Time-Domain Analysis of Floating Bodies with Forward Speed

2002 ◽  
Vol 124 (2) ◽  
pp. 66-73 ◽  
Author(s):  
Gu¨nther F. Clauss ◽  
Katja Stutz
Keyword(s):  
Nonlinear Wave ◽  
Time Domain ◽  
Degrees Of Freedom ◽  
Wave Structure ◽  
Nonlinear Effects ◽  
Time Domain Analysis ◽  
Offshore Structures ◽  
Driving Mechanism ◽  
Forward Speed ◽  
Transient Wave

Broaching, surf-riding, and capsizing of ships and offshore structures are transient wave-structure interactions which imply high risks for crew, vessel and cargo. As nonlinear effects are of great importance, time-domain investigations are indispensable. For unveiling the associated driving mechanism of these critical motions, it is desirable to analyze the cause-reaction chains in detail: Depending on the transient wave elevation, we obtain an instationary pressure distribution on the wetted surface of the cruising vessel. Resulting forces and moments excite vessel motions in six degrees of freedom. Based on the linear panel-method program for transient wave-body interactions, TiMIT [Korsmeyer et al. (1999)], this paper investigates seakeeping characteristics of offshore structures with forward speed. Results are presented in frequency and time domain. The procedure allows to identify critical seaways, and to analyze cause-reaction chains in deterministic wave sequences where critical and steep wave packets are embedded in random seas. The detailed evaluation reveals that large roll and pitch motions are easily reduced by variation of course and speed. For investigating the mechanism of wave/structure interactions, this paper introduces the relevant time-domain methodology, and indicates how nonlinear wave characteristics can be introduced in the time-stepping analysis. In subsequent steps nonlinear wave/structure interactions will also be considered.

Time Domain Analysis of Springing and Whipping Responses Acting on a Large Container Ship

2011 ◽  
Author(s):  
Yongwon Lee ◽  
Zhenhong Wang ◽  
Nigel White ◽  
Spyros E. Hirdaris
Keyword(s):  
Time Domain ◽  
Nonlinear Effects ◽  
Time Domain Analysis ◽  
Container Ship ◽  
Wave Loads ◽  
Ocean Engineering ◽  
Hull Girder ◽  
Engineering Research ◽  
Large Container ◽  
Element Method

As part of WILS II (Wave Induced Loads on Ships) Joint Industry Project organised by MOERI (Maritime and Ocean Engineering Research Institute, Korea), Lloyd’s Register has undertaken time domain springing and whipping analyses for a 10,000 TEU class container ship using computational tools developed in the Co-operative Research Ships (CRS) JIP [1]. For idealising the ship and handling the flexible modes of the structure, a boundary element method and a finite element method are employed for coupling fluid and structure domain problems respectively. The hydrodynamic module takes into account nonlinear effects of Froude-Krylov and restoring forces. This Fluid Structure Interaction (FSI) model is also coupled with slamming loads to predict wave loads due to whipping effects. Vibration modes and natural frequencies of the ship hull girder are calculated by idealising the ship structure as a Timoshenko beam. The results from springing and whipping analyses are compared with the results from linear and nonlinear time domain calculations for the rigid body. The results from the computational analyses in regular waves have been correlated with those from model tests undertaken by MOERI. Further the global effects of springing and whipping acting on large container ships are summarised and discussed.

Computation of Wave-Induced Drift Forces Introduced by Displaced Position of Compliant Offshore Platforms

1992 ◽  
Vol 114 (3) ◽  
pp. 175-184 ◽  
Author(s):  
Y. Li ◽  
A. Kareem
Keyword(s):  
Time Domain ◽  
Phase Relationship ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Offshore Structures ◽  
Time Dependent ◽  
Wave Loads ◽  
Nonlinear Feedback ◽  
The Time Domain ◽  
Drift Forces

The wave forces computed at the displaced position of offshore structures may introduce additional drift forces. This contribution is particularly significant for compliant offshore structures that are configured by design to experience large excursions under the environmental load effects, e.g., tension leg platform. In a random sea environment, this feature can be included in the time domain analysis by synthesizing drag and diffraction forces through a summation of a large number of harmonics with an appropriate phase relationship that reflects the platform displaced position. This approach is not only limited to the time domain analysis, but the superposition of a large number of trigonometric terms in such an analysis requires a considerable computational effort. This paper presents a computationally efficient procedure in both the time and frequency domains that permits inclusion of the time-dependent drift forces, introduced by the platform displacement, in terms of linear and nonlinear feedback contributions. These time-dependent feedback forces are expressed in terms of the applied wave loads by linear and quadratic transformations. It is demonstrated that the results obtained by this approach exhibit good agreement with the procedure based on the summation of trigonometric functions.

Transient Motions of Floating Bodies at Zero Forward Speed

1987 ◽  
Vol 31 (03) ◽  
pp. 164-176 ◽  
Author(s):  
Robert F. Beck ◽  
Stergios Liapis
Keyword(s):  
Integral Equations ◽  
Impulse Response ◽  
Response Function ◽  
Time Domain ◽  
Linear Time ◽  
Impulse Response Function ◽  
Time Domain Analysis ◽  
The Body ◽  
Floating Body ◽  
Forward Speed

Linear, time-domain analysis is used to solve the radiation problem for the forced motion of a floating body at zero forward speed. The velocity potential due to an impulsive velocity (a step change in displacement) is obtained by the solution of a pair of integral equations. The integral equations are solved numerically for bodies of arbitrary shape using discrete segments on the body surface. One of the equations must be solved by time stepping, but the kernel matrix is identical at each step and need only be inverted once. The Fourier transform of the impulse-response function gives the more conventional added-mass and damping in the frequency domain. The results for arbitrary motions may be found as a convolution of the impulse response function and the time derivatives of the motion. Comparisons are shown between the time-domain computations and published results for a sphere in heave, a sphere in sway, and a right circular cylinder in heave. Theoretical predictions and experimental results for the heave motion of a sphere released from an initial displacement are also given. In all cases the comparisons are excellent.

Stochastic Response Analysis of Deepwater Structures in Short-Crested Random Seas

1999 ◽  
Vol 121 (3) ◽  
pp. 181-186 ◽  
Author(s):  
P. Teigen ◽  
A. Naess
Keyword(s):  
Time Domain ◽  
Large Volume ◽  
Time Domain Analysis ◽  
Order Theory ◽  
Offshore Structures ◽  
Response Analysis ◽  
Random Waves ◽  
Stochastic Response ◽  
Cpu Time ◽  
Random Seas

The paper discusses the problem of estimating the response statistics of moored large-volume offshore structures subjected to short-crested random waves. A general second-order theory is described that makes it possible to carry out the entire analysis in the frequency domain, which is computationally more efficient than time domain analysis, which generally requires considerably more CPU time to reach the same level of accuracy.

Effect of Wind Loads on the Performance of Free-Fall Lifeboats

2014 ◽  
Author(s):  
Thomas Sauder ◽  
Eloise Croonenborghs ◽  
Sebastien Fouques ◽  
Nabila Berchiche ◽  
Svein-Arne Reinholdtsen
Keyword(s):  
Numerical Simulations ◽  
Degrees Of Freedom ◽  
Free Fall ◽  
Offshore Structures ◽  
Water Entry ◽  
Forward Speed ◽  
Cfd Simulations ◽  
Six Degrees Of Freedom ◽  
Complete Set ◽  
Water Exit

The paper presents a model describing the launch of free-fall lifeboats from offshore structures in strong environmental wind. Six-degrees-of-freedom numerical simulations of the lifeboat launch are performed using the free-fall lifeboat simulator VARUNA with a complete set of wind coefficients for the lifeboat. Those wind coefficients are obtained by CFD simulations validated against wind tunnel tests. The lifeboat launch simulations are then verified against time-domain CFD simulations of the whole launch in air until water entry. It is shown by means of numerical simulations that wind-induced loads on the lifeboat have a strong influence on its kinematics until water entry, and subsequently on the acceleration loads experienced by the occupants, on the structural loads on the lifeboat, and on its forward speed after water exit. It is concluded that the effect of wind-induced loads on the lifeboat performances should in general be investigated when establishing the operational limits for a given offshore installation.

A Theoretical Study on the Second-Order Wave Forces for Two-Dimensional Bodies

1989 ◽  
Vol 111 (1) ◽  
pp. 37-42 ◽  
Author(s):  
G. P. Miao ◽  
Y. Z. Liu
Keyword(s):  
Nonlinear Wave ◽  
Incident Wave ◽  
Theoretical Study ◽  
Nonlinear Effects ◽  
Offshore Structures ◽  
Second Order ◽  
Wave Forces ◽  
Two Dimensional ◽  
Regular Waves ◽  
Steady Wave

Nonlinear wave forces on fixed or floating offshore structures have attracted much attention recently. This paper deals with the nonlinear effects of regular waves on fixed two-dimensional bodies up to second-order terms. The second-order diffraction potential is solved consistently and the second-order steady wave forces and the biharmonic wave forces with frequency corresponding to the double of the incident wave frequency are obtained.

scholarly journals A theory of nonlinear wave loading on offshore structures

1981 ◽  
Vol 4 (3) ◽  
pp. 589-613 ◽  
Author(s):  
Lokenath Debnath ◽  
Matiur Rahman
Keyword(s):  
Nonlinear Wave ◽  
Vertical Cylinder ◽  
Nonlinear Effects ◽  
Diffraction Theory ◽  
Offshore Structures ◽  
The Body ◽  
Wave Number ◽  
Wave Forces ◽  
Wave Loading ◽  
Nonlinear Contribution

A theoretical study is made of the nonlinear wave loading on offshore structures using the diffraction theory of hydrodynamics. A nonlinear modification of the classical Morison equation,D≡Fℓ+FDfor estimating wave forces on offshore structures is suggested in this paper. The modified equation is found in the formD≡Fℓ+Fnℓ+FDwhereFnℓ≡Fd+Fw+Fqis the nonlinear contribution made up of the dynamic, waterline, and the quadratic forces associated with the irrotational-flow part of the wave loading on structures. The study has then been applied to calculate the linear and the nonlinear wave loadings on a large vertical cylinder partially immersed in an ocean of arbitrary uniform depth. All the linear and nonlinear forces exerting on the cylinder are determined explicitly. A comparison is made between these two kinds of forces. Special attention is given to the nonlinear wave loadings on the cylinder. It is shown that all nonlinear effects come from the interaction between the body's responses to the oncoming wave's fluctuating velocity and its fluctuating extension. It is found that the nonlinear effects are dominated by the sum of the dynamic and waterline forces. The nonlinear correction to Morison's equation increases with increasingkbwherebis the characteristic dimension of the body andkis the wave number. This prediction is shown to be contrary to that of the linear diffraction theory which predicted that the Morison coefficient decreases with increasingkb. Several interesting results and limiting cases are discussed in some detail.

scholarly journals Development of a Blended Time-Domain Program for Predicting the Motions of a Wave Energy Structure

2019 ◽  
Vol 8 (1) ◽  
pp. 1 ◽  
Author(s):  
Hao Wang ◽  
Abhilash Somayajula ◽  
Jeffrey Falzarano ◽  
Zhitian Xie
Keyword(s):  
Wave Energy ◽  
Time Domain ◽  
Linear Time ◽  
Large Body ◽  
Nonlinear Effects ◽  
Time Domain Analysis ◽  
Energy Structure ◽  
Floating Structures ◽  
External Forces ◽  
Inertia Forces

Traditional linear time-domain analysis is used widely for predicting the motions of floating structures. When it comes to a wave energy structure, which usually is subjected to larger relative (to their geometric dimensions) wave and motion amplitudes, the nonlinear effects become significant. This paper presents the development of an in-house blended time-domain program (SIMDYN). SIMDYN’s “blend” option improves the linear option by accounting for the nonlinearity of important external forces (e.g., Froude-Krylov). In addition, nonlinearity due to large body rotations (i.e., inertia forces) is addressed in motion predictions of wave energy structures. Forced motion analysis reveals the significance of these nonlinear effects. Finally, the model test correlations examine the simulation results from SIMDYN under the blended option, which has seldom been done for a wave energy structure. It turns out that the blended time-domain method has significant potential to improve the accuracy of motion predictions for a wave energy structure.

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