Evaluation of bending moments and shear forces at unit connections of very large floating structures using hydroelastic and rigid body analyses

This paper investigates the characteristics of bending moments, shear forces and stresses at unit connections of very large floating structures (VLFS) under wave loads. The responses of VLFS are calculated by solving multi-body motion equation considering hydroelasticity and connection stiffness. Hydroelastic responses are calculated by the direct method. Higher-order boundary element method (HOBEM) is used for fluid analysis and finite element method (FEM) is introduced for structural analysis. The equation of motion is modified to describe the unit connections by employing spring elements. Bending moments and shear forces at the connections are obtained from the dynamic equilibrium condition for pressures and inertia forces. Two types of VLFS units such as tandem arranged units and side-by-side arranged units are considered in the numerical examples. The influences of connection stiffness, wave frequency and heading angle on responses of VLFS are investigated through the numerical examples. Rigid body analysis along with hydroelastic analysis is also carried out in the numerical analysis and comparison of those two approaches is discussed.     

Rare earth element fingerprints in Korean coastal bay sediments: Association with provenance discrimination

Rare earth elements (REEs: La-Lu) in surface sediments collected from the mouth and middle tidal flats of Gomso Bay, South Korea, in August 2011 and May 2012 were analyzed to investigate the fine-grained sediment provenance. The upper continental crust (UCC)-normalized light REEs (LREEs: La to Nd) were more enriched than the middle REEs (MREEs: Sm to Dy) and heavy REEs (HREEs: Ho to Lu), resulting in large (La/Yb)_UCC (1.9 ± 0.4) to (Gd/Yb)_UCC (1.4 ± 0.2) ratios. The monthly (La/Yb)_UCC values differed between the mouth and middle tidal flats due to deposition of fine-grained sediments that originated from distant rivers (the Geum and Yeongsan) and the Jujin Stream, located on the southern shore of the inner bay. We observed relative reductions in the (La/Yb)_UCC value and REE content in the sediments from the mouth of the bay compared with those from Jujin Stream sediments. Confined to the middle tidal flat around the KH Line of Jujin Stream, the sediments, most enriched in LREEs but depleted in Eu, were distributed in August as strong Jujin Stream runs. Here, we suggest that an increase in LREE/HREE and decrease in MREE/LREE ratios can be used as a proxy to identify the Jujin Stream provenance in mixed riverine sediments and to trace Jujin Stream sediments within the Gomso Bay tidal flat, especially in the summer rainy season.     

Reconstruction of regular time series from bi-monthly temperature data in the Yellow Sea and the northwestern East China Sea

Reconstruction of evenly-spaced, regular time series from routine survey serial data was investigated for precise analysis of spatio-temporal variations in a temperate sea at mid-latitudes where the seasonality dominates the interannual variability. Considering that the annual range of sea surface temperature in the Yellow Sea and the northwestern East China Sea can be as large as 15–20 °C, temperature data collected bi-monthly in these temperate seas were used for the assessment of reconstruction methodology. The cubic spline interpolation with a sampling interval of 0.5 months reconstructed the regular time series closest to the in-situ measurements among various interpolation schemes. Also, two computation methods for the interannual anomaly were compared; the residual method that the long-term monthly means are subtracted from the monthly serial data and the filtering method that high-frequency variations are removed using a low-pass filter. In that the high-frequency variations in frequencies greater than 1 cycle per year are comparable in magnitude to the interannual variation, the residual method proves inadequate in the temperate sea. High-frequency noises can be effectively removed through the use of a proper low-pass filter with bell-shaped weights.     

Photochemical characteristics of high and low ozone episodes observed in the Taehwa Forest observatory (TFO) in June 2011 near Seoul South Korea

We present a comprehensive discussion on what cause high ozone episodes at a suburban photochemical observation site of the Seoul Metropolitan Area (population ～23 million). The observational site, Taehwa Research Forest (TRF), is situated ～30 km from the center of Seoul. In June 2011, we observed two very distinctive ozone periods-high ozone (peak up to 120 ppbv) and low ozone (peak up to 60 ppbv) in the mid and early month, respectively. The trace gas measurement dataset, especially CO and NO _X clearly indicate that less anthropogenic influences during the high ozone period. Volatile organic compound (VOC) measurement results show that at the observational site, biogenic VOCs (mostly isoprene) contribute most of chemical reactivity towards OH, although toluene from anthropogenic activities was observed in higher concentrations. Back-trajectory analysis indicates that air-masses from the forest part of Korea Peninsula were dominant influences during the high ozone episode event. On the other hand, Aged air masses from China were the dominant influence during the low ozone episode event. Model calculations conducted using the University of Washington Chemical Mechanism (UWCM) box model, also consistently show that BVOC, especially isoprene photochemistry, can be the significantly contribution to local ozone formation in the given photochemical environments of TRF. These research results strongly suggest that ozone control strategy in the Eastern Asian megacities, mostly situated in surrounding forest areas should be based on the comprehensive scientific understanding in BVOC photochemistry and interplays between anthropogenic and biogenic interactions.     

Oceanic response to Typhoon Nari (2007) in the East China Sea

The oceanic response to a typhoon in the East China Sea (ECS) was examined using thermal and current structures obtained from ocean surface drifters and a bottom-moored current profiler installed on the right side of the typhoon’s track. Typhoon Nari (2007) had strong winds as it passed the central region of the ECS. The thermal structure in the ECS responded to Typhoon Nari (2007) very quickly: the seasonal thermocline abruptly collapsed and the sea surface temperature dropped immediately by about 4°C after the typhoon passed. The strong vertical mixing and surface cooling caused by the typhoon resulted in a change in the thermal structure. Strong near-inertial oscillation occurred immediately after the typhoon passed and lasted for at least 4–5 days, during which a strong vertical current existed in the lower layer. Characteristics of the near-inertial internal oscillation were observed in the middle layer. The clockwise component of the inertial frequency was enhanced in the surface layer and at 63 m depth after the typhoon passed, with these layers almost perfectly out of phase. The vertical shear current was intensified by the interaction of the wind-driven current in the upper layer and the background semi-diurnal tidal current during the arrival of the typhoon, and also by the near-inertial internal oscillation after the typhoon passage. The strong near-inertial internal oscillation persisted without significant interfacial structure after the mixing of the thermocline, which could enhance the vertical mixing over several days.     

Forward induced seismic hazard assessment: application to a synthetic seismicity catalogue from hydraulic stimulation modelling

The M _w 3.2-induced seismic event in 2006 due to fluid injection at the Basel geothermal site in Switzerland was the starting point for an ongoing discussion in Europe on the potential risk of hydraulic stimulation in general. In particular, further development of mitigation strategies of induced seismic events of economic concern became a hot topic in geosciences and geoengineering. Here, we present a workflow to assess the hazard of induced seismicity in terms of occurrence rate of induced seismic events. The workflow is called Forward Induced Seismic Hazard Assessment (FISHA) as it combines the results of forward hydromechanical-numerical models with methods of time-dependent probabilistic seismic hazard assessment. To exemplify FISHA, we use simulations of four different fluid injection types with various injection parameters, i.e. injection rate, duration and style of injection. The hydromechanical-numerical model applied in this study represents a geothermal reservoir with preexisting fractures where a routine of viscous fluid flow in porous media is implemented from which flow and pressure driven failures of rock matrix and preexisting fractures are simulated, and corresponding seismic moment magnitudes are computed. The resulting synthetic catalogues of induced seismicity, including event location, occurrence time and magnitude, are used to calibrate the magnitude completeness M _c and the parameters a and b of the frequency-magnitude relation. These are used to estimate the time-dependent occurrence rate of induced seismic events for each fluid injection scenario. In contrast to other mitigation strategies that rely on real-time data or already obtained catalogues, we can perform various synthetic experiments with the same initial conditions. Thus, the advantage of FISHA is that it can quantify hazard from numerical experiments and recommend a priori a stimulation type that lowers the occurrence rate of induced seismic events. The FISHA workflow is rather general and not limited to the hydromechanical-numerical model used in this study and can therefore be applied to other fluid injection models.     

Electrokinetic Removal of Petroleum Hydrocarbon from Residual Clayey Soil Following a Washing Process

This study investigates total petroleum hydrocarbon (TPH) removal from residual clayey soil, after a washing procedure, using an electrokinetic process. Eight electrokinetic experiments were carried out to investigate the characteristics of TPH removal. When 0.1 M MgSO₄ or 0.1 M NaOH was used as an electrolyte, the electric current rapidly increased within the first 100 or 200 h, respectively. A negatively charged soil surface resulted in a more negative zeta potential and greater electroosmotic flow toward the cathode. Therefore, the accumulated electroosmotic flow (EOF) when using 0.1 M NaOH as the anolyte‐purging solution was higher than when using 0.1 M MgSO₄. Although the energy consumption for the two purging solutions was similar, the efficiencies of TPH removal when 0.1 M MgSO₄ and 0.1 M NaOH with surfactant were used were 0 and 39%, respectively, because the electroosmotic flow rate increased with TPH removal efficiency. When 5% isopropyl alcohol (IPA) was used as a circulation solution, the electric current increased but the TPH removal was similar to that using water. In terms of energy consumption, the use of a surfactant‐enhanced electrokinetic process with NaOH as electrolyte was effective in removing TPHs from low‐permeability soil.     

Ecological significance of alkaline phosphatase activity and phosphatase-hydrolyzed phosphorus in the northern part of Gamak Bay, Korea

We investigated the ecological significance of alkaline phsophatase (APase) and alkaline phosphatase-hydrolyzable phosphorus (APHP) in the northern part of Gamak Bay, Korea. APase activity was detectable throughout the year, and dissolved inorganic phosphorus (DIP) concentration and APase activity are highly correlated and can be regarded as an indicator of DIP-limiting conditions. Also, a strong linear positive correlation between APase activity and Chl a concentration indicated that the major part of APase activity may have been induced by phytoplanktons. The APHP proportion in dissolved organic phosphorus (DOP) was above ca. 30% from winter to spring and below ca. 15% from summer to autumn due to freshwater discharge and uptake by phytoplankton. APHP may play an important role in species competition in coastal area such as northern part of Gamak Bay where DIP is limiting. Thus, APase induction by phytoplanktons may be ecologically significant, allowing dominance by these organisms under DIP-limiting conditions.     

Stress-induced evolution of anisotropic thermal conductivity of dry granular materials

Various factors, such as the volumetric fraction of constituents, mineralogy, and pore fluids, affect heat flow in granular materials. Although the stress applied on granular materials controls the formation of major pathways for heat flow, few studies have focused on a detailed investigation of its significance with regard to the thermal conductivity and anisotropy of the materials. This paper presents a numerical investigation of the stress-induced evolution of anisotropic thermal conductivity of dry granular materials with supplementary experimental results. Granular materials under a variety of stress conditions in element testing are analyzed by the three-dimensional discrete element method, and quantitative variations in their anisotropic effective thermal conductivity are calculated via the network model and conductivity tensor measurements. Results show that the directional development of contact area and fabric under anisotropic stress conditions leads to the evolution of anisotropy in thermal conductivity. The anisotropy induced in thermal conductivity by shear stress is higher than that induced by compressive stress because shear stress causes more significant changes in microstructural configurations and boundary conditions. The shear-stress-induced evolution of anisotropy between principal thermal conductivities depends on dilatancy as well as shearing mode, and the shear-driven discontinuity localizes the conductivity. Factors involved in the stress-induced evolution and their implications on the thermal conductivity characterization are discussed.