• Title/Summary/Keyword: Wave Drift Forces

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Mean Drift Force Acting on a Floating OWC Wave Power Device (부유식 OWC 파력발전 챔버의 파 표류력해석)

  • HONG Do-Chun;HONG Sa-Young;HONG Seok-Won
    • Proceedings of the KSME Conference
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    • 2002.08a
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    • pp.373-376
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    • 2002
  • The drift force acting on a floating OWC chamber in waves is studied taking account of fluctuating air pressure in the air chamber. A velocity potential in the water due to the free surface oscillating pressure patch is added to the conventional radiation-diffraction potential problem. The potential problem inside the chamber is formulated by making use of the Green integral equation associated with the Rankine Green function while the outer problem with the Kelvin Green function. The drift forces as well as the chamber motions are calculated taking account of the air pressure in the chamber.

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A Far Field Solution of the Slowly Varying Drift Force on an Offshore Structure in Bichromatic Waves - Two Dimensional Problems

  • Lee, Sang-Moon
    • Journal of Ocean Engineering and Technology
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    • v.22 no.2
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    • pp.7-12
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    • 2008
  • A far field solution of the slowly varying force on an offshore structure by gravity ocean waves was shown as a function of the reflection and transmission of the body disturbed waves. The solution was obtained from the conservation of the momentum flux, which simply describes various wave forces, while making it unnecessary to compute complicated integration over a control surface. The solution was based on the assumption that the frequency difference of the bichromatic incident waves is small and its second order term is negligible. The final solution is expressed in term of the reflection and transmission waves, i.e. their amplitudes and phase angles. Consequently, it shows that not only the amplitudes but also the phase differences make critical contributions to the slowly varying force. In a limiting case, the slowly varying force solution gives the one of the mean drift force, which is only dependent on the reflection wave amplitude. An approximation is also suggested in a case where only the mean drift force information is available.

A Study on the Deck Wetness of the FPSO (원유 생산.저장.하역선의 갑판침수에 관한 연구)

  • 임춘규;이호영
    • Journal of the Society of Naval Architects of Korea
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    • v.41 no.1
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    • pp.8-14
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    • 2004
  • As the number of offshore structure is glowing in deep waters, there have been increased damages of it. These floating structures in offshore locations exposed to harsh environmental conditions. In recent years, there has been a slowing attention around damages on bow and deck on FPSO caused by waves in steep storm condition. This paper describes a study of the water on deck due to the dynamic behavior of a FPSO with turret mooring system. The nonlinear motions of the FPSO are simulated under external forces due to wave, current, wind, and mooring forces in the time domain. The direct integration method is employed to estimate low frequency drift wave forces. The current forces are calculated by using slow motion maneuvering equations in the horizontal plane. The coefficients of a model for wind forces are calculated from Isherwood's experimental data and the variation of wind speed is estimated by wind spectrum according to the guidelines of API-RP2A.

Transient Motion Analyses for a Ship Advancing in Irregular Waves (불규칙파 중에서 전진하는 선박에 대한 시간영역 운동해석)

  • Ho-Young Lee;Hong-Shik Park;Hyun-Kyoung Shin
    • Journal of the Society of Naval Architects of Korea
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    • v.38 no.3
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    • pp.47-53
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    • 2001
  • When a ship advancing in waves is subjected to impact forces or irregular forces, the motion analyses for ship are convenient for being calculated in the time domain. The added mass, wave damping coefficients, wave exciting forces and mean drift forces are calculated by 3-Dimensional panel method used the translating pulsating Green function in the frequency domain and the motion equations which are considered by the memory effect due to waves are numerically solved by using the Newmark-$\beta$ method in the time domain. The motion analyses are carried out for a Series 60($C_B=0.7$) moving in irregular waves. The items of calculation are 6-degree motions, accelerations at the fore and after position, numbers of deck wetness and numbers of exposure at ship-bottom, etc. Moreover, the thrust addition in waves is examined by considering the time mean drift forces in the motion equations of time domain.

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Drift Forces on a Freely-Floating Sphere in Water of Finite Depth(I) -Momentum Theorem Method- (유한수심(有限水深)의 해상(海上)에서 규칙파(規則波)에 놓인 구(球)에 작용(作用)하는 표류력(漂流力)(I) -운동량(運動量) 이론(理論) 방법(方法)-)

  • H.S.,Choi;T.M.,Oh
    • Bulletin of the Society of Naval Architects of Korea
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    • v.20 no.4
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    • pp.33-40
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    • 1983
  • The drift force acting on a freely-floating sphere in water of finite depth is studied within the framework of a linear potential theory. A velocity potential describing fluid motion is determined by distribution pulsating sources and dipoles on the immersed surface of the sphere. Upon knowing values of the potential, hydrodynamic forces are evaluated by integrating pressures over the immersed surface of the sphere. The motion response of the sphere in water of finite depth is obtained by solving the equation of motion. From these results, the drift force on the sphere is evaluated by the momentum theorem, in which a far-field velocity potential is utilized in forms of Kochin function. The drift force coefficient Cdr of a fixed sphere increases monotononically with non-dimensional wave frequency ${\sigma}a$. On the other hand, in freely-floating case, the Cdr has a peak value at ${\sigma}a$ of heave resonance. The magnitude of the drift force coefficient Cdr in the case of finite depth is different form that for deep water, but the general tendency seems to be similar in both cases. It is to note that Cdr is greater than 1.0 when non-dimensional water depth d/a is 1.5 in the case of freely-floating sphere.

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Steady Drift Forces on Very Large Offshore Structures Supported by Multiple Floating Bodies in Waves(II) (다수의 부체로 지지된 초대형 해양구조물에 작용하는 정상표류력(II))

  • J.S. Goo;H.J. Jo;S.Y. Hong;C.H. Lee;K.T. Kim
    • Journal of the Society of Naval Architects of Korea
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    • v.33 no.4
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    • pp.150-161
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    • 1996
  • A numerical procedure is described for predicting steady drift forces on very large offshore structures supported by a large number of the floating bodies of arbitrary shape dimensional source distributing method, the wave interaction theory, the far-field method of using momentum theory and the finite element method for structurally treating the space frame elements. Numerical results are compared with the experimental or numerical ones, which are obtained in the literature, of steady drift forces on a offshore structure supported by the 33(3 by 11) floating composite vertical cylinders in waves. The results of comparison confirmed the validity of the proposed approach.

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Steady Drift Forces on Very Large Offshore Structures Supported by Multiple Floating Bodies in Waves(I) (다수의 부체로 지지된 초대형 해양구조물에 작용하는 정상표류력(I))

  • H.J. Jo;J.S. Goo;S.Y. Hong;C.H. Lee
    • Journal of the Society of Naval Architects of Korea
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    • v.32 no.4
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    • pp.123-135
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    • 1995
  • A numerical procedure is described for predicting steady drift forces an multiple three-dimensional bodies of arbitrary shape freely floating in waves. The developed numerical approach is based on combination of a three-dimensional source distribution method, wave interaction theory art the far-field method using momentum theory. Numerical results are compared with the experimental or numerical ones, which are obtained in the literature, of steady drift forces on 33(3 by 11) floating composite vertical cylinders in waves. The results of comparison confirmed the validity of the proposed approach. Finally, the interaction effects are examined in the case of an array of 40(4 by 10) freely floating rectangular bodies in shallow water.

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A Study on the Slowly Varying Wave Drift Force Acting on a Semi-Submersible Platform in Waves (반잠수식 시추선에 작용하는 장주기 표류력에 관한 연구)

  • S.Y.,Hong;P.M.,Lee;D.C.,Hong
    • Bulletin of the Society of Naval Architects of Korea
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    • v.26 no.2
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    • pp.49-63
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    • 1989
  • Wave drift forces which are small in magnitudes compared to the first order wave exciting forces can cause very large motion of a vessel in waves. In this paper a theoretical and experimental analysis is made of the mean and slowly varying wave dirft forces on the semi-submersible platform. Theoretical calculations are performed by using near field method with three dimensional diffraction theory and model tests are carried out in regular and irregular waves with a 1/60 semi model. Test results are compared with theoretical calculations and the mooring spring effects in the test are discussed.

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Nonlinear Motion Analysis of FPSO and Shuttle Tanker in a Tandem Configuration (탠덤 배치된 FPSO와 셔틀탱커의 비선형 운동 해석)

  • Lim, Choon-Gyu;Lee, Ho-Young;Shin, Hyung-Young
    • Journal of the Society of Naval Architects of Korea
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    • v.43 no.5 s.149
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    • pp.560-567
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    • 2006
  • FPSO and shuttle tanker are connected to each other by a mooring hawser and a loading hose through which cargo oil is off-loaded. Even in mild sea-state. environmental loads can cause unstable large drift motions between two vessels in tandem off-loading operations, which may result in collision incidents. Accordingly. the analysis on the relative motion between two vessels due to the environmental loads should be investigated in initial design stage. In this study, the low speed maneuvering equation is employed to simulate nonlinear motions of FPSO and shuttle tanker. Low frequency wave drift forces including hydrodynamic interactions between two vessels are evaluated by near field approaches. Current loads are determined by mathematical model of MMG and wind loads are calculated by employing the wind spectrum according to the guidelines of API-RP2A. Mooring forces produced by turret mooring lines and a flexible hawser are modeled quasi-statically by catenary equations. The effect of environmental loads that affect nonlinear motion is investigated through variation in their magnitudes and the nonlinear motions between FPSO and shuttle tanker are simulated under wave, current and wind in time domain.

A Study on the Captive Model Test of KCS in Regular Waves (KCS 선형의 규칙파 중 구속모형시험에 대한 연구)

  • Choi, Hujae;Kim, Dong Jin;Kim, Yeon Gyu;Yeo, Dong Jin;Yun, Kunhang;Lee, Gyeongjung
    • Journal of the Society of Naval Architects of Korea
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    • v.59 no.5
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    • pp.296-305
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    • 2022
  • In order to investigate maneuvering characteristics of KCS in waves, captive model test in regular waves was conducted. Purpose of the test is measuring maneuvering hull forces when ship is maneuvering in waves. Model test was carried out using CPMC (Computerized Planar Motion Carriage) of ocean engineering basin in KRISO (Korea Research Institute of Ships and Ocean engineering). Total 6 degrees-of-freedom motion were fixed by two points supporting captive model test device, which is specially designed for this test. This system estimates 6 degrees-of-freedom forces and moments through 12 strain gauge signals. Mapping matrix from strain gauge signals to 6 degrees-of-freedom forces and moments was derived by a well-organized calibration test. Static drift test was conducted in calm sea and in regular waves with various wave conditions. Hydrodynamic coefficients related to drift angle were extracted for each wave conditions, and the effect of waves on course stability was analyzed.