Wave Impact Loads Research Articles

Experimental Investigations of Acceleration on Slender Cylindrical Member under Breaking Waves

Many of the coastal and offshore structures consist of slender cylindrical member as the fundamental component. Since the structural damages and failures are catastrophic under wave impact, it is essential to understand the influence of breaking wave impact on the structural members. Past literature dictates that the magnitude of impact pressures is of the order of ten times more than the non breaking wave induced pressure. The standard codal provisions for the structural design of those members under breaking wave impact are scarce. In the present study, an experimental investigation has been carried out to measure the response of the slender vertical cylinder under breaking waves. Acceleration measurements were made under the incidence of breaking waves of different intensities varying from Plunging to Spilling. Deflections are found out from the measured acceleration. The maximum acceleration observed under severe plunging event is 0.5 times that of moderate plunging event, the deflection induced by the moderate plunging is two order higher than that of severe plunging. Acceleration is maximum not only above still water level (SWL) but also lies in the impact zone. Hence moderate plunging events with higher impulse and larger pressure rise time is crucial in the design of offshore and coastal structural member under breaking wave impact load.

Investigation of uplift impact forces on a vertical wall with an overhanging horizontal cantilever slab

A vertical wall with an overhanging horizontal cantilever slab is tested in a small scale test set-up under wave impact (impulsive) loads. Tested waves are regular and irregular. A single approaching wave creates two individual impacts which occur sequentially on the wall and slab of the structure. Results from wave impact tests are used to analyze the uplift impact forces under breaking waves. Then, a set of parameters which is responsible for the uplift impact forces is investigated under regular waves and results are compared with the results of irregular wave tests. Incident wave height at the toe of the foreshore (H1), water depth at the structure toe (hs) and incident wave period (T) are the main parameters governing the uplift impact forces. In addition, the influence of different geometric properties, described by dimensionless ratios like c'/hs (c' is the clearance between still water level and slab of the structure) should be taken into account. Based on the experimental investigations on breaking wave kinematics and impact loads, a new prediction model for uplift impact forces, occurring on the overhanging horizontal cantilever slab, has been derived as a function of the rise time tr. tr is the time duration between points t1 and t2, see Fig. 2. The formula is tested within the range 0.45≤H1/hs≤1.2, 2.0s≤T≤2.8s and 0.75m≤hs≤1.65m. The formula is developed based on the scaled test results. Therefore, a correction of the formula is required for the prototype dimension. In addition, the formula considers the maximum uplift impact forces which are more intense but shorter lasting. Thus, the structural response of the cantilever should be taken into account in design process. This paper is a completion of previously published works (Kisacik et al., 2012a and Kisacik et al., 2012b) on this type of structure.

Impulsive wave loads on rigid structures, an experimental approach

Marzeddu, A., Gironella, X., 2013. Impulsive wave loads on rigid structure, an experimental approach In: Conley, D.C., Masselink, G., Russell, P.E. and O’Hare, T.J. (eds.), Proceedings 12 th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 332-337, ISSN 0749-0208. Within the European project ‘Hydralab IV’, HyRes we aim to improve the characterization of wave loads on rigid structures and the associated response by carrying out some laboratory experiments. Wave loads on rigid structures are divided into quasi static loads and impact loads. If the physics of quasi-static loads due to waves is well known, this cannot be said the same for wave impact loads. A comprehensive method to design maritime rigid structures under impact loads does not exist yet and the actual design method suggests avoiding scenarios where impact loads can take place. In the last decade, some laboratory experiments have been carried out; however some questions remain still unanswered. The use of different sensors can lead to significant changes in the results and an “exhaustive comparison” between dissimilar types of sensors has not been done yet. Even the magnitude of these forces can be underestimated during a laboratory test just for the choice of sample frequencies which are too low. This paper describes the experiments performed on a small scale flume at UPC on a scaled vertical breakwater in order to compare the results of pressure transducers and force load cells. Moreover, a high frequency sampler (up to 20 KHz) was used in order to understand the importance of sample frequency on the magnitude of the results. A simplified scenario has been set up in order to make the data analysis easier.

Simulating structural response to water impact

Structural response to water impact is important for several areas, including the aerospace and marine industries. Aircraft must be designed to cope with ditching and offshore structures are subject to extreme wave impact and green water loading. The goal is a reliable technique for predicting the structural response to extreme water loading. This is a complex problem involving the interaction of non-linear fluid behaviour (breaking waves, fluid impact) with non-linear structural behaviour (large deformations, contact, material plasticity and damage). This paper gives an overview of the coupled FE/SPH approach for modelling water impact on structures and discusses specific issues related to the analysis of floating structures. The capabilities of the method are illustrated first through comparison with published numerical results for simple problems involving the equilibrium of rigid floats and an initially submerged cylindrical float. Then by comparison with experimental data for the collapse of a metallic helicopter floor structure due to impact with water. Finally the response of a moored buoy to an extreme wave is modelled. The sensitivity of the model to spatial resolution is investigated for the rising cylinder and helicopter floor cases.

Dynamic response and sliding distance of composite breakwaters under breaking and non-breaking wave attack

Over the last 15 years improved awareness of wave impact induced failures has focused attention on the need to account for the dynamic response of maritime structures to wave impact load. In this work a non-linear model is introduced that allows evaluating the effective design load and the potential sliding of caisson breakwater subject to both pulsating and impulsive wave loads. The caisson dynamics is modelled using a time-step numerical method to solve numerically the equations of motion for a rigid body founded on multiple non-linear springs having both horizontal and vertical stiffness. The model is first shown to correctly describe the dynamics of caisson breakwaters subject to wave attack, including nonlinear features of wave–structure–soil interaction. Predictions of sliding distances by the new method are then compared with measurements from physical model tests, showing very good agreement with observations. The model succeeds in describing the physics that stands behind the process and is fast, accurate and flexible enough to be suitable for performance design of caisson breakwaters.

An assessment of the effect of hull girder vibration on the statistical characteristics of wave loads

For the assessment of statistical characteristics of wave loads in the real sea state, the probability distribution of wave loads are computed based on the sufficient duration of computations in irregular waves. First of all, the estimation of wave impact loads is well modified applying the displacement potential formulation, which was proposed by one of authors, for solving Wagner's flow model. Consequently, the present computation method is also modified. Prior to the computation in irregular waves, preliminary computation to determine the adequate number of realization of irregular waves is examined. The effect of hull girder vibration on the statistical characteristics is examined by means of the computation with/without hull girder vibration. It is found that hull girder vibration has a certain effect on the probability of occurrence of wave loads. Furthermore, computations taking account of the effect of operation, that is the effects of ship speed and course change, is conducted for the rational evaluation of the effects of hull girder vibration. It is clarified that the effect of operation on the statistical characteristics of wave loads is significant. It is verified that the evaluation without the effect of operation may overestimate the effect of hull girder vibration.

A fully nonlinear wave model to account for breaking wave impact loads on offshore wind turbines

This paper presents a numerical model capable of simulating offshore wind turbines exposed to extreme loading conditions. External condition-based extreme responses are reproduced by coupling a fully nonlinear wave kinematic solver with a hydro-aero-elastic simulator. The transient nonlinear free surface problem of water waves is formulated assuming the potential theory and a higher-order boundary element method (HOBEM) is used to discretize Laplace’s equation. For temporal evolution a second-order Taylor series expansion is implemented. The code is successfully adopted to simulate overturning plunging breakers, which give rise to dangerous impact loads when they break against wind turbine substructures. Emphasis is also placed on the development of a global simulation framework that aims at embedding the wave simulator into a more general stochastic environment. Indeed, first a linear irregular sea is generated by a spectral approach, then only on critical sub-domains, where wave impacts are expected, the fully nonlinear solver is invoked for a more refined simulation. This permits to systematically account for dangerous effects on the structural response (which would be missed by adopting linear or weakly nonlinear wave theories alone) without penalizing the computational effort.

Simulation of wave impact on a horizontal deck based on SPH method

A numerical model was established for simulating wave impact on a horizontal deck by an improved incompressible smoothed particle hydrodynamics (ISPH). As a grid-less particle method, the ISPH method has been widely used in the free-surface hydrodynamic flows with good accuracy. The improvement includes the employment of a corrective function for enhancement of angular momentum conservation in a particle-based calculation and a new estimation method to predict the pressure on the horizontal deck. The simulation results show a good agreement with the experiment. The present numerical model can be used to study wave impact load on the horizontal deck.

Evaluation of wave impact loads on caisson breakwaters based on joint probability of impact maxima and rise times

When waves break against seawalls, vertical breakwaters, piers or jetties, they abruptly transfer their momentum into the structure. This energy transfer is always spectacular and perpetually unrepeatable but can also be very violent and affect the stability and the integrity of coastal structures. Over the last 15 years, increasing awareness of wave-impact induced structural failures of maritime structures has emphasised the need for a more complete approach to dynamic responses, including effects of impulsive loads. At the same time, movement of design standards toward probabilistic approaches requires new statistical tools able to account for uncertainties in the variability of wave loading processes. This paper presents a new approach to the definition of loads for use in performance design of vertical coastal structures subject to breaking wave impacts. Based on conservation of momentum and joint probability of non-dimensional wave impact maxima and rise times from large-scale test measurements, a new set of equations have been derived to characterise design impact loads at different non-exceedance probability levels and guidance is given for the estimation of the static-equivalent design loads to be used in early-stage feasibility studies. Predictions of static equivalent design loads and corresponding safety factor against sliding using the proposed methodology are found to be in very good agreement with both predictions by most established deterministic methods and field observations reported in literature.

Breaking Wave Impact Pressure on a Vertical Wall

Offshore and coastal structures experience wave impact loads particularly due to breaking waves throughout its design period. In the present paper, an experimental program has been devised to improve our perceptive of the physics and the characteristics of impact pressures due to breaking waves on a vertical wall. The salient features of the study such as model details, breaking wave simulation, the impact and dynamic pressure variation along the depth of the wall are presented. The maximum impact pressure magnitude is found to be of the order of ten times that of the non-breaking dynamic pressure and it always occurred above the still water level for different intensities of breaking waves considered. The minimum pressure rise time is observed to be 0.235 ms.

Evolution of the air cavity during a depressurized wave impact. I. The kinematic flow field

This paper describes a systematic experimental study of the role of the ambient pressure on wave impact events in depressurized environments. A wave impact event of “mode (b)” [see Lugni et al., “Wave impact loads: The role of the flip-through,” Phys. Fluids 18, 122101 (2006)] causes entrapment of an air cavity. Here the topological and kinematic aspects of its oscillation and evolution toward collapse into a mixture of water and air bubbles are studied, while Part II [Lugni et al., “Evolution of the air cavity during a depressurized wave impact. II. The dynamic field,” Phys. Fluids 22, 056102 (2010)] focuses on the dynamic features of the flow. Four distinct stages characterize the flow evolution: (1) the closure of the cavity onto the wall, (2) the isotropic compression/expansion of the cavity, (3) its anisotropic compression/expansion, and (4) the rise of the cavity up the wall. The first two stages are mainly governed by the air leakage, the last two by the surrounding hydrodynamic flow, which contributes to compressing the bubble horizontally and to convecting it up the wall. Ullage pressure affects the ratio between the minimum and maximum cavity areas. An ullage pressure of 2.5% of the atmospheric pressure leads to an area ratio of about 360% of the equivalent ratio at atmospheric conditions.

Scaling wave impact pressures on vertical walls

It is widely recognized that the use of Froude similarity for scaling up wave impact pressures recorded during physical model tests may lead to over-estimation of impact maxima. Based on reviewing historical work dating back to the 30s and further developments in the 60s and 80s, a general method is presented that is suitable for scaling up impact pressures and rise times measured during small scale physical model tests. The method accounts for the effect of air leakage and is applicable to most wave impact loads. The model is applied to scale wave impact pressures on vertical walls and similar structures, and consistent correction factors for the Froude scaling law are derived.

The occurrence and effects of wave impacts

The determination of design parameters for coastal structures has historically often involved the definition of maximum water levels, the highest significant wave height and the longest wave periods. The design then proceeds with these values using numerical models, direct design methods or physical model studies. Design studies have often been conducted with maximum water levels, using random seas with previously determined parameters, a chosen wave spectrum and a sufficiently large number of waves. Parametric analysis methods using multiple parameters have identified that different combinations of water level; wave condition; perhaps fluvial discharge; or sea bed condition; may all influence the response. Recent research into the most important responses of vertical structures (plunging wave impact, toe scouring, overtopping and overtopping-induced loading) indicates that these mechanisms are all linked to the occurrence of violent wave impacts. Wave breaking is a depth-limited phenomenon, with plunging breakers occurring only for a limited range of wave height to water depth ratios. For given storm wave heights, but tidally varying water levels, this wave-breaking regime will often only prevail for a limited amount of time, leading to the conclusion that the duration of maximum exposure, namely the length of time that a coastal structure is exposed to the most important wave-induced damage mechanisms, is a function of the combined probability of (at least) water level (tidal + storm surge) and wave height/storm profile characteristics. This may have significant consequences for the definition of critical design parameters and the parameter variations employed in model tests.

Wave impact loads: The role of the flip-through

The impact of waves upon a vertical, rigid wall during sloshing is analyzed with specific focus on the modes that lead to the generation of a flip-through [M. J. Cooker and D. H. Peregrine, “A model for breaking wave impact pressures,” in Proceedings of the 22nd International Conference on Coastal Engineering (ASCE, Delft, 1990), Vol. 2, pp. 1473–1486]. Experimental data, based on a time-resolved particle image velocimetry technique and on a novel free-surface tracking method [M. Miozzi, “Particle image velocimetry using feature tracking and Delaunay tessellation,” in Proceedings of the 12th International Symposium on Applications of Laser Techniques to Fluid Mechanics (2004)], are used to characterize the details of the flip-through dynamics while wave loads are computed by integrating the experimental pressure distributions. Three different flip-through modes are observed and studied in dependence on the amount and modes of air trapping. No air entrapment characterizes a “mode (a) flip-through,” engulfment of a single, well-formed air bubble is typical of a “mode (b)” event, while the generation of a fine-scale air-water mixing occurs for a “mode (c)” event. Upward accelerations of the flip-through jet exceeding 1500g have been measured and the generation/collapse process of a small air cavity is described in conjunction with the available pressure time histories. Predictions of the vertical pressure distributions made with the pressure-impulse model of Cooker and Peregrine [M. J. Cooker and D. H. Peregrine, “Pressure-impulse theory for liquid impact problems,” J. Fluid Mech. 297, 193 (1995)] show good agreement with the experimental data.

Finite-element computation of wave impact load due to a violent sloshing

A nonlinear sloshing problem is numerically simulated. During excessive sloshing, the sloshing-induced impact load can cause a critical damage on the tank structure. Recently the problem becomes an important research topic in LNG (Liquefied Natural Gas) Tanker and FPSO (Floating Production Storage Offloading) design. In this study, the wave impact load on the structure is obtained numerically by imposing the exact nonlinear free surface conditions and compared with that predicted by Morison's formula. As a theoretical model, a three-dimensional free surface flow in a tank is formulated in the scope of potential flow theory with the exact nonlinear free-surface conditions. A finite-element method based on Hamilton's principle is employed as a numerical method. The problem is treated as an initial-value problem. The nonlinear problem is numerically solved through an iterative scheme at each time step.

Numerical Prediction of Wave Impact Loads on Multiple Rectangular Beams

Wave impact on one and two structural beams with rectangular cross section is simulated with a two-dimensional finite volume method, solving the unsteady Euler equations and employing a VOF-type method for the description of the free surface. Four different test series are carried out, each corresponding to a wave impact scenario in the experimental database of Sterndorff [2002]. For the case of wave impact on a single structural element the numerical results show good agreement, with measured force time histories. In the computations featuring two beams, the prediction of the shadowing effect of the first beam on the second is in reasonable agreement with the experimental data. However, the force peak on the second beam is somewhat over-predicted. The calculations successfully predict a second peak in the force time series of the second beam, which is caused by airborne water shipped over the first beam. Throughout the work, spurious spikes of very short duration appear in the computed load time series, originating from the changing of the flow separation location along the lower edge of the beams.

Dynamic analysis of a vertical plate exposed to breaking wave impact

From the experimental studies in recent years, it has become known that when a wave breaks directly on a vertical faced coastal structure, high magnitude impact pressures are produced. The theoretical and experimental studies show that the dynamic response of such structures under wave impact loading is closely dependent on the magnitude and duration of the load history. The dynamic analysis and design of a coastal structure can be succeeded provided the design load history for the wave impact is available. Since these types of data are very scarce, it is much more convenient to follow a method which is based on static analysis for the dynamic design procedure. Therefore, to facilitate the dynamic design of a vertical plate that is exposed to breaking wave impact, a multiplication factor called “dynamic magnification factor” is herein presented which is defined as the ratio of the maximum value of the dynamic response to that found by static analysis. The computational results of the present study show that the dynamic magnification factor is a useful ratio to transfer the results of static analysis to the dynamic design of a coastal plate for the maximum impact pressure conditions of p max/ γH 0≤18.

Evaluation of methods to remove inertial force from measured model wave impact force signals

The subject of wave impact on offshore structures and their components is important to vessel designers and operators for many reasons. They are often required to quantify these impact loads. Standard methods for wave load prediction will underestimate the forces on these structures due to intermittent loading. This necessitates the use of physical model tests to establish wave impact loads. The model measurement systems are designed to have high stiffness. This ensures that the natural frequency of the structure is above the wave frequency. However, it is widely believed that impacting waves contain high-frequency energy components that cause the structure to vibrate at its modal frequencies. This impact-induced vibration is recorded by the measuring system as a force (inertial force), and corrupts the actual applied force measurement. Before scaling can occur, the inertial force must be removed from the measured signal. A number of techniques for removing inertial force from measured signals are described in the published literature. Three methods are discussed, implemented and compared in this paper. The algorithms and procedures are presented. Each technique contains inherent and unique problems, while some are common to all methods. Neither of the techniques produced results that are fully satisfactory. The main problem is unwanted high-frequency content after the application of the methods. While neither method offers the perfect solution, the use of digital filtering techniques is recommended based on their relative performance and ease of implementation.

Theoretical and experimental investigation of a vertical wall response to wave impact

The laboratory and field experiments so far have shown that when a wave breaks directly on a vertical faced coastal structure, the resulting impact pressures may become very severe in magnitude and short in duration. Some experimental evidence in the literature suggests that the structural response to the extremely high magnitude impact forces is only limited. This study is mainly concerned with the comparison of the theoretical and experimental results of a vertical wall response under the wave impact loading. In the dynamic analysis of the wall the classical elastic plate theory is used and the numerical results for the dynamic values of the transverse displacement are obtained by employing the method of finite elements. In the theoretical analyses the experimental pressure histories are used and the theoretical wall deflection histories are compared with the experimental results. The computational and experimental deflection histories exhibit similar patterns. The theoretical maximum wall deflections are mostly found to be slightly smaller than the experimental values.

Vibratory Response of Ship Hulls to Wave Impact Loads

The present paper describes a method to predict the stresses induced on the ship hull when the forward bottom impacts in water. The relative motion between the ship and waves is determined using a linear strip method. The time history of the relative motion indicates the conditions for the occurrence of slamming. Several empirical methods for the evaluation of the slamming loads based on experimental results are compared. The vibratory response of the ship is calculated by modelling it with finite elements. The response is obtained by modal superposition using the central differences for the time integration. Results are presented of an example on a container ship. It as shown that very large differences are obtained for the slam induced vertical bending moment amidships depending on the method used to calculate the slamming forces.