The Effect of Aircushion Division on the Structural Loads of Large Floating Offshore Structures

2007 ◽  
Author(s):  
J. L. F. van Kessel ◽  
J. A. Pinkster
Keyword(s):  
Water Surface ◽  
Three Dimensional ◽  
Potential Method ◽  
Offshore Structures ◽  
Bending Moments ◽  
Shear Forces ◽  
Floating Structures ◽  
The Mean ◽  
Wave Induced

The effect of aircushion division on the structural loads of large floating offshore structures is described and compared with that of a rectangular barge having the same dimensions. Calculations are based on a linear three-dimensional potential method using a linear adiabatic law for the air pressures inside the cushions. The water surface within the aircushions and the mean wetted surface are modelled by panel distributions representing oscillating sources. In the presented cases the structural loads include the wave induced bending moments and shear forces along the length of the structure. Aircushions significantly influence the behaviour of large floating structures in waves and consequently reduce the bending moments. The internal loads of different configurations of aircushion supported structures are described and compared with those of a rectangular barge having the same dimensions. The significant reduction of the bending moments shows that aircushion support can be of interest for large floating structures.

Numerical and Experimental Study on Aircushion Supported Structures

2008 ◽  
Author(s):  
J. L. F. van Kessel
Keyword(s):  
Water Surface ◽  
Three Dimensional ◽  
Numerical Approach ◽  
Potential Method ◽  
Model Tests ◽  
Numerical Calculations ◽  
Floating Structures ◽  
University Of Technology ◽  
The Mean ◽  
Very Large Floating Structures

The use of aircushions for very large floating structures has been investigated in recent years at Delft University of Technology. Model tests were performed to validate the results of numerical calculations based on a linear three-dimensional potential method. A linear adiabatic law was used in the numerical approach to describe the air pressures inside the cushions. It is assumed that air cannot escape from the cavity underneath the structure. The water surface within the aircushions and the mean wetted surface are modelled by panel distributions representing oscillating sources. Experimental results and numerical calculations of two configurations of aircushion supported structures at zero speed are presented in this paper. The results show that model tests of different aircushion supported structures can be well predicted by means of 3D diffraction calculations.

The Effect of Aircushion Division on the Motions of Large Floating Structures

2007 ◽  
Author(s):  
J. L. F. van Kessel ◽  
J. A. Pinkster
Keyword(s):  
Water Surface ◽  
Three Dimensional ◽  
Potential Method ◽  
Wave Fields ◽  
Floating Structures ◽  
Different Types ◽  
The Mean ◽  
Motion Characteristics ◽  
The Stability ◽  
Drift Forces

The effect of aircushion division on the motions of large floating structures is studied by means of calculations based on a linear three-dimensional potential method. A linear adiabatic law is used to describe the air pressures inside the cushions. The water surface within the aircushions and the mean wetted surface are modelled by panel distributions representing oscillating sources. The behaviour of different types of aircushion supported structures is described and compared with that of a rectangular barge having the same dimensions. Successively, the aircushion theory, motion characteristics, wave frequency forces and moments, mean second order drift forces and surrounding wave fields are discussed. The results show that aircushions significantly influence the stability and behaviour of large floating structures.

Bending Moments and Shear Forces in Ships Sailing in Irregular Waves

1981 ◽  
Vol 25 (04) ◽  
pp. 243-251
Author(s):  
J. Juncher Jensen ◽  
P. Terndrup Pedersen
Keyword(s):  
Linear Part ◽  
Vertical Motion ◽  
Bending Moment ◽  
Irregular Waves ◽  
Bending Moments ◽  
Shear Forces ◽  
Strip Theory ◽  
Wave Heights ◽  
Exciting Forces ◽  
Wave Induced

This paper presents some results concerning the vertical response of two different ships sailing in regular and irregular waves. One ship is a containership with a relatively small block coefficient and with some bow flare while the other ship is a tanker with a large block coefficient. The wave-induced loads are calculated using a second-order strip theory, derived by a perturbational procedure in which the linear part is identical to the usual strip theory. The additional quadratic terms are determined by taking into account the nonlinearities of the exiting waves, the nonvertical sides of the ship, and, finally, the variations of the hydrodynamic forces during the vertical motion of the ship. The flexibility of the hull is also taken into account. The numerical results show that for the containership a substantial increase in bending moments and shear forces is caused by the quadratic terms. The results also show that for both ships the effect of the hull flexibility (springing) is a fair increase of the variance of the wave-induced midship bending moment. For the tanker the springing is due mainly to exciting forces which are linear with respect to wave heights whereas for the containership the nonlinear exciting forces are of importance.

A Method to Upgrade Iceberg Velocity Statistics to Include Wave-Induced Motion

1987 ◽  
Vol 109 (3) ◽  
pp. 278-286 ◽  
Author(s):  
J. H. Lever ◽  
D. Sen
Keyword(s):  
Three Dimensional ◽  
Interaction Model ◽  
Environmental Parameters ◽  
Offshore Structures ◽  
Grand Banks ◽  
Drilling Rig ◽  
Velocity Statistics ◽  
Impact Risk ◽  
Induced Motion ◽  
Wave Induced

Iceberg impact design loads for offshore structures can be estimated by incorporating an ice/structure interaction model in a probabilistic framework, or risk analysis. The relevant iceberg and environmental parameters are input in statistical form. Iceberg velocity statistics are usually compiled from drilling rig radar reports, and hence represent estimates of average hourly drift speeds. Yet it is the instantaneous ice velocity which is the relevant input to the simulation of the iceberg/structure collision process. Thus, risk analyses based on mean drift speed distributions will only yield valid results for the subset of conditions where wave-induced iceberg motion is negligible. This paper describes a method which, for the first time, systematically accounts for wave-induced motion in iceberg impact risk analyses. A linear three-dimensional potential flow model is utilized to upgrade iceberg velocity statistics to include the influence of Grand Banks sea-state conditions on instantaneous ice motion. The results clearly demonstrate the importance of including wave-induced motion in iceberg impact risk analyses.

Drogue-cluster and dye-dispersion measurements

1987 ◽  
Vol 14 (3) ◽  
pp. 320-326 ◽  
Author(s):  
Merv D. Palmer ◽  
Rob Jarvis ◽  
Larry Thompson
Keyword(s):  
Water Surface ◽  
Center Of Mass ◽  
Three Dimensional ◽  
Vertical Direction ◽  
Dispersion Data ◽  
Batch Release ◽  
Surface Dispersion ◽  
The Mean ◽  
The Difference ◽  
Lagrangian Data

Near the water surface, dispersion and transport were extensively measured in the coastal regions of Lake Ontario using dye patches and clusters of water sail and surface drogues. The measurements were carried out for 6–8 h. Each method produced different measurements of dispersion magnitudes with the largest dilution occurring for the dye, followed by sail drogue clusters (40% of the dye's value) and then surface drogue clusters (25% of the dye's value). Both the sail and surface drogues measured the two-dimensional dispersion. The mean surface dispersion was about 50% less than the dispersion 1.5 m below the water surface. The dilution characteristics decreased as the water surface was approached. The sail dispersion was about half of the dye-dispersion data. It was not known how much of the difference was attributable to the dye being three-dimensional and affected by dispersion in the vertical direction; consequently, as time progressed, the dye patch was measuring dispersion at greater depth than the water sail drogues, which were set for a depth 1.5 m below the water surface. The statistical increase of the variance with time was computed for each method of measuring dispersion, and the results were compared. A method for predicting dilution envelopes for a location using the path lines of the drogue-cluster centroids or center of mass of the dye patch was developed for both a batch release and a continuous discharge. These dilution envelopes are based entirely on Lagrangian data for both the velocity and dispersion estimates. Key words: lake, dispersion, drogue clusters, dye, surface streaking.

scholarly journals A Divergence-Form Wave-Induced Pressure Inherent in the Extension of the Eliassen–Palm Theory to a Three-Dimensional Framework for All Waves at All Latitudes

2015 ◽  
Vol 72 (7) ◽  
pp. 2822-2849 ◽  
Author(s):  
Hidenori Aiki ◽  
Koutarou Takaya ◽  
Richard J. Greatbatch
Keyword(s):  
Three Dimensional ◽  
Conservation Equation ◽  
Divergence Form ◽  
Water Model ◽  
Gradient Term ◽  
Mean Flow ◽  
Classical Energy ◽  
Linear Waves ◽  
The Mean ◽  
Wave Induced

Classical theory concerning the Eliassen–Palm relation is extended in this study to allow for a unified treatment of midlatitude inertia–gravity waves (MIGWs), midlatitude Rossby waves (MRWs), and equatorial waves (EQWs). A conservation equation for what the authors call the impulse-bolus (IB) pseudomomentum is useful, because it is applicable to ageostrophic waves, and the associated three-dimensional flux is parallel to the direction of the group velocity of MRWs. The equation has previously been derived in an isentropic coordinate system or a shallow-water model. The authors make an explicit comparison of prognostic equations for the IB pseudomomentum vector and the classical energy-based (CE) pseudomomentum vector, assuming inviscid linear waves in a sufficiently weak mean flow, to provide a basis for the former quantity to be used in an Eulerian time-mean (EM) framework. The authors investigate what makes the three-dimensional fluxes in the IB and CE pseudomomentum equations look in different directions. It is found that the two fluxes are linked by a gauge transformation, previously unmentioned, associated with a divergence-form wave-induced pressure [Formula: see text]. The quantity [Formula: see text] vanishes for MIGWs and becomes nonzero for MRWs and EQWs, and it may be estimated using the virial theorem. Concerning the effect of waves on the mean flow, [Formula: see text] represents an additional effect in the pressure gradient term of both (the three-dimensional versions of) the transformed EM momentum equations and the merged form of the EM momentum equations, the latter of which is associated with the nonacceleration theorem.

scholarly journals The geometry of structural equilibrium

2017 ◽  
Vol 4 (3) ◽  
pp. 160759 ◽  
Author(s):  
Allan McRobie
Keyword(s):  
Three Dimensional ◽  
Natural Generalization ◽  
Bending Moments ◽  
Shear Forces ◽  
Generalized Polygons ◽  
Analysis And Design ◽  
Oriented Area ◽  
Graphic Analysis ◽  
Reciprocal Diagrams ◽  
Structural Equilibrium

Building on a long tradition from Maxwell, Rankine, Klein and others, this paper puts forward a geometrical description of structural equilibrium which contains a procedure for the graphic analysis of stress resultants within general three-dimensional frames. The method is a natural generalization of Rankine’s reciprocal diagrams for three-dimensional trusses. The vertices and edges of dual abstract 4-polytopes are embedded within dual four-dimensional vector spaces, wherein the oriented area of generalized polygons give all six components (axial and shear forces with torsion and bending moments) of the stress resultants. The relevant quantities may be readily calculated using four-dimensional Clifford algebra. As well as giving access to frame analysis and design, the description resolves a number of long-standing problems with the incompleteness of Rankine’s description of three-dimensional trusses. Examples are given of how the procedure may be applied to structures of engineering interest, including an outline of a two-stage procedure for addressing the equilibrium of loaded gridshell rooves.

scholarly journals Relationship between Three-Dimensional Radiation Stress and Vortex-Force Representations

2021 ◽  
Vol 9 (8) ◽  
pp. 791
Author(s):  
Duoc Tan Nguyen ◽  
Ad J. H. M. Reniers ◽  
Dano Roelvink
Keyword(s):  
Three Dimensional ◽  
Ocean Model ◽  
Radiation Stress ◽  
Mean Motion ◽  
Numerical Ocean Model ◽  
The Mean ◽  
Ocean Models ◽  
Wave Induced ◽  
Mean Current

In numerical ocean models, the effect of waves on currents is usually expressed by either vortex-force or radiation stress representations. In this paper, the differences and similarities between those two representations are investigated in detail in conditions of both conservative and nonconservative waves. In addition, comparisons between different sets of equations of mean motion that apply different representations of wave-induced forcing terms are included. The comparisons are useful for selecting a suitable numerical ocean model to simulate the mean current in conditions of waves combined with currents.

A general linear hydroelasticity theory of floating structures moving in a seaway

1986 ◽  
Vol 316 (1538) ◽  
pp. 375-426 ◽  
Keyword(s):  
Fluid Flow ◽  
Finite Elements ◽  
Linear Theory ◽  
Three Dimensional ◽  
Elastic Beam ◽  
Offshore Structures ◽  
Elastic Structure ◽  
Complicated Shape ◽  
Floating Structures ◽  
General Method

The dynamics of an elastic beam floating on the surface of disturbed water has formed the basis of a fairly comprehensive linear theory of hydroelastic behaviour of ships in waves. The existing theory cannot easily be extended to floating vehicles of more complicated shape (such as semi-submersibles), or to fixed offshore structures. A general method is presented, by which finite elements permit any three-dimensional elastic structure to be admitted in a linear hydroelastic theory. Sinusoidal waves provide the excitation of the structure and the fluid flow is three-dimensional. Some examples are given which illustrate the use of the theory and expose behaviour that has not been encountered hitherto.

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