Offshore Stations Research Articles

Improved Communication-Free Coordinated Control of VSC-MTDC Integrated Offshore Wind Farms for Onshore System Frequency Support

To reduce the adverse influence of communication time delay, the DC voltage of offshore converters is usually employed as input signals of existing communication-free frequency support schemes of offshore wind farms (OWFs). However, the offshore DC voltage varies with the output power of OWFs, this distortion will deteriorate the frequency support performance. To address this issue, an improved communication-free coordinated control (ICFCC) scheme of VSC-MTDC integrated OWFs for frequency support is proposed. The ICFCC scheme involves the onshore frequency variation into the DC voltage deviation once frequency events occur, and an estimator is proposed to obtain the onshore DC voltage in offshore VSC stations to provide fast frequency support and reduce the distortion influence. Meanwhile, to obtain better support performance, the frequency support power is allocated reasonably by regulating droop coefficients adaptively, more power will be injected via the station closer to the disturbing bus. After frequency support, the offshore wind turbines will recover their rotor speed with asymptotic control to reduce second frequency drop. Case studies are carried out on the 4-terminal and 5-terminal test systems, respectively, while considering the parameter uncertainties and noise disturbance. Different control methods are compared to illustrate the effectiveness of the ICFCC scheme.

Molecular size variations of chromophoric dissolved organic matter (CDOM) along a salinity gradient in the Changjiang River estuary

Molecular size is a critical parameter for characterizing dissolved organic matter in aquatic systems. Four cruises were conducted in October 2018, March 2019, July 2019 (with a normal flood), and August 2020 (with an extreme flood), along a transect from the Changjiang (Yangtze) River freshwater to the shelf area with salinities near or > 30. During each cruise surface water samples at 5–6 stations were collected along the salinity gradient, and absorption spectra (to quantify and qualify chromophoric dissolved organic matter (CDOM)) were measured. In addition, ultrafiltration was conducted so that the size partitions including four factions of <1 kDa, 1–3 kDa, 3–10 kDa and 10 kDa–0.2 μm were obtained. In the different seasons, CDOM molecular sizes always showed decreasing trends from land to sea, which was consistent to most estuaries worldwide. In the freshwaters, those with sizes >1 kDa generally contributed 20%–40% of the bulk CDOM in different seasons, but these large colloidal molecules always disappeared at the offshore stations. The extreme flood in August 2020 brought more waters with lower salinity and higher CDOM concentration into the transects, and the extra CDOM materials were generally of larger molecular sizes. In addition to suspended particulate matter (SPM) resuspension, the flocculation of organic matter could also be another important reason for localised increased CDOM molecular size near the river mouth.

EXPRESS ANALYSIS OF PROBABILISTIC CHARACTERISTICS OF WIND POWER TATIONS AS A SOURCE OF ENERGY FOR SEAWATER DESALINATION IN THE AZOV-BLACK SEA REGION OF UKRAINE

Ukraine takes part in the implementation of the European program «2´40 GW Green Hydrogen Initiative». It is planned to build 10 GW of electrolyser capacity on the territory of Ukraine for the production of «green» hydrogen using the energy of wind power stations in the Azov-Black Sea region. Territorial sea waters of Ukraine are characterized by a significant potential of offshore wind energy, which is currently developing rapidly in the world. One of the alternative options for supplying fresh water for the operation of electrolysers when powered by offshore wind power stations involves using desalinated seawater. The operation of 10 GW of electrolyser capacity will ensure the production of «green» hydrogen in the amount of 1.65 million tons per year and will require the consumption of prepared fresh water of about 24 million m3/year. Seawater desalination is an established technology that is widely used worldwide to reduce freshwater scarcity. The use of energy from wind power stations for seawater desalination is an alternative option within the scope of this Program. However, the mismatch between the random nature of wind power generation and the constant energy demand for desalination makes the integration between the two technologies all the more important. The goal of this paper is to estimate the probabilistic annual ensuring of given levels of power and volume of electricity production by wind power stations in the coastal and offshore territories of the Azov-Black Sea region of Ukraine. The probabilistic annual distributions of wind speed and energy indicators of wind power stations for a height of 100 meters were studied, the average annual wind speed of about 7.5 m/s was substantiated. An achievable value of the utilization rate of the installed capacity of wind power stations was obtained at the level of 35 %, provided that the generated energy is fully integrated into the technological schemes of water desalination. The duration of production of full and half volumes of electricity production and power generation is determined. Algorithms for calculating the probability of exceeding various required power levels and volumes of electricity production by wind power stations have been developed. Bibl. 48, Fig. 6, Tab. 4.

Aggregation of monitoring datasets for functional diversity estimation

Long-term monitoring data is central for the analysis of biodiversity change and its drivers. Time series allow a more accurate evaluation of diversity indices, trait identification and community turnover. However, evaluating data collected across different monitoring programs remains complicated because of data discrepancies and inconsistencies. Here we propose a method for aggregating datasets using diffusion maps. The method is illustrated by aggregating long-term phytoplankton abundance data from the Wadden Sea and Southern North Sea gathered by two institutions located in Germany and The Netherlands. The aggregated data allowed us to infer species traits, to reconstruct the main trait axis which drives community functionality, ultimately quantifying functional diversity of the individual samples, having used only the co-occurrence of species in samples. Although functional diversity varies greatly among sampling stations, we detect a slight positive trend in German stations, which contrasts with the clear decreasing trend observed in most of the Dutch Wadden Sea stations. At the Terschelling transect, in Southern North Sea, the stations also showed contrasting estimations of functional diversity between off-shore and in-shore stations. Our research provides further evidence that traits and functional diversity can be robustly reconstructed from monitoring data alone, showing that data aggregation can increase the accuracy of this reconstruction, being able to aggregate heterogeneous datasets.

Mass Mortality of Asari Clams (Ruditapes philippinarum) Triggered by Wind-Induced Upwelling of Hypoxic Water Masses

To investigate the mass mortality of the macrobenthos community, primarily asari clams, triggered by upwelling-driven hypoxia, we conducted continuous observations of temperature, salinity, and DO, and monthly macrobenthos monitoring on the Rokujo tidal flat in Mikawa Bay, central Japan, from 2014 to 2016. Additionally, laboratory experiments were conducted using sediments on a tidal flat containing macrobenthos to examine the possibility of hydrogen sulfide formation in tidal flats. The bottom layer at the offshore station was intermittently hypoxic, and the station of the tidal flat was occasionally hypoxic in August and September for three years. Hypoxia was mostly observed on the tidal flat when constant easterly winds were recorded offshore. The biomass of asari clams decreased considerably from September to October 2016 when hypoxia was intermittent. Hypoxia persisted for approximately one week from 20 September, which was associated with the calm weather and stagnation of tidal currents owing to the neap tide. Conversely, the hydrogen sulfide concentration in the water directly above the sediment exceeded 30 mg L−1 after 3 days of incubation in the laboratory experiment. Therefore, the possibility of oxygen consumption on tidal flats due to hydrogen sulfide formed by biological die-offs was considered in the long-term persistence of hypoxia.

Transdimensional ambient-noise surface wave tomography of the Reykjanes Peninsula, SW Iceland

SUMMARY Ambient noise seismic tomography has proven to be an effective tool for subsurface imaging, particularly in volcanic regions such as the Reykjanes Peninsula (RP), SW Iceland, where ambient seismic noise is ideal with isotropic illumination. The primary purpose of this study is to obtain a reliable shear wave velocity model of the RP, to get a better understanding of the subsurface structure of the RP and how it relates to other geoscientific results. This is the first tomographic model of the RP which is based on both on- and off-shore seismic stations. We use the ambient seismic noise data and apply a novel algorithm called one-step 3-D transdimensional tomography. The main geological structures in the study area (i.e. covered by seismic stations) are the four NE–SW trending volcanic systems, orientated highly oblique to the plate spreading on the RP. These are from west to east; Reykjanes, Eldvörp-Svartsengi, Fagradalsfjall and Krýsuvík, of which all except Fagradalsfjall host a known high-temperature geothermal field. Using surface waves retrieved from ambient noise recordings, we recovered a 3-D model of shear wave velocity. We observe low-velocity anomalies below these known high-temperature fields. The observed low-velocity anomalies below Reykjanes and Eldvörp-Svartsengi are significant but relatively small. The low-velocity anomaly observed below Krýsuvík is both larger and stronger, oriented near-perpendicular to the volcanic system, and coinciding well with a previously found low-resistivity anomaly. A low-velocity anomaly in the depth range of 5–8 km extends horizontally along the whole RP, but below the high-temperature fields, the onset of the velocity decrease is shallower, at around 3 km depth. This is in good agreement with the brittle–ductile transition zone on the RP. In considerably greater detail, our results confirm previous tomographic models obtained in the area. This study demonstrates the potential of the entirely data-driven, one-step 3-D transdimensional ambient noise tomography as a routine tomography tool and a complementary seismological tool for geothermal exploration, providing an enhanced understanding of the upper crustal structure of the RP.

Evaluating ERA5 reanalysis predictions of low wind speed events around the UK

Low wind speed events represent one of the biggest challenges in fully de-carbonising the electricity system due to the growing proportion of wind energy in the UK energy mix. While reanalysis products are a useful tool to study the spatio-temporal characteristics of these occurrences, their performance and limitations should be understood prior to usage. In this study, hourly 10 m ERA5 reanalysis wind speed data were evaluated against in-situ wind speed measurements from 205 onshore and offshore observation stations around the UK. It was found that ERA5 has biases in mean wind speed of 0.166 m/s and −0.136 m/s for onshore and offshore domains respectively, and biases in hourly wind speed standard deviation of −0.354 m/s and −0.425 m/s for onshore and offshore domains respectively. Both errors are more pronounced in autumn and winter. These errors lead to an underestimation of the percentage frequency of short-duration low wind speed events. Furthermore, the findings suggest that the largest errors are from sites which are situated in coastal and mountainous regions where short-range topographical variability and local wind effects may not be resolved by ERA5. Despite these shortcomings, ERA5 nevertheless outperforms its global reanalysis counterparts in the UK domain and therefore, can provide valuable information in the context of low wind speed events prediction.

Composition and distribution of diazotrophs in the Baltic Sea

Nitrogen (N2) fixation rates in the brackish Baltic Sea are among the highest per unit of area in the world. However, beyond the filamentous heterocyst-forming cyanobacteria, knowledge about the composition and distribution of N2-fixing microbes (diazotrophs) is limited. To address this, we investigated nitrogenase gene (nifH) composition and expression at coastal (<10 km offshore) and offshore (>10 km offshore) stations, at surface (avg. 1.8 m) and at depth (avg. 24 m) and in free-living (0.2–3.0 μm) and particle-associated size fractions (>3 μm). Surprisingly, nifH genes affiliated with Pseudanabaena and non-cyanobacterial diazotrophs (NCDs) dominated the composition whereas filamentous heterocyst-forming cyanobacteria accounted for almost 80% of the nifH transcripts. Salinity had a minor influence on the composition, but Aphanizomenon and Nodularia showed increased relative nifH gene expression at low and higher salinity, respectively. Pseudanabaena only accounted for up to 5% of the nifH transcripts and nifH gene expression by Candidatus Atelocyanobacterium thalassa (sublineage UCYN-A2) was mainly observed in the most saline western part of the Baltic. The only notable expression by NCDs (up to 15% of nifH transcripts at a given station) coincided with an upwelling event at the southern coast and was largely accounted for by a Pseudomonas-like nifH phylotype, recurrently found in the Baltic Sea. NCD relative abundances were dominant in coastal stations, presumably driven by sediment resuspension as evidenced by higher turbidity and DOC levels and the recovery of sediment diazotrophs in the pelagic zone. This study reveals the heterogeneity of the composition and activity of diazotrophs in the Baltic Sea, and underscores the need for future N2 fixation studies that include coastal and offshore Baltic waters.

Quantifying the effect of rock armour scour protection on eigenfrequencies of a monopile supported OHVS

It is common practice to install scour protection systems during the construction of offshore wind farms. This is because the guidelines allow for a more optimal structural design, as the potential effects of scouring may be ignored. However, multiple sources, as well as measurements, suggest that there is an additional gain in stiffness which is not being taken into account by current guidelines. Scour protection has the ability to provide extra foundational stiffness to the monopile, which results in higher natural eigenfrequencies and mode shapes. In this paper the focus will be on an offshore high voltage station (OHVS) located in the North Sea and the impact of a rubble riprap scour protection on the foundational stiffness of the structure. Furthermore, a proposal is made on how to take these effects into account when calculating for design. This site was equipped with a mobile measurement system containing a multi-axial accelerometer, which was active in a period both before and after installation of the scour protection rock armour layer. In case of similar environmental circumstances, the eigenfrequencies of the OHVS were noticeably higher after the scour protection was installed, thus substantiating the claim of increased foundational stiffness. To perform numerical studies on the impact of the scour protection system on structural dynamics, an OHVS support structure finite element model was built. As no formally agreed upon formulation to include scour protection exists, two possible methods are proposed. One is adding a global accretion layer to model the scour protection as an additional soil layer. The other calculates an overburden pressure due to the scour protection weight, which translates to the stiffening of the upper soil layers of the foundation. Both methods have their own strengths and weaknesses, but more research on different locations and offshore structures is needed to formalize the definitive description for modelling scour protection.

Regional offshore ground motion prediction model from a referenced empirical approach: A case study in the Japan Trench area

With increasing resource exploitation and engineering construction offshore, the seismic safety of offshore structures has received attention from researchers and designers, and an offshore ground motion prediction model (GMPM) can be used to estimate the possible earthquake response of structures and provide the foundation for seismic fortification of structures. Because there are few strong motion records recorded by offshore stations, there are insufficient empirical data in the magnitude-distance range to directly develop a GMPM using empirical regression techniques. To develop a reliable GMPM for data-poor offshore regions, we used a referenced empirical approach based on an onshore empirical prediction model. We collected 2605 records from 138 crustal earthquake events, 2669 records from 125 slab earthquake events and 5693 records from 267 interface earthquake events recorded by S-net stations. We used the existing Japanese onshore GMPM as a reference model. Then, we analyzed the residuals of the reference model for offshore data and calculated its adjustment factors. Finally, we developed a GMPM for the Japan Trench area and compared it with the existing onshore models. The proposed model fit the data subsets at hypocentral distances of 20–300 km reasonably well. There were different spectral accelerations (SAs) for different source types, with considerably larger SAs for short periods of offshore motions from crustal events than for the motions from the other two types, and the difference becomes more obvious with the increase of magnitude. At short periods, the SAs of the offshore motions from slab events were slightly larger than those of the motions from the interface events. The discrepancy of the offshore and onshore GMPM was significant, and the peak ground accelerations (PGAs) and SAs of the offshore ground motions were apparently greater than those of onshore ground motions of the same magnitude and distance. The offshore GMPM established by the reference empirical approach contribute to studying the characteristics of offshore strong ground motions, and this approach can be applied to data-poor regions.

INVESTIGATION OF OFFSHORE DC STATION WITH DISTRIBUTED POWER COLLECTION AND CASCADED BOOSTING-VOLTAGE TOPOLOGY: A COMPREHENSIVE STUDY

In order to collect DC power and boost DC voltage for offshore DC stations, the present research uses a distributed power- collection and cascaded boosting voltage (DPCB) topology made up of several cascaded single active bridges (SABs). In contrast, HVDC stations use the conventional concentrated power-collection and boosting voltage (CPCB) framework. It has high DC voltage boosting powers, is simple to build, is affordable, and offers a low system error rate. The next three variables are looked at. After first determining the DPCB topology with its circuit equations, the phase-shifting control approach is applied to further investigate the operational mechanism. The system minimal signal model is then used to construct a fuzzy control strategy. Third, a fault tolerance strategy with projected power ratio is developed to stop the DPCB station from failing if part of the SABs and generators fail. A significant power differential exists due to windward and wind turbine siting factors. The viability and efficacy of the designed DPCB station with its strategy are finally validated using simulation findings.INDEX TERMS: - SAB, fault tolerance, distributed power collecting, cascaded boosting voltage, and topology.

Seasonal and spatial comparisons of microzooplankton grazing and phytoplankton growth in the Bohai Bay, China

The dilution experiment technique was used in two cruises in July-August (summer) and October-November (autumn) 2020, with a total of 14 stations. The grazing impact of microzooplankton on phytoplankton in the interior of Bohai Bay was comprehensively investigated. We compared phytoplankton growth rates (μ0) and microzooplankton grazing rates (m) spatially (distance between experimental stations and shore far vs. near) and seasonally (summer vs. autumn). Both m and μ0 values were significantly higher in summer than in autumn, and the phytoplankton growth rate μ0 was positively correlated to temperature. Offshore stations showed higher values. There is no significant spatial and seasonal differences in the ratio of microzooplankton grazing rate and phytoplankton growth rate (m/μ0) indicating that daily consumption of primary production by microzooplankton was similar in the two seasons. Therefore, our research showed a close coupling between microzooplankton grazing with phytoplankton growth in the Bohai Bay.

Community dynamics and co-occurrence relationships of pelagic ciliates and their potential prey at a coastal and an offshore station in the ultra-oligotrophic Eastern Mediterranean Sea.

Ciliates have been recognized as one of the major components of the microbial food web, especially in ultra-oligotrophic waters, such as the Eastern Mediterranean Sea, where nutrients are scarce and the microbial community is dominated by pico- and nano-sized organisms. For this reason, ciliates play an important role in these ecosystems since they are the main planktonic grazers. Regardless the importance of these organisms, little is known about the community structure of heterotrophic and mixotrophic ciliates and how they are associated to their potential prey. In this study, we used 18S V4 rRNA gene metabarcoding to analyze ciliate community dynamics and how the relationship with potential prey changes according to different seasons and depths. Samples were collected seasonally at two stations of the Eastern Mediterranean Sea (HCB: coastal, M3A: offshore) from the surface and deep chlorophyll maximum (DCM) layers. The ciliate community structure varied across depths in HCB and across seasons in M3A, and the network analysis showed that in both stations, mixotrophic oligotrichs were positively associated with diatoms and showed few negative associations with ASVs annotated as marine Stramenopiles (MAST). On the other hand, heterotrophic tintinnids showed negative relationships in both HCB and M3A stations, mostly with Ochrophyta and Chlorophyta. These results showed, in first place that, although the two stations are close to each other, the ciliate dynamics differed between them. Moreover, mixotrophic and heterotrophic ciliates may have different ecological niches since mixotrophic ciliates may be more selective compared to heterotrophic species regarding their prey. These findings are the first glimpse into an understanding of the dynamics between heterotrophic and mixotrophic ciliates and their role in microbial assemblages and dynamics of ultra-oligotrophic environments.

Abundance, composition and distribution of carnivorous gelatinous zooplankton in the Northern Gulf of Alaska

AbstractAbundance, biomass, size and distribution of macro-jellyfish were measured in the Northern Gulf of Alaska (NGA). Nearly 1000 kg dispersed among ~13 800 jellyfish were collected using a 5-m2 Methot net. We present length-weight regressions for seven most-common taxa. Catches were dominated by the hydrozoan Aequorea victoria and the scyphozoan Chrysaora melanaster. During 2018, epipelagic macro-jellyfish biomass averaged 1.46 ± 0.36 g WW m−3 for July and 1.14 ± 0.23 g WW m−3 for September, while during 2019 they averaged 0.86 ± 0.19 g WW m−3 for July and 0.72 ± 0.21 g WW m−3 by September. Despite similar biomass among seasons within a year, July abundances were fivefold greater than abundances in September, with July catches dominated by smaller-sized jellyfish over the inner shelf, while during September larger jellyfish were more prominent and most predominant at offshore stations. Comparison to 20 years of data from standard towed nets allowed determination of the relative magnitude of the dominant carnivorous zooplankton components: scyphozoans, hydrozoans and chaetognaths in the NGA. The biomass of these smaller epipelagic predators (5.4 mg WW m−3 for hydrozoans and 10.5 mg WW m−3 for chaetognaths) is a low percentage of the macro-jellyfish, despite their much higher numerical abundance.

Influence of seawater depth on offshore ground motion characteristics and seismic responses of sea-crossing cable-stayed bridges

This paper studies the effect of seawater depth on offshore earthquake motion characteristics and seismic response of bridges. By analyzing the offshore seismic data recorded by the K-NET during 2006–2021, the horizontal amplification coefficient spectrum and V/H spectral ratio of offshore ground motions were compared for various offshore stations at different seawater depths. Moreover, a numerical model simulating the propagation process of seismic waves in the seabed was developed to investigate the effects of seawater depth and site conditions on offshore ground motions. Finally, a site-seawater-pile foundation-cable-stayed bridge coupling model was created to study the influence of seawater depth on the seismic responses of sea-crossing cable-stayed bridges. The results reveal that the vertical response spectrum of offshore ground motion in deeper seawater is lower for periods longer than 0.3 s. The numerical models reveal that seawater has a suppression effect on the P wave near the resonance frequency of the seawater layer. Due to the hydrodynamic effect, the longitudinal bridge seismic response is significantly amplified. The amplification effect becomes more obvious with an increase in seawater depth. Therefore, it is important to consider the influence of the seawater layer in the seismic design of sea-crossing bridges.

Seasonal and spatial dynamics of phylogenetic structure of fish communities in coastal waters

Understanding community assembly has long attracted the attention of ecologists. There are a lot of studies on how community assembly mechanisms work through phylogenetic analysis. Nevertheless, phylogenetic structures have generally been assumed to be stable over time in most studies, and little attention has been paid to seasonal shifts in phylogenetic structure, which is important to mobile animals. In this study, phylogenetic distance was measured as the mean pairwise distance (MPD) and as mean nearest taxon distance (MNTD). The phylogenetic structures of fish communities in different regions, subregions and at sampling stations were examined using the standardized effect sizes of MPD and MNTD (SES.MPD and SES.MNTD) under null model independent swap over four seasons in the coastal waters of Shandong Province, China. Generalized additive model (GAM) was used to detect which environmental factors have an important impact on phylogenetic structure indices at each sampling station. The fish communities were phylogenetically clustered in most seasons in the whole survey waters, indicating environmental filtering tended to shape the whole fish assemblage. Two phylogenetic indices indicated that the phylogenetic structure (phylogenetic over-dispersion or phylogenetic clustering) was inconsistent on the different regions and subregions. For fish assemblages at each sampling station, the phylogenetic structure of fish communities presented patchy distribution patterns in different seasons. GAM analysis showed that phylogenetic structure responded differently to different environmental factors over four seasons, and the change in phylogenetic structure indices was significantly associated with environmental factors, especially in autumn and winter. SES.MPD can better respond to environmental changes than SES.MNTD. Spatial factors (longitude and latitude) were important factors influencing fish community assembly, indicating that fish communities at offshore stations with low latitude exhibit a relatively high degree of phylogenetic overdispersion. It was difficult to draw consistent conclusions about seasonal changes in phylogenetic structure of fish communities on different temporal and spatial scales. It is possible that seasonal variations and spatial scales in assembly mechanisms contribute to maintaining marine fish diversity in temperate waters, resulting in different phylogenetic structures in fish community.

Aerosol and atmospheric dynamics over the tropical Indian ocean: A study from the IInd international Indian Ocean expedition

The study addresses the aerosol distribution, boundary layer dynamics and associated shortwave radiative forcing over the Tropical Indian Ocean. The observations were carried out onboard Ocean Research Vessel Sagar Nidhi in December 2015 as a part of the International Indian Ocean Expedition-II (IIOE-II). The dominance of continentally originated aerosols was observed near the coast because of northeasterly winds, while marine aerosols abounded offshore. The uneven distribution of aerosols in the study region was due to varying atmospheric conditions triggered by various convective processes, wherein the convective available potential energy and convective inhibition energy ranged between 0.0274 and 2535.2 J kg−1 and 0 and -387.5 J kg−1, respectively. The atmospheric forcing varied in tandem with aerosol loading. It was evident from the high forcing values (17.98 W m−2) and corresponding heating rate of 0.504 K day−1 observed near the coast. The average heating rate in the far offshore (stations 6–10) region was 0.063 ± 0.017 K day−1, which amounted to an ∼87% decrease from the coastal location.

Failure Analysis of Fatigue Failed M20 Class 8.8 Galvanized Steel Bolt

An M20 Class 8.8 galvanized steel bolt (derrick bolt) was found to fail after five years of service in an offshore drilling station. A comprehensive failure analysis investigation was carried out to reveal the root cause of the detected failure. Visual inspection coupled with complete microstructural characterization as well as mechanical testing were performed in the investigation. The results showed that the failure was mainly caused by fatigue failure. The initiation of the fatigue crack was supposedly promoted by the existence of non-metallic inclusions that were present at both the surface and the interior of the microstructure. Furthermore, unbalanced preload forces resulted in bolt loosening which caused fretting wear of the galvanized layer thickness. The detected drop in the galvanized layer thickness by 42.20% at the primary origin of the fatigue crack along with the presence of non-metallic inclusions were thought to be the reason for the initiation of the fatigue crack. The crack was then found to propagate through a transitional complete cleavage to quasi-cleavage fracture as appeared from fracture surface topography studied by scanning electron microscopy (SEM). The final stage of the fatigue failure was found to be caused by ductile fracture of the overloaded zone.

Short-term solar radiation forecasting using hybrid deep residual learning and gated LSTM recurrent network with differential covariance matrix adaptation evolution strategy

Developing an accurate and robust prediction of long-term average global solar irradiation plays a crucial role in industries such as renewable energy, agribusiness, and hydrology. However, forecasting solar radiation with a high level of precision is historically challenging due to the nature of this source of energy. Challenges may be due to the location constraints, stochastic atmospheric parameters, and discrete sequential data. This paper reports on a new hybrid deep residual learning and gated long short-term memory recurrent network boosted by a differential covariance matrix adaptation evolution strategy (ADCMA) to forecast solar radiation one hour-ahead. The efficiency of the proposed hybrid model was enriched using an adaptive multivariate empirical mode decomposition (MEMD) algorithm and 1+1EA-Nelder–Mead simplex search algorithm. To compare the performance of the hybrid model to previous models, a comprehensive comparative deep learning framework was developed consisting of five modern machine learning algorithms, three stacked recurrent neural networks, 13 hybrid convolutional (CNN) recurrent deep learning models, and five evolutionary CNN recurrent models. The developed forecasting model was trained and validated using real meteorological and Shortwave Radiation (SRAD1) data from an installed offshore buoy station located in Lake Michigan, Chicago, United States, supported by the National Data Buoy Centre (NDBC). As a part of pre-processing, we applied an autoencoder to detect the outliers in improving the accuracy of solar radiation prediction. The experimental results demonstrate that, firstly, the hybrid deep residual learning model performed best compared with other machine learning and hybrid deep learning methods. Secondly, a cooperative architecture of gated recurrent units (GRU) and long short-term memory (LSTM) recurrent models can enhance the performance of Xception and ResNet. Finally, using an effective evolutionary hyper-parameters tuner (ADCMA) reinforces the prediction accuracy of solar radiation.

Techno-economic assessment of offshore wind energy potential at selected sites in the Gulf of Guinea

Offshore wind power has been found to stand out among the most dynamic renewable energy technologies. With its long coastal line, Nigeria has an overwhelming advantage in developing marine energy resources to relieve the power crisis effectively. This work analyzed and characterized observation data of sea-surface wind speed and direction at 30-minute intervals between 1979 and 2015 at five synoptic offshore stations in the Gulf of Guinea. The seasonal variations in hourly surface wind speed and directions as well as the Weibull distribution of wind speed and wind power at 100 m hub height were examined. The wind shears, capacity factors, and accumulated energy outputs for seven offshore wind turbine types were determined for the selected locations. In addition, the economic analysis of the selected offshore turbines using levelized cost of energy was carried out, while sensitivity analysis of the total levelized cost of energy to key input parameters was further determined. The results revealed large spatial and temporal variations in wind speed and wind power in the Gulf of Guinea. The most viable offshore site for wind energy exploitation was Agbami (the deepest offshore site), while Bonny (the shallow coastal site) had the least. The findings established very good fits (having mean bias (between −0.08 ms−1 and −2.44 ms−1), percentage bias (between −0.47% and −13.98%), correlation coefficients (between 0.97 and 0.98), Chi-square (between 0.2 and 1.2), and root mean square error (between 1.2 ms−1 and 3.1 ms−1)) between Weibull distribution and the actual wind data. The wind turbines with the highest and the lowest wind power densities, capacity factors, and power outputs across the seasons and sites were V236-15.0 MW and Siemens SWT113, respectively. The levelized cost of energy was considered for the deep waters due to the moderately high-capacity factors. The highest values ranged between 101.48 and 137.12 USD/MWh at Sea Eagle with V236-15 MW and V117-4.2 MW, respectively, while the lowest ranged from 52.29 to 69.66 USD/MWh at Agbami with V236-15 MW and Siemens (SWT113), respectively. The exploitation of Nigeria’s offshore wind resources could also be dedicated to producing renewable hydrogen and can serve to meet the country’s ambitious targets set for carbon neutrality by 2060.