Jinho Woo

Drag Coefficients of Stock and Stockless Anchors

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 in unnecessarily heavy designs for the protective structures. Therefore, reliable calculations of the drag coefficients are required to reduce the design, manufacture, and installation costs of the protective structures. Here, the authors describe calculations of the drag coefficients of 2,000-kg stock and stockless anchors using numerical flow analyses. The authors show that the drag coefficients of the anchors converge to 1.0 and 0.8, respectively, regardless of the initial velocity and the Reynolds number.

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.

Effect of Cutout Orientation on Stress Concentration of Perforated Plates with Various Cutouts and Bluntness

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 analyses of perforated plates with not only various cutouts and bluntness but also different cutout orientations. Especially, the effect of cutout orientation on stress concentration is emphasized since structural members have become more complicated recently. To obtain stress concentration patterns, a finite element program, ANSYS, is used. For the designated goal, three parameters are considered as follows: the shapes of polygonal cutouts (circle, triangle, and square), bluntness (a counter measure of radius ratio, r/R), and rotation of cutouts (). From the analyses, it is shown that, in general, as bluntness increases, the stress concentration increases, regardless of the shape and rotation. A more important finding is that the stress concentration increases as the cutouts become more oriented from the baseline, which is the positive horizontal axis (+x). This fact demonstrates that the orientation is also a relatively significant design factor to reduce stress concentration. In detail, in the case of the triangle cutout, orienting one side of the triangle cutout to be perpendicular to the applied tensile forces is preferable. Similarly, in the case of the square cutout, it is more advantageous to orient two sides of square cutout to be perpendicular to the applied tensile force. Therefore, at the design stage, determining the direction of a major tensile force is required. Then, by aligning those polygon cutouts properly, we can reduce stress concentration.

Anchor Dragging Analysis of Rock-Berm Using Smoothed Particle Hydrodynamics Method

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 associated software package—ANSYS-AUTODYN. Second, 2-ton stockless anchor was modeled as a rigid body and eventually dragging external force was obtained. Then, the dragging velocity (1 and 2 m/s) was considered as a parameter to investigate the effect of its variation on the responses. Finally, the dragging tensile forces of the anchor cable were obtained and compared according to the dragging velocities. It is shown that the four-layer rock-berm gives the safety margin to the submarine power cable according to the unaffected gauge points near the cable. This safety is accomplished by the four layers (related to rock-berm height) and the number of rock particles at each layer (related to rock-berm widths).

Output-only Modal Analysis Approach for Time-Unsynchronization Signals in Wireless Sensor Network

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 effect of time unsynchronization on the modal analysis method is mathematically derived. Thirdly, a new modal analysis approach using complex mode-shapes is proposed to extract modal parameters from unsynchronized signals. Finally, the proposed approach is evaluated by numerical tests and experimental tests.

Anchor Collision Analysis of Stone-filled Bags for Submarine Power Cable Protection using a Smoothed Particle Hydrodynamics Method

ABSTRACT 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, Special Issue, No. 75, pp. 720–724. Coconut Creek (Florida), ISSN 0749-0208. We present a method for the safety analysis of stone-filled bags used to protect submarine power cables from anchor collision, using a smoothed particle hydrodynamics (SPH) method capable of dealing with discrete rocks. For this purpose, stone bags were modelled using the SPH method. Using the terminal velocities of five anchors, we simulated the transient dynamic behavior that occurs when the anchors collide with the stone bags. The response was sensitive to the collision velocity, while the displacement responses were also sensitive to the shape of the anchor heads. Stockless anchors provided more sensitive responses, especially stresses, to the larger stone bag size. Our results indicated that the safety performance of the 3.0 m × 3.0 m × 3.0 m stone bag provided sufficient protection for all of the collision cases. It should be noted that the collision velocities corresponded to the maximum (terminal) velocity. For shallower water depths, resulting in anchor velocities less than the terminal velocity, the 1.5 m × 1.5 m × 1.5 m stone bag would also provide suitable protection against anchor collision.

Three-Dimensional Flow Response Analysis of Subsea Riser Transporting Deep Ocean Water

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 an inlet, outlet, and symmetric boundary conditions. As a result, the responses (drag coefficients and pressure fields) were obtained and the pressure fields were applied for the structural analysis. Finally, the structural responses (displacements and equivalent stresses) of the risers were measured to demonstrate the efficiency of the riser connectors.

Finite Element Simulation of Elastic Waves for Detecting Defects and Deteriorations in Underwater Steel Plates

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 using a general purpose finite element program. Second, two types of damages (mechanical defects and deteriorations) were applied to the underwater steel plate and three parameters (defect location, defect width, and depth) were considered to adjust the severity of the applied damages. Third, elastic waves were generated using the oblique incident method with a Gaussian tone burst, and the response signals were obtained at the receiving point for each defect or deterioration case. In addition, the received time domain signals were analyzed, particularly by measuring the magnitudes of the maximum amplitudes. Finally, the presence and severity of each type of damage were identified by the decreasing ratios of the maximum amplitudes. The results showed that the received signals for the models had the same global pattern with minor changes in the amplitudes and phases, and the decreasing ratio generally increased as the damage area increased. In addition, we found that the defect depth was more critical than the width in the decrease of the amplitude. This mainly occurred because the layout of the depth interfered with the elastic wave propagation in a more severe manner than the layout of the width. An inverse analysis showed that the proposed method is applicable for detecting mechanical defects and quantifying their severity.

Rockfall Impact Analysis of Typical Roadway Using Finite Element Simulation

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 impact velocity to save computational time. The roadway model is comprised of an asphalt layer, base layer, sub-base layer, and sub-grade layer. In this paper, the asphalt is modeled using a linear elastic model. The base layer, sub-base layer, and sub-grade layer are modeled using a Mohr-Coulomb model. From the analyses, the effects of the drop height on the damages and stresses are examined and discussed.

Nondestructive inspection of reinforced concrete reefs submersed in seawater

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 observed from 1997 to 2002. Secondly, four reinforced concrete reefs recovered from different sites in Tongyeong waters were tested through various nondestructive tools such as visual inspection, composition test, tensile strength test, compressive strength test, absorption rate and apparent density test, and pore volume test. Thirdly, those test results are analyzed to see the physical deteriorations. Based on the observations and test results, it is shown that, in global, the reinforced concrete reefs have sound physical properties and their originally estimated service life is secured enough for a further service period in the water depth of 28 to 32 m.