• Ocean Systems Engineering
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• v.7 no.1
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• pp.53-74
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• 2017

Ship shaped FPSO (Floating Production, Storage and Offloading) units are the most commonly used floating production units to extract hydrocarbons from reservoirs under the seabed. These structures are usually much larger than general cargo ships and have their natural frequency outside the wave frequency range. This results in the response to first order wave forces acting on the hull to be negligible. However, second order difference frequency forces start to significantly impact the motions of the structure. When the difference frequency between wave components matches the roll natural frequency, the structure experiences a significant roll motion which is also termed as second order roll. This paper describes the theory and numerical implementation behind the calculation of second order forces and motions of any general floating structure subjected to waves. The numerical implementation is validated in zero speed case against the commercial code OrcaFlex. The paper also describes in detail the popular approximations used to simplify the computation of second order forces and provides a discussion on the limitations of each approximation.

• International Journal of Ocean System Engineering
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• v.1 no.3
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• pp.155-164
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• 2011

The second-order motion characteristics of a semi-submersible are investigated in regular waves. A higher-order boundary element method in a frequency domain and a finite element method in a time-domain were applied to the numerical analysis of the nonlinear hydrodynamic force and motion characteristics of semi-submersibles in view point of potential flow. Various aspects of nonlinear effects on the heave and roll of a semi-submersible were numerically investigated and some selected cases were compared with the model test data.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.1
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• pp.34-43
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• 1994

In order to improve thickness profile of cold rolled steel strip, a computer simulation and actual cold rolling experiments on a 4-high tandem cold mill were carried out. In this study, tapered barrel end at top work roll and incurved barrel body at bottom work roll were examined for reducing edge drop and threading of stripe. Also, the most effective stand of tandem cold mill and magnitude of optimal taper crown at several stand were investigated under a typical rolling condition. From actual rolling experiments, it was verified that thickness deviation over strip width could be improved up to 1.06% by applying tapered and incurved work roll only at the second and up to 0.89% by applying them at both the second and the third stand.

• Journal of Advanced Research in Ocean Engineering
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• v.2 no.2
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• pp.81-85
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• 2016

Nonlinear wave loads can lead to resonant responses of offshore structures in sum or difference frequencies. In this study, the roll motion of an FPSO with a low natural frequency is simulated in the time domain. To generate the time signals of wave loads, the quadratic transfer functions of the second-order excitations are calculated in the frequency domain. The equations of motions based on the time memory functions are used to evaluate the roll responses in irregular waves. The roll damping in empirical form is accounted for in the simulation.

• Journal of Advanced Research in Ocean Engineering
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• v.1 no.3
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• pp.168-175
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• 2015

This study consists of an experimental investigation of the reduction of the second-order roll motion of a semi-submersible platform in head sea conditions by adding hull damping. The second-order heave drift force and roll drift moment are known to be the main triggers that induce the list angle (Hong et al., 2010). Hong et al. (2013) used numerical calculations to show the possibility of reducing the list angle by changing the pontoon shape and adding a damping device on the hull. One of their findings was that the reduction in the list angle due to the increase in pontoon surface damping was significant. A series of model tests were carried out with a 1:50 scaled model of semi-submersible at the KRISO wave basin. The experiments indicated that adding damping on the hull surface effectively suppressed the list angle.

• Korean Journal of Materials Research
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• v.23 no.7
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• pp.392-398
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• 2013

A two-pass differential speed rolling(DSR) was applied to a deoxidized low-phosphorous copper alloy sheet in order to form a homogeneous microstructure. Copper alloy with a thickness of 3 mm was rolled to 75 % reduction by two-pass rolling at $150^{\circ}C$ without lubrication at a differential speed ratio of 2.0:1. In order to introduce uniform shear strain into the copper alloy sheet, the second rolling was performed after turning the sample by $180^{\circ}$ on the transverse direction axis. Conventional rolling(CR), in which the rotating speeds of the upper roll and lower roll are identical to each other, was also performed by two-pass rolling under a total rolling reduction of 75 %, for comparison. The shear strain introduced by the conventional rolling showed positive values at positions of the upper roll side and negative values at positions of the lower roll side. However, samples processed by the DSR showed zero or positive values at all positions. {100}//ND texture was primarily developed near the surface and center of thickness for the CR, while {110}//ND texture was primarily developed for the DSR. The difference in misorientation distribution of grain boundary between the upper roll side surface and center regions was very small in the CR, while it was large in the DSR. The grain size was smallest in the upper roll side region for both the CR and the DSR. The hardness showed homogeneous distribution in the thickness direction in both CR and DSR. The average hardness was larger in CR than in DSR.

• Journal of the Korean Society for Precision Engineering
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• v.21 no.8
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• pp.112-119
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• 2004

This paper develops a computer model of a cabover type large truck for estimating the effects of the mounted method of frame on handling performance. The computer model considers two mounted methods of frame; flange mounted and web mounted. Frame is modeled by finite elements using MSC/NASTRAN in order to consider the flexibility of frame. The reliability of the developed computer model is verified by comparing the actual vehicle test results with the simulation results. The actual vehicle test is performed in a double lane change course, and lateral acceleration, yaw rate, and roll angle are measured. To estimate the effects of the mounted method of frame on handling performance, simulations are performed with the flange mounted and web mounted frame. Simulation results show that the web mounted frame＇s variations of roll angle, lateral acceleration, and yaw rate are larger than the flange mounted frame＇s variations, especially in the high test velocity and the second part of the double lane course. Also, simulation results show that the web mounted frame＇s tendencies of roll angle, lateral acceleration, and yaw rate advance the flange mounted frame＇s tendencies, especially in the high test velocity and the second part of the double lane course.

• Bulletin of the Society of Naval Architects of Korea
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• v.23 no.4
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• pp.25-34
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• 1986

A two-dimensional linear method has been developed for the motion and the second-order steady force arising from the hydrodynamic coupling between waves and currents in the presence of a body of arbitrary shape. Interaction between the incident wave and current in the absence of the body lies in the realm beyond our interest. A Fredholm integral equation of the second kind is employed in association with the Haskind's potential for a steadily moving source of pulsating strength located in or below the free surface. The numerical calculations at the preliminary stage showed a significant fluctuation of the hydrodynamic forces on the surface-piercing body. The problem is approximately solved by using the asymptotic Green function for $U^2{\rightarrow}0$. The original Green function, however, is applied for the fully submerged body. Numerical calculations are made for a submerged and for a half-immersed circular cylinder and extensively for the mid-ship section of a Lewis-form. Some of the results are compared with other analytical results without any available experimental data. The current has strong influence on roll motion near resonance. When the current opposes the waves, the roll response are generally negligible in the low frequency region. The current has strong influence on roll motion near resonance. When the current opposes the wave, the roll response decreases. When the current and wave come from the same direction, the roll response increases significantly, as the current speed increases. The mean drift forces and moment on the submerged body are more affected by current than those on the semi-immersed circular cylinder or on the ship-like section in the encounter frequency domain.

• Journal of the Korean Chemical Society
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• v.17 no.4
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• pp.298-305
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• 1973

Following results were obtained for the mechanical degradation of polystyrene (for polystyrene itself and when blended with rubber) by roll mastication. 1) The rate of mechanical degradation for polystyrene itself can be represented by the second-order rate equation proposed by Goto. $-\frac{dP_t}{dt} = k_s(P_t-P_{\infty})^2$ Where Pt is the degree of polymerization of the degraded polymer at t minutes and $P{\infty}$ is the final degree of polymerization. 2) The mechanical degradation of polystyrene component in the polystyrene-rubber (SBR, BR) blend system occurred similarly as that of polystyrene itself. 3) Under the experimental conditions the mechanical degradation rate of the polystyrene component of the polystyrene-rubber, (SBR, BR) blend system followed approximately the same second-order equation as that for polystyrene itself.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.15 no.1
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• pp.59-69
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• 2001

Ths paper presents an algorithm of detecting eccentricity signal and diagnosing the cause in rolling process. A designed technique uses only the outputs of load cell and tachorreter, which are generally utilized in rolling process, and need not any acklitional sensors. Detecting signals related to eccentricity is done by combining outputs of 6 digital bandPass filters whose center frequency are varied with roll's rotating frequency. A function of diagnosing the cause of occentricity from extracted data having occentricity components is implemented by defining reasonable variables. In order to show validity of an algorithm designed in this paper, simulation was accomplished for three groups. The first is that some portion of roll in the direction of roll circumference is distorted. The second is that rotating axis and roll center is not identical. The third is that composite cause is occmed Simulation results show that every kind of eccentricity can be diagnosed in terms of algorithm proposed.oposed.