Won-Bae Na

Pukyong National University · Department of Ocean Engineering

Marine vegetation is increasingly viewed as a living shoreline that protects coastal communities and ecosystems from the damaging effects of wave energy. Many studies have explored the potential of marine vegetation in terms of reducing wave height, but more work is needed. Here, we used particle image velocimetry, fluid–structure interaction simul...

Oyster reefs are currently at risk of severe decline due to dangerous human interference and its aftermath; hence, artificial oyster reefs (AORs) have been utilized for their restoration. AORs with high vertical reliefs interact with the surrounding flow, constitute a reverse flow, and create a wake region in which concentrated nutrients and food o...

Human intervention is required to preserve wild animals due to human-induced problems such as climate change and altered habitats [...]

Like aquatic habitats in nature, artificial reefs (ARs) have been investigated to determine whether their complexity affects species diversity. Moreover, studies have been carried out to investigate wake regions of ARs, which provide a shelter, feeding ground, rest area, and temporary stopover for marine species; however, few have examined the corr...

Considering the movement of quarried rocks falling freely in water is important, because these are often used in marine habitat enhancement structures such as artificial reefs. Few studies have accounted for the horizontal movement of these rocks due to water flow, which is likely to be significant in oceans. Consideration of horizontal water flows...

Macroalgae are invaluable constituents of marine forest environments and important sources of material for human needs. However, they are currently at risk of severe decline due to global warming and negative anthropogenic factors. Restoration efforts focus on beds where macroalgae previously existed, as well as the creation of new marine forests....

Artificial reef-installed seabeds may have significantly different scouring patterns depending on the ground conditions, such as the soil particle size, even though the flow velocity and water depth are similar. In this study, the scour characteristics of the ground were determined using Flow-3D to evaluate the geotechnical stability of artificial...

Several studies have identified stability issues related to artificial reefs (ARs) installed on the seabed. However, initial AR settlement during free-fall installation has not been well examined. We performed a laboratory-scale experiment, regression analysis, and numerical simulation to evaluate the initial settlements of two cube-type AR models...

Many studies have evaluated the performance of turbulence models used for computational fluid dynamics (CFD) analysis of artificial reefs (ARs), but an optimal model remains elusive, particularly in terms of wake length and areal estimation. Only a few models were used in previous studies and no report has yet investigated wake shape. We compared f...

Some artificial reefs (ARs) have experienced an initial settlement primarily owing to their collision to the seabed during an installation process, particularly by a free-fall. This study presents a numerical estimation of the seabed settlement of a box-type artificial reef during the initial installation. For the purpose, the following works were...

Consider social value (or contribution) of community-based education of the Busan Green Environment Center (BGEC), we analyzed the outcomes of environmental education programs in the last three years (2015~2017). Accordingly, we developed ten indices (E1~E10) and proposed equations to calculate the environmental education social value (EESV) of eac...

Marine environmental education projects have been operated by O-RCC (ocean keeping regional collaboration center) of Pukyong National University, for regional community innovation. These projects can be classified into either of two models i.e. stepping stone–living lab or community innovation project. Of 79 undergraduate and graduate students who...

From the perspective of saving energy of marine species and creating feeding areas, the wake volume of an artificial reef (AR) should be considered as a parameter in any wake region estimation. Wake regions of AR modules (reef ball, cylinder reef, and cube reef) and sets were numerically estimated considering tropical seawater temperatures and wate...

Seaweed plays a central role in supporting good habitats and spawning grounds for a number of fisheries resources. Artificial seaweed reefs (ASRs) should provide firm, stable substrates for seaweed. Hence, it is desirable to propose a physical measure of seaweed spore settlement, which can be characterized by the surface area of an ASR and its wake...

Marine forest artificial reefs (MFARs) have been used in relatively shallow seawaters to provide light intensity and firm, stable substrates for seaweeds. Their design should be oriented to enlarge surface areas with relatively small reef sizes and to spread spores with a productive reef placement. Regarding such productive placement, it is necessa...

It is not easy to properly manage the fishery resources mainly because we do not know how the invaluable resources act with social, economic, and environmental fluctuations in detail. Nonetheless, we have installed artificial reefs (underwater man-made structures) in the coastal waters and tried to take advantages over. One of the main trials is la...

The present study presents efficient placement models of labyrinth-type artificial concrete reefs (LT-ACRs) to support natural submerged aquatic vegetation beds or enhance macrophyte beds [Undaria pinnatifida (Harv.) Suringar, Laminaria sp., Ecklonia stolonifera Okamura, and Sargassum fulvellum (Turner) C. Agardh] in the coastal waters of South Kor...

The wake and upwelling regions of artificial reefs (ARs), and sets, groups, and complexes thereof, are critical factors in engineering AR design and estimating fishing grounds, which are often facilitated by ARs. This study proposes six AR set placement models, constructed based on previous field observations, to characterize their wake and upwelli...

Recently, artificial reefs (ARs) have been frequently used primarily owing to the development in AR materials and projects for relatively complicated, large ARs. Among several engineering issues of ARs, wake region of an AR has been characterized because these regions have a high probability of recruiting seaweed spores, providing an energy saving...

Two field tests were carried out to measure the lifting forces required to salvage a sunken vessel and caisson, and force histories were obtained to assess their response to bottom friction (BF), surface tension (ST), buoyancy release (BR), water capture (WC) and water release (WR). The test results for the two fully sunken objects showed rather di...

To evaluate the wake regions of six artificial reefs (ARs) frequently used in the marine forest creation project in Korea, we consider the effect of water flow directions on the wake regions and accordingly propose a wake region diagram, which is characterized by parameters such as wake volume fluctuations, averaged wake volume, fundamental symmetr...

The wake region of an artificial reef (AR) is defined as the space consisting of the recirculating water flow immediately behind the AR. This study proposes three performance indices to evaluate the wake region and pinpoints how to determine the indices for ARs. First, we established dimensionless performance indices, such as the so-called wake reg...

Considering the artificial reef (AR) canyon intervals facilitated by flatly distributed placement models, the wake volumes of 25 AR sets were characterized through the following works. First, twenty-five different canyon intervals were established to investigate how the intervals affect the wake volumes of the AR placement models, each with nine cu...

Woo, J.; Kim, D., and Na, W.B., 2016. Anchor collision analysis of stone-filled bags for submarine power cable protection using a smoothed particle hydrodynamics method. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research...

In this study, effective usable and burial volumes were used to estimate the current state of nine cube-type artificial reefs (AR) constructed between 1987 and 2002. Three volume coefficients (k1, k2, and k3) were proposed for estimation of the total loss, effective usable volume, and burial volume of ARs, based on the recorded facility standard vo...

The dynamic characteristics of a tunnel structure used to protect underwater power cables, the so-called A-duct, were determined for anchor collisions to provide a procedure for damage assessment and recommendations. The required physical quantities of five target anchors, including the drag coefficient, were obtained using an element-based finite-...

A procedure for obtaining the dynamic characteristics of rock berms in the event of an anchor collision is developed, a safety analysis is performed, and associated recommendations are made. The drag coefficients of five target anchors were obtained through finite element flow analysis, and their terminal velocities calculated to maximise colliding...

This study presents dynamic responses of rock-berm structural system under anchor dragging and accordingly provides the characteristics of the stresses and displacements obtained. For the purpose, first, a rock-berm was modeled by the SPH (smoothed particle hydrodynamics) method and piecewise Drucker-Prager material model by facilitating the associ...

This study presents a 3-dimensional flow-structure interaction analysis of subsea risers in water flows. Two structural connectors (flat and circular couplers) were intentionally devised and numerically tested using ANSYS CFX to investigate how these couplers behave under the water flows. In the flow analysis, the water field was constructed with a...

Submersed rock-berm structures arefrequently used for protection of underwater lifelines such as pipelines and power cables. During the service life, the rock-berm structure can experience several accidental loads such as anchor collision. The consequences can be severe with a certain level of frequency; hence, the structural responses should be ca...

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 press...

Abstract In recent years, collisions between anchors and submarine power cables have attracted increasing research interest because of the rapid growth of offshore wind farms. Since the drag coefficients have not been calculated, conservative values have been assumed and used to simulate collisions. This conservative estimation is likely to result...

It is not easy to analyze the behavior of a structural body composed of particles such as rocks using the finite element method facilitating typical element meshes because we cannot ignore the interactions among particles. In the study, we investigated the applicability of smooth particle hydrodynamics (SPH) element method for collision analysis of...

This study presents the effects of geometric characteristics on the structural responses of three target risers (simple riser, flat coupled riser, and circular coupled riser). For the purpose, a coupled analysis was carried out to initially calculate the pressure fields on the target structures in water flow fields and then obtain structural respon...

Eco-friendly and sustainable seaweed biomass energy have been under the spotlight as the future of renewable energy. However, seaweed culture is primarily conducted inshore, with the research on offshore culture still in an early stage. For massive biomass production, a systematic engineering approach is required to devise offshore seaweed culture...

This paper presents the results of finite element simulations of elastic wave propagation in an underwater steel plate and the verification of a proposed method utilizing elastic wave-based damage detection. For the simulation and verification, we carried out the following procedures. First, three-dimensional finite element models were constructed...

A safety assessment of mattress type submarine power cable protectors under dragging forces of a 2-ton anchor was carried out through field tests. These cable protectors were designed to adjust their size by controlling the number of reinforced concrete blocks. In the field tests, soil properties on a target site were simulated and the safety asses...

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...

A capstone design is regarded as one of cap courses in undergraduate engineering education because it requires most prerequisites and makes students experience real engineering design processes. There have been case studies to show how this subject should be organized, practiced, and optimized. This study shows one of the case studies by focusing o...

By carrying out numerical analyses and model experiments, this paper presents the attenuation characterization of an L(0,2) guided ultrasonic wave propagating in a buried steel pipe. From this investigation, we first find that the L(0,2) mode has a better attenuation property. Second, it is shown from the numerical analyses that the attenuation inc...

An undergraduate course named computational structural analysis becomes more significant in recent years because of its important role in industries and the recent innovation in computer technology. Typically, the course consists of introduction to finite element method, utilization of general purpose finite element software, and examples focusing...

This study presents numerical simulations of elastic waves for detecting symmetrically or non-symmetrically aligned defects in a thin epoxy adhesive layer connecting two aluminum plates. For the numerical simulations, a two dimensional plane model is constructed, and an element size and time increment are properly selected to acquire stable explici...

This study presents stress concentration factor estimation for a composite plates using finite element and inverse analysis. For the finite element analysis, so-called implicit-explicit sequential solution is modified to static-explicit sequential solution for applying static compressive surface forces and dynamic ton-burst Gaussian pulses to an al...

This study presents a rockfall impact analysis of a typical roadway. Dynamic finite element analyses using ANSYS AUTODYN are conducted to determine the effect of the drop heights (5 m, 10 m) on the damage to a roadway model. The Rockfall is modeled as a spherical shape with a weight of 400 kg, and each drop height is converted to a corresponding im...

Perforated plates with cutouts (or holes) are widely used in structural members. These cutouts provide stress concentration in plates. Extensive studies have been carried out on stress concentration in perforated plates, which consider cutout shapes, boundary conditions, bluntness of cutouts, and more. This study presents stress concentration analy...

Attenuations of the fundamental longitudinal guided wave mode propagating in buried steel pipes, which transport different fluids, are numerically investigated. From the attenuation dispersion curves, two specific frequency values (1 MHz, 2.65 MHz) are selected to capture attenuation characteristics and, then the previously introduced parametric de...

This study presents a nondestructive test for underwater PVC pipes. To use guided ultrasonic waves, specially denoted by cylindrical Lamb waves, a test setup was made in a water tank using the pitch and catch mode and specimens were made to give artificial cutouts located in the circumferential direction of the pipes. Total three states of damaged...

A study was conducted to investigate curvature effect on guided wave propagation in curved steel plates and hollow steel cylinders. The results, including the dispersion curves of phase, group velocities, and detailed quantitative values at certain frequencies, provided information about the curvature effect by showing how curvature affects phase a...

This study presents an explicit dynamic analysis of an adhesively bonded single-lap joint under an impact load. The finite element software, ANSYS LS-DYNA, was used for the analysis and Von Mises stresses were obtained from the analysis. To model the adherents, solid elements were used and a rigid body was assumed for impactor modeling. Three impac...

This study presents the attenuation characteristics of the first guided longitudinal wave mode propagating in water-filled, buried steel pipes in order to investigate the effects of soil saturation and compaction on the attenuation patterns. For numerical calculation of attenuation, 10 different combinations of S-wave velocity, P-wave velocity, and...

This study presents a stability analysis procedure for caisson structures and a case study for a honeycomb slit-caisson. CADMAS-SURF was used to calculate the wave pressures based on an irregular wave with a 50-year period and the data for three regular waves obtained from a target site. Then, the irregular and regular wave pressures were used to o...

Recently, many techniques have been developed for the inspection of pipelines using guided waves. However, few researches have been made on the application of those techniques for buried underground pipes. Guided wave motions in the buried pipes are somewhat different from those of on-ground pipes which have traction-free (air) boundary condition o...

In this study, characteristics of the fundamental longitudinal guided wave mode, L(0,1), which is a usual mode employed in the inspection of the above-ground pipe, of the buried pipe were numerically investigated considering property changes in the surrounding soil. Results showed that soil conditions are significantly affecting the attenuation of...

This study presents the results of collision and lift analyses of a cylinders-connected protector under stock anchor colliding and dragging. For the analyses, the terminal velocity of the stock anchor was obtained first, and, then, the velocity was used to calculate the falling distance of the stock anchor in air. In addition, two other falling dis...

This study presents a detection method for mechanically fabricated defects on underwater steel pipes, using ultrasonic guided waves. Three different diameters (60, 90, and 114 mm) of 1000-mm long steel pipes were considered, along with several experimental design factors such as incident angles, incident distances, and the degrees of defects, to in...

Acceleration and impedance signatures extracted from a structure are appealing features for a prompt diagnosis on structural condition since those are relatively simple to measure and utilize. However, the feasibility of using them for damage monitoring is limited when their changes go undisclosed due to uncertain temperature conditions, particular...

In this study, an output-only modal analysis approach for wireless sensor nodes is proposed on the basis of assumption that a target structure is a linear system. In order to achieve the objective, the following approaches are implemented. Firstly, an output-only modal analysis method is selected for the wireless sensor networks. Secondly, the effe...

A vibration-impedance-based monitoring method is proposed to predict the loss of prestress forces in prestressed concrete (PSC) girder bridges. Firstly, a global damage alarming algorithm using the change in frequency responses is formulated to detect the occurrence of damage in PSC girders. Secondly, a local damage detection algorithm using the ch...

In 2006, Jeju Island in South Korea experienced a crisis, no electricity for three hours anywhere in the entire island. This incident was caused by a domino effect that occurred after one of the submarine power cables connecting the island to Haenam, a coastal city on the mainland, was damaged by an external load, probably from a ship anchor or a s...

In this paper, hybrid health monitoring techniques using acceleration-impedance features are newly proposed to detect two damage-type in steel plate-girder bridges, which are girder's stiffness-loss and support perturbation. The hybrid techniques mainly consists of three sequential phases: 1) to alarm the occurrence of damage in global manner, 2) t...

This study presents a finite element simulation of elastic waves for detecting anti-symmetric damages in an adhesively-bonded single lap joint. Plane strain elements were used for modeling adherents (aluminum) and adhesives (epoxy). Three types of damage were introduced: thickness reduction, elasticity deterioration, and voids in the adhesive layer...

This paper presents a numerical simulation of guided ultrasonic waves propagating in aluminum-epoxy-aluminum adhesive plates. In particular, this study investigated the effect of the epoxy thickness on the dispersive patterns, such as the phase velocity and group velocity of guided ultrasonic waves. In addition to investigating the dispersive curve...

Two-point elastic wave excitation method is proposed to detect the damages (crack and deterioration). This method does not require any baseline signal to distinguish the damages. For verifying the proposed method, finite element simulations were carried out. Two models including 2-D plane and 3-D solid models were constructed, and non-reflecting bo...

As part of a marine habitat enhancement project, the physical and chemical deterioration of reinforced concrete reefs that were fully immersed in Tongyeong waters of Korea was investigated. For the investigation, marine environmental factors such as seawater, salinity, pH, dissolved oxygen, sea-bottom materials, and water depth of the targeted site...

The chemical degradation characteristics of reinforced concrete reefs, which had been fully immersed in Tongyeong waters of South Korea for 18–25 years, were studied. In order to investigate the marine environmental impact on the concrete reefs, environmental factors, for example pH, have been observed between 1997 and 2002. Then, four reinforced c...

In this paper, a hybrid vibration-impedance approaches is newly proposed to detect the occurrence of damage, the location of damage, and extent of damage in steel plate-girder bridges. Firstly, theoretical backgrounds of the hybrid structural health monitoring are described. The hybrid scheme mainly consists of three sequential phases: 1) to alarm...

Among many damage types, prestress-loss in tendon is the major one that should be monitored in its early stage in order to secure the safety of PSC girder bridges. This damage-type obviously change vibration characteristics, but with apparent difference depending on sensing mechanism as well as information analysis. Recently, there have been resear...

Two points source method was proposed for damage detection in concrete structures. To validate the proposed method, finite element simulations were carried out. In 3-D models, non-reflecting boundary conditions were considered to model an infinite medium so that the models prevented artificial dilatational stress wave reflections generated at the b...

YOON, H.S., YOO, C.I., NA, W.B., LEE, I.C. AND RYU, C.R., 2007. Geomorphologic evolution and mobility of sand barriers in the Nakdong estuary, South Korea. Journal of Coastal Research, SI 50 (Proceedings of the 9th International Coastal Symposium), 358 - 363. Gold Coast, Australia, ISSN 0749.0208 This study discusses the characteristics of the spat...

The sound intensities required to sustain a certain interval between two ultrasonic sensors that are installed outside a steep pipe for structural health monitoring are numerically investigated. Two fundamental guided wave modes, longitudinal and flexural, are considered for the calculation of their attenuations via the concept of parametric densit...

Parametric density concept is proposed for a long-range pipeline health monitoring. This concept is designed to obtain the attenuation of ultrasonic guided waves propagating in underwater pipelines without complicated calculation of attenuation dispersion curves. For the study, three different pipe materials such as aluminum, cast iron, and steel a...

To develop a promising hybrid structural health monitoring (SHM) system, which enables to detect damage by the dynamic response of the entire structure and more accurately locate damage with denser sensor array, a combined use of structural vibration and electro-mechanical (EM) impedance is proposed. The hybrid SHM system is designed to use vibrati...

The physical deterioration of reinforced concrete reefs, which were fully immersed in Tongyeong waters of South Korea for 19, 21, 23, and 25 years, respectively, were investigated. Firstly, the marine environmental factors such as sea temperature, salinity, pH, dissolved oxygen, sea bottom materials, and water depth of target water sites were obser...

Guided wave techniques have been used for pipeline inspection because of their long-range inspection capability. One of main concerns of these techniques is how ones decide axial interval of sensors. This question is related to the characteristics of attenuation of cylindrical guided waves. Parametric density concept is proposed for a long-range pi...

Attenuations of the first (or fundamental) longitudinal guided wave modes propagating in liquid-filled steel pipes are numerically investigated. Several filling liquids transported by the steel pipe are considered in the investigation. In the numerical modeling stage, a sink is considered for abandoning standing wave modes caused by the internal li...

Finite element simulation of elastic wave propagation in a concrete plate was carried out to investigate its modeling and damage detection procedures. For the numerical stability three criteria were introduced and tested. With a proper element size and time increment, two different kinds of damage scenarios (crack and deterioration) were applied to...

Physical and chemical deterioration of reinforced concrete reefs are characterized using scientific destructive and nondestructive testing tools. From the test results, the reinforced concrete reefs elected from Tongyeong waters are evaluated, and the new minimum concrete depth is proposed.

This paper presents hybrid structural damage monitoring system which performs both global damage assessment of structure and damage detection of local structural joints. Hybrid damage monitoring system is composed of vibration-based technique and electro/mechanic impedance technique. Vibration-based technique detects global characteristic change ot...

To develop a promising hybrid structural health monitoring system, which enables to detect damage by the dynamic response of the entire structure and more accurately locate damage with denser sensor array, a combined use of mechanical vibration and electro-mechanical impedance is proposed. For the verification of the proposed healthmonitoring schem...

Guided-waves-based mortar-filled steel pipe inspection is carried out through using EMAT (Electro magnetic acoustic transducer) and wavelet transform. Possibly existing anomalies such as separation (or void) and inclusion are made in the fabricated mortar-fled steel pipes: these anomalies are infected. Since guided waves have the long range inspect...

A typical composite breakwater is composed of upright caisson structure and rubble mound which again consists of core and revetments. For the purpose of efficient disaster prevention or harbor tranquility, a proper distribution of sections for each part is an essential factor in design and economic construction of composite breakwater. Most of the...