An Experimental Study of Wave Loads on Deck in Irregular Waves

2012 ◽  
Author(s):  
Ravikiran S. Kota ◽  
Torgeir Moan
Keyword(s):  
Structural Integrity ◽  
Irregular Waves ◽  
Wave Loads ◽  
Short Term ◽  
Wave Impact ◽  
Integrity Verification ◽  
Non Gaussian ◽  
Floating Platforms ◽  
The Waves ◽  
Impact Events

Global loads on deck-structures due to air-gap loss in irregular seas are discussed. Estimation of such loads is important in structural integrity verification of fixed and floating platforms. The current study presents results from wave-tank experiments in which global forces and wetted lengths arising from wave impact on a rigid deck-box from several pseudo-random realizations of long-crested waves of moderate steepness are recorded. Short-term statistics and distributions of these quantities are estimated and compared against estimates from semi-analytical results previously reported by the authors for von Karman-type impact events in Gaussian waves. The scope of work presented here does not address wave loads on decks in extreme seas where the waves are steep and predominantly non-Gaussian.

Long-Term Area Statistics for Maximum Crest Height Under a Fixed Platform Deck

2018 ◽  
Author(s):  
Øistein Hagen ◽  
Jørn Birknes-Berg ◽  
Ida Håøy Grue ◽  
Gunnar Lian ◽  
Kjersti Bruserud ◽  
...  
Keyword(s):  
Time Series ◽  
North Sea ◽  
Extreme Values ◽  
Second Order ◽  
Irregular Waves ◽  
Scatter Diagram ◽  
Short Term ◽  
Wave Impact ◽  
Crest Height

As offshore reservoirs are depleted, the seabed may subside. Furthermore, the extreme crests estimates are now commonly higher than obtained previously due to improved understanding of statistics of non-linear irregular waves. Consequently, bottom fixed installations which have previously had sufficient clearance between the deck and the sea surface may be in a situation where wave impact with the deck must be considered at relevant probability levels. In the present paper, we investigate the long-term area statistics for maximum crest height under a fixed platform deck for 2nd order short crested and long crested sea based on numerical simulations as a function of platform deck dimension for jackets. The results are for one location in the northern North Sea, but some key results are also reported and verified for a more benign southern North Sea location. Time domain simulations for long crested and short crested waves over a spatial domain with dimension of a platform deck are performed, and relevant statistics for airgap assessment determined. Second order waves are simulated for the different cells in the (Hs, Tp) scatter diagram for Torsethaugen two-peak wave spectrum for long-crested and short-crested sea. A total of 1000 3-hour sea states are generated per cell, and time series generated for 160 spatial points under a platform deck. Short-term and long-term statistics are established for the maximum crest height as function of platform dimension; inline and transverse to the wave direction, and over the area. Results are given for the linear sea and for the second order time series. The annual q-probability estimates for the maximum crest height over area as a function of platform dimension is determined for a location at the Norwegian Continental Shelf by weighting the short-term statistics for the individual cells in the scatter diagram with the long-term probability of occurrence of the sea state. To reduce the number of numerical second order simulations, the effect of excluding cells that have a negligible effect on the long term extreme crest estimate is discussed. The percentiles in the distribution of maximum crest (over area) in design sea states that corresponds to the extreme values obtained from the long-term analysis are determined for long crested and short crested sea. The increase in the extreme crest over an area compared to the point in space estimate is estimated for both linear and second order surface elevation.

Extreme Value Analyses of Dynamic Response Parameters of a Wind Tower Structure Under Short-Term Nonlinear Irregular Seastate

2017 ◽  
Author(s):  
Leonardo Nascimento ◽  
Luis Sagrilo ◽  
Gilberto Ellwanger
Keyword(s):  
Nonlinear Behavior ◽  
Nonlinear Effects ◽  
Extreme Value ◽  
Irregular Waves ◽  
Wind Loading ◽  
Extreme Value Analysis ◽  
Short Term ◽  
Gaussian Stochastic Process ◽  
Value Analysis ◽  
Non Gaussian

In the assessment of marine structures in shallow waters domain it is important to take into account the nonlinear (or non-Gaussian) nature of the irregular waves when predicting short and long-term responses of such structures. Other sources of nonlinearities in the response are also present due to some nonlinear effects such as: wet-dry surface effects, wind force on dry parts of the structure, drag term in Morison hydrodynamic force equation, etc. The estimation of the characteristic short-term extreme responses requires the extreme value analysis of a non-Gaussian stochastic process. There are many approaches available in literature which can be employed, such as: Hermite-based model, Weibull-fitting model, etc. In this paper two distinct Weibull fitting models (one based on the first two and other based on the first three moments of the response peaks sample) and Hermite-based models using both conventional and linear moments (L-moments) are investigated for the prediction of extreme short-term response of mono-column wind tower installed in a water depth of 20m and subject to wave, current and wind loading. The tower responses (load effects) time-histories are obtained by means of a time-domain finite element-based program using 3-D geometric nonlinear beam elements developed for the dynamic analysis of this type of structure. In this program, the nonlinear behavior of the irregular waves is modelled by means of the second order Sharma and Dean theory [1] and the wind forces are represented by a very simplified load model based on wind velocity simulated time-series and the obstruction area of the tower and blades.

scholarly journals Short-term Variations in Strain and Surface Tilt on Storglaciären, Kebnekaise, Northern Sweden

1989 ◽  
Vol 35 (120) ◽  
pp. 201-208 ◽  
Author(s):  
Peter Jansson ◽  
Roger LeB. Hooke
Keyword(s):  
Water Pressure ◽  
High Water ◽  
Ice Thickness ◽  
Gradual Change ◽  
Short Term ◽  
Glacier Surface ◽  
Vertical Movements ◽  
Bed Topography ◽  
The Waves ◽  
Local Uplift

AbstractTiltmeters that can detect changes in slope of a glacier surface as small as 0.1 μ rad have been used on Storglaciären. The records obtained to date have been from the upper part of the ablation area, where the bed of the glacier is overdeepened. A total of 82 d of records has been obtained for various time periods between early June and early September.There is generally a gradual change in inclination of the glacier surface over periods of several days, but these changes do not appear to be systematic. In particular, they are not consistent with vertical movements of stakes located 2–3 ice thicknesses away from the tiltmeters. This suggests that the tiltmeters are sensing disturbances over areas with diameters comparable to the local ice thickness.Superimposed on these trends are diurnal signals suggesting rises and falls of the surface just up-glacier from the riegel that bounds the overdeepening on its down-glacier end. These may be due to waves of high water pressure originating in a crevassed area near the equilibrium line. If this interpretation is correct, the waves apparently move down-glacier at speeds of 20–60 m h−1and become sufficiently focused, either by the bed topography or by conduit constrictions, to result in local uplift of the surface. Also observed are abrupt tilts towards the glacier center line shortly after the beginning of heavy rainstorms. These appear to be due to longitudinal stretching as the part of the glacier below the riegel accelerates faster than that above. Water entering the glacier by way of a series of crevasses over the riegel is believed to be responsible for this differential acceleration. In June 1987, a dramatic event was registered, probably reflecting the initial summer acceleration of the glacier.

A CFD Study of Focused Extreme Wave Impact on Decks of Offshore Structures

2014 ◽  
Author(s):  
Xin Lu ◽  
Pankaj Kumar ◽  
Anand Bahuguni ◽  
Yanling Wu
Keyword(s):  
Real World ◽  
Offshore Structures ◽  
Air Gap ◽  
Irregular Waves ◽  
Linear Wave ◽  
Extreme Wave ◽  
Wave Impact ◽  
Loading Test ◽  
Wide Range ◽  
Wave Maker

The design of offshore structures for extreme/abnormal waves assumes that there is sufficient air gap such that waves will not hit the platform deck. Due to inaccuracies in the predictions of extreme wave crests in addition to settlement or sea-level increases, the required air gap between the crest of the extreme wave and the deck is often inadequate in existing platforms and therefore wave-in-deck loads need to be considered when assessing the integrity of such platforms. The problem of wave-in-deck loading involves very complex physics and demands intensive study. In the Computational Fluid Mechanics (CFD) approach, two critical issues must be addressed, namely the efficient, realistic numerical wave maker and the accurate free surface capturing methodology. Most reported CFD research on wave-in-deck loads consider regular waves only, for instance the Stokes fifth-order waves. They are, however, recognized by designers as approximate approaches since “real world” sea states consist of random irregular waves. In our work, we report a recently developed focused extreme wave maker based on the NewWave theory. This model can better approximate the “real world” conditions, and is more efficient than conventional random wave makers. It is able to efficiently generate targeted waves at a prescribed time and location. The work is implemented and integrated with OpenFOAM, an open source platform that receives more and more attention in a wide range of industrial applications. We will describe the developed numerical method of predicting highly non-linear wave-in-deck loads in the time domain. The model’s capability is firstly demonstrated against 3D model testing experiments on a fixed block with various deck orientations under random waves. A detailed loading analysis is conducted and compared with available numerical and measurement data. It is then applied to an extreme wave loading test on a selected bridge with multiple under-deck girders. The waves are focused extreme irregular waves derived from NewWave theory and JONSWAP spectra.

Structural Safety Assessment of a 1100 TEU Container Ship, Based on a Enhanced Long Term Fatigue Analysis

2014 ◽  
Vol 1036 ◽  
pp. 935-940
Author(s):  
Leonard Domnisoru ◽  
Ionica Rubanenco ◽  
Mihaela Amoraritei
Keyword(s):  
Safety Assessment ◽  
Safety Evaluation ◽  
Irregular Waves ◽  
Structural Safety ◽  
Random Wave ◽  
Wave Loads ◽  
Strength Assessment ◽  
Integrated Method ◽  
Hydroelastic Response

This paper is focused on an enhanced integrated method for structural safety assessment of maritime ships under extreme random wave loads. In this study is considered an 1100 TEU container test ship, with speed range 0 to 18 knots. The most comprehensive criteria for ships structural safety evaluation over the whole exploitation life is based on the long term ship structures analysis, that includes: stress hot-spots evaluation by 3D/1D-FEM hull models, computation of short term ship dynamic response induced by irregular waves, long term fatigue structure assessment. The analysis is enhanced by taking into account the ships speed influence on hydroelastic response. The study includes a comparative analysis on two scenarios for the correlation between the ships speed and waves intensity. The standard constant ship speed scenario and CENTEC scenario, with total speed loss at extreme waves condition, are considered. Instead of 20 years ship exploitation life estimated by classification societies rules from the long term structural safety criteria, the enhanced method has predicted more restrictive values of 14.4-15.7 years. The numerical analyses are based on own software and user subroutines. The study made possible to have a more realistic approach of ships structural strength assessment, for elastic and faster ships as container carriers, in compare to the standard one based only on naval rules, delivering a method with higher confidence in the designed structural safety.

Analysis on the Performance of Deckwetness of Sandglass-type and Cylindrical Floating Bodies

2016 ◽  
Vol 60 (03) ◽  
pp. 145-155
Author(s):  
Ya-zhen Du ◽  
Wen-hua Wang ◽  
Lin-lin Wang ◽  
Yu-xin Yao ◽  
Hao Gao ◽  
...  
Keyword(s):  
Transfer Functions ◽  
Linear Response Theory ◽  
Second Order ◽  
Irregular Waves ◽  
Wave Loads ◽  
Floating Bodies ◽  
Drift Motion ◽  
First Order ◽  
Series Of Experiments ◽  
Deck Wetness

In this paper, the influence of the second-order slowly varying loads on the estimation of deck wetness is studied. A series of experiments related to classic cylindrical and new sandglass-type Floating Production, Storage, and Offloading Unit (FPSO) models are conducted. Due to the distinctive configuration design, the sand glass type FPSO model exhibits more excellent deck wetness performance than the cylindrical one in irregular waves. Based on wave potential theory, the first-order wave loads and the full quadratic transfer functions of second-order slowly varying loads are obtained by the frequency-domain numerical boundary element method. On this basis, the traditional spectral analysis only accounting for the first-order wave loads and time-domain numerical simulation considering both the first-order wave loads and nonlinear second-order slowly varying wave loads are employed to predict the numbers of occurrence of deck wetness per hour of the two floating models, respectively. By comparing the results of the two methods with experimental data, the shortcomings of traditional method based on linear response theory emerge and it is of great significance to consider the second-order slowly drift motion response in the analysis of deck wetness of the new sandglass-type FPSO.

Hull Deformation Effect on Membrane-Type LNG Containment Systems

2016 ◽  
Author(s):  
Bo Wang ◽  
Yung-Sup Shin ◽  
Eric Norris
Keyword(s):  
Structural Integrity ◽  
Water Pressure ◽  
Local Deformation ◽  
Fatigue Tests ◽  
Wave Loads ◽  
Insulation System ◽  
Structural Fatigue ◽  
Hull Structure ◽  
Membrane Type ◽  
Global Deformation

The objective of this study is to investigate the relationship between the maximum allowable hull deformation, which includes global elongation and local deflection, and the capacity of the CCS in membrane-type LNG vessels. The LNG CCS mainly consists of the primary barrier (e.g. a corrugated membrane for GTT MK III system and an invar membrane for GTT NO 96 system) and the insulation panel which is attached to the inner hull through mastics or couplers. The excessive hull elongation due to dynamic wave loads may cause fatigue damage of the primary barrier. Thus, the maximum allowable hull elongation (global deformation) can be determined based on the fatigue strength of the primary barrier. On the other hand, the excessive hull deflection due to cargo or ballast water pressure may cause failure of the insulation panel and the mastic. Therefore, the maximum allowable hull deflection (local deformation) in the hull design can be determined based on the strength of the insulation panel and the mastic. In the present paper, the determination of fatigue life vs. strain curves of materials has been summarized for the primary barrier. Fatigue curves based on either structural fatigue tests or standard specimen tests can be applied in fatigue assessment of a primary barrier. As an example, the finite element (FE) analysis has been conducted on the MK III CCS with the hull structure under pressure loads. Two different load cases including full load and ballast load conditions have been considered to evaluate the structural integrity of the insulation system in numerical simulations. FE results show that the mechanical behavior of the insulation system and the mastic under the maximum allowable hull deflection has been examined based on the yielding strength of each individual component. Finally, the complete procedure to determine the maximum allowable hull elongation and the maximum allowable hull deflection has been developed for meeting the requirements of containment system design for membrane-type LNG carriers.

Subsidence of the Ekofisk Platforms: Wave in Deck Impact Study — Various Wave Models and Computational Methods

2002 ◽  
Author(s):  
Bogdan Iwanowski ◽  
Henrik Grigorian ◽  
Ingar Scherf
Keyword(s):  
Computational Methods ◽  
Stokes Equations ◽  
Type Equation ◽  
Navier Stokes ◽  
Stokes Wave ◽  
Wave Loads ◽  
Wave Impact ◽  
Navier Stokes Equations ◽  
Wave Models ◽  
Displacement Approach

Subsidence of the Ekofisk platforms creates several operational challenges. For safety of the platforms, it is of great importance to find the wave impact loads acting on the platforms’ decks. The paper describes how such loads can be computed. Three theoretical wave models are discussed in the paper: the Airy wave, Airy wave modified through Wheeler stretching and the 5th order non-linear Stokes wave. The wave loads for these wave models are computed by various methods. The method based on momentum displacement approach and Morison-type equation developed by Dr. Kaplan is used as a reference point. The loads are also computed through a solution of complete Navier-Stokes equations, with the Volume of Fluid (VOF) method used to trace motion of the fluid’s free surface. Results of different wave models and different computational methods are compared and discussed.

scholarly journals NON-BREAKING AND BREAKING WAVE LOADS ON A COOLING WATER OUTFALL

1978 ◽  
Vol 1 (16) ◽  
pp. 148
Author(s):  
G.R. Mogridge ◽  
W.W. Jamieson
Keyword(s):  
Water Level ◽  
Cooling Water ◽  
High Water ◽  
Breaking Waves ◽  
Irregular Waves ◽  
Scale Model ◽  
Breaking Wave ◽  
Wave Loads ◽  
Eastern Canada ◽  
Spilling Breakers

Cooling water from a power generating station in Eastern Canada is pumped to an outfall and distributed into the ocean through discharge ports in the sidewalls of a diffuser cap. The cap is essentially a shell-type structure consisting of a submerged circular cylinder 26.5 ft in diameter and 14 ft high. It is located in 25 ft of water at low water level and 54 ft at high water level. Horizontal forces, vertical forces and overturning moments exerted by waves on a 1:36 scale model of the diffuser cap were measured with and without cooling water discharging from the outfall. Tests were run with regular and irregular waves producing both non-breaking and breaking wave loads on the diffuser cap. The overturning moments measured on the diffuser cap were up to 150 percent greater than those on a solid submerged cylinder sealed to the seabed. Unlike sealed cylinders, all of the wave loads measured on the relatively open structure reached maximum values at approximately the same time. The largest wave loads were measured on the diffuser structure when it was subjected to spilling breakers at low water level. For a given wave height, the spilling breakers caused wave loads up to 100 percent greater than those due to non-breaking waves.

scholarly journals WAVE LOADS ON HORIZONTAL CYLINDERS

1978 ◽  
Vol 1 (16) ◽  
pp. 147
Author(s):  
P. Holmes ◽  
J.R. Chaplin
Keyword(s):  
Oscillatory Flow ◽  
Vertical Plane ◽  
Vertical Cylinder ◽  
Irregular Waves ◽  
Wave Loads ◽  
Incident Flow ◽  
Cylinder Axis ◽  
Straight Line ◽  
Horizontal Cylinders ◽  
Wave Induced

The problem of predicting wave induced loads on cylinders is an enormously complex one. It is clear from the scatter present in most experimental determinations of force coefficients that there are many individual factors which influence the mechanisms of flow induced loading. Among these are some, for instance Reynolds number, separation and periodic vortex shedding, which are inter-related and whose influences cannot be studied in isolation. Others, such as shear flow, irregular waves and free surface effects, can at least be eliminated in the laboratory, in order to approach an understanding of the more fundamental characteristics of the flow. A vertical cylinder in uniform waves experiences an incident flow field which can be described in terms of rotating velocity and acceleration vectors, always in the same vertical plane, containing also the cylinder axis, whose magnitudes are functions of time and of position along the length of the cylinder. Some of the essential features of this flow can be studied under two-dimensional oscillatory conditions, in which either the cylinder or the fluid is oscillated relative to the other along a straight line (planar oscillatory flow). The incident velocity and acceleration vectors are then always concurrent, normal to the cylinder axis, and oscillating in magnitude with time.

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