBM) recovery. Coal permeability is mainly controlled by the structure of the coal. In this study, based on the pore-elasticity principle and the coal matrix-fracture interactions, we developed an analytical coal permeability model and its five forms under the corresponding specific boundary conditions, and four of them are validated by matching the corresponding field data or experimental data. The study presents a discussion of the permeability evolution for the different forms. The results show that the coal permeability under constant volume and constant effective stress conditions are controlled by the matrix sorption deformation and that the coal permeability under uniaxial strain and constant external stress conditions is governed by both the effective stress and the matrix sorption deformation. The constant volume model may be a special form of the constant effective stress model. For the same pressure drawdown, the permeability that is predicted under constant external stress conditions decreases more than that the permeability that is predicted under uniaxial strain conditions, and the permeability change induced by the matrix sorption deformation depended on the values of the internal swelling partition. The boundary conditions are important to the model, and different boundary conditions will lead to different evolutions of coal permeability. The permeability model can only be used to predict the evolution of permeability under the corresponding boundary conditions.

Simulation of wave damping during a cold front over the muddy Atchafalaya shelf

The Atchafalaya Shelf off the Louisiana coast in the United States is characterized by fine grained sediments dispersing into the shelf from the lower Atchafalaya River and Wax Lake outlets. Rapid seaward flushing of the sediment-laden river plumes, due to water level set-down during cold front passages forms a fluid mud layer close to the bottom, which effectively dampens the wave energy. In this study, the performance of a phase-averaged spectral wave model was skill assessed based on the wave data recorded close to the southern periphery of the mud zone during several days in March 2009. Separation of wave spectra into sea and swell partitions showed that the wave model overestimated the sea waves generated by northerly wind during the cold front passage. A non-stationary scheme was needed to solve the wave action balance equation to include the wind dynamics during the cold front passages over the study area. A recently developed mud-induced dissipation term was improved by modifying its algorithm for solving the implicit dispersion equation. The modified model became efficient enough to be used for non-stationary calculations. The thickness, density, kinematic viscosity of the mud layer, and its offshore extent were determined by trial and error. The mud-induced energy dissipation term enabled the model to reproduce the energy attenuation of short waves by the fluid mud. A high value of mud viscosity (0.01–0.1m2/s) was required to obtain good agreement with in situ measurements when the maximum wave height occurred. However, the model using lower values of mud viscosity (0.001–0.01m2/s) was more successful in reproducing the measured wave spectra from a few hours after the maximum wave activity. The simulation results also showed that presence of fluid mud with high value of mud viscosity hindered the wave growth in shallow water due to suppression of high frequency waves.

FPSO Conference—Estimating Wind-Sea and Swell for FPSO Operability

The motion response of an FPSO is sensitive to the relative intensities and directions of the wind-sea and swell components in a sea state, and the operability of the FPSO is a function of the long-term variation in these components. Estimations of the operability are therefore dependent both on how the sea state is described in terms of its constituent wind-sea and swell components, and on how the long-term variability of the sea state is captured. However, there is currently no consensus on how either the sea state or its long-term variability should be described. We investigate these issues by means of a study of the responses of a typical FPSO to the wave fields at a location offshore Namibia and a location off the west coast of New Zealand. We make use of a state-of-the-art program for splitting a directional wave spectrum into wind-sea and swell components, and we examine the effect on the motion responses of allowing the spectra to be split into many swell partitions or constraining the spectral split to a maximum of two partitions, as is often assumed in response calculations. The resulting decompositions are used to examine the effects of swell on hull motions and, hence, to identify methods for generating sea state criteria for operability. In addition, one-year metocean conditions are estimated; these are relevant for analysis of the limits on operations.