• Journal of the Korean Society for Marine Environment & Energy
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• v.19 no.2
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• pp.99-110
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• 2016

When a riser is damaged, the oil spills to sea. Oil spills cause huge economic losses as well as a destruction of the marine environment. To reduce losses, it is needed to predict spilled oil volume from risers and the excursion of the oil. The present paper simulated the oil spill for a damaged riser using open source libraries, called Open-FOAM. To verify numerical methods, jet flow and Rayleigh-Taylor instability were simulated. The oil spill was simulated for various damaged leak size, spilled oil volume rates, damaged vertical locations of a riser, and current speeds. From results, the maximum excursion of the spilled oil at the certain time was predicted, and a forecasting model for various parameters was suggested.

• Journal of Korea Foundry Society
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• v.28 no.2
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• pp.74-78
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• 2008

An integrated riser heating equipment has been developed to control shrinkage defects originated from casting of a marine propeller. The integrated riser heating equipment is composed of up/down moving parts, heating power source parts and an integrated controller. Heat capacity putting into the riser was calculated quantitatively on the base of a heat transfer analysis, which consisted of the establishment of heating model and the theoretical analysis for heat transfer. The riser heating equipment was evaluated through arc heating and electro-slag heating method. With the results, the arc type heating method was selected by considering high thermal efficiency, inexpensive cost, and convenient workship. This equipment improves the quality of a propeller casting and the poor working environment.

• Journal of the Korean Society for Marine Environment & Energy
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• v.18 no.1
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• pp.29-35
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• 2015

The concept design of a riser for Ocean Thermal Energy Conversion in 10 MW is proposed and its dynamic behaviour characteristics is analyzed with numerical method. A riser pipe with a hollow along its thickness in the cross-section to increase the effective modulus of its cross-section is designed considering the manufacture. The riser pipe without hollows along its thickness needs a lumped weight at the bottom end of a riser in order to keep its vertical hanging configuration from large buoyancy and the strong current. The riser is designed to control its density by inserting materials in high or low density into a hollow. The dynamic behaviour characteristics of the two designed risers is evaluated with the developed numerical analysis tool. The combined stress of the riser with a lumped weight is showed to be dominated by weight of a lumped mass. The riser with no hollow shows large combined stress near sea surface by strong current. Local structural analysis for the cross-section of a hollow riser is needed in detail.

• Journal of the Korean Society for Marine Environment & Energy
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• v.18 no.1
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• pp.22-28
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• 2015

The design on a riser in 1MW OTEC system is performed. The minimum diameter of the riser is decided depending on intake quantity of deep-sea water to supply an OTEC cycle. An applicable pipe material is selected from analyzing the properties of commercial pipes. The selected HDPE pipe with the low density and strength is reinforced with a lumped block attached at the end of and wire ropes along the riser. A lumped block, connected to a floating structure by wire ropes, with 25% and 50% weight of a GFRP riser is designed to be attached the end of a riser. The structural safety of the HDPE riser with wire rope supporting axial loads induced by a lumped block is analyzed under the harsh ocean environmental condition near Hawaii ocean with the numerical method. The final dimension of the riser and accessories is determined considering the economic point of view. The designed riser will be applicable to the construction of the 1 MW OTEC pilot plant.

• Journal of Ocean Engineering and Technology
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• v.24 no.1
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• pp.153-160
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• 2010

This paper deals with the topology optimized design of the riser support structures for floating production storage and offloading units (FPSOs) under global and local loading conditions. For a preliminary study and validation of the numerical approach, a simplified plate under static loading is first evaluated with the representative topology optimization methods, the Homogenization Design Method (HDM) and Density Method (DM) or Simple Isotropic Material with Penalization (SIMP). In the context of the corresponding riser support structures, the design problem is formulated such that structure shapes based on design domain variables are determined by minimizing the compliance subject to a mass target, considering the stress criterion. An initial design model is generated based on an actual FPSO riser support configuration. The topology optimization results present improved design performances under various loading conditions, while staying within the allowable limit of the offshore area.

• Journal of Ocean Engineering and Technology
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• v.33 no.5
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• pp.387-393
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• 2019

In this study, the free motion of a riser due to vortex shedding was numerically simulated with Large Eddy Simulation (LES) and Detached Eddy Simulation (DES) turbulence models. A numerical simulation program was developed by applying the Rhie-Chow interpolation method to the pressure correction of the OpenFOAM standard solver pimpleDyMFoam. To verify the developed program, the vortex shedding around the fixed riser at Re = 3900 was calculated, and the results were compared with the existing experimental and numerical data. Moreover, the vortex-induced vibration of a riser supported by a linear spring was numerically simulated while varying the spring constant. The results are compared with published direct numerical simulation (DNS) results. The present calculation results show that the numerical method is appropriate for simulating the vortex-induced motion of a riser, including lock-in phenomena.

• Structural Monitoring and Maintenance
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• v.6 no.4
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• pp.365-387
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• 2019

This study aims to provide useful information on the fatigue assessment of a top-tensioned riser (TTR) subjected to vortex-induced vibration (VIV) by performing parametric study. The effects of principal design parameters, i.e., riser diameter, wall thickness, water depth (related to riser length), top tension, current velocity, and shear rate (or shear profile of current) are investigated. To prepare the base model of TTR for parametric studies, three (3) riser modelling techniques in the OrcaFlex were investigated and validated against a reference model by Knardahl (2012). The selected riser model was used to perform parametric studies to investigate the effects of design parameters on the VIV fatigue damage of TTR. From the obtained comparison results of VIV analysis, it was demonstrated that a model with a single line model ending at the lower flex joint (LFJ) and pinned connection with finite rotation stiffness to simulate the LFJ properties at the bottom end of the line model produced acceptable prediction. Moreover, it was suitable for VIV analysis purposes. Findings from parametric studies showed that VIV fatigue damage increased with increasing current velocity, riser outer diameter and water depth, and decreased with increasing shear rate and top tension of riser. With regard to the effects of wall thickness, it was not significant to VIV fatigue damage of TTR. The detailed outcomes were documented with parametric study results.

• Proceedings of the Korean Society of Marine Engineers Conference
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• 2006.06a
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• pp.275-276
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• 2006

선박 및 해양기자재의 안전 진단을 위한 기존의 기술은 접 센서(point sensor)를 이용한 변형률 측정 기술이 대부분이라 할 수 있다. 본 논문은 기존의 기술보다 효율적으로 응용될 수 있는 광섬유 센서를 이용한 분포 개념의 온도 및 변형률 측정(DTSS: Distributed Temperature & Strain Sensing) 기술에 대해 소개하고 있다. 이 기술은 선체 응력 모니터링, 해양 구조물 안전진단, subsea flowline 모니터링, platform의 riser 안정성, umbilical 모니터링 등에 활용될 수 있다.

• Ocean Systems Engineering
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• v.1 no.2
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• pp.171-194
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• 2011

A method to design a boundary controller for global stabilization of three-dimensional nonlinear dynamics of flexible marine risers is presented in this paper. Equations of motion of the risers are first developed in a vector form. The boundary controller at the top end of the risers is then designed based on Lyapunov's direct method. Proof of existence and uniqueness of the solutions of the closed loop control system is carried out by using the Galerkin approximation method. It is shown that when there are no environmental disturbances, the proposed boundary controller is able to force the riser to be globally exponentially stable at its equilibrium position. When there are environmental disturbances, the riser is stabilized in the neighborhood of its equilibrium position by the proposed boundary controller.

• Journal of Coastal Disaster Prevention
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• v.1 no.4
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• pp.149-155
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• 2014

Flexible risers have been used extensively in recent years for floating and early production systems. Such risers offer the advantage of having inherent heave compliance in their catenary thereby greatly reducing the complexity of the riser-to-rig and riser-to subsea interfaces. Another advantage with flexible risers is their greater reliability. Concerns about fatigue life, gas permeation and pigging of lines have been overcome by extensive experience with these risers in production applications. In this paper, flexible riser analysis results were compared through coupled and uncoupled dynamic analyses methods. A time domain coupled analysis capability has been developed to model the dynamic responses of an integrated floating system incorporating the interactions between vessel, moorings and risers in a marine environment. For this study, SPM (Single Point Mooring) system for an FSU in shallow water was considered. This optimization model was integrated with a time-domain global motion analysis to assess both stability and design constraints of the flexible riser system.