Longfei Xiao

Wind-wave induced dynamic response analysis for motions and mooring loads of a spar-type offshore floating wind turbine

Due to the energy crisis and the environmental issues like pollution and global warming, the exploration for renewable and clean energies becomes crucial. The offshore floating wind turbines (OFWTs) draw a great deal of attention recently as a means to exploit the steadier and stronger wind resources available in deep water seas. This paper studies the hydrodynamic characteristics of a spar-type wind turbine known as the OC3-Hywind concept and the dynamic responses of the turbine. Response characteristics of motions and mooring loads of the system under different sea states are evaluated and the effects of the loads induced by the wind and the wave on the system are discussed. The calculations are carried out with the numerical simulation code FAST in the time domain and the frequency analysis is made by using the FFT method. The results and the conclusions from this paper might help better understand the behavior characteristics of the floating wind turbine system under actual ocean environments and provide valuable data in design and engineering practice.

Large-eddy simulation of the flow past both finite and infinite circular cylinders at Re = 3900

The flow past a finite circular cylinder with a height-to-diameter ratio of 1.5 and an infinite circular cylinder of the same diameter at a Reynolds number Re = 3900 is investigated using the large eddy simulation (LES). The objective of the present study is to explore the differences of the flow mechanisms between the finite and infinite circular cylinders. It is shown that the free end of the finite circular cylinders affects the wake region significantly. The mean drag coefficient and the fluctuating lift coefficient of the finite circular cylinder are smaller than those of the infinite circular cylinder. The three-dimensional separation and the separated shear layer instability of the finite circular cylinder can obviously be observed. The existence of an arch vortex in the average flow downstream of the free end is demonstrated

Low-frequency drift forces and horizontal motions of a moored FPSO in bi-directional swell and wind-sea offshore West Africa

Frequencies of floating production, storage and offloadings (FPSOs) horizontal motions are much smaller than the natural frequencies of waves and thus 2nd-order difference-frequency wave drift forces could excite large response. In this paper, the 2nd-order wave drift forces and low-frequency (LF) horizontal motions of a multi-point moored FPSO are computed in bi-directional swell and wind-sea conditions offshore West Africa (WOA). Newmans approximation has been applied as the computation theory. Numerical approximation compares reasonably well with measured results of model tests. Under environmental conditions in WOA, it is concluded that swell induces larger drift loads than wind-sea at the surge/sway natural frequency, and they both are indispensable to the LF motion. However, the coupling interacting drift loads between different incident angles could be neglected due to the large peak period deviation between swell and wind-sea. Simplified approximation will be effective by superposition of drift force contributions from uni-directional long-crested swell and wind-sea. Newmans approximation provides a tool to accurately simulate drift loads on FPSO in the bi-directional waves in WOA. An important reference has been obtained for both LF motion prediction and mooring system design in bi-directional swell and wind-sea WOA.

Experimental Investigation of Effects of Inner-Tank Sloshing on Hydrodynamics of an FLNG System

The present research focuses on experimentally clarifying the effect of inner-tank sloshing on the hydrodynamics of an Floating Liquefied Natural Gas (FLNG) system. Through the comparisons of the results obtained from the model tests carried out with the vessel model ballasted with liquid and solid cargo separately, the effects of the inner-tank sloshing on the hydrodynamics of an FLNG system are highlighted and presented. Statistical languages of the maximum, minimum, mean values and the standard deviations and power density spectra calculated with the help of the algorithm of Fast Fourier Transformation (FFT) are provided. It is concluded that the effects of the inner-tank sloshing on the responses of the FLNG system are sensitive to wave excitation frequencies, and that the effects of the inner-tank sloshing play an important role, particularly in the roll motion of the FLNG hull. The outcome of the proposed technique would offer constructive feedback, which can lead to more practical applications and can serve as a reference for the verification of the potential numerical simulations by other researchers.

Effects of wave excitation force prediction deviations on the discrete control performance of an oscillating wave energy converter

Oscillating-body converters are widely used in offshore engineering, to capture wave energies. In such a case, discrete control including both latching and declutching controls is adopted to improve the power capture performance of oscillating wave energy converters (WECs). A reliable prediction of wave excitation forces on the WEC is essential for the discrete control strategy. In this study, a time domain model is developed to calculate the hydrodynamic responses of a hemispherical oscillating WEC with discrete control in regular waves. A state space model is used to deal with the convolution term in the time domain equation, taking into account the memory effects of wave surface. Based on the developed numerical model, the effects of prediction deviations, such as the amplitude and phase of wave excitation force, have been studied. It is observed that the amplitude prediction deviation exhibits very few effects on the control performance. However, the phase prediction deviation performances significantly influence on the control performance. In some conditions, the phase prediction deviation will reduce the efficiency of the discrete control.

Experimental investigation of flow characteristics around four square-cylinder arrays at subcritical Reynolds numbers

Abstract The Deep Draft Semi-Submersible (DDS) concepts are known for their favourable vertical motion performance. However, the DDS may experience critical Vortex-Induced Motion (VIM) stemming from the fluctuating forces on the columns. In order to investigate the current-induced excitation forces of VIM, an experimental study of flow characteristics around four square-section cylinders in a square configuration is presented. A number of column spacing ratios and array attack angles were considered to investigate the parametric influences. The results comprise flow patterns, drag and lift forces, as well as Strouhal numbers. It is shown that both the drag and lift forces acting on the cylinders are slightly different between the various L/D values, and the fluctuating forces peak at L/D = 4.14. The lift force of downstream cylinders reaches its maximum at around α = 15°. Furthermore, the flow around circular-section-cylinder arrays is also discussed in comparison with that of square cylinders.

Damping ratio identification using a continuous wavelet transform to vortex-induced motion of a Truss Spar

The paper presents an application of the continuous wavelet transform (CWT), a time-frequency method based on Gabor wavelet function and widely used in many engineering fields, in ocean engineering for the first time. The advantage of the CWT over commonly used methods like time-domain and frequency-domain methods is its ability to decompose signals simultaneously both in time and frequency. The essential information is the wavelet ridge where the modal parameters like the damping ratio and frequency can be extracted. This paper developed an algorithm to extract the wavelet ridge from the wavelet transform. The method is validated by a numerical example which is similar to the experimental data. Finally the method is applied to estimate the damping ratio and frequency of a Truss Spar in vortex-induced motion. The results indicate that the CWT method performs remarkably well.

Influence of Wave Group Characteristics on the Motion of a Semisubmersible in Freak Waves

In the last few decades, the hydrodynamic performance of offshore structures has been widely studied to ensure their safety as well as to achieve an economical design. However, an increasing number of reported accidents due to rough ocean waves call for in-depth investigations on the loads and motions of offshore structures, particularly the effect of freak waves.The aim of this paper is to determine the sea conditions that may cause the maximum motion responses of offshore structures, which have a significant effect on the loads of mooring systems because of their tight relationship. As a preliminary step, the response amplitude operators (RAOs) of a semisubmersible platform of 500 meters operating depth are obtained with the frequency-domain analysis method. Subsequently, a series of predetermined extreme wave sequences with different wave group characteristics, such as the maximum crest amplitude and the time lag between successive high waves, are adopted to calculate the hydrodynamic performance of the semisubmersible with mooring systems in time-domain. The paper shows that the maximum motion responses not only depend on the largest wave crest amplitude but also the time lags between successive giant waves. This paper will provide an important reference for future designs which could consider the most dangerous wave environment.Copyright

Numerical Study of Air Gap Response and Wave Impact Load on a Moored Semi-Submersible Platform in Predetermined Irregular Wave Train

The importance of understanding air gap response and potential deck impact is well-known in the design stage of semi-submersible platform. The highly non-linear nature of wave elevation around large structures in steep waves makes it difficult to accurately predict wave field under the deck and wave run up along the columns. Present engineering tools for the prediction of air gap response generally based on simplified models. Even the models accounting for nonlinear wave diffraction is not free of uncertainties. A method adopted here couples a Navier-Stokes solver, VOF technique capturing violent free surface and DNV/Seasam predicting motions of moored semi-submersible platform. Air gap response at different locations of the hull was evaluated in predetermined irregular wave train. Wave run up was also measured by wave probes near the columns. Load cells were mounted under the deck of the platform to trace potential deck impact. The predetermined irregular wave train was simulated in a numerical wave tank and verified against physical tank results. Analysis of the air gap response, wave run up and impact loads on the semi-submersible platform were conducted.Copyright

Experimental and numerical studies on the excitation loads and vortex structures of four circular section cylinders in a square configuration

A variety of concepts of deep draft semi-submersibles (DDS) have been developed in order to improve the vertical motions. However, the DDS may experience critical vortex-induced motions (VIMs) stemming from the fluctuating forces on the columns. Aiming to investigate the excitation loads and the fluid mechanism of VIM, this paper presents experimental and numerical studies of the three-dimensional (3-D) flow around four circular section cylinders in a square configuration at subcritical Reynolds numbers (Re). The experimental measurements were carried out by using the piezoelectric load cells in a wind tunnel, while the numerical simulations were conducted by employing the Large Eddy Simulation (LES) method. The effects of three spacing ratios (L/D) and four array attack angles (α) ranging from 0° to 45° in 15° increments were investigated. The results show that the vortex structures are the key to understand the behaviours of the forces and the Strouhal number (St). Consequently, the downstream cylinders are normally subjected to smaller mean drags but more serious fluctuating forces than the upstream cylinders, due to the influence of unsteady wake vortices. Furthermore, at the present range of L/D ratios, the effect of spacing ratio on the excitation forces is not significant.