Effects Of Wave Loads Research Articles

Numerical and experimental study on the scale effect of internal solitary wave loads on spar platforms

Based on laboratory experiments and numerical simulations, the scale effect of Internal Solitary Wave (ISW) loads on spar platforms is investigated. First, the waveforms, loads, and torques on the spar model at a laboratory obtained by the experiments and simulations agree well with each other. Then, a prototype spar platform is simulated numerically to elucidate the scale effect. The scale effect for the horizontal forces is significant owing to the viscosity effect, whereas it is insignificant and can be neglected for the vertical forces. From the similarity point of view, the Froude number was the same for the scaled model and its prototype, while the Reynolds number increased significantly. The results show that the Morison equation with the same set of drag and inertia coefficients is not applicable to estimate the ISW loads for both the prototype and laboratory scale model. The coefficients should be modified to account for the scale effect. In conclusion, the dimensionless vertical forces on experimental models can be applied to the prototype, but the dimensionless horizontal forces of the experimental model are larger than those of the prototype, which will lead to overestimation of the horizontal force of the prototype if direct conversion is implemented.

Distributed plasticity approach for the nonlinear structural assessment of offshore wind turbine

This study provides an insight of the nonlinear behavior of the Offshore Wind Turbine (OWT) structure using the distributed plasticity approach. The fiber section beam-column element is applied to construct the finite element model. The accuracy of the proposed model is verified using linear analysis via the comparison of the dynamic characteristics. For collapse risk assessment of OWT, the nonlinear effects considering the earthquake Incident Angle (IA) have been evaluated first. Then, the Incremental Dynamic Analysis (IDA) has been executed using a set of 20 near-fault records. Lastly, fragility curves are developed to evaluate the vulnerability of structures for different limit states. Attained results justify the accuracy of the proposed approach for the structural response against the ground motions and other environmental loads. It indicates that effects of static wind and wave loads along with the earthquake loads should be considered during the risk assessment of the OWT structure.

The interaction of laser radiation with tissue in the aspect of generating the process of decellularization in the preparation of animal origin autologous tissue

The aim of the work was to create an appropriate substrate for organ transplantation using bioactive tissue-based scaffold populated by cells of the graft recipient. The purpose of the modeling was to investigate the mechanical effects of wave loading of aortic and pulmonary tissue material. The biological properties of tissues of aortic and pulmonary valves were modified by the process of decellularization. The host cells were removed by various physical methods with focus on minimal degradation of the extracellular matrix. Thus, the decellularization process was controlled by histological methods. The tissue decellularization process was simulated by finite element modelling. The mechanical results represented by a displacement at the center of the sample were coherent and the heterogeneity of the distribution of the caves on the surface of the samples was confirmed, both by experiment and in the simulation by the alternate occurrence of local minima and maxima. The latter results from the uneven removal of cells from the effect of the wave causing decellularization were also predicted by the numerical model. Laser radiation had a destructive effect on the components of the extracellular matrix (e.g., collagen and elastic fibers), mainly depending on the fluence and number of pulses in a single exposure. The differences between the valve tissue materials were shown, and the impact of the process of decellularization on the properties of the tissues was analyzed. It should be emphasized that due to low absorption and high scattering, laser radiation can deeply penetrate the tissue, which allows for effective decellularization process in the entire volume of irradiated tissue.

Uplift soil resistance to a shallowly-buried pipeline in the sandy seabed under waves: Poro-elastoplastic modeling

A reasonable evaluation of the vertical soil resistance to a buried pipeline is crucial for upheaval buckling analyses. Under wave troughs, the effective stress in the seabed could be reduced significantly due to the upward seepage caused by vertical pore-pressure gradients. To investigate the influence of wave loading on the vertical soil resistance to a buried pipe, a plane-strain poro-elastoplastic model is proposed and verified with the existing pipe-soil interaction test results and DNV GL predictions. The model is capable of sequentially simulating the pore-pressure generation in a sandy seabed under waves and the plastic-zone development while uplifting the pipe. Numerical results show that the presence of wave troughs could not only reduce the soil effective stress, but also generate an additional uplift force on the buried pipe due to non-uniform distribution of pore-pressures along the pipe periphery. The normalized additional uplift force generally increases linearly with normalized amplitude of wave pressures at seabed mudline. Parametric study is then performed to examine the influence of wave parameters and soil properties. Moreover, to qualitatively characterize the effect of wave loading, a resistance-reduction coefficient is introduced and found to decrease linearly with the increase of normalized amplitude of wave pressure at seabed mudline.

Research on the dynamical responses of H-type floating VAWT considering the rigid-flexible coupling effect

Offshore floating wind turbines are classified into floating horizontal axis wind turbines (HAWTs) and floating vertical axis wind turbines (VAWTs) according to the orientation of rotating shaft. Compared with the traditional floating HAWTs, main advantages of floating VAWTs are the independence of wind direction and the lower position of power generating system, which reduce the installation and maintenance costs. Dynamical characteristics of floating VAWTs were widely studied in the preceding researches by tackling the wind turbine as a rigid body. However, the increasing size of wind turbine is beneficial to achieve higher cost performance. The elastic deformations of slender structures including blades and tower should be considered when the dynamical characteristics of the floating wind turbine are studied. In this paper, rigid-flexible coupling equations were established to investigate the dynamic behavior of H-type floating VAWT based on multi-body kinematics and dynamics. Natural motion performances of the wind turbine structures were presented and the phenomenon of “dynamic stiffening” was analyzed. Motions of the floater and vibrations of the tower and blades were computed under complex environmental loads. Time domain and frequency domain simulations were carried out to study the dynamic response of floater, tower and blades, respectively. Besides, the effect of aerodynamic loads and wave loads on the floating VAWT was researched. Load transfer performance through rigid and flexible structures was investigated.

Comparative Study of Effect of Wind and Wave Load on Floating PV: Computational Simulation and Design Method

Comparative Study of Effect of Wind and Wave Load on Floating PV: Computational Simulation and Design Method

A study on fully nonlinear wave load effects on floating wind turbine

The influence of fully nonlinear wave effects on floating wind turbine has been studied in this paper by comparing both floater motions and structural responses of tower base and mooring lines exposed to linear and fully nonlinear long-crested irregular waves. Wave kinematics of the linear and nonlinear wave are calculated in a 2D Harmonic Polynomial Cell wave tank. The wave kinematics are further processed by a polynomial fitting method to scale down the data size so that it fulfills the memory requirement of HAWC2 where coupled dynamic analysis is carried out. The external DLL used to provide wave kinematics to HAWC2 is extended from one dimensional (Wkin.dll 2.4) to two dimensional (Wkin.dll 2D) wave field so that fully nonlinear wave can be implemented on floating wind turbine through reading pre-generated wave kinematics manner. The whole work procedure including wave generation, polynomial fitting and implementation in HAWC2 has been verified with a linear regular wave case. Two extreme irregular wave conditions are focused to study the nonlinear wave effects regarding critical responses, such as wave elevation, floater motions and mooring line tension. The results have not only proved the accuracy and applicability of the polynomial fitting method and the extended Wkin.dll 2D for HAWC2 but also revealed the importance to consider fully nonlinear wave model in hydrodynamic analysis compared with linear wave theory especially for high sea states in shallow and intermediate water. As a result of development of computer capacity and numerical wave tank, this paper has demonstrated that fully nonlinear wave effect can be considered in an engineering manner with acceptable efficiency.

An innovative mooring system for floating storage tanks and stochastic dynamic response analysis

A modular floating concept for oil storage is proposed. The concept is composed of modular floating tanks and barges. The floating tanks are self-stable and designed for storage of oil and hydrocarbon material. The barges are connected together and moored by mooring dolphins. Mooring system is designed between barges and the tanks enclosed by the barges. An innovative mooring system that consists of several mooring ropes besides the fender system is proposed and investigated in this study. The mooring ropes are stretchy and can carry the mooring loads in all the loading and environmental conditions. Single floating tank is investigated under different filling levels, i.e. empty, full and partially filled conditions. It is assumed in this study that the barges are fixed, and multi-body hydrodynamic interaction is neglected. Linear and nonlinear mooring rope properties are investigated. Failure condition of one of the mooring rope is also considered. Wind, wave and current load effects under 100-year storm are also investigated, as well as the uncertainty analysis in terms of viscous effects.

Analysis of Offshore Wind Turbine Towers with Different Designs by Finite Elements Method

It is known that the use of renewable energy has an increasing trend in whole world. Wind energy is one of the renewable energy types, as well is among the cleanest and most economical energy sources. Nowadays, in order to provide much more energy from wind, turbine towers are being built higher and the turbine blades have begun to be manufactured longer. Due to these applications, tower and turbine weights are continuously increasing. For this reason, it is necessary to optimize the materials used as well as the dimensions of the turbine towers. In the present study, behavior of TLP floating wind turbine towers with three different designs under wave, hydrostatic and static loads were investigated. In order to clarify the effect of these loads, turbine designs were analyzed in the ratio of 1/5 using finite elements method. Steel, reinforced concrete and hybrid (reinforced concrete and steel) wind turbine towers tied to sea floor at a depth of 10 meters rigidly by TLP floating method. In this context, 10-meter-high turbine towers having three different designs which static analyzed previously were used for investigate effects of wave and hydrostatic loads. Turbine structures analyzed with ABAQUS finite elements model. The deformations and stress values of underwater turbine structures were obtained and compared with each other. As it can be seen from analysis results, compared to the reinforced concrete design, the displacement of steel tower design decreased 77.84%. It is seen that the torsion effect was dominant in the steel tower design. However, the decreasing displacement value for steel design was recorded as 44.43% compared to the hybrid tower design.

Effect of Wave Load on the Member Force of Steel Structure of Floating Buildings

For floating buildings may float on the water for a long time, they are constantly affected by various environmental loads such as wind and wave loads. In this study to find the wave effect on the floating building, five models are designed using steel moment resisting frame. It is assumed that the lower part of the floating building is a reinforced concrete pontoon, while the upper part is a three-story steel frame. To analyze floating buildings affected by wind and wave loads, hydro-dynamic and substructure analysis are performed. As input loads, this study set limits that the mean wind velocity is 35 m/s and the significant wave height is 0.5 m for the residential building. From the hydrodynamic analysis, the time-history acceleration of building is obtained and transformed into a base ground input for a substructure analysis of the superstructure of the building. Finally the mean of the maximum from 30 dynamic analysis of the floating buildings are used to be compared with the results of the same model on the ground. It was shown that the dynamic results with wind and wave loads are not always lesser than the static results which are calculated with static equivalent wind load for a building that is located on the ground.

Research on Dynamic Response Characteristics of 6MW Spar-Type Floating Offshore Wind Turbine

A 6MW spar-type floating offshore wind turbine (FOWT) model is put forward and a fully coupled aero-hydro-servo-elastic time domain model is established in the fatigue, aerodynamics, structures and turbulence (FAST) code. Influence rules of wind load and wave load on the characteristics of 6MW spar-type FOWT are investigated. Firstly, validation of the model is carried out and a satisfactory result is obtained. The maximal deviations of rotor thrust and power between simulation results and reference values are 4.54% and −2.74%, respectively. Then the characteristics, including rotor thrust, rotor power, out-of-plane blade deflection, tower base fore-aft bending moment, and mooring line tension, are researched. The results illustrate that the mean value of dynamic response characteristics is mainly controlled by the wind-induced action. For characteristics of tower base fore-aft bending moment and platform pitch motion, the oscillation is dominated by the wave-induced action during all conditions considered. For characteristics of out-of-plane blade tip deflection and mooring line tension, the oscillation is commanded by combination effect of wave and wind loads when the wind speed is lower than the rated wind speed (hereinafter referred to as below rated wind speed) and is controlled by the wave-induced action when the wind speed is higher than the rated wind speed (hereinafter referred to as above rated wind speed). As to the rotor thrust and power, the oscillation is dominated by the wind induced action at below rated wind speed and by the combination action of wind and wave loads at above rated wind speed. The results should be useful to the detailed design and model basin test of the 6MW spar-type FOWT.

Bivariate Extreme Value Statistics of Offshore Jacket Support Stresses in Bohai Bay

This paper presents a generic Monte Carlo-based approach for bivariate extreme response prediction for fixed offshore structures, particularly jacket type. The bivariate analysis of extremes is often poorly understood and generally not adequately considered in most practical measurements/situations; that is why it is important to utilize the recently developed bivariate average conditional exceedance rate (ACER) method. According to the current literature study, there is not yet a direct application of the bivariate ACER method to coupled offshore jacket stresses. This study aims at being first to apply bivariate ACER method to jacket critical stresses, aiming at contributing to safety and reliability studies for a wide class of fixed offshore structures. An operating jacket located in the Bohai bay was taken as an example to demonstrate the proposed methodology. Satellite measured global wave statistics was used to obtain realistic wave scatter diagram in the jacket location area. Second-order wave load effects were taken into account, while simulating jacket structural response. An accurate finite element ANSYS model was used to model jacket response dynamics, subject to nonlinear hydrodynamic wave and sea current loads. Offshore structure design values are often based on univariate statistical analysis, while actually multivariate statistics is more appropriate for modeling the whole structure. This paper studies extreme stresses that are simultaneously measured/simulated at two different jacket locations. Due to less than full correlation between stresses in different critical jacket locations, application of the multivariate (or at least bivariate) extreme value theory is of practical engineering interest.

Numerical analysis of interaction between seabed and mono-pile subjected to dynamic wave loadings considering the pile rocking effect

Pile rocking is the motion of a mono-pile foundation as a result of cyclic loads caused by wind and wave, and the effect of upper structures such as offshore wind turbines. The rocking effect further complicates the interaction between a mono-pile and the seabed and results in instability of offshore structures. In this research, a three-dimensional integrated numerical model, combining the effect of wave loads with the seabed and mono-pile, is adopted to investigate wave-structure-seabed interaction (WSSI) considering the pile rocking effect. The present model is validated by available laboratory experiment data and analytical results. In addition, a detailed comparison with a non-rocking single pile is conducted to examine the effect of pile rocking on soil response more clearly. Finally, the momentary liquefaction depth is investigated and a parametric study on pore pressure response has been carried out. Results indicate that local pressure enlargement occurs near the mudline and the rate of pore pressure decrease in depth is slowed down due to pile rocking, leading to a relatively high pore pressure in the upper soil range. Furthermore, the pile rocking effect has a significant influence on the liquefaction distribution and enlarges the liquefaction zone in the vicinity of mono-pile foundation.

Numerical and experimental investigation into the dynamic response of a floating wind turbine spar array platform

The station keeping ability of a platform is the most fundamental guarantee to securing the dynamic stability of a Floating offshore wind turbine (FOWT) when being subjected to complex marine environment. These are usually evaluated in terms of rigid body dynamic response of a floating platform which supports whole FOWT. To investigate the dynamic response of array of FOWT, we put forward the concept of a large FOWT farm which consists of nine spar-type wind turbine array and meanwhile shares mooring line system by connecting those FOWT to one another. The whole FOWT is established based on OC3-Hywind spar buoy platform with NREL 5MW wind turbine. Considering effects of wind, wave and current loads, the dynamic response of the proposed FOWT was conducted based on FEM software ANSYS AQWA under both operational condition and survival condition. Besides, comparisons of dynamic response between the original single FOWT and the proposed FOWT farm were also made. Several interesting conclusions were gained through the investigation. For the original single FOWT, RAO in surge, heave and pitch are concentrated in low frequency, while the peak value of frequency is about 0.2 rad/s. In addition, FOWT (P1, P3, P7 and P9) located on the four vertices of 3×3 have higher sway motion which range from about -0.15 m~ 0.15 m, while sway motion of the rest can be neglected. With the worsening of environmental condition, surge motion of the original single FOWT increases significantly, while it’s not significant for the proposed FOWT farm. At the same time, statistic values of pitch motion of the proposed FOWT farm are quite close to those of the original single FOWT. While, under the survival condition, range of pitch motion of the proposed FOWT farm is significantly smaller than that of the original single FOWT. In all, the results verify the effectiveness of the proposed FOWT farm.

Effect of cyclic wave loading on scouring stability of geotube dams

Cyclic wave loading on geotube dams creates a cyclic flow in seams between geotubes regime distinct from the uni-directional flow that has been previously studied. A new laboratory apparatus was used to investigate the effects of hydraulic conditions on the core sand of geotube dams with seams between tubes. With this apparatus, a series of tests was conducted. Various maximum pressure heads (0.25, 0.3, 0.35, and 0.4 m), periods of cyclic wave loading (6, 7, 8, and 10 s) and particle porosities (34%, 35%, 40%, and 45%) were used in this test series. In the test series, the erosion development process of sand was analyzed. The results also indicate that the sand in the dam core can still perform satisfactorily after being scoured into the seam and extending a certain length. This critical length was found to be a function of the sand porosity, cycle period and maximum pressure head.

Modeling of submarine initial pipe-laying process and its real-time semi-physical virtual reality system

The mathematical model of the initial pipeline and cable process is created with regard to the ocean current load and wave load; and the mathematical model is solved by the numerical quasi-Newton method to investigate the effects of the current and wave load on the shape and force of the pipeline and cable during the initial pipe-laying process. On this basis, semi-physical simulation system is built to visualize the initial pipe-laying process. The simulation results are compared with OFFPIPE’s results to verify the mathematical model. The current and wave load effects on the shape and tension of the pipeline and cable are analyzed. This real-time virtual reality system will help engineering project in risk management and prediction as well as staff training, which is of vital significance for minimizing the risk of the actual pipe laying and improving the efficiency of the pipe-laying work.

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.

Research on analytical method of fatigue characteristics of Soft Yoke Mooring System based on full-scale measurement

By focusing on the vulnerability of the structure of marine equipments, together with considering the randomness of meta-ocean load in statistics, a kind of analytical method of fatigue characteristics of marine structure based on full-scale and actual measurement of prototype is proposed. On the basis of short-term field measurement results of structural response, research is carried out on the fatigue analysis of hinge joints at the upper part of the Soft Yoke single point Mooring System (SYMS) by simultaneously monitoring the environmental load and considering the design criteria of offshore structure. Through analysis of finite element modeling, the time-histories of typical stress response are obtained, and then the assessment of fatigue damage at key hinge joints is conducted. The simulation results indicate that the proposed method can accurately analyze the fatigue damage of offshore engineering structure caused by the effect of wave load. The present analytical method of fatigue characteristics can be extended on other offshore engineering structures subjected to meta-ocean load.

Effects of wave loads on the horizontal bracing strength of a semi-submersible platform

The effect of hydrodynamic loads from ocean waves on the strength of horizontal brace structures in semi-submersible platforms was studied. Fifteen different wave loads were considered, including seven used by the classification societies the American Bureau of Shipping and Det Norske Veritas. The results indicate that the torsional moments and shearing forces generated by the waves are the main loads affecting the strength of horizontal brace structures. The eight wave loads not used by the classification societies were found to have a significant effect on the strength of the brace structures and thus need to be considered when designing such structures.

Stability and dynamic response analysis of a submerged tension leg platform for offshore wind turbines

This paper proposed a submerged tension leg platform (STLP) for an offshore wind turbine in moderate water depth (70–150m). During the transportation, the platform is semi-submersible and self-stabilized because it has a relatively large water plane area. Therefore, it can be wet-towed out together with the wind turbine from the quayside to the offshore installation site. During the operation phase, the platform is submerged with a relatively small water plane area, which improves its hydrodynamic performance. Hence, this STLP wind turbine requires low transportation and installation cost. It can also achieve a potentially good dynamic behavior during the operation phase. In this paper, the stability of the STLP wind turbine during the transportation phase is first assessed without considering the mooring lines. The results show that the STLP wind turbine has a good stability to ensure a safe wet towing. The dynamic responses of the STLP wind turbine during the operation phase are then studied with emphasis on the effect of second-order wave loads, wind-wave misalignment and water depth. Based on fully coupled time domain simulations, it is found that the effect of second-order wave loads on the dynamics of the STLP wind turbine is slightly larger in an extreme sea state than that in a moderate sea state. The standard deviations of the surge, sway, roll and pitch motions and the tower base bending moment are dependent on the wind-wave misalignment, while those of the heave and yaw motions, the blade root bending moment are not. In addition, a larger water depth leads to larger standard deviations of the platform motions and a smaller standard deviation of the tower base bending moment. The effect of water depth on the blade root and tower top bending moments is negligible.