Prediction Of Wave Loads Research Articles

Faster than real-time, phase-resolving, data-driven model of wave propagation and wave–structure interaction

A machine learning time-series prediction approach is proposed for wave propagation and wave load prediction. Under unidirectional wave conditions and variable bathymetry, given a wave gauge upstream, a model is shown to reproduce wave elevation or wave forces downstream under irregular steep and either non-breaking or breaking conditions. Attempts to perform the opposite calculation, predicting upstream conditions from downstream measurements, results in higher error, likely due to information loss under breaking conditions. For choice of machine learning approach, comparisons show that the Time-series Dense Encoder (TiDE) approach results in a good balance between model complexity, stability, computational time, and error. Over a flat bottom, time-series of wave elevation can be predicted up to 10 wavelengths away, though with a degraded accuracy compared to shorter distances. Similar results are shown for time-series of forces on a vertical cylinder, showing better results than a simple Morison approach, as used in engineering tools such as OpenFAST, but with a similarly fast computational time. Generalizations show that training on irregular wave data permit extrapolations to periodic wave cases. Finally, the same method also is also demonstrated at field-scale, comparing results between two offshore buoys.

Analysis of Wave Load Characteristics of Hovercraft Based on Model Test

The prediction of the wave load on a hovercraft is essential for the design of the hull structure and safety. However, theoretical methods for the prediction of wave loads are still not mature enough due to the unique and complex nature of the air cushion structure, and numerical modeling and simulation are challenging due to the complexity of the gas-solid-liquid three-phase coupling, so the study of wave loads on hovercrafts still relies on experimentation. In this study, we aim to analyze the wave load response characteristics of a four-chamber hovercraft by conducting a wave load model test under medium/low sea states. The load components and amplitude-frequency response characteristics were thoroughly analyzed based on the acquired data of the cushion pressure, acceleration, and bending moment. The main characteristics of the wave-induced response of the hovercraft were described in detail, and an analytical relationship between the cushion pressure and hull acceleration was derived. The reliability of the experimental results was confirmed through a comparison with the derived results. The relationship between the cushion pressure and cushion volume was investigated in terms of the observed geometric volume of the air chamber, and the relationship between the cushion pressure and flow rate was analyzed to validate the derivation of the theory of wave loads on hovercrafts.

Intelligent prediction of wave loads based on multi-source data-driven methods

ABSTRACT High-fidelity wave-load tests is pivotal in the design and development of modern ship structures. Nonetheless, the prohibitive cost of conducting such tests limits their applicability. A wave-load data fusion method was implemented using the model test data from a typical displacement-type ship to generalise the small sample test, and a multi-source data characterisation and model parameter optimisation method were established to achieve efficient wave-load (wave frequency and slamming superposition) prediction. Using multi-source data, a multi-fidelity wave-load prediction (MFWLP) model was constructed by combining numerical computation, regular wave tests (RWT), and irregular wave test (IWT) data based on transfer learning with a secondary correction wave load prediction under irregular wave conditions. The model was effectively employed in a ship wave-load engineering case. This study reveals that the adopted MFWLP model demonstrates adequate extrapolation and prediction capabilities, even with limited high-fidelity test samples. The discrepancy between the nonlinear wave load prediction and model test was found to be within 15%. Therefore, the proposed method can serve as a foundation for rapidly assessing ship wave loads across various sea conditions, especially under high sea states.

Review of the uncertainties associated to hull girder hydroelastic response and wave load predictions

This paper reviews the importance of uncertainties in hull girder loads influenced by flexible fluid structure interactions. The focus is on developments in the field of hydroelastic modelling, simulation and model tests of practical relevance to the prediction hull girder wave load predictions and their validation. It is concluded that whereas hydroelastic methods for use in design development and assessment become increasingly useful, challenges in realizing and modelling uncertainties can be attributed to: (1) the limitations of numerical methods to suitably model nonlinearities; (2) the ambiguity of model tests; and (3) the systematic use of data emerging from computational, model- or full-scale methods. An approach is recommended to assess the uncertainty in the hydroelastic responses to wave loading and an example is provided to demonstrate the application of the procedure.

Wave Effects on Vortex-Induced Vibrations of a Top-Tensioned Riser

Abstract Offshore drilling risers and many production risers are top-tensioned risers (TTRs), connecting the vessel and seabed via joints. External loads such as currents, waves, and vessel motions introduce cyclic loads and motions on riser sections and associated components, such as universal joint, flex joints, and well head at the bottom, which may shorten the service life due to accumulated fatigue damage. Dynamic responses under combined currents and waves are more complicated than vortex-induced vibrations (VIVs) caused by pure currents, and it is not fully understood. Several model test campaigns on TTR have been carried out at SINTEF Ocean (former MARINTEK) during the past decades. Currents, waves, and vessel motions were modeled, and the riser model responses were measured. In this study, selected cases from such model tests are analyzed, and used to validate a semi-empirical time domain VIV prediction tool—VIVANA-TD. A better understanding of the dynamic responses of TTR under combined currents and waves has been achieved. By comparing the results from numerical simulation using VIVANA-TD and model test measurements, validity and limitation of the time domain tool have been investigated. Important features that need to be considered are discussed. The experience gained from the present study establishes a good basis for VIV and wave load prediction of full-scale TTRs under combined currents and waves where the uncertainty of combined wave and VIV prediction is further reduced.

Design Wave Calculation of a Passenger Catamaran Under Multiple Load Control Parameters

Abstract To determine the loading conditions considering the action of both bending and torque moment for a passenger catamaran moving among waves, a method for calculating equivalent design waves under multiple load control parameters was derived based on wave load prediction results using three-dimensional potential flow theory. The method was developed by defining the wave amplitude discrepancy factors between the primary and second load of the combined bending and torquing equivalent design wave. The primary goal was to find a reasonable design wave. Finally, the design waves of a target passenger catamaran ship were calculated using the proposed method, and each load component of every design wave for the target hull was recalculated. The average error compared with the object load component was less than 1%, which verifies the effectiveness of the method and offers an effective engineering evaluation method for a catamaran.

Numerical study of trimaran motion and wave load prediction based on time-domain Rankine-Green matching method

The theoretical prediction of trimaran motion and load in complex environment has always been a hot issue in the field of ship and ocean engineering. Based on the calculation principle of Rankine source and free-surface Green function for solving ship-wave interaction, a new kind of nonlinear three-dimensional time-domain Rankine-Green matching method for simulating the trimaran motion and wave load in head seas is studied. In the numerical simulation, the influence of factors such as wave slamming, green water, panel space and density, are investigated systematically. And the elevation of surrounding wave surface caused by speed effect and side hull interference is also considered to modify the instantaneous impact load. In order to verify the reliability of Rankine-Green matching method, a kind of segmented trimaran model with water jet propulsion system was designed and taken in regular and irregular waves. Finally, the numerical forecasting values based on Rankine-Green matching method are compared with the results of model test and traditional frequency-domain Green function method to illustrate its advantages and application scope.

Prediction of nonlinear vertical bending moment using measured pressure distribution on ship hull

Accurate prediction of wave loads on ships and floating structures is paramount in the structural design stage. Use of a segmented ship model is a common method to quantify the wave loads. Nevertheless, the value could be measured only at segmented sections. To obtain the wave loads at any longitudinal position and to account for nonlinear features in the wave loads more precisely, local quantities of the pressure on the whole ship-hull surface need to be measured along with ship motions in waves. In this paper, an unprecedented experiment using a bulk carrier model has been carried out to measure the spatial distribution of wave-induced unsteady pressure by means of a large number of Fiber Bragg Gratings (FBG) pressure sensors affixed on the whole ship-hull surface, and at the same time the wave-induced ship motions and ship-side wave profile have been measured. In order to see hydrodynamic characteristics in nonlinear and forward-speed effects on measured and analyzed results, some computations with the linear frequency-domain Rankine Panel Method (RPM) and the nonlinear Computational Fluid Dynamics (CFD) method solving the Reynolds-Averaged Navier-Stokes (RANS) equations are made. Favorable agreement is found for the pressure distribution and resulting vertical bending moment between the results of the experiment and corresponding numerical computations. Validation of the measured pressure distribution has also been made through a comparison of the wave-exciting force and moment between the two independent results obtained by integration of the measured pressure over the entire wetted surface of a ship and by direct measurement using a dynamometer. Very good agreement is confirmed in this case, too. As another validation for the wave loads, a comparative study is made with the benchmark test data of a 6750-TEU container ship used for the ITTC-ISSC joint workshop in 2014; which also demonstrates remarkable agreement. The present study may provide a new research technique, especially in the experiment, for predicting the wave-load distribution and for studying local hydrodynamic features in wave-related unsteady phenomena.

Coupling voyage and weather data to estimate speed loss of container ships in realistic conditions

The purpose of this paper is to investigate the speed loss based on the real-life voyage data and weather reanalysis. A methodology for coupling weather and voyage data is presented. Speed loss is investigated for a series of container ships of different dimensions which are then compared with the existing methods for speed loss. Another approach in estimating voluntary speed reductions in high seas is having first hand experience of the captains. For this purpose, we have designed and handed a questionnaire to ship masters. This has provided additional insight to the decision-making process during the voyages in rough weather conditions. This analysis sets the ground for a more comprehensive study, including various ship types, sizes and weather conditions leading to more accurate prediction of wave loads, fuel consumption or emissions from ships.

Application of the 3D time-domain Green's function for finite water depth in hydroelastic mechanics

Based on the boundary element method and the superposition principle of structural elastic modes, the three-dimensional time-domain Green's function in finite water depth is applied to hydroelasticity, and the theoretical basis of three-dimensional time-domain hydroelasticity is established. To address the difficulty and divergence of the three-dimensional time-domain Green's function in finite water depth, a numerical solution method that has high accuracy and good stability is developed using series expansion, asymptotic methods and fourth-order differential equations. Taking a large bulk carrier as an example, the corresponding predictions of resonance frequency, motion, wave load and hydroelastic response with speed in finite water depth are carried out. The numerical results are compared with the results of the three-dimensional frequency domain method and the time-domain method of inner and outer region matching and are verified by the towing model test. The three-dimensional time-domain hydroelastic theory and numerical method established in this paper are of great significance for fluid coupling analysis, structural dynamic response research and wave load prediction of complex floating bodies with speed.

Investigation of Longitudinal Vibrations and Slamming of a Trimaran in Regular Waves

Slamming and longitudinal vibration are two important factors during the preliminary structural design of trimarans. In order to better understand wave-induced loads of trimarans in regular waves, hydroelastic experiments were carried out to observe the wave slamming and longitudinal vibration using a segmented self-propelled model. Details of the model design and the experimental method are provided. Characteristics of the slamming pressure and the wave-induced response were measured under different wave lengths, wave heights, and sailing speeds. Strain and pressure gauges attached to the model in different locations were employed to investigate the distribution of the wave loads and pressures. The high-frequency and low-frequency components of the measured loads were separated using Fourier filter, and the influence of environment parameters on these load components were analyzed. In the high-frequency load, the effects of whipping and springing were found, and their characteristics were illustrated. Based on the experimental results obtained, some considerations for the structural design of trimarans are discussed. 1. Introduction Due to the special structure design of trimaran, the slamming and hull vibration that the trimaran is subjected to are always more complicated than the load response of an ordinary ship. Besides the bow slamming, the slamming at cross-bridge and wave load change caused by mutual interference of the main hull and demihull add to the challenges encountered by research on trimaran hull vibration and slamming. In order to obtain a comprehensive understanding of trimaran vibration and slamming, various model tests have been conducted by many researchers over the years. Hampshire et al. (2004) undertook a series of segmented model tests in QinetiQ Rosyth, and the experiment results provided data support for wave load prediction methodology. Kennell (2004) designed a back-spline structure in segmented model and conducted the model experiment under different speeds to determine the extent to which the sailing speed influenced wave load and slamming. Chao (2008) used a partial trimaran model in water entry test to observe the phenomenon of cross-bridge slamming and studied the influence of air-cushion and water splash effect on the pressure peak. Wang et al. (2010) undertook a model towing experiment in different wave azimuths and speeds, observing the characteristics of trimaran load response under different wave conditions. In a different study, Hu et al. (2010) carried out a set of irregular wave experiments on a trimaran model. The test results were used to determine the design wave load for the trimaran preliminary design. Meanwhile, Peng et al. (2011) studied the cross-bridge slamming by using a small-scale trimaran model water entry test. Through comparative analysis of numerical prediction and experimental results, the relevant parameters of numerical simulation were determined. Yu et al. (2014) observed the hull girder high-frequency vibration caused by slamming in the trimaran model test and validated its importance for the whole hull girder vibration.

URANS predictions of wave induced loads and motions on ships in regular head and oblique waves at zero forward speed

The paper presents predictions on the hydrodynamics of a conceptual floating liquefied natural gas (FLNG) and a liquefied natural gas (LNG) carrier in regular head and oblique sea waves using an unsteady Reynolds-Averaged Navier–Stokes (URANS) solver. Initially, a verification and validation study is performed on estimating the numerical uncertainties in the presented URANS simulations. Using the verified numerical setup, ship hydrodynamic properties including wave induced loads as well as ship motion responses are investigated for different wave conditions. The computed time history results are decomposed by Fourier Series to estimate the wave load and ship motion transfer functions. These results show good correlation with predictions from model tests and potential flow calculations. From the computations, it is observed when increasing the wave length, wave diffraction around the ship becomes less profound and the water depth starts to influence the transfer functions. Full scale computations in head and oblique sea waves are also presented and compared with model scale predictions for investigating possible scale effects. The differences are found to be close to the numerical uncertainties, indicating minor influences of scale effects on the prediction of wave loads and ship motion responses for the tested cases.

Benchmarking of a Computational Fluid Dynamics-Based Numerical Wave Tank for Studying Wave Load Effects on Fixed and Floating Offshore Structures

A computational fluid dynamics (CFD) based numerical wave tank (NWT) is developed and verified to study wave load effects on fixed and free floating offshore structures. The model is based on solving Navier–Stokes equations on a structured grid, level set method for tracking the free surface, and an immersed boundary method for studying wave–structure interaction. This paper deals with establishing and verifying a CFD-based NWT. Various concerns that arise during this establishment are discussed, namely effects of wave reflection which might affect the structure response, damping of waves in downstream, and three-dimensional (3D) effects of the waves. A method is described and verified to predict the time when incoming waves from wave generator are affected by reflecting waves from the structure which can help in better designing the dimensions of NWT. The model is then used to study sway, heave, and roll responses of a floating barge which is nonuniform in density and limited in sway direction by a spring and damper. Also, it is used to study wave loads on a fixed, large diameter, surface piercing circular cylinder. The numerical results are compared with the experimental and other numerical results, and in general very good agreement is observed in all range of studied wave frequencies. It is shown that for the studied fixed cylinder, the Morison equation leads to promising results for wavelength to diameter ratio larger than 2π (kD < 1), while for shorter wavelengths results in considerable over prediction of wave loads, due to simplification of wave diffraction effects.

Wave Load Analysis of the Corvette Ship in the Sea Water of Indonesia

Wave load prediction of ship should be considered in the design stage of ship’s construction. The excessive of wave load might cause a structural failure of ship during operational in seaway. The bending moment might be experienced on the ship structure that it contributes to stress concentration on the particular part of construction. This paper describes the prediction of bending moment and shear forces of the warship corvette type considered to the sea condition of Indonesia. The 3D diffraction theory was adopted to analyze ship’s motion responses, bending moments, and shear forces numerically. In this numerical simulation, the variations of speed and heading angle of ship were performed with respect to environmental condition of sea state 4, 5, 6 and 7. The simulation results had shown that the maximum shear force and bending moment was occurred on the area of mid-ship.

The effect of small forward speed on prediction of wave loads in restricted water depth

Wave load prediction at zero forward speed using finite depth Green function is a well-established method regularly used in the offshore and marine industry. The forward speed approximation in deep water condition, although with limitations, is also found to be quite useful for engineering applications. However, analysis of vessels with forward speed in finite water depth still requires efficient computing methods. In this paper, a method for analysis of wave induced forces and corresponding motion on freely floating three-dimensional bodies with low to moderate forward speed is presented. A finite depth Green function is developed and incorporated in a 3D frequency domain potential flow based tool to allow consideration of finite (or shallow) water depth conditions. First order forces and moments and mean second order forces and moments in six degree of freedom are obtained. The effect of hull flare angle in predicting added resistance is incorporated. This implementation provides the unique capability of predicting added resistance in finite water depth with flare angle effect using a Green function approach. The results are validated using a half immersed sphere and S-175 ship. Finally, the effect of finite depth on a tanker with forward speed is presented.

Two- and three-dimensional springing analysis of a 16,000 TEU container ship in regular waves

In recent years, the increase in world trade has resulted in a large expansion of sea traffic. As a result, market demands are leading to the development of Ultra Large Container Ships (ULCSs), with lengths of up to 400 m and increased flexibility of operational requirements. The multicellular open-decked thin-walled structural design of these ships means that flexible hull girder dynamics become important for the prediction of wave loads. This paper investigates the importance of various hydroelastic modelling approaches on the global symmetric and anti-symmetric response of a 16,000 twenty-foot equivalent unit (TEU) ULCS design. Two- and three-dimensional linear and weakly non-linear flexible fluid–structure interaction models that respectively combine Vlasov beam and three-dimensional finite element analysis (FEA) structural dynamics with a B-spline Rankine panel and Green's function hydrodynamics are assessed and compared. Comparisons between rigid body and hydroelastic predictions demonstrate the importance of considering the effects of hull flexibility on the dynamic response and the suitability of different idealisations at preliminary or detailed design stages.

Conditional Stochastic Processes Applied to Wave Load Predictions*

The concept of conditional stochastic processes provides a powerful tool for evaluation and estimation of wave loads on ships and offshore structures. This article first considers conditional waves with a focus on critical wave episodes. Then the inherent uncertainty in the results is illustrated with an application where measured wave responses are used to predict the future variation in the responses within the next 5‐30 seconds. The main part of the article is devoted to the application of the First Order Reliability Method for derivation of critical wave episodes for different nonlinear wave-induced responses. A coupling with Monte Carlo simulations is shown to be able to give uniform accuracy for all exceedance levels with moderate computational time, even for rather complex nonlinear problems. The procedure is illustrated by examples dealing with overturning of jackup rigs, parametric rolling of ships, and slamming and whipping vibrations.

Conditional Stochastic Processes Applied to Wave Load Predictions

The concept of conditional stochastic processes provides a powerful tool for evaluation and estimation of wave loads on ships and offshore structures. This article first considers conditional waves with a focus on critical wave episodes. Then the inherent uncertainty in the results is illustrated with an application where measured wave responses are used to predict the future variation in the responses within the next 5–30 seconds. The main part of the article is devoted to the application of the First Order Reliability Method for derivation of critical wave episodes for different nonlinear wave-induced responses. A coupling with Monte Carlo simulations is shown to be able to give uniform accuracy for all exceedance levels with moderate computational time, even for rather complex nonlinear problems. The procedure is illustrated by examples dealing with overturning of jackup rigs, parametric rolling of ships, and slamming and whipping vibrations.

Effects of Cnoidal Wave-Induced Seepage on Vertical Breakwater Resting on Permeable Elastic Seabed

Theoretical investigations on cnoidal waves interacting with breakwater resting on permeable elastic seabed are presented in this paper. Based on the shallow water reflected wave theory and Biot consolidation theory on wave-induced seepage pressure, the analytical solutions to first order cnoidal wave reflection and wave-induced seepage pressure are obtained by the eigenfunction expansion approach. Numerical results are presented to show the effects of depth of water, breakwater geometry on cnoidal wave-induced seepage uplift force and overturning moment. Compared with Airy wave theory, in certain shallow water conditions, the shallow water wave theory can more effectively illustrate wave nonlinearity effect in wave load prediction.

Cnoidal Wave-Induced Seepage Forces on Large Porous Vertical Circular Cylinder Resting on Porous Elastic Seabed

The influence of the porosity of the structure on the shallow water wave-Induced seepage force on the bottom of porous vertical circular cylinder resting on porous elastic seabed has been investigated. Based on the shallow water diffracted wave theory and Biot consolidation theory on wave-induced seepage pressure, the analytical solutions to first order cnoidal wave diffraction by porous vertical circular cylinder and wave-induced seepage pressure are obtained by the eigenfunction expansion approach. Numerical results show that cnoidal wave-induced uplift and moment may have same order of magnitude as the horizontal cnoidal wave force and moment , and the body porosity of the structure may lead to a reduction both in direct cnoidal wave forces and in the cnoidal wave induced seepage moment. Compared with Airy wave theory, in certain shallow water conditions, the shallow water wave theory can more effectively reflect wave nonlinearity effect in wave load prediction.