• Title/Summary/Keyword: Irregular waves

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Parametric pitch instability investigation of Deep Draft Semi-submersible platform in irregular waves

  • Mao, Huan;Yang, Hezhen
    • International Journal of Naval Architecture and Ocean Engineering
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    • v.8 no.1
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    • pp.13-21
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    • 2016
  • Parametric pitch instability of a Deep Draft Semi-submersible platform (DDS) is investigated in irregular waves. Parametric pitch is a form of parametric instability, which occurs when parameters of a system vary with time and the variation satisfies a certain condition. In previous studies, analyzing of parametric instability is mainly limited to regular waves, whereas the realistic sea conditions are irregular waves. Besides, parametric instability also occurs in irregular waves in some experiments. This study predicts parametric pitch of a Deep Draft Semi-submersible platform in irregular waves. Heave motion of DDS is simulated by wave spectrum and response amplitude operator (RAO). Then Hill equation for DDS pitch motion in irregular waves is derived based on linear-wave theory. By using Bubnov-Galerkin approach to solve Hill equation, the corresponding stability chart is obtained. The differences between regular-waves stability chart and irregular-waves stability chart are compared. Then the sensitivity of wave parameters on DDS parametric pitch in irregular waves is discussed. Based on the discussion, some suggestions for the DDS design are proposed to avoid parametric pitch by choosing appropriate parameters. The results indicate that it's important and necessary to predict DDS parametric pitch in irregular waves during design process.

Numerical Simulation of Irregular Waves Over a Shoal Using Parabolic Wave Model (포물형 파랑모형을 이용한 수중천퇴상 불규칙파의 수치모의)

  • 윤성범;이정욱;연영진;최병호
    • Journal of Korean Society of Coastal and Ocean Engineers
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    • v.13 no.2
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    • pp.158-168
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    • 2001
  • A numerical model based on the wide-angle parabolic approximation equation is developed for the accurate simulation of the directional spreading and partial breaking of irregular waves. This model disintegrates the irregular waves into a series of monochromatic wave components, and the simultaneous calculations are made for each wave component. Then, the computed wave components are superposed to get the wave height of irregular waves. To consider the partial breaking of irregular waves in the computation the amount of energy dissipation due to breaking is estimated using the superposed wave height. The accuracy of the developed model is tested by comparing the numerical results with the experimental measurements reported earlier. In the case of non-breaking waves a considerable accuracy of the model is observed for both regular and irregular waves. On the contrary it is found that the accuracy is significantly degenerated for the case of breaking waves. Some analyses for the accuracy degeneration are presented.

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A time-domain simulation of an oscillating water column with irregular waves

  • Koo, Weoncheol;Kim, Moo-Hyun
    • Ocean Systems Engineering
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    • v.2 no.2
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    • pp.147-158
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    • 2012
  • A time-domain simulation of a land-based Oscillating Water Column (OWC) with various irregular waves as a form of PM spectrum is performed by using a two-dimensional fully nonlinear numerical wave tank (NWT) based on the potential theory, mixed Eulerian-Lagrangian (MEL) approach, and boundary element method. The nonlinear free-surface condition inside the OWC chamber was specially devised to describe both the pneumatic effect of the time-varying pressure and the viscous energy loss due to water column motions. The quadratic models for pneumatic pressure and viscous loss are applied to the air and free surface inside the chamber, and their numerical results are compared with those with equivalent linear ones. Various wave spectra are applied to the OWC system to predict the efficiency of wave-energy take-off for various wave conditions. The cases of regular and irregular waves are also compared.

Time Domain Analysis of a Tension Leg Platform in Multi-Directional Irregular Waves (다방향 불규칙파중의 인장계류식 해양구조물의 시간영역 해석)

  • Lee, Chang-Ho;Kim, Chuel-Hyun
    • Journal of Ocean Engineering and Technology
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    • v.20 no.5
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    • pp.36-41
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    • 2006
  • The main object of this study is to develop an accurate and convenient method for the response analysis of offshore structures in real sea states. A numerical procedure is described for predicting the motion responses and tension variations of the ISSC TLP in multi-directional irregular waves. The developed numerical approach in the frequency domain is based on acombination of the three dimensional source distribution method, the dynamic response analysis method, and the spectral analysis method. Frequency domain analysis in the multi-directional irregular waves is expanded to a time domain analysis by using a convolution integral after obtaining the impulse response by Fourier transformation. The results of the comparison between responses in the frequency and time domain confirmed the validity of the proposed approach.

A Study of the Numerical Model on the Interaction between Irregular Waves and Permeable Coastal Structures (투수성해안구조물과 불규칙파의 상호작용에 관한 수치모델 연구)

  • 김종욱;남인식;윤한삼;류청로
    • Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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    • pp.186-195
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    • 2001
  • The purpose of this study is to develop the time-dependent, one-dimensional numerical model on the interaction between irregular waves and two-layer permeable coastal structures, by extending and modifying the numerical model PBREAK(Wurjanto and Kobayashi, 1992) which is applicable only to one-layer permeable coastal structures. The two-layer permeable coastal structure consists of two permeable underlayers with different permeable media resting on an impermeable slope and an armor layer covering the permeable underlayer. The numerical model of this study simulates the wave over rough permeable underlayer of arbitrary geometry as well as the waves inside two-permeable underlayers of arbitrary thickness for specified normally-incident irregular waves. The utility of the numerical model is founded from comparing with PBREAK and the four hydraulic model tests under irregular waves. The sensitivities of computed results according to typical parameters(porosity, stone diameter, horizontal width of the permeable underlayer) and major factors(friction factor of primary armor layer etc.) discussed.

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A Study on the Assessment for the Auto-pilot System of a Ship in Waves (파랑중 선박의 자동조타 시스템의 평가에 관한 연구)

  • S.K. Lee;K.W. Lee;T.K. Kim
    • Journal of the Society of Naval Architects of Korea
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    • v.35 no.1
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    • pp.40-45
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    • 1998
  • There are two kinds of methods in the analysis of ship motion in irregular waves. The one is the spectral method in which the ship motion is assessed with spectral of irregular waves times R.A.O. of a ship. The other is, so called, time domain analysis, in which the irregular waves are used directly in the equation of ship motion to calculate the responses. In this paper, both methods are applied for the calculation of course keeping motion of a ship in irregular waves with auto-pilot control. And, the differences and useful1ness of the two methods in the assessment of auto-pilot system are compared.

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Numerical and experimental analysis of hydroelastic responses of a high-speed trimaran in oblique irregular waves

  • Chen, Zhanyang;Gui, Hongbin;Dong, Pingsha;Yu, Changli
    • International Journal of Naval Architecture and Ocean Engineering
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    • v.11 no.1
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    • pp.409-421
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    • 2019
  • Investigation of hydroelastic responses of high-speed vessels in irregular sea state is of major interest in naval applications. A three dimensional nonlinear time-domain hydroelastic method in oblique irregular waves is developed, in which the nonlinear hydrostatic restoring force caused by instantaneous wetted surface and slamming force are considered. In order to solve the two technical problems caused by irregular sea state, the time-domain retardation function and Proportional, Integral and Derivative (PID) autopilot model are applied respectively. Besides, segmented model tests of a high-speed trimaran in oblique waves are performed. An oblique wave testing system for trimarans is designed and assembled. The measured results of main hull and cross-decks are analyzed, and the differences in distribution of load responses between trimarans and monohull ships are discussed. Finally, from the comparisons, it is confirmed that the present concept for dealing with nonlinear hydroelastic responses of ships in oblique irregular waves is reliable and accurate.

On Damping of Irregular Waves Passing over a Permeable Seabeds (해저투수층을 통과하는 불규칙파의 파랑감쇠에 관한 연구)

  • Hur, Dong-Soo;Choi, Dong-Seok;Lee, Woo-Dong;Bae, Ki-Seong
    • Journal of Ocean Engineering and Technology
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    • v.21 no.6
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    • pp.34-41
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    • 2007
  • The present study investigates numerically damping characteristics of irregular waves passing over a permeable seabeds. At first, the numerical model, which is able to consider the flow through a porous medium with inertial, laminar and turbulent resistance terms and determine the eddy viscosity with LES turbulent model, is validated by comparing with existing experimental data. And then, the numerical test on irregular wave damping over a permeable seabeds is performed in case that wave and flume conditions are changed. It is revealed from the numerical results that the more porosity and mean grain are increased, the more wave damping is increased. Also, the effect of wave period on damping of irregular waves over a permeable seabed is discussed.

Downtime cost analysis of offloading operations under irregular waves in Malaysian waters

  • Patel, M.S.;Liew, M.S.;Mustaffa, Zahiraniza;Abdurasheed, Abdurrasheed Said;Whyte, Andrew
    • Ocean Systems Engineering
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    • v.10 no.2
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    • pp.131-161
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    • 2020
  • The objective of this study was to evaluate the downtime cost of side-by-side offloading operations in Malaysian waters. With the help of a numerical time domain tool, the structure and cable response of moored FPSO vessel was simulated for heading and beam sea-states under irregular waves. The weather downtime was assessed by comparing the response under operational wave condition with the pre defined industrial safe offloading criteria. Additionally, two cases of cable failure were simulated for each sea-state. The novel study on downtime cost was presented for three different location of Malaysia subcontinent for which the location specific wave scatter diagram facilitated to estimate the probability of occurrence of operational wave condition. It was concluded that an unpredictable increment in wave height by 0.5 m can significantly impact the production cost.

Effects of the Multi-directional Irregular Waves on the Motion Responses and Tension Variations of ISSC-TLP (ISSC-TLP의 운동응답 및 변동장력에 미치는 다방향 불규칙파의 영향)

  • Lee, Chang-Ho
    • Journal of Ocean Engineering and Technology
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    • v.20 no.4
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    • pp.70-75
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    • 2006
  • A numerical procedure is described for estimating the effects of the multi-directional irregular waves on the motion responses and tension variations of the ISSC-TLP. The numerical approach is based on a three-dimensional source distribution method and a spectral analysis technique of directional waves. The spectral description for the linear system of ISSC-TLP in the frequency domain is sufficient to completely define the motion responses and tension variations. This is because both the wave inputs and responses are stationary Gaussian random processes, of which the statistical properties in the amplitude domain are well known. The numerical results for the linear motion responses and tension variations in regular waves are compared with the experimental and numerical ones, which are obtained in the literature. The results of comparison confirmed the validity of the proposed approach.