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Abstract It has recently been pointed out that the vertical shear stresses at pile-soil interface play an important role in static responses of offshore monopiles with large diameters, and traditional p - y curves cannot fully capture this influence. However, the influences of the vertical shear stresses on the dynamic response of monopiles are still not clear. This is the main focus of this study. The coupled horizontal and rocking vibration of a monopile embedded in a fully saturated poroelastic seabed is studied in this paper. The focus of the study is the difference between the responses with and without the vertical shear stresses on the pile. The monopile dynamic response is studied by the integral equation method, with both 3D elastodynamic theory and Euler-Bernoulli beam theory. Selected numerical results for highlighting the influence of the vertical shear stresses on the dynamic contact load distributions, pile displacements, and lateral dynamic impedance functions are examined for different length to diameter (l/d) ratios as well as poroelastic materials and frequencies of excitation. These results confirm that exclusion of the vertical shear stresses will lead to potentially very conservative design for monopiles with l / d < 6 in soft soil, and monopiles with l / d < 10 in medium to stiff soils. The computed dynamic responses of the monopile when the vertical shear stress is ignored can be up to 30% larger than the case when the vertical shear stress is considered. Highlights • The influence of vertical shear stresses on the dynamic responses of monopiles is studied. • A rigorous integral equation method is used. • Exclusion of vertical shear stresses will lead to a potentially very conservative design. • When the vertical shear stress is ignored, dynamic responses can be up to 30% larger. [ABSTRACT FROM AUTHOR]

Abstract In this study, an efficient time-domain prediction model is developed to predict unsteady flow vortex-induced vibrations (VIV) of flexible risers. The hydrodynamic forces on flexible risers are calculated on the basis of forced oscillation experiments on rigid cylinders. A period identification criterion, based on the spatial and temporal variations of reduced velocity, is proposed to divide the entire vibration process into exciting and damping periods of each exited mode. In exciting periods, assuming that VIV enters an ideal lock-in stage, a non-iterative solving model is established under modal space for response calculations, which efficiently predicts time domain VIV responses. In damping periods, free-decay vibration theory based recurrence formulas are established under modal space, and they get solved stepwise for modal responses. After some slight response adjustments to smooth period transitions, the VIV response time history can be obtained efficiently. This model is validated by steady flow VIV prediction cases, and further applied to predict oscillatory flow VIV experimental results. The prediction cases reveal that this model is able to realize high-speed VIV predictions with satisfactory results and no convergence problems. This model, with high efficiency and stability, is highly suitable for unsteady flow VIV prediction in engineering applications. Highlights • An efficient time-domain prediction model is proposed for high-speed VIV predictions under unsteady flows. • A period identification criterion is developed to distinguish exciting- and damping-dominant periods for each exited mode. • In exciting periods, a non-iterative response solver is further developed to calculate time-domain VIV responses. • In damping periods, free-decay vibration theory based recurrence formulas are established for response calculations. • The model is validated by experimental results in steady flow and oscillatory flow VIV prediction cases. [ABSTRACT FROM AUTHOR]

Abstract The paper presents a comparison of two vibration-based methods for the damage detection of a laboratory scale model of a tripod. Tripods are a part of the supporting structures for offshore wind turbines. The tripod model structure allows the investigation of the propagation of a circumferential representative crack in one of the cylindrical upper braces of the tripod itself. The first damage detection method addresses the use of acceleration signals in a genuine experimental modal analysis (i.e. input-output modal analysis) while the second one is based on operational modal analysis (i.e. output only modal analysis). The progressive damage is monitored by the calculation of the modal parameters and following their deviations. Both methods were performed on the undamaged and damaged structure for different support conditions and excitations (shaker, hammer, in water basin under wave excitation). The results suggest that both the methods can be considered useful tools for damage detection in dry and in-water conditions for offshore support structures. The presented technique proves to be effective for detecting and assessing the presence of representative cracks. Highlights • Vibration-based methods for damage detection of wind turbine tripod. • Experimental modal analysis of a damage tripod in dry condition. • Operational modal analysis of a damage tripod with wave motion in a tank. • Useful tools for damage detection in dry and in-water conditions for tripod. [ABSTRACT FROM AUTHOR]

Abstract Power flow method (PFM) is firstly applied to the structural analysis of autonomous underwater vehicle (AUV). By finite element analysis, the numerical model of AUV is established. The propulsive force of the propeller is calculated and the dynamic responses of AUV are discussed. The characteristics of power flow distribution in AUV are studied, and the optimization of AUV shape by PFM is developed. The results show that the performances of AUV's anti-vibration are improved. The present work may lay the foundation for the propagation characteristics of structural vibration power flow under the multi-source load excitations, and also provide a novel optimization approach for AUV shape. Highlights • Power flow method (PFM) is firstly applied to the structural analysis of autonomous underwater vehicle (AUV). • The distribution characteristics of vibration power flow of AUV are investigated. • The scheme of optimizing the AUV shell by PFM is proposed. • The performances of AUV's anti-vibration are improved. [ABSTRACT FROM AUTHOR]

The effects of surface roughness on the vortex-induced vibration (VIV) performances of a circular cylinder were studied numerically. The VIV response amplitude, response frequency, vortex force, vortex phase and vortex shedding flow pattern with different degrees of surface roughness were compared. The numerical results show that the VIV response amplitude decreases with increased surface roughness. For a smooth cylinder and a cylinder with small surface roughness, the VIV response could be divided into three branches: initial branch, upper branch and lower branch. In the initial and upper branches, the vortex shedding flow pattern displays a 2S mode. However, the VIV response produces a 2P mode vortex shedding pattern in the lower branch. For a cylinder with large surface roughness, the VIV response can be divided into only two branches: an initial branch and a lower branch. In the initial branch, the vortex shedding flow pattern displays a 2S mode, but in the lower branch, the pattern changes to the 2P mode. The transition of 2S mode to 2P mode is caused by the abrupt change of the vortex phase between the vortex force and the VIV response. [ABSTRACT FROM AUTHOR]

According to Jauvtis and Williamson's experiment, when the mass ratio is 2.6, the maximum transverse amplitude of the cylinder reaches 1.5 D in the super-upper branch. Many researchers have tried to capture the maximum transverse amplitude in numerical simulation. However, according to the existing references, few of the numerical results can reach such amplitude and the maximum vibration amplitudes are obviously smaller than the experimental values. In order to get more accurate results, a modified SST turbulence model is applied for the numerical simulations based on OpenFOAM. The influence of the magnitude of inflow acceleration in numerical simulation is also investigated. Firstly, the performance of modified SST model is tested by example of flow around a circular cylinder. Secondly, the appropriate inflow acceleration magnitude is determined by analyzing the numerical response under different acceleration magnitudes, finding that the inflow acceleration must be less than 0.017 per normalization time in order to capture the maximum transverse amplitude. Then, the two degrees of freedom vortex induced vibration of a cylinder with a mass ratio of 2.6 and reduced velocity from 2 to 14 is simulated. The numerical results are compared in detail with the experimental data and the maximum transverse amplitude, mutations of phase angle between lift force and displacement, the characteristics and change trends of the trajectory as well as the “2T” vortex model are captured clearly. The results show that the maximum transverse amplitude can be captured more accurately with modified SST turbulence model and appropriate inflow acceleration value. [ABSTRACT FROM AUTHOR]

Offshore wind turbines in regions of active seismicity are under a potential threat caused by the joint earthquake and ocean wave action. Taking NREL-5MW monopile wind turbine as the prototype, this study is devoted to probe the joint action of strong earthquakes and moderate sea conditions. A series of shake table model tests of scale 30:1 have been carried out by the contemporary world unique facility (Earthquake, Wave and Current Joint Simulation System) to investigate the structural response in dry flume, low and high calm water levels, with and without regular or random waves. Particular interest has been in the nacelle acceleration subjected to strong ground motions as well as to random waves. The experimental results of nacelle peak accelerations and corresponding dynamic amplification factors show that the joint earthquake and wave action is important for the proper evaluation of structural response. Ignoring the effect of wave action in seismic analysis will lead to underestimation of structural response, especially when the monopile foundation is dynamically sensitive. The coupling of earthquake and wave actions is sourced not only from the relative velocity of vibration but also from the initial condition induced by waves. It has been also derived that the peak acceleration excited by a moderate sea condition is comparable to that by a moderate earthquake. [ABSTRACT FROM AUTHOR]

Vortex-induced vibration (VIV) of a flexible cylinder in oscillatory flow was experimentally investigated in an ocean basin. An intermittent VIV was confirmed to have occurred during the tests. The fatigue damage caused by VIV was calculated based on rainflow counting and a standard S–N curve. There are 3 main observations for fatigue damage from VIV in oscillatory flow: 1) the damage varied significantly with the KC number, which is a unique feature for VIV in oscillatory flow. 2) Fatigue damage at small KC number cases was found to be larger compared to those at large KC numbers owing to the fact that number of vortex shedding cycles per half of the motion cycle is low, and damping within half of the motion cycle will hence become low. The fact that vortices from the previous cycle still are active during the next may also contribute to the large response at small KC numbers. 3) ‘Amplitude modulation’ and ‘mode transition’, two specific features for VIV in oscillatory flow, were found to have a strong influence on fatigue. Fatigue damage has also been calculated by an empirical VIV prediction model assuming that all cases have steady flow at an equivalent velocity. Finally, a simplified method for calculating fatigue damage from VIV in oscillatory flow based on steady flow conditions is proposed. A modification factor diagram is presented, but the scope of the present study is too limited to provide a good basis for a general model for this factor. A general model for how to apply results from constant current analysis to predict fatigue in oscillatory flow will therefore need further research. [ABSTRACT FROM AUTHOR]

A large-scale model test of a truncated steel catenary riser (SCR) was performed in an ocean basin to investigate the vortex-induced vibration (VIV) and its fatigue damage under pure top vessel motion. The top end of the test model was forced to oscillate at given vessel motion trajectories. Fiber Bragg grating (FBG) strain sensors were used to measure both in-plane and out-of-plane responses. Four different factors have been discussed to understand the VIV responses and fatigue damage results: instantaneous shedding frequency, touch down point (TDP) variation, tension variation and traveling waves. Out-of-plane VIV associated with strong time-varying features was confirmed to have occurred under pure vessel motion. Both KC number and maximum shedding frequency were investigated and indicated that the middle part of the truncated model riser was the ‘power-in’ region for out-of-plane VIV. Meanwhile, fatigue damage caused by out-of-plane VIV was found to be strongly dependent on both top motion amplitude and period. The probability distribution of the maximum damage exhibits 3 critical locations in the test model: TDP, upper sag-bend and top of the SCR. Strong traveling waves, TDP variation and end wave reflection have been proven to cause the maximum damage locations to shift from the ‘power-in’ region to these three positions. Finally, a maximum fatigue damage diagram with top motion amplitude, period and maximum shedding frequency was constructed. [ABSTRACT FROM AUTHOR]

The paper presents two estimation methods to calculate the natural torsional vibration mode of marine power transmission system. Additionally, the method of forced vibration analysis (torsional stress level amplitude estimation) is discussed. A typical propulsion system of merchant ship is made up of slow speed main engine connected directly to the propeller by a relatively short shaft line. All classification societies require calculation of the propulsion system operating parameters, but they have no simplified formulas. Torsional vibrations of the marine power transmission system are usually most dangerous for the shaft line and the crankshaft. Numerical algorithms based mostly on Finite Element Method (FEM) are unobtainable and not easy to use by the ship crew and unspecialized engineers. Estimation method of the torsional vibration of the system was being investigated by the authors. Modelling method of more difficult propulsion system parameters like propeller added water polar moment of inertia was presented. Short description of the advantages and disadvantages of the undercritical and overcritical propulsion system was introduced. A discussion about the calculation results was included in the final part of the paper. The presented calculation method was verified by comparison with the detailed FEM calculation and measurements on real ships. [ABSTRACT FROM AUTHOR]

Abstract: The fatigue damage of a long deepwater riser undergoing in-line and cross-flow vortex-induced vibration (VIV) in deepwater is numerically studied using pseudo-excitation method (PEM) in present paper. For evaluating the fluid–structure interaction problem of vortex-induced vibration of deepwater riser at high Reynolds number, the strip theory is employed in this paper, and the discrete vortex method (DVM) is used to calculate the VIV of each strip to obtain the load spectrum as the pseudo-excitation, while the finite volume method (FVM) is employed to evaluate the structure dynamics of a deepwater riser. The VIV is considered as a stationary random process. The response of riser to vortex induced excitation is calculated using pseudo-excitation method. The DVM model and pseudo-excitation method are both validated by comparing their numerical results with experiments. The fatigue damage of one deepwater riser is evaluated based on the Palmgren–Miner Rule. [Copyright &y& Elsevier]

Abstract: A vibration control scheme is proposed in this paper based on the principle of the tuned mass damper. An anti-vibration device is designed and installed on a similar model of a jacket platform. There are two structural types for controlling the horizontal and vertical vibrations. The parametric study is performed in order to control the vibration effectively. The results show that satisfactory control effects can be achieved when the parameters of the anti-vibration device take the values in optimal parameter range. [Copyright &y& Elsevier]

Abstract: Jacket platforms in the Bohai Gulf are often victims of random ice-induced vibrations. Their design originally did not account for dynamic ice loading on the jacket structures and significant ice-induced vibrations have been observed during winters. So it is urgent to find out as to how to consider the dynamic ice loading in the design stage of an ice-resistant jacket platform in the Bohai Gulf. In this paper, based on the state-of-art developments of dynamic ice load and ice–structure interaction, the authors proposed two different approaches to determine the short-term dynamic ice cases for dynamic analysis of ice-resistant jacket platform in the Bohai Gulf, named as the failure probability-based approach and the expected loss-based approach. Considering the variation of the ice environments and the variability of ice-resistant structure''s properties, the random ice spectrum and the pseudo-excitation method (PEM) are employed to improve the efficiency of the procedure. Through finite element method (FEM) modeling of the real ice-resistant jacket platforms in the Bohai Gulf, such as MSW, MUQ and NW, the characteristics of the two proposed approaches have been investigated and compared. This paper provides an open and universal framework to assist designers and owners for determining the short-term dynamic ice cases in the Bohai Gulf, in which more information can be considered in the procedure to update and modify the results if available. [Copyright &y& Elsevier]

Abstract: The paper presents an outline of the advanced theory of thin-walled girders. The improvement includes shear influence on torsion as an extension of shear influence on bending. The analogy between bending and torsion is recognized and pointed out throughout the paper. Complete differential equations of coupled flexural and torsional vibrations for a prismatic girder are derived. In addition, the 8 d.o.f. beam finite element, utilizing the energy approach, is constituted with stiffness and mass matrices, and load vectors. The paper describes determining of geometrical properties of multi-cell open cross-sections by employing the strip element method. Numerical procedures for vibration analyses are outlined. Furthermore, dry natural vibrations of a VLCS (Very Large Container Ship) are analysed by 1D FEM model as a prerogative for hydroelastic analyses of these relatively flexible vessels. Influence of transverse bulkheads is taken into account by increasing torsional stiffness of the ship hull proportionally to their deformation energies. Validation of 1D FEM model is checked by correlation analysis with the vibration response of the fine 3D FEM model. [Copyright &y& Elsevier]

Abstract: In this paper, the dynamic response of a scale model of a jacket offshore structure is investigated both theoretically and experimentally. The experiments were conducted both in air and in water. The in-water experiments were done in the towing tank of Memorial University to simulate the realistic operating conditions. The model was subjected to random wave loads. Froude''s law of modeling was used to obtain the dimensions of the scale model based on the dimensions of an existing structure. The effects of varying the structure''s weight, and the characteristics of the wave loading were investigated. The structure''s weight was changed by adding weights to the structure''s deck. A finite element model was designed to determine the dynamic response of the model. Excellent agreement between the experimental and theoretical results was obtained. The reaction force at the foundation was estimated from strain measurements and compared with the finite element calculations. Fair agreement was obtained. This work is the first stage of a project whose objective is to develop a method for structural damage detection using the free vibration response of the structure. The free vibration response will be obtained from the stationary random response of the structure using the random decrement method. Having an accurate model to describe the dynamic response of the structure is the first step in this study. [Copyright &y& Elsevier]

This paper presents a practical procedure for creating finite element (FE) model for vibration analysis of cruise ships. The most preferable FE modelling approaches are studied and discussed through case studies of common ship structures, which cover the range from low to high frequencies. The application of homogenized equivalent single layer (ESL) theory based equivalent element for stiffened panel is extended to local forced vibration analysis, where inertia induced interaction between plate and stiffener occurs. Modal method is used with an energy-based correction for accounting the plate-stiffener interaction into modal properties. Case study results reveal that mesh density of one 4-node element per web frame spacing is suitable for global FE-model when vibration analysis is limited to global hull girder modes. For such modes it is sufficient to only include the membrane stiffness of stiffened panels. For investigating the response at higher frequencies, bending properties of stiffened panel should be included and mesh density should be at least two elements per web frame spacing. Then forced vibration analysis can be performed with an excellent accuracy up to frequencies about one third of the local plate natural frequencies between the stiffeners. Beyond that, the influence of the local plate vibration becomes more significant in panel vibration, making the ESL-theory based element limited. With the applied correction method, the validity of the ESL-model can be extended to approximately two thirds of the local plate natural frequency. [Display omitted] • Practical modelling procedure for ship vibration analysis is presented. • Finite element model is based on Equivalent Single Layer (ESL) theory. • A correction for local plate vibration is extended to forced vibration analysis. • Presented method is compared to other commonly used modelling techniques. • Range of applicability of each modelling technique is presented. [ABSTRACT FROM AUTHOR]