Seawater Layer Research Articles

Redox speciation of copper in the estuarine environment: Towards better understanding of copper water chemistry

An adapted solid-phase extraction method was used for Cu redox speciation in the dissolved fraction in the stratified Krka River estuary in March and July 2023 at a “clean” and a “polluted” site, while anodic stripping voltammetry in differential pulse mode was used for Cu determination in the Cu(I), Cu(II) and CuT fractions. Overall, [Cu(I)]/[CuT] was significant in the Krka River estuary and varied between 20.7 and 88.7 %. At the “clean” site E1, the surface maximum of [Cu(I)]/[CuT] was observed in March. This was assumed to be related to the biological activity of the freshwater biota, as the temperature rose in March (compared to the previous winter months) and the salinity was low. On the other hand, at E1 in July, the maximum of [Cu(I)]/[CuT] was observed at the halocline and corresponded to the highest biological production thus we hypothesised it to be the result of Cu(I) complexation with ligands of biological origin. The minimum of [Cu(I)]/[CuT] was observed at the surface at E1 in July where terrestrial humic-like substances were present with higher affinity for Cu(II) than for Cu(I). At the “polluted site” E2 high [Cu(I)]/[CuT] values were observed at both the surface and the halocline in March. However, [Cu(I)]/[CuT] maximum was observed at the halocline at E2 in July and was associated with the highest biological production, as also observed at E1 in July. Nevertheless, the stabilisation of Cu(I) by chloride should not be disregarded as it maintained relatively high [Cu(I)]/[CuT] in the seawater layer at both stations in March and July. The fluctuations in [Cu(I)]/[CuT] occurred during the day at the surface at E2 in July, with the minimum of [Cu(I)]/[CuT] observed in the early morning, followed by an increase in [Cu(I)]/[CuT] during the day, indicating the importance of photochemical reactions in the Cu(I) formation and stabilisation.

Diversity and Potential Metabolic Characteristics of Culturable Copiotrophic Bacteria That Can Grow on Low-Nutrient Medium in Zhenbei Seamount in the South China Sea

Oligotrophs are predominant in nutrient-poor environments, but copiotrophic bacteria may tolerate conditions of low energy and can also survive and thrive in these nutrient-limited conditions. In the present study, we isolated 648 strains using a dilution plating method after enrichment for low-nutrient conditions. We collected 150 seawater samples at 21 stations in different parts of the water column at the Zhenbei Seamount in the South China Sea. The 648 isolated copiotrophic strains that could grow on low-nutrient medium were in 21 genera and 42 species. A total of 99.4% (644/648) of the bacteria were in the phylum Pseudomonadota, with 73.3% (472/644) in the class Gammaproteobacteria and 26.7% (172/644) in the class Alphaproteobacteria. Among the 42 representative isolates, Pseudoalteromonas arabiensis, Roseibium aggregatum, and Vibrio neocaledonicus were present in all layers of seawater and at almost all of the stations. Almost half of these species (20/42) contained genes that performed nitrate reduction, with confirmation by nitrate reduction testing. These isolates also contained genes that functioned in sulfur metabolism, including sulfate reduction, thiosulfate oxidation, thiosulfate disproportionation, and dimethylsulfoniopropionate degradation. GH23, CBM50, GT4, GT2, and GT51 were the main carbohydrate-active enzymes (CAZymes), and these five enzymes were present in all or almost all of the isolated strains. The most abundant classes of CAZymes were those associated with the degradation of chitin, starch, and cellulose. Collectively, our study of marine copiotrophic bacteria capable of growing on low-nutrient medium demonstrated the diversity of these species and their potential metabolic characteristics.

Importance of dissolved organic nutrients on nutrient stoichiometry in the Yellow Sea and the East China Sea

We investigated seasonal variations in the dissolved nutrients and organic matter (DOM) in the Yellow Sea and the East China Sea shelf waters from 2017 to 2021. Dissolved organic nitrogen and phosphorus (DON and DOP) concentrations were the highest in the upper layer (<30 m) during the time of spring and autumn bloom and were the lowest in summer. In spring, DON and DOP contributed up to approximately 90% and 80% of the total dissolved nitrogen (TDN) and phosphorus (TDP), respectively. The C:N:P stoichiometry of the DOM pool in spring and autumn indicates that freshly in-situ produced DOM is N-rich compared to the DOM observed in summer and winter, resulting in a dissolved inorganic nitrogen (DIN)-limited environment in the upper layer of seawater during spring and autumn. Conversely, in summer, the highest C:N ratios (∼27) of the DOM pool revealed the DOM remained after the degradation and formation of the refractory pool by marine bacteria. In winter, all nutrients were vertically uniform due to active vertical mixing. This study highlights notable seasonal variations in the cycling of DON and DOP, leading to changes in their bioavailability and subsequent ecosystem responses in the shelf sea.

Vanadium redox speciation in the acid-extractable phase of Krka River estuary surface sediment

This study investigated the redox speciation and mobility of V in the acid-extractable fraction of surface sediments from the Krka River estuary using an optimized IC-UV/Vis analytical method. The separation of V(IV) and V(V) redox species was done using anion-exchange based chromatographic method, while pseudo-total V concentrations were measured using HR ICP-MS analytical instrumentation. Extracted V concentrations from the sediment fraction (pH = 5, HCl) and determined pseudo-total V concentrations were used to calculate the Enrichment Factor (EF) and Risk Assessment Code (RAC), indicating potential anthropogenic influence and environmental risk. A simple PHREEQC model was developed to asses V speciation in the oxic bottom seawater layer simulating possible remobilization of the leached sediment phase. The results of the study show that minor fraction of V is present in the acid-extractable phase across the surface sediment of Krka River estuary. Higher V mobility is mostly observed at locations rich with clay minerals, terrigenous input, and carbonates. Anthropogenic influence was linked to higher enrichment but lower mobility, suggesting binding to less mobile sediment phases (reducible, organic and residual fractions). The predominance of reduced V(IV) species in the acid-extractable sediment fraction indicates a potentially low V toxicity risk in the sediments of Krka River estuary, even in cases of high potential remobilization of V. However, the model predicted complete oxidation of V(IV) to V(V) upon remobilization into the oxic bottom water layer. This highlights the complexity of V behavior in natural estuarine systems, where the toxicity risks of possible V remobilization still remain unclear. Results of this study demonstrate the need for the strengthening efforts in speciation of V in the mobile sediment phase to obtain a cohesive outlook on its potential toxicity and biogeochemical cycling.

Analytical approach for lateral dynamic responses of offshore wind turbines supported by cement-soil composite steel pipe pile embedded in gassy marine sediment layer

This study aims to clarify and gain an insight into the lateral dynamic response of offshore wind turbines (OWTs) supported by cement-soil composite steel (CCS) pipe pile embedded in gassy marine sediment layer by developing an analytical approach. In the proposed approach, the CCS pipe pile-supported OWT system is divided into three parts including the tower part, the seawater-immersed steel pipe pile part, and the embedded CCS pipe pile part. The lateral dynamic behaviours of the CCS pipe pile-supported OWT system have been determined and then validated by solving the analytical solution of lateral displacement for the above three parts. Numerical discussions are finally conducted to analysis the influence of some physical parameters on the lateral displacement and the first-order natural frequency of the CCS pipe pile-supported OWT system. The main findings can be summarized as: (i) the offshore deep cement-soil mixing (ODCM) pile is an effective reinforcement method to reduce the lateral vibration of conventional steel pipe pile-supported OWTs; (ii) the increase in tower height and seawater layer thickness will have a serious negative impact on the lateral dynamic response of CCS pipe pile-supported OWTs; (iii) the increase in gas saturation of gassy marine soil will lead to a negative influence on the maximum lateral displacement of CCS pipe pile-supported OWTs.

Resource diversity disturbs marine Vibrio diversity and community stability, but loss of Vibrio diversity enhances community stability.

Vibrio is a salt-tolerant heterotrophic bacterium that occupies an important ecological niche in marine environments. However, little is known about the contribution of resource diversity to the marine Vibrio diversity and community stability. In this study, we investigated the association among resource diversity, taxonomic diversity, phylogenetic diversity, and community stability of marine Vibrio in the Beibu Gulf. V. campbellii and V. hangzhouensis were the dominant groups in seawater and sediments, respectively, in the Beibu Gulf. Higher alpha diversity was observed in the sediments than in the seawater. Marine Vibrio community assembly was dominated by deterministic processes. Pearson's correlation analysis showed that nitrite (-N), dissolved inorganic nitrogen (DIN), ammonium (-N), and pH were the main factors affecting marine Vibrio community stability in the surface, middle, and bottom layers of seawater and sediment, respectively. Partial least-squares path models (PLS-PM) demonstrated that resource diversity, water properties, nutrients, and geographical distance had important impacts on phylogenetic and taxonomic diversity. Regression analysis revealed that the impact of resource diversity on marine Vibrio diversity and community stability varied across different habitats, but loss of Vibrio diversity increases community stability. Overall, this study provided insights into the mechanisms underlying the maintenance of Vibrio diversity and community stability in marine environments.

Biodiversity and distribution patterns of blooming jellyfish in the Bohai Sea revealed by eDNA metabarcoding

BackgroundThe mass occurrence of scyphozoan jellyfish severely affects marine ecosystems and coastal economies, and the study of blooming jellyfish population dynamics has emerged in response. However, traditional ecological survey methods required for such research have difficulties in detecting cryptic life stages and surveying population dynamics owing to high spatiotemporal variations in their occurrence. The environmental DNA (eDNA) technique is an effective tool for overcoming these limitations.ResultsIn this study, we investigated the biodiversity and spatial distribution characteristics of blooming jellyfish in the Bohai Sea of China using an eDNA metabarcoding approach, which covered the surface, middle, and bottom seawater layers, and sediments. Six jellyfish taxa were identified, of which Aurelia coerulea, Nemopilema nomurai, and Cyanea nozakii were the most dominant. These three blooming jellyfish presented a marked vertical distribution pattern in the offshore regions. A. coerulea was mainly distributed in the surface layer, whereas C. nozakii and N. nomurai showed a upper-middle and middle-bottom aggregation, respectively. Horizontally, A. coerulea and C. nozakii were more abundant in the inshore regions, whereas N. nomurai was mainly distributed offshore. Spearman’s correlation analysis revealed a strong correlation between the eDNA of the three dominant blooming jellyfish species and temperature, salinity, and nutrients.ConclusionsOur study confirms the applicability of the eDNA approach to both biodiverstiy evaluation of blooming jellyfish and investigating their spatial distribution, and it can be used as a supplementary tool to traditional survey methods.

Ocean bottom dual-sensorQ-compensated elastic least-squares reverse time migration based on acoustic and separated-viscoelastic coupling equations

The ocean bottom dual-sensor (OBD) observation system places water detectors (pressure sensors) and land detectors on the irregular seabed interface to receive acoustic pressure fields and elastic wavefields, respectively. These data are usually applied to effectively remove ghost waves and ringing interference in the seawater layer, rather than being jointly used for migration. We propose an ocean bottom dual-sensor viscoacoustic and separated-viscoelastic coupled Q-compensated least-squares reverse time migration (OBD-VSV- QLSRTM) in curvilinear coordinates. In this method, to solve the Q-compensated elastic inverse problem, we jointly use the recorded acoustic pressure and elastic multicomponent seismic data from the OBD observation system. A new misfit function in the sense of least-squares inversion is constructed based on viscoacoustic and separated viscoelastic wavefields. Acoustic and separated viscoelastic equations are used to describe the acoustic wavefield in the acoustic medium and the Q-compensated P and S wavefields in the underlying viscoelastic medium, respectively. On the acoustic-viscoelastic interface, we implement a stable governing equation to ensure continuous and steady transmission of Q-compensated stresses in the subseabed and pressure in the seawater. To address the irregular seabed problem, we mesh the acoustic-viscoelastic models onto curvilinear grids and apply coordinate transformations to map the models and equations to a new curvilinear coordinate system. We derive viscoacoustic and separated-viscoelastic coupled adjoint and demigration operators in the curvilinear coordinates, enabling linear acoustic-viscoelastic modeling and stable receiver-side back-propagated wavefield continuation. Numerical examples conducted on two typical acoustic-viscoelastic models demonstrate that our proposed OBD-VSV- QLSRTM in curvilinear coordinates can produce highly accurate P- and S-wave images efficiently.

Study on the onset mechanism of bio-blister degradation of polyolefin by diatom attachment in seawater

It is essential to develop a mechanism for lowering the molecular weight of polyolefins to achieve biodegradation in seawater. In this study, a polypropylene/polylactic acid blend sample was first subjected to photodegradation pretreatment, and it was confirmed that in pure water, the acid generated promotes the polypropylene degradation (autoxidation), while in alkaline seawater, the promotion was inhibited by a neutralization reaction. In the autoxidation of polyolefins in alkaline seawater, aqueous Cl− was also the inhibitor. However, we found that autoxidation could be initiated even in seawater by lowering the pH and using dissociation of ClOH (called blister degradation). The blister degradation mechanism enabled autoxidation, even in seawater, by taking advantage of the ability of diatoms to secrete transparent exopolymer particles (TEP) to prevent direct contact between the surface layer of polyolefins and alkaline seawater. We named blister degradation in seawater with diatoms as bio-blister degradation and confirmed its manifestation using linear low-density polyethylene (LLDPE)/starch samples by SEM, IR, DSC and GPC analysis.

Study on the seismic characteristics of piers considering site conditions and hydrodynamic effect

Sea-crossing bridges face complex site environments compared to onshore bridges, with the marine environment significantly influencing seismic responses. Despite this, current seismic design for these bridges relies on onshore earthquake records. Therefore, investigating the impact of seawater layers and site conditions on sea-crossing bridge seismic performance is crucial. This paper investigates the influence of characteristics of offshore ground motion, site conditions, and hydrodynamic effect on the seismic performance of piers. Initially, based on the validated finite element model, 5 offshore and onshore strong ground motion records from K-NET were selected for assessing the seismic performance of piers. Subsequently, the effects of the water depth and site conditions on the seismic responses of piers were investigated. Finally, the effects of the section size, section shape, and boundary conditions on the piers were investigated. The result shows that the seismic response of piers under offshore ground motion exceeds that under onshore ground motion. In addition, the seismic response of the pier increases with greater water depth, while they exhibit a slight decrease with increasing soil depth. Notably, the larger the section size, result in higher the hydrodynamic pressure, and square piers experience greater hydrodynamic pressure compared to circular piers.

An analytical solution for the dynamic response of a seawater‐sloping seabed‐bedrock system under an oblique incident SV wave

AbstractThe characteristics of seabed ground motion are the theoretical basis for the study of seawater‐soil‐structure interactions and seismic‐induced geological hazards. There are numerous offshore marine structures located on sloping seabeds. Therefore, the sloping seabed's seismic response requires to be investigated. Engineers and scholars are more interested in the SV waves because they carry more energy than the P waves in earthquakes. An analytical solution for the steady‐state response of the inclined seafloor under an oblique incident SV wave is offered. The impacts of the seafloor angle on the fluid pressure and displacement of the saturated sediment layer are investigated under different incident wave frequencies, incident angles, and thicknesses of the seawater layer and sediment layer. By observing along the seabed surface, we discovered that the slope angle might cause considerable variances between different positions even at a small site. While examining the seabed at varying depths, we discovered that the incline angle has distinct impacts on different areas of the seabed. It has an amplifying effect at some depths and a reducing effect at other depths. Generally, as the slope angle increases, the distinction between the responses of the inclined and horizontal seabed becomes more prominent. Especially when the incidence angle of the SV waves is near the critical angle, the impacts of the slope angle become more significant. Even if the horizontal seabed model is employed with the same seawater depth, it is not suitable for analyzing the seismic response of a sloping seabed. Otherwise, significant errors would arise.

Decadal variability of oxygen uptake, export, and storage in the Labrador Sea from observations and CMIP6 models

The uptake of dissolved oxygen from the atmosphere via air-sea gas exchange and its physical transport away from the region of uptake are crucial for supplying oxygen to the deep ocean. This process takes place in a few key regions that feature intense oxygen uptake, deep water formation, and physical oxygen export. In this study we analyze one such region, the Labrador Sea, utilizing the World Ocean Database (WOD) to construct a 65–year oxygen content time series in the Labrador Sea Water (LSW) layer (0–2200 m). The data reveal decadal variability associated with the strength of deep convection, with a maximum anomaly of 27 mol m–2 in 1992. There is no long-term trend in the time series, suggesting that the mean oxygen uptake is balanced by oxygen export out of the region. We compared the time series with output from nine models of the Ocean Model Intercomparison Project phase 1 in the Climate Model Intercomparison Project phase 6, (CMIP6-OMIP1), and constructed a “model score” to evaluate how well they match oxygen observations. Most CMIP6-OMIP1 models score around 50/100 points and the highest score is 57/100 for the ensemble mean, suggesting that improvements are needed. All of the models underestimate the maximum oxygen content anomaly in the 1990s. One possible cause for this is the representation of air-sea gas exchange for oxygen, with all models underestimating the mean uptake by a factor of two or more. Unrealistically deep convection and biased mean oxygen profiles may also contribute to the mismatch. Refining the representation of these processes in climate models could be vital for enhanced predictions of deoxygenation. In the CMIP6-OMIP1 multi-model mean, oxygen uptake has its maximum in 1980–1992, followed by a decrease in 1994–2006. There is a concurrent decrease in export, but oxygen storage also changes between the two periods, with oxygen accumulated in the first period and drained out in the second. Consequently, the change in oxygen export (5%) is much less than that in uptake (28%), suggesting that newly ventilated LSW which remains in the formation region acts to buffer the linkage between air-sea gas exchange and oxygen export.

Cathodic protection of carbon steel in thin layers of seawater

Purpose The purpose of this study is to investigate cathodic protection (CP) efficiency in the tidal zone and its associated processes. Design/methodology/approach Specific features of CP in the tidal zone, that is, persistence of a thin seawater film and insufficient cathodic potential due to ohmic drop, were addressed. In this preliminary study, carbon steel electrodes were polarized at two cathodic potentials (correct or insufficient protection) while immersed in 1 mm or 5 mm thick natural seawater layers. After CP interruption, the protective ability of the layers covering the steel electrodes was studied using various electrochemical methods, including electrochemical impedance spectroscopy. The layers were characterized by XRD. Findings The protective ability of calcareous deposits was increased in thin seawater films. Insufficient CP could promote protective aragonite/corrosion products layer. Originality/value The combined effects of thin seawater film and applied potential were never addressed, and the conclusions drawn from this preliminary study give new insight on the efficiency of CP in the tidal zone.

The Diversity and Metabolism of Culturable Nitrate-Reducing Bacteria from the Photic Zone of the Western North Pacific Ocean

To better understand bacterial communities and metabolism under nitrogen deficiency, 154 seawater samples were obtained from 5 to 200 m at 22 stations in the photic zone of the Western North Pacific Ocean. Total 634 nitrate-utilizing bacteria were isolated using selective media and culture-dependent methods, and 295 of them were positive for nitrate reduction. These nitrate-reducing bacteria belonged to 19 genera and 29 species and among them, Qipengyuania flava, Roseibium aggregatum, Erythrobacter aureus, Vibrio campbellii, and Stappia indica were identified from all tested seawater layers of the photic zone and at almost all stations. Twenty-nine nitrate-reducing strains representing different species were selected for further the study of nitrogen, sulfur, and carbon metabolism. All 29 nitrate-reducing isolates contained genes encoding dissimilatory nitrate reduction or assimilatory nitrate reduction. Six nitrate-reducing isolates can oxidize thiosulfate based on genomic analysis and activity testing, indicating that nitrate-reducing thiosulfate-oxidizing bacteria exist in the photic zone. Five nitrate-reducing isolates obtained near the chlorophyll a-maximum layer contained a dimethylsulfoniopropionate synthesis gene and three of them contained both dimethylsulfoniopropionate synthesis and cleavage genes. This suggests that nitrate-reducing isolates may participate in dimethylsulfoniopropionate synthesis and catabolism in photic seawater. The presence of multiple genes for chitin degradation and extracellular peptidases may indicate that almost all nitrate-reducing isolates (28/29) can use chitin and proteinaceous compounds as important sources of carbon and nitrogen. Collectively, these results reveal culturable nitrate-reducing bacterial diversity and have implications for understanding the role of such strains in the ecology and biogeochemical cycles of nitrogen, sulfur, and carbon in the oligotrophic marine photic zone.

Numerical simulations of seawater spraying on launch platform during CZ-8X offshore launching

In this paper, the cooling effect of seawater spraying on the offshore launching platform used by the CZ-8X rocket at different takeoff heights is studied. The rocket plume model using the realizable k- ε turbulence model is established, which is based on the three-dimensional Navier-Stokes equations. The injected seawater for cooling the deck surface is described by the Eulerian dispersed phase (EDP) model. The numerical results are compared with the experimental data in the literature to verify the validity and accuracy of the numerical model. The coupled flow field of the CZ-8X rocket plume and seawater at different takeoff heights and different spray velocities are simulated. The results show that the thermal environment on the deck surface of the platform is the worst when the takeoff height reaches 30 m. The seawater spraying velocity shall be moderate before the takeoff height reaches 30 m. A low spraying velocity will lead to poor cooling effect on deck surface, and a high spraying velocity could lead to increased diffusion of plume flow in non-spraying direction. After the takeoff height rises to 40 m, a low water spraying velocity makes the injection position of seawater far away from the jet core area, and the seawater evaporation rate is low, which is conducive to the accumulation of seawater layer on the deck surface. The cooling effect of thermal barrier layer is very obvious. The research shows that when the rocket is launched at sea, the launch platform can be cooled by pumping and spraying seawater, and the water spraying rate can be adjusted appropriately according to the takeoff height to achieve the best cooling effect.

Результаты измерения тяжелых металлов в водах Баренцева и Гренландского морей

The results of observations of the content of dissolved forms of Fe, Mn, Cr are presented in the surface layer of sea water in the eastern part of the Barents Sea and in the area of the Greenland Sea near the Svalbard archipelago. The observations were made during the complex expeditions of the MMBI RAS in 2021 and 2022. The concentration of these microelements was measured by atomic absorption spectroscopy using a “KVANT.Z” spectrometer. Maps of the distribution of these elements of the study areas are presented, the average concentration of heavy metals did not exceed the maximum allowable concentrations for marine fishery water bodies in the waters of the studied areas. In general, the content of HMs in the waters of the Isfjord and its branches is lower than in the waters of the Barents Sea. The maximum range of heavy metals content variability is also observed in the Barents Sea. The largest spatial variation characterizes in concentrations the distribution of dissolved forms of Fe in the studied basins.

Study on multidimensional and multipoint seafloor spatial ground motion simulation under an ice-water layer and the dynamic response of a cross-sea bridge

A combined ice-water reflection coefficient is developed to measure the effect of saltwater and floating ice on seabed ground motions to explore the effect of floating ice on seafloor ground motions. The combined reflection coefficient is made up of three parts: the reflection coefficient of the seawater layer, the transmission coefficient of the water layer, and the reflection coefficient of the ice layer. The combined reflection coefficient analyzes the reflection, transmission, and refraction pathways of seismic waves from the soil layer to the water layer, then to the ice layer and air, and then reflects the attenuation principle of seismic waves in an ice-water layer based on the mechanical parameters of the soil layer, water layer, ice layer, and air. The spatial coherence effect, traveling wave effect, and local site effect are then addressed using the trigonometric series method, and the ice-water layer combined reflection coefficient is utilized to analyze the influence of offshore complex sites on seafloor spatial ground motion. Under the floating ice and seawater layer, an artificial synthetic approach of multidimensional multipoint spatial ground motion is described. The random ice load time history was artificially produced using the ice load spectrum theory and the recorded sea ice parameters. The random response of a long-span cable-stayed bridge is investigated under the combined action of seafloor spatial ground motion and random ice load, and the structural responses of the bridge and tower vary substantially under the combined action of ground motion and ice load.

Theoretical modeling of seafloor spatial earthquake ground motion with random fluctuation sea surface

With the growing number of offshore structures, the damage degree and probability of offshore structures under earthquake increases. However, the influence of the fluctuating seawater layer on the propagation of seafloor ground motion is generally ignored in seismic design of offshore structures or ocean ground motion researching. Numerous studies have revealed that the spatial earthquake ground motion on the seafloor is notably affected by the saturation in the soil and the seawater layer overlaying the seabed. A new seawater transfer function to estimate the effect of random fluctuations in the sea surface on the behavior of ground motion was proposed in this paper. The seawater transfer function was derived by incorporating the reflection coefficient of the seawater layer and the scattering coefficient corresponding to the random fluctuation of the sea surface. The reflection of the P wave in the seawater layer between the lower medium and upper medium was considered by a reflection coefficient. The scattering coefficient was utilized to facilitate the estimation of the P wave attenuation due to random sea surface fluctuations. Moreover, the effects of the soil layer and soil saturation under the sea on ground motion are also considered. Taking the above factors into consideration, the ground motions affected by random fluctuations in the sea surface can be synthesized. A water layer transfer function proposed by Crouse and Quilter is used to validate the proposed seawater transfer function. To demonstrate the effect of the new seawater transfer function on ground motion, three example cases are presented. Comparisons among these three case results show that the new seawater transfer function exhibits attenuation at relatively high frequencies. Furthermore, the higher a sea wave is, the more severe the attenuation in the seawater transfer function is. The results also suggest that ground motion energy can be greatly influenced by random fluctuations in the sea surface. Lastly, the simulation method presented in this paper, which considers the fluctuating sea surface, offers a valuable reference for the study of ocean ground motion and presents a more rational approach to seismic simulations for offshore structure research.

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.

Coupled analysis of monopile offshore wind turbine under modified three-dimensional seafloor ground motions

The safe operation of offshore wind turbines (OWTs) is significantly influenced by the combined effect of wind, waves, and offshore earthquakes. However, limited seafloor ground motion data have been used to determine the dynamic characteristics and responses of OWTs subjected to offshore earthquakes. Hence, synthetic seafloor ground motions are particularly important for seismic analysis of OWTs. This paper proposes a new method to simulate seafloor ground motions. In the study, the dynamic shear modulus of the water-bearing soil layer and the damping effect of the soil–seawater layers were introduced to modify the simple Crouse and Quilter (C-Q) model in seawater and P-SV and SH transfer function models at offshore sites. Further, three-dimensional seismic waves were synthesized based on the modified transfer function models at a typical offshore site with bedrock–soil–seawater layers, and a fully coupled numerical analysis model of OWTs, including the rotor nacelle assembly, tower structure, and substructure with pile–soil interaction (PSI) under wind, wave, and seafloor ground motion, was established by incorporating the earthquake module and PSI module into the updated FAST v8 software. Moreover, based on the established coupled OWT model, the measured seafloor ground motion records and the seafloor ground motions synthesized using the proposed method were compared, and a coupled analysis of the OWT under wind, wave, and offshore earthquake loadings was carried out. Furthermore, the coupled mechanisms of the OWT under synthesized and recorded seafloor ground motions were determined.