Effects Of Wave Loads Research Articles

Fatigue damage characteristics of a semisubmersible-type floating offshore wind turbine at tower base

Fully coupled analysis of a semisubmersible-type floating offshore wind turbine is carried out for the fatigue damage investigation at tower base by using the code FAST. By combining the axial force and the bending moment, the stress around the circumference can be calculated. This study investigates the effect of second order wave force, wind load, mooring model, and tower elasticity on the stress response at tower base. There are three peaks in the stress amplitude spectrum, corresponding to pitch resonance, wave frequency, and first-order natural mode of the tower, respectively. The first and third peaks are mainly induced by wind load. The effect of the second order wave force and mooring model on the stress seems to be neglectable based on the numerical results. By using the rainflow counting method, the fatigue damage is calculated. The obtained fatigue damage under several environmental conditions indicates that the misalignment and coupling effect of wind and wave loads may significantly affect the total fatigue damage. The wind and wave induced responses are calculated separately (uncoupled method). Next, two superposition methods for fatigue damage calculation are analyzed: fatigue damage superposition and stress superposition. Compared with the fully coupled analysis considering wind and wave together, the former gives lower prediction but the latter seems to provide acceptable results. Finally, considering the long-term wind and wave distribution, the fatigue damage under different environmental conditions is calculated, and the fatigue damage obtained from stress superposition method shows reasonable agreement with that obtained from a fully coupled analysis.

Nonlinear random motion analysis of coupled heave-pitch motions of a spar platform considering 1st-order and 2nd-order wave loads

In this study, we consider first- and second-order random wave loads and the effects of time-varying displacement volume and transient wave elevation to establish motion equations of the Spar platform’s coupled heave-pitch. We generated random wave loads based on frequency-domain wave load transfer functions and the Joint North Sea Wave Project (JONSWAP) wave spectrum, designed program codes to solve the motion equations, and then simulated the coupled heave-pitch motion responses of the platform in the time domain. We then calculated and compared the motion responses in different sea conditions and separately investigated the effects of second-order random wave loads and transient wave elevation. The results show that the coupled heave-pitch motion responses of the platform are primarily dominated by wave height and the characteristic wave period, the latter of which has a greater impact. Second-order mean wave loads mainly affect the average heave value. The platform’s pitch increases after the second-order low frequency wave loads are taken into account. The platform’s heave is underestimated if the transient wave elevation term in the motion equations is neglected.

Effects of transient slamming and harmonic swings on the marine compound NC machine tool

To study structural seakeeping stability of the marine compound NC machine tool in the ship environment, this research analyses the effects of extreme wave loads including transient slamming and harmonic swings on the machine tool, and the coupling methods of transient reaction, static structural analysis and harmonic response are used to solve the problems. The results show that the major guide pairs of the column are the weakest parts in the machine tool in a ship environment, so the column working deflection angle of the machine tool should be consistent with the ship heading angle to reduce influence of wave loads on dynamic stability of the machine. The data indicate that the minimum safety factor is 3.43 under an extreme surge acceleration of 41.8m/s2, and the normal safety factor of 3.0 shows that the guide pairs are able to endure extreme slamming wave loads of H1/3=7.6m at the maximum velocity of the ship. In addition, the machine tool has a maximum deformation of 0.032mm and a peak stress of 5.07MPa under extreme harmonic wave excitation. Importantly, the machine's cutting accuracy was suitable for the rapid manufacturing requirement to maintain a ship's broken components.

Comparison of wave load effects on a TLP wind turbine by using computational fluid dynamics and potential flow theory approaches

Tension Leg Platform (TLP) is one of the concepts which shows promising results during initial studies to carry floating wind turbines. One of the concerns regarding tension leg platform wind turbines (TLPWTs) is the high natural frequencies of the structure that may be excited by nonlinear waves loads. Since Computational Fluid Dynamics (CFD) models are capable of capturing nonlinear wave loads, they can lead to better insight about this concern. In the current study, a CFD model based on immersed boundary method, in combination with a two-body structural model of TLPWT is developed to study wave induced responses of TLPWT in deep water. The results are compared with the results of a potential flow theory-finite element software, SIMO-RIFLEX (SR). First, the CFD based model is described and the potential flow theory based model is briefly introduced. Then, a grid sensitivity study is performed and free decay tests are simulated to determine the natural frequencies of different motion modes of the TLPWT. The responses of the TLPWT to regular waves are studied, and the effects of wave height are investigated. For the studied wave heights which vary from small to medium amplitude (wave height over wavelength less than 0.071), the results predicted by the CFD based model are generally in good agreement with the potential flow theory based model. The only considerable difference is the TLPWT mean surge motion which is predicted higher by the CFD model, possibly because of considering the nonlinear effects of the waves loads and applying these loads at the TLPWT instantaneous position in the CFD model. This difference does not considerably affect the important TLPWT design driving parameters such as tendons forces and tower base moment, since it only affects the mean dynamic position of TLPWT. In the current study, the incoming wave frequency is set such that third-harmonic wave frequency coincides with the first tower bending mode frequency. However, for the studied wave conditions a significant excitation of tower natural frequency is not observed. The high stiffness of tendons which results in linear pitch motion of TLPWT hull (less than 0.02 degrees) and tower (less than 0.25 degrees) can explain the limited excitement of the tower first bending mode. The good agreement between CFD and potential flow theory based results for small and medium amplitude waves gives confidence to the proposed CFD based model to be further used for hydrodynamic analysis of floating wind turbines in extreme ocean conditions.

Time domain calculation of connector loads of a very large floating structure

Loads generated after an air crash, ship collision, and other accidents may destroy very large floating structures (VLFSs) and create additional connector loads. In this study, the combined effects of ship collision and wave loads are considered to establish motion differential equations for a multi-body VLFS. A time domain calculation method is proposed to calculate the connector load of the VLFS in waves. The Longuet–Higgins model is employed to simulate the stochastic wave load. Fluid force and hydrodynamic coefficient are obtained with DNV Sesam software. The motion differential equation is calculated by applying the time domain method when the frequency domain hydrodynamic coefficient is converted into the memory function of the motion differential equation of the time domain. As a result of the combined action of wave and impact loads, high-frequency oscillation is observed in the time history curve of the connector load. At wave directions of 0° and 75°, the regularities of the time history curves of the connector loads in different directions are similar and the connector loads of C1 and C2 in the X direction are the largest. The oscillation load is observed in the connector in the Y direction at a wave direction of 75° and not at 0°. This paper presents a time domain calculation method of connector load to provide a certain reference function for the future development of Chinese VLFS

Wave Loads Computation for Offshore Floating Hose Based on Partially Immersed Cylinder Model of Improved Morison Formula

Aimed at wave load computation of floating hose, the paper analyzes the morphologic and mechanical characteristics of offshore hose by establishing the partially immersed cylender model, and points out that the results of existing Morison equation to calculate the wave loads of floating hose is not precise enough. Consequently, the improved Morison equation has been put forward based on its principle. Classical series offshore pipeline has been taken as example which applied in the water area of different depth. The wave loads of pipeline by using the improved Morison equation and compared the calculation results with the existing Morison equation. Calculations for wave loads on pipelines in different depth were accomplished and compared by the improved Morison equation and the existing Morison equation. Results show that the improved Morison equation optimizes the accuracy of the computation of wave load on floating hose. Thus it is more suitable for analyzing the effects of wave loads on floating hose and useful for mechanic analysis of offshore pipeline.

Optimum Design of Bridge Cross Section with Low Clearance Considering Wave Load Effects Based on Numerical Wave-Tank

Zhang, X.; Chen X.; Huang, J.; Zhou, H., and Wang, Q., 2015. Optimum design of bridge cross section with low clearance considering wave load effects based on numerical wave-tank.Bridges with low clearance crossing sea are generally partially or fully submerged by surge or wave. The safety of bridge girder in the splash zones is closely related to wave impact. Using the 2D numerical wave-tank established by ANSYS FLUENT, authors make comparative analysis of the numerical simulation results of wave forces acting on three kinds of twin-deck girders (circular arc box girder, trapezoid box girder and T-shaped girder) of bridge with low clearance crossing sea. Wave forces on bridge girder under different circumstances were analyzed. The circumstances include different bridge deck elevation, different seabed elevation, and the influence of revetment. Then authors got economically reasonable, safe and reliable optimal bridge girder cross-section form. It proves that the box girder with arc-shaped bottom is considered to be the best form. For coastal twin-deck bridges with low clearance, there is an optimum deck elevation from sea level.

Experimental and numerical investigation of ship structure subjected to close-in underwater shock wave and following gas bubble pulse

One of the major topics confronted by the designers of naval vessels is to qualify the ship integrity exposed to underwater explosions (UNDEX). The far-field UNDEX & contact UNDEX problems are investigated by a few researchers previously. However, when a ship is subjected to close-in non-contact UNDEX environment, the failure mechanism of structures under the combined effect of shock wave load and bubble pulsations is rarely discussed. Thus, the dynamic response of ship structures subjected to close-in non-contact UNDEX is mainly concerned in this paper. Four UNDEX experimental tests of ship-type box structure (STBS, which can be fundamentally viewed as surface ships) are conducted in the study by using charge explosion. With reasonable finite element models, the coupled acoustic-structure algorithm in ABAQUS/Explicit code is utilized for the simulations. The wet vibration modes, accelerations, acoustic fluid pressure, and velocities from simulation results are compared with experimental data. The numerical simulation results agree well with those of experimental tests. Then, the detailed discussions are given to explore the damage processes of STBS subjected to the shock wave and bubble integrated loadings. The experiment and numerical simulation results reveal that global longitudinal strength collapse combined with the local wrinkling, the plastic deformation of hull between bulkheads and the plastic deformation of local bottom hull are the mainly three failure modes. And if the UNDEX bubble effects are not incorporated in the analysis, the damage severity will be underestimated. Moreover, other two practical numerical simulation examples are also discussed to illustrate the failure modes in depth.

On the Modeling of Spar-Type Floating Offshore Wind Turbines

In this paper an overview about floating offshore wind turbines (FOWT) including operating conditions, property and applicability of the barge, tension-leg, and spar floating platforms is described. The spar-floating offshore wind turbines (S-FOWT) have advantages in deepwater and their preliminary design, numerical modeling tools and integrated modeling are reviewed. Important conclusions about the nacelle and blade motions, tower response, effects of wind and wave loads, overall vibration and power production of the S-FOWT are summarized. Computationally-simplified models with acceptable accuracy are necessary for feasibility and pre-engineering studies of the FOWT. The design needs modeling and analysis of aero-hydro-servo dynamic coupling of the entire FOWT. This paper also familiarizes authors with FOWT and its configurations and modeling approaches.

Some aspects of structural modelling and restoring stiffness in hydroelastic analysis of large container ships

The increase in world trade has largely contributed to the expansion of sea traffic. As a result, the market demand is leading to ultra-large container ships (ULCS), with expected capacity up to 18,000 TEU (twenty-foot equivalent unit) and length about 400 m, without changes in the operational requirements (speed up to 27 knots). The particular structural design of the container ships leads to open midship sections, resulting in increased sensitivity to torsional and horizontal bending loads that is much more complex to model numerically. At the same time, due to their large dimensions, the structural natural frequencies of ULCS become significantly lower so that the global hydroelastic structural responses (springing and whipping) can become a critical issue in the ship design and should be properly modelled by the simulation tools since the present classification rules do not cover described operating stages completely. There are several research projects worldwide aiming at solving this problem, and one of them is the EU FP7 project TULCS (tools for ultra-large container ships) for development of the integrated design tools, based on numerical procedures, model tests and full-scale measurements. This paper is based on research activities and results of the project, with particular emphasis on the part that deals with global hydroelastic loading and response. Special attention is paid to beam structural model based on the advanced beam theory. It includes shear influence on bending and torsion, contribution of transverse bulkheads to hull stiffness and an appropriate modelling of relatively short engine room structure of ULCS. Along with that, a hydrodynamic model is presented in a condensed form. Further on, a fully consistent formulation of restoring stiffness, which plays an important role in the hydrostatic model, is described. Theoretical contributions are illustrated within the numerical example, which includes a complete hydroelastic analysis of an 11,400 TEU container ship. In this case, validation of the one-dimensional (1D) finite-element method (FEM) model is done by a correlation analysis with the vibration response of the fine three-dimensional (3D) FEM model. The procedure related to determination of engine room effective stiffness is checked by a 3D FEM analysis of a ship-like pontoon, which has been made according to the 7800 TEU container ship properties. The obtained results confirm that the sophisticated beam model is a very useful numerical tool for the designer and represents a reasonable choice for determining wave load effects on ULCS, in preliminary design stage.

파압에 의한 해안구조물-해저지반의 침하거동에 대한 수치해석

Seabed settlement underneath a coastal structure may occur due to wave loading generated by storm surge. If the foundation seabed consists of sandy soil, the possibility of the seabed settlement may be more susceptible because of generation of residual excess pore-water pressure and cyclic mobility. However, most coastal structures, such as breakwater, quay wall, etc., are designed by considering wave load assumed to be static condition as an uniform load and the wave load only acts on the structure. In real conditions, however, the wave load is dynamically applied to seabed as well as the coastal structure. In this study, therefore, a real-time wave load is considered and which is assumed acting on both the structure and seabed. Based on a numerical analysis, it was found that there exists a significant effect of wave load on the structure and seabed. The deformation behavior of the seabed according to time was simulated, and other related factors such as the variation of effective stress and the change of effective stress path in the seabed were clearly observed.

탄성 다물체계 동역학을 기반으로 한 부유식 해상 풍력 발전기 타워의 구조 해석

In this study, we perform the structural analysis of a floating offshore wind turbine tower by considering the dynamic response of the floating platform. A multibody system consisting of three blades, a hub, a nacelle, the platform, and the tower is used to model the floating wind turbine. The blades and the tower are modeled as flexible bodies using three-dimensional beam elements. The aerodynamic force on the blades is calculated by the Blade Element Momentum (BEM) theory with hub rotation. The hydrostatic, hydrodynamic, and mooring forces are considered for the platform. The structural dynamic responses of the tower are simulated by numerically solving the equations of motion. From the simulation results, the time history of the internal forces at the nodes, such as the bending moment and stress, are obtained. In conclusion, the internal forces are compared with those obtained from static analysis to assess the effects of wave loads on the structural stability of the tower.

Influence of sweeping detonation-wave loading on shock hardening and damage evolution during spallation loading in tantalum

Widespread research over the past five decades has provided a wealth of experimental data and insight concerning the shock hardening, damage evolution, and the spallation response of materials subjected to square-topped shock-wave loading profiles. However, fewer quantitative studies have been conducted on the effect of direct, in-contact, high explosive (HE)-driven Taylor wave (unsupported shocks) loading on the shock hardening, damage evolution, or spallation response of materials. Systematic studies quantifying the effect of sweeping-detonation wave loading are yet sparser. In this study, the shock hardening and spallation response of Ta is shown to be critically dependent on the peak shock stress and the shock obliquity during sweeping-detonation-wave shock loading. Sweeping-wave loading is observed to: a) yield a lower spall strength than previously documented for 1-D supported-shock-wave loading, b) exhibit increased shock hardening as a function of increasing obliquity, and c) lead to an increased incidence of deformation twin formation with increasing shock obliquity.

High-Strain-Rate Brain Injury Model Using Submerged Acute Rat Brain Tissue Slices

Blast-induced traumatic brain injury (bTBI) has received increasing attention in recent years due to ongoing military operations in Iraq and Afghanistan. Sudden impacts or explosive blasts generate stress and pressure waves that propagate at high velocities and affect sensitive neurological tissues. The immediate soft tissue response to these stress waves is difficult to assess using current in vivo imaging technologies. However, these stress waves and resultant stretching and shearing of tissue within the nano- to microsecond time scale of blast and impact are likely to cause initial injury. To visualize the effects of stress wave loading, we have developed a new ex vivo model in which living tissue slices from rat brain, attached to a ballistic gelatin substrate, were subjected to high-strain-rate loads using a polymer split Hopkinson pressure bar (PSHPB) with real-time high-speed imaging. In this study, average peak fluid pressure within the test chamber reached a value of 1584±63.3 psi. Cavitation due to a trailing underpressure wave was also observed. Time-resolved images of tissue deformation were collected and large maximum eigenstrains (0.03-0.42), minimum eigenstrains (-0.33 to -0.03), maximum shear strains (0.09-0.45), and strain rates (8.4×10³/sec) were estimated using digital image correlation (DIC). Injury at 4 and 6 h was quantified using Fluoro-Jade C. Neuronal injury due to PSHPB testing was found to be significantly greater than injury associated with the tissue slice paradigm alone. While large pressures and strains were encountered for these tests, this system provides a controllable test environment to study injury to submerged brain slices over a range of strain rate, pressure, and strain loads.

EFFECT OF WAVES ON THE BEHAVIOR OF OFFSHORE PILES

This paper studies the effect of wave loads on the submerged piles. The submerged piles are used widely in harbors, and offshore structures.The bottom part of the pile is buried in soil under the mud line while the upper part (free height of the pile above the mud line) is subjected to waves. The internal forces induced in the pile depend on many factors such as, pile diameter, soil type, water depth, wave period and height. A numerical model for a submerged pile was developed using a commercial Finite Element program to study the internal forces induced on submerged piles which subjected to wave action. The factors affecting the internal forces induced in the pile, due to the wave load, were studied. In the present research, range of different values for each factor was taken into consideration to establish the effect of each factor on the internal forces induced in the pile. The results were summarized in curves to highlight the effect of each factor.

Long-term fatigue analysis of multi-planar tubular joints for jacket-type offshore wind turbine in time domain

Long-term fatigue analysis of welded multi-planar tubular joints for a fixed jacket offshore wind turbine designed for a North Sea site in a water depth of 70 m is performed. The dynamic response of the jacket support structure due to wind and wave loads is calculated by using a decoupled procedure with good accuracy (Gao et al., 2010). Hot-spot stresses at failure-critical locations of each reference brace for 4 different tubular joints (DK, DKT, X-type) are derived by summation of the single stress components from axial, in-plane and out of plane action, the effects of planar and non-planar braces are also considered. Both a 2-parameter Weibull function and generalized gamma function are used to fit the long-term statistical distribution of hot-spot stress ranges by a combination of time domain simulation for representative environmental conditions in operational conditions of the wind turbine. A joint probabilistic model of mean wind speed U w , significant wave height H s and spectral peak period T p in the northern North Sea is used to obtain the occurrence frequencies of representative environmental conditions (Johannessen, 2002). In order to identify the contributions to fatigue damage from wind loads, wave loads and the interaction effect of wind and wave loads, 3 different load cases are analyzed: wind loads only; wave loads only; a combination of wind and wave loads. The representative environmental condition corresponding to the maximum contribution to fatigue damage is identified. Characteristic fatigue damage of the selected joints for different models is predicted and compared. The effect of brace thickness on the characteristic fatigue damage of the selected joints is also analyzed by a sensitivity study. The conclusions obtained in this paper can be used as the reference for the design of future fixed jacket offshore wind turbines in North Sea.

Shock response of 3D woven composites: An experimental investigation

A modified shock tube was used to determine the effect of shock wave loading on 3D woven composite panels . The shock wave, which produces a short duration steeply rising pressure pulse when impacting the panel, was used to load the panels. The out of plane deformation response was measured using a full field Digital Image Correlation (DIC) technique. The results allow for measurements of full field displacements and strains in the samples. Three distinct textile composite architectures, corresponding to different amounts of Z-fiber (fiber tows that bind the different textile layers together) were investigated. Two separate shock intensities were used. Matrix micro-cracking was observed to be the mechanism by which failure is initiated, and this micro-cracking was found to occur closest to the center of the panel where the outer-surface straining is highest. Fiber tow failure was absent in the shock strengths studied in the present work. The results suggest that the 6% Z-fiber architecture provided the largest panel stiffness and the least amount of damage. This result suggests that this may be the optimal architecture and density for orthogonally woven Z-fiber reinforced composites, however due to the complex nature of the problem the same architecture with a different tow (and fiber) volume fraction may yield different results.

Damage Assessment of Structures due to the 2006 Java Tsunami

The paper describes damage assessment of structural components such as wall and column in a masonry infilled house due to a tsunami, assessing the damage data of structures in the south coast of Java due to the 2006 Giant Earthquake and Tsunami. The effect of wave loads with failure mode of subject structural components was verified, inversely computing the dominant parameter ‘α’ representing the amplification of tsunami wave pressure against a structural component. It was found that its crack mode might occur when parameter α takes the value ranging from 4 to 5, and that major failure mode such as collapse or shear crack might occur although parameter a takes more than 3 dependent upon tsunami affection.

반강접 접합부를 적용한 초대형 부유식 구조물 상부구조체에 대한 동적해석

초대형 부유식 구조물의 경우 지진하중보다 파랑하중에 의한 영향이 크게 작용하기 때문에 파랑하중에 의한 하부부체의 변형이 상부구조물에 부가모멘트를 발생시키는 요인이 된다. 이러한 부가모멘트를 저감시키기 위해 본 연구에서는 강접합과 핀접합 사이의 거동을 하는 반강접 접합부를 적용하였다. 초대형 부유식 구조물의 상부구조체에 반강접 접합부를 적용할 경우 보의 부가모멘트를 감소시킬 수 있으며, 더욱 경제적인 설계가 가능하다. 본 논문에서는 4경간 3층 예제구조물에 대하여 파랑하중에 의한 영향을 분석하고, 구조물의 반강접 접합부 적용에 따른 효율성을 검토하였다. 접합부는 각형강관 외-다이아프램 접합부를 적용하였으며 파랑하중에 의한 동적 특성을 분석하기 위하여 시간 이력해석을 수행하였다. 초대형 부유식 구조물의 상부구조물의 경우 파랑하중에 의해서 정하중의 최대모멘트 응답이 강접 구조물에서는 <TEX>$33\%$</TEX> 증가하였으며, 스프링 모델을 이용한 반강접 구조물에서는 <TEX>$26\%$</TEX> 증가하였다. The additional moment occurs because the superstructures of VLFS are influenced by wave loads instead of earthquake loads. In order to reduce the additional moment, this study used the semi-rigid connections which lie between fully rigid and pinned. If the semi-rigid connections are used for superstructures of VLFS, the moment of beams can be reduced and more economical construction will be possible. This study aims to show the effect of wave loads on structure and the efficiency of the semi-rigid connections due to wave loads by analyzing the time history responses. The dynamic behaviors of the rigid frame are compared with those of the semi-rigid frame considering of static loads, wave loads and combination loads for a four-bay, three-story frames. The semi-rigid connection type is a steel tubular column with square external-diaphragm connections and the time history analysis is used for the dynamic responses. The additional moment responses due to wave loads increase <TEX>$33\%$</TEX> in the rigid frame, <TEX>$26\%$</TEX> in the semi-rigid frame with the spring model.

Combination of global still-water and wave load effects for reliability-based design of floating production, storage and offloading (FPSO) vessels

The purpose of this paper is to establish probabilistic models for still-water loads, based on design data, and the combined still-water and wave load effects for semi-probabilistic and probabilistic design of floating production, storage and offloading vessels (FPSO). A new still-water load model for FPSOs is proposed, based on a Poisson square-wave model, with a modified Weibull distribution for load intensity, which accounts for load control during operation. The long-term variation of wave-induced load effects is modelled by a Poisson square-wave process. A new solution for the combined effect is derived. A procedure for determining characteristic extreme values for individual and combined load effects, and load combination factors, is established. The methodology is used to illustrate load combination factors suitable for typical FPSOs. This approach is also shown to be useful in obtaining realistic load models, in terms of random variables, for use in reliability formulations.