• Coupled systems mechanics
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• v.2 no.1
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• pp.111-126
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• 2013

The structural design requirements of an offshore platform subjected to wave induced forces and moments in the jacket can play a major role in the design of the offshore structures. For an economic and reliable design; good estimation of wave loadings are essential. A nonlinear response analysis of a fixed offshore platform under structural and wave loading is presented, the structure is discretized using the finite element method, wave plus current kinematics (velocity and acceleration fields) are generated using 5th order Stokes wave theory, the wave force acting on the member is calculated using Morison's equation. Hydrodynamic loading on horizontal and vertical tubular members and the dynamic response of fixed offshore structure together with the distribution of displacement, axial force and bending moment along the leg are investigated for regular and extreme conditions, where the structure should keep production capability in conditions of the 1-yr return period wave and must be able to survive the 100-yr return period storm conditions. The result of the study shows that the nonlinear response investigation is quite crucial for safe design and operation of offshore platform.

• Journal of the Earthquake Engineering Society of Korea
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• v.18 no.2
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• pp.89-94
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• 2014

Spatial variations of a seismic wave are mainly wave passage and wave scattering. Wave passage effect is produced by changed characteristics of exciting seismic input motions applied to the bedrock. Modified input motions travel horizontally with time differences determined by apparent shear wave velocity of the bedrock. In this study, wave passage effect on the seismic response of a structure-soil system is investigated by modifying the finite element software of P3DASS (Pseudo 3-Dimensional Dynamic Analysis of a Structure-soil System) to apply inconsistent (time-delayed) seismic input motions along the soft soil-bedrock interface. Study results show that foundation size affected on the seismic response of a structure excited with inconsistent input motions in the lower period range below 0.5 seconds, and seismic responses of a structure were decreased considerably in the lower period range around 0.05 seconds due to the wave passage. Also, shear wave velocity of the bedrock affected on the seismic response of a structure in the lower period range below 0.3 seconds, with significant reduction of the seismic response for smaller shear wave velocity of the bedrock reaching approximately 20% for an apparent shear wave velocity of 1000m/s at a period of 0.05 seconds. Finally, it is concluded that wave passage effect reduces the seismic response of a structure in the lower period range when the bedrock under a soft soil is soft or the bedrock is located very deeply, and wave passage is beneficial for the seismic design of a short period structure like a nuclear container building or a stiff low-rise building.

• Earthquakes and Structures
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• v.8 no.5
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• pp.1241-1254
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• 2015

Spatially variable ground motions can be significant on the seismic response of a structure due to the incoherency of the incident wave. Incoherence of the incident wave is resulted from wave passage and wave scattering. In this study, wave passage effect on the response spectrum of a building structure built on a soft soil layer was investigated utilizing a finite element program of P3DASS (Pseudo 3-dimensional Dynamic Analysis of a Structure-soil System). P3DASS was developed for the axisymmetric problem in the cylindrical coordinate, but it is modified to apply anti-symmetric input earthquake motions. Study results were compared with the experimental results to verify the reliability of P3DASS program for the shear wave velocity of 250 m/s and the apparent shear wave velocities of 2000-3500 m/s. Studied transfer functions of input motions between surface mat foundation and free ground surface were well-agreed to the experimental ones with a small difference in all frequency ranges, showing some reductions of the transfer function in the high frequency range. Also wave passage effect on the elastic response spectrum reduced the elastic seismic response of a SDOF system somewhat in the short period range.

• Ocean Systems Engineering
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• v.2 no.2
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• pp.115-135
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• 2012

The very low natural frequencies of tension leg platforms (TLP's) have raised the concern about the significance of the action of hydrodynamic wave forces on the response of such platforms. In this paper, a numerical study using modified Morison equation was carried out in the time domain to investigate the influence of nonlinearities due to hydrodynamic forces and the coupling effect between surge, sway, heave, roll, pitch and yaw degrees of freedom on the dynamic behavior of TLP's. The stiffness of the TLP was derived from a combination of hydrostatic restoring forces and restoring forces due to cables and the nonlinear equations of motion were solved utilizing Newmark's beta integration scheme. The effect of wave characteristics such as wave period and wave height on the response of TLP's was evaluated. Only uni-directional waves in the surge direction was considered in the analysis. It was found that coupling between various degrees of freedom has insignificant effect on the displacement responses. Moreover, for short wave periods (i.e., less than 10 sec.), the surge response consisted of small amplitude oscillations about a displaced position that is significantly dependent on the wave height; whereas for longer wave periods, the surge response showed high amplitude oscillations about its original position. Also, for short wave periods, a higher mode contribution to the pitch response accompanied by period doubling appeared to take place. For long wave periods, (12.5 and 15 sec.), this higher mode contribution vanished after very few cycles.

• Structural Monitoring and Maintenance
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• v.1 no.4
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• pp.411-425
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• 2014

A relatively low frequency Lamb wave-based damage identification method called damage imaging method for rectangular composite plate is presented. A damage index (DI) is generated from the delay matrix of the Lamb wave response signals, and it is used to indicate the location and approximate area of the damage. The viability of this method is demonstrated by analyzing the numerical and experimental Lamb wave response signals from rectangular composite plates. The technique only requires the response signals from the plate after damage, and it is capable of performing near real time damage identification. This study sheds some light on the application of Lamb wave-based damage detection algorithm for plate-type structures by using the relatively low frequency (e.g., in the neighborhood of 100 kHz, more suitable for the best capability of the existing fiber optic sensor interrogator system with the sampling frequency of 500 kHz) Lamb wave response and a reference-free damage detection technique.

• Journal of Power System Engineering
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• v.13 no.2
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• pp.63-70
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• 2009

This paper discusses the influence on long-tenn predictions of the ship response in ocean by using the Global Wave Statistics data, GWS, and wave information from the remote sensing satellites. GWS's standard scatter diagrams of significant wave height and zero-crossing wave period are suggested to be corrected to a round number of 0.01/1000 fitted with a statistical analytic model of the conditional lognormal distribution for zero-crossing wave period. The GEOSAT satellite data are utilized which presented by I. R. Young and G. J. Holland (1996, named as GEOSAT data). At first, qualities of this data are investigated, and statistical characteristic trends are studied by means of applying known probability distribution functions. The wave height data of GEOSAT are compared to the data observed onboard merchant ships, the data observed by measure instrument installed on the ocean-going container ship and so on. To execute a long-tenn prediction of ship response, joint probability functions between wave height and wave period are introduced, therefore long-term statistical predictions are executed by using the functions.

• Journal of the Society of Naval Architects of Korea
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• v.44 no.2 s.152
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• pp.148-153
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• 2007

Two typical impact loadings, shock wave and gas bubble pulse, due to UNDEX(UNDerwater EXplosion), should be considered together for the closest response analysis of structure subjected to UNDEX to a reality. Since these two impact loadings have different response time bands, however, their response characteristics of structure are different from each other. It is impossible to consider these effectively under the current computational environment and the mathematical model has not yet been developed. Whereas Hicks model approximates the fluid-structure interaction due to gas bubble pulse as virtual mass effect, treating the flow by the response of gas bubble after shock wave as incompressible ideal fluid contrary to the compressible flow due to shock wave, Geers-Hunter model could make the closest response analysis of structure under UNDEX to a real one as a mathematical model considering the fluid-structure interaction due to shock wave and gas bubble pulse together using acoustic wave theory and DAA(Doubly Asymptotic Approximation). In this study, the application and effectiveness of integrated dynamic response analysis of submerged structure was examined with the analysis of the shock wave and gas bubble pulse together.

• Proceedings of the Earthquake Engineering Society of Korea Conference
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• 2001.09a
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• pp.155-162
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• 2001

In this study, it was performed that the seismic response analysis using long period earthquake wave and short period earthquake wave on dynamic behavior of concrete gravity dam. The results showed that if the same magnitude earthquake waves acted on concrete dam, the maximum displacement and stress at dam crest of long period wave(0funato wave) were about 30 % larger than those of short period wave(Hachinohe wave). And the response acceleration of dam crest was amplified about 5 times in long period earthquake wave and about 3 times in short period earthquake wave.

• Journal of Ocean Engineering and Technology
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• v.10 no.4
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• pp.10-16
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• 1996

The vibration due to progressive ocean waves is analyzed for a typical footing-type offshore airport platform. The platform is modelled as a spring-supported Euler beam and buoyancy change due to wave is considered as excitation force, under the assumption that the wave propagates without distortion by the structure. The results show that the natural frequencies of this structure are distributed very closely and are little affected by boundary conditions and that the response charateristics due to ocean waves are quite different according to the wave frequency. In this study, the wave frequencies are divided into three regions; the resonance region at which the response is governed by the resonance between the natural mode at the wave frequency and the corresponding modal component of the wave excitation force, the bending governed region at which the response is governed by the bending stiffness, and the spring (buoyancy) governed region at which the response is governed by the spring constant ahd therefore is same as the incident wave form.

• Proceedings of the Earthquake Engineering Society of Korea Conference
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• 2006.03a
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• pp.141-148
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• 2006

This study describes a generation of artificial earthquake wane compatible with seismic design spectrum. In seismic response analysis of building structures, the input ground accelerations have considerable effect on dynamic characteristics of structures. Therefore, it is important to properly select input ground motions for seismic response analysis. In this paper, the artificial earthquake wave are generated according to previously recorded earthquake waves in past earthquake events. The artificial wave have identical phase angles to the recorded earthquake wane, and their overall response spectra are compatible with seismic design spectrum with 5% of critical viscous damping. Each simulated earthquake wave has a identical phase angles to the original recorded ground acceleration, and match to design response spectra in the range of period from 0.02 to 10.0 seconds. It is concluded that the artificial earthquake waves simulated in this paper ate applicable as input ground motions for a seismic response analysis of building structures.