Time-Domain Simulation of Motions of Large Structures in Nonlinear Waves

2002 ◽  
Author(s):  
Debabrata Sen
Keyword(s):  
Incident Wave ◽  
Time Domain ◽  
Nonlinear Waves ◽  
Large Amplitude ◽  
Time Integration ◽  
Offshore Structures ◽  
Boundary Integral ◽  
Boundary Integral Method ◽  
Computational Scheme ◽  
Irregular Waves

In this paper, we discuss development of a time-domain motion simulation method for studying the interaction of nonlinear waves with large offshore structures. The computational algorithm follows a simplified numerical wave-tank approach based upon a boundary-integral method and time-integration of boundary conditions. The simplifying approximations include linearization of the interaction hydrodynamic effects (radiation and diffraction) while the incident wave effects are considered in full. The main aim is to develop a method that will consider all important nonlinear effects associated with a large-amplitude incident wave, and yet practical enough to be applied routinely by the industry. In the time-integration of motion equations, numerical instabilities usually arise if difference rules are applied for determining pressures, due to coupling between forces and motions. To avoid this, an algorithm has been developed for the pressure evaluation. The resulting computational scheme is numerically stable for all conditions. The method can incorporate effects of other forces such as Morison forces, forces from mooring lines etc. which can be nonlinear. After providing a description of computational scheme, force and motion results for the interaction of large amplitude regular waves as well as irregular waves with two practical semisubmersible configurations are presented.

An Improved Body-Exact Method to Predict Large Amplitude Ship Roll Responses

2018 ◽  
Author(s):  
Rahul Subramanian ◽  
Naga Venkata Rakesh ◽  
Robert F. Beck
Keyword(s):  
Incident Wave ◽  
Large Amplitude ◽  
Nonlinear Models ◽  
Body Position ◽  
Computational Cost ◽  
Offshore Structures ◽  
The Body ◽  
Surface Boundary ◽  
Strip Theory ◽  
New Formulation

Accurate prediction of the roll response is of significant practical relevance not only for ships but also ship type offshore structures such as FPSOs, FLNGs and FSRUs. This paper presents a new body-exact scheme that is introduced into a nonlinear direct time-domain based strip theory formulation to study the roll response of a vessel subjected to moderately large amplitude incident waves. The free surface boundary conditions are transferred onto a representative incident wave surface at each station. The body boundary condition is satisfied on the instantaneous wetted surface of the body below this surface. This new scheme allows capturing nonlinear higher order fluid loads arising from the radiated and wave diffraction components. The Froude-Krylov and hydrostatic loads are computed on the intersection surface of the exact body position and incident wave field. The key advantage of the methodology is that it improves prediction of nonlinear hydrodynamic loads while keeping the additional computational cost small. Physical model tests have been carried out to validate the computational model. Fairly good agreement is seen. Comparisons of the force components with fully linear and body-nonlinear models help in bringing out the improvements due to the new formulation.

scholarly journals WAVE INTERCEPTION BY SEA-BALLOON BREAKWATER

1986 ◽  
Vol 1 (20) ◽  
pp. 173 ◽  
Author(s):  
Takahiko Uwatoko ◽  
Takeshi Ijima ◽  
Yukimitsu Ushifusa ◽  
Haruyuki Kojima
Keyword(s):  
Incident Wave ◽  
Fluid Motion ◽  
Fluid Pressure ◽  
Vertical Axis ◽  
Boundary Integral ◽  
Long Waves ◽  
Boundary Integral Method ◽  
Wave Crest ◽  
Airflow Resistance ◽  
Incident Waves

When a submerged, flexible bag is filled with air about 60~T0 % of its full volume ( it is called " sea-balloon " ), it has a stable shape with vertical axis of symmetry, on which several vertical wrinkles appear with folds of membrane. If two or more such sea-balloons are arranged to the direction of wave travel and connected pneumatically, balloons are deformed periodically and the air flows reciprocally in connecting pipe, following to the fluid pressure fluctuation due to incident waves. Such a system of sea-balloon intercepts incident waves effectively ( it is called " sea-balloon breakwater "). The wave interception by the breakwater is analyzed numerically by three-dimensional boundary integral method, assuming that the fluid motions both in- and out-side of the balloon are potential and that the tension in balloon membrane is proportional to the apparent elongation of membrane with virtual elastic constant. After analysis and experiments, it is made clear that in relatively long waves the incident wave is canceled by the radiation wave which is generated by volumetric change of sea-balloons, being affected by airflow resistance in connecting pipe. In short waves, sea-balloons seem to behave like as rigid piles and the incident wave is absorbed by airflow resistance in pipe and by the turbulence of fluid motion around balloons. Moreover, the effect of gaps between sea-balloons along wave crest on wave interception for relatively long waves is expressed by a simple empirical formula, by which the transmission coefficients at various types of sea-balloon breakwater is easily estimated by twodimensional computation. For the improvement of wave interception effect and from the point of practical use, the effects of other sea-balloon breakwater system are investigated by two-dimensional computation and experiments.

Computing Large Amplitude Motions of a Floating Offshore Structure in the Time Domain

2008 ◽  
Author(s):  
Wei Qiu ◽  
Hongxuan Peng
Keyword(s):  
Time Domain ◽  
Large Amplitude ◽  
Offshore Structures ◽  
The Body ◽  
Surface Boundary ◽  
Offshore Structure ◽  
Floating Structure ◽  
Time Step ◽  
Large Amplitude Motions ◽  
The Time Domain

Based on the panel-free method, large-amplitude motions of floating offshore structures have been computed by solving the body-exact problem in the time domain using the exact geometry. The body boundary condition is imposed on the instantaneous wetted surface exactly at each time step. The free surface boundary is assumed linear so that the time-domain Green function can be applied. The instantaneous wetted surface is obtained by trimming the entire NURBS surfaces of a floating structure. At each time step, Gaussian points are automatically distributed on the instantaneous wetted surface. The velocity potentials and velocities are computed accurately on the body surface by solving the desingularized integral equations. Nonlinear Froude-Krylov forces are computed on the instantaneous wetted surface under the incident wave profile. Validation studies have been carried out for a Floating Production Storage and Offloading (FPSO) vessel. Computed results were compared with experimental results and solutions by the panel method.

The Interaction of Nonlinear Waves With the Submerged Caissons of a Gravity Based Structure

2008 ◽  
Author(s):  
Marios Christou ◽  
Jannicke S. Roos ◽  
Chris Swan ◽  
Ove T. Gudmestad
Keyword(s):  
Wave Field ◽  
Incident Wave ◽  
Nonlinear Waves ◽  
Wave Structure ◽  
Boundary Integral ◽  
Numerical Predictions ◽  
Water Region ◽  
Geometric Discontinuities ◽  
The Waves ◽  
Wave Structure Interaction

This paper concerns the numerical description of nonlinear waves propagating over the storage caissons of a gravity based structure. This process produces a steepening of the incident wave-field, which occurs when the waves propagate into the shallower water region above the storage caissons, resulting in the focussing of wave energy. A fully nonlinear Multiple-flux Boundary Element Method (MF-BEM) is applied to simulate this effect. The MF-BEM differs from traditional boundary integral approaches in two important respects: first, a multiple-flux approach is employed to overcome the problem of geometric discontinuities; and, second, no filtering, smoothing, re-gridding or redistribution of the nodes is performed at any stage during the simulations. These two aspects are believed to play an important role in accurately predicting the steepening of the incident wave-field. The numerical predictions are compared to new laboratory observations that examine the extent of this wave-structure interaction and, particularly, the steepening of the incident wave-field.

Environmental Load Effect Analysis of Guyed Towers

1985 ◽  
Vol 107 (1) ◽  
pp. 24-33 ◽  
Author(s):  
O. Mo ◽  
T. Moan
Keyword(s):  
Time Domain ◽  
Structural Model ◽  
Dimensional Space ◽  
Time Integration ◽  
Offshore Structures ◽  
Random Waves ◽  
Significant Discrepancy ◽  
Effect Analysis ◽  
Load Effect ◽  
Fluid Field

A general method for dynamic load effect analysis of slender offshore structures subjected to short crested random waves, current and wind, is given. The structure is represented by a three-dimensional space frame model utilizing dash-pots and linear or nonlinear spring elements to represent guy lines and coupling between structure and foundation. The component mode synthesis formulation is adopted for reduction of the number of degrees of freedom. The hydrodynamic forces are computed by Morison’s equation, accounting for finite wave elevation, directionality, and relative fluid-structure motion. Various kinematic models for the fluid field in the splashing zone are compared. To get a reasonable representation of nonlinearities in the loading and the structural model, a Monte Carlo approach is adopted. Starting with simulated samples of the random fluid field and wind forces, time series of structural responses are found by numerical time integration utilizing the Newmark β-family of time integration operators. Numerical results for a guyed tower at 450-m water depth are presented. The statistical uncertainties associated with the stochastic time domain simulations are discussed. A significant discrepancy is found between linearized frequency domain solutions and the present nonlinear time domain formulation. The importance of an adequate representation of superharmonic responses is particularly discussed. The differences in results due to various solution methods are found to vary significantly with sea-state conditions.

A Multipole Accelerated Desingularized Method for Computing Nonlinear Wave Forces on Bodies

1998 ◽  
Vol 120 (2) ◽  
pp. 71-76 ◽  
Author(s):  
S. M. Scorpio ◽  
R. F. Beck
Keyword(s):  
Boundary Conditions ◽  
Nonlinear Wave ◽  
Mixed Boundary ◽  
Offshore Structures ◽  
Computational Effort ◽  
Boundary Integral ◽  
Boundary Integral Method ◽  
Wave Forces ◽  
Surface Boundary ◽  
Time Step

Nonlinear wave forces on offshore structures are investigated. The fluid motion is computed using a Euler-Lagrange time-domain approach. Nonlinear free surface boundary conditions are stepped forward in time using an accurate and stable integration technique. The field equation with mixed boundary conditions that result at each time step are solved at N nodes using a desingularized boundary integral method with multipole acceleration. Multipole accelerated solutions require O(N) computational effort and computer storage, while conventional solvers require O(N2) effort and storage for an iterative solution and O(N3) effort for direct inversion of the influence matrix. These methods are applied to the three-dimensional problem of wave diffraction by a vertical cylinder.

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