Han-Sam Yoon

Multiscale Acceleration-Dynamic Strain-Impedance Sensor System for Structural Health Monitoring

A multiscale wireless sensor system is designed for vibration- and impedance-based structural health monitoring. In order to achieve the objective, the following approaches are implemented. Firstly, smart sensor nodes for vibration and impedance monitoring are designed. In the design, Imote2 platform which has high performance microcontroller, large amount of memory, and flexible radio communication is implemented to acceleration and impedance sensor nodes. Acceleration sensor node is modified to measure PZTs dynamic strain along with acceleration. A solar-power harvesting unit is implemented for power supply to the sensor system. Secondly, operation logics of the multi-scale sensor nodes are programmed based on the concept of the decentralized sensor network. Finally, the performance of the multi-scale sensor system is evaluated on a lab-scale beam to examine the long-term monitoring capacities under various weather conditions.

Flow and Structural Response Characteristics of a Box-type Artificial Reef

We carried out flow and structural response analysis of a box-type artificial reef (AR), which is made of concrete and structural steel. From the flow analysis, the wake region and drag coefficient were evaluated and accordingly, the structural analysis was performed to evaluate the stress and deformation of the target reef by considering the pressure field obtained from the flow analysis. The concept of wake volume was presented to quantitatively estimate the wake region and its variation according to flow direction and velocity. From the results, it is shown that the flow responses are only sensitive to the flow direction; the structural responses are sensitive to both of the flow velocity and direction although the magnitudes are negligible; and the wake volume became 3.52 times the AR volume with an optimum installation condition (30 o , flow direction) of the target unit.

Anchor Drop Tests for a Submarine Power-Cable Protector

Submarine power cables are widely used for power transmission, such as between mainlands and offshore islands and from offshore wind farms to on-land substations. There are several ways to protect power cables from accidental loads. Protection includes concrete blankets, sand bags, bundles, tunnel-type protectors, and trenching. However, no design standard for power-cable protectors is currently available because of the varieties of cable protection solutions and man-made or natural hazards to submarine power cables. Thus, this paper presents anchor drop tests for a newly designed, matrix-type submarine power-cable protector assembled with reinforced concrete blocks, to make a safety assessment. Marine environments were surveyed at the target site and simulated in the test set-up. A 2-ton stock anchor was selected as the colliding object, and a 25-ton crane was prepared to drop the anchor. Preliminary tests were performed to investigate the effect of soil composition and protector arrangements on the test results. Finally, four field anchor drop test scenarios were designed, carried out, and analyzed, and a safety assessment was made for the submarine power cable. From the tests, it was found that, in addition to falling distances, the soil composition and saturation were significant factors for the settlement depth and damaged areas. Considering the settlement depth of soils, the damaged areas of the concrete blocks, and the damaged state of the pipes (safety zone), all of the test results showed that the mattress failed to protect the power cable from the anchor collision. The deformation, damage, and breakage of the pipe, which simulated the safety zone of the power cable, gave clues as to the reasons for the failure.

Field Implementation of Wireless Vibration Sensing System for Monitoring of Harbor Caisson Breakwaters

A wireless sensing system for structural health monitoring (SHM) of harbor caisson structures is presented. To achieve the objective, the following approaches were implemented. First, a wave-induced vibration sensing system was designed for global structural health monitoring. Second, global SHM methods which are suitable for damage monitoring of caisson structures were selected to alarm the occurrence of unwanted behaviors. Third, an SHM scheme was designed for the target structure by implementing the selected SHM methods. Operation logics of the SHM methods were programmed based on the concept of the wireless sensor network. Finally, the performance of the proposed system was globally evaluated for a field harbor caisson structure for which a series of tasks were experimentally performed by the wireless sensing system.

Vibration Characteristics of Gravity-Type Caisson Breakwater Structure with Water-Level Variation

Vibration characteristics of gravity-type caisson breakwater structures which have water-level variations are experimentally examined by using wave load excitations. To achieve the objective, the following approaches are implemented. Firstly, vibration analysis methods are selected to examine the dynamic characteristics of the lab-scale caisson. Secondly, vibration test on a lab-scale caisson which is installed in a two-dimensional wave flume is performed under several excitation sources and water levels. Thirdly, the compatibility of the wave-induced vibration responses is evaluated by comparison with the impact vibration responses, and the appropriate vibration analysis method is selected. Finally, the water-level effects on the caisson breakwater are examined based on the modal parameters which are experimentally measured for the 2D wave flume tests.

Characteristics of Incident Waves on Seaweed Farm Field Around Gumil-up Sea, Wando

Wave field measurements were made over a period of 18 days to study the spatial distribution of incident wave on seaweed tarm field around Gumil-up Sea, Wando, Korea. These measured data were compared with data from the Geomun-do ocean weather/wave observation buoy. A numerical simulation model that combined the offshore design wave with the seasonal normal incoming wave was used to study the incident wave distribution surrounding a seaweed farm. The results are summarized as follows. (1) On-site wave measurements showed that the major relationship between maximum and significant wave height was

Variation Characteristics of the Groundwater Level of Natural Vegetation and Sandy Beaches

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Wave-Induced Vibration Monitoring for Stability Assessment of Harbor Caisson

The variation of groundwater by wave, tide and precipitation conditions is closely related to the vegetation environment at the natural vegetation and sandy based beach, and it has a significant impact on the vegetation development and ground stabilization. In this study, the water temperature, electrical conductivity, and pressure were monitored at five observational stations normal to the Jinu-do(Island) shoreline of Nakdong river estuary from March 2012 to September 2014 (approximately 799 days) with the aim of measuring the variation in groundwater-table characteristics. The purpose of the study was to identify factors (tide, wave etc.) affecting groundwater-table variation using time series and correlation analysis, and to record spatial variations in the groundwater level and electrical conductivity as a result of storm events. The observational station in the intertidal zone was strongly affected by wave period and tide level. During the storm period, the groundwater-table and electrical conductivity were stabilized at the edge of sand dunes, vegetation, and areas of transition between freshwater and seawater.

Vegetation roles on changes of soil properties and microbial activities in a coastal sand dune. A case study

Up-to-date structural health monitoring (SHM) studies have been focused mostly on in-land structures such as bridge and building. Only a few research efforts have been made for harbor structures such as caisson-type breakwater. For stability assessment of harbor caisson structures, it is very essential to monitor vibration responses with limited accessibility, to analyze the vibration features, and to specify the sensitive motions with respect to damage in the caisson-foundation’s interface. In this paper, a wireless sensing system for SHM of harbor caisson structures is presented. To achieve the objective, the following approaches were implemented. First, a wave-induced vibration sensing system was designed for global structural health monitoring. Second, global SHM methods which are suitable for damage monitoring of caisson structures were selected to alarm the occurrence of unwanted behaviors. Third, a SHM scheme was designed for the target structure by implementing the selected SHM methods. Operation logics of the SHM methods were programmed based on the concept of the wireless sensor network. Finally, the performance of the proposed system was globally evaluated for a field harbor caisson structure for which a series of tasks were experimentally performed by the wireless sensing system.

Use of Groundwater-table To Establish a Buffer Zone In a Barrier Island, Nakdong River Estuary, South Korea

The vegetation effects on changes of soil physicochemical properties and microbial activities in the costal sand dune were investigated to understand the roles of vegetation on sand dune ecosystem. Eight sites from six vegetation zones and two bare zones in the dune front, dune crest, and dune back regions were selected. Soil microbial enzyme activities of β-glucosidase, acid phosphatase, arylsulfatase, and dehydrogenase, and soil physicochemical properties of each site were evaluated. The results showed that all the enzyme activities were higher in the mixed vegetation sites with native sand dune plants and naturalized plants and in Pinus thunbergii community site both located in the dune back regions where the accumulation of organic matter and nitrogen were more prominent. The results demonstrated that soil organic matter and nutrients are the primary determinants of the microbial activity in sand dune where are exposed to a gradient of physicochemical stress such as high salinity, moisture and salt spray. Therefore, the conservation of vegetation that generates more soil organic matter and nutrients is important factor in controlling the soil microbial activities and biogeochemical cycles in the coastal sand dune systems.