Wenyong Tang

Time Domain Analysis Approach for Riser Vortex-Induced Vibration Based on Forced Vibration Test Data

As oil and gas exploration and production are pushed into deepwater area, the offshore industry is facing more challenges for riser vortex induces vibration (VIV). Although frequency domain approach has been widely used for the riser VIV prediction and fatigue design, several assumptions need to be made. In addition, frequency domain approach cannot account for the variable current and riser nonlinear boundary conditions, such as top boundary response, the interaction between riser and guides in the hull and soil-SCR interaction.Considering above cases, several time domain codes have been developed for riser cross-flow (CF) VIV prediction. This paper presents a time domain approach based on forced algorithm. The exciting force is derived from the non-dimensional amplitude and frequency dependent lift coefficients from forced vibration test. The hydrodynamic damping model consists of empirical model and the extension of the lift curves. At each step, the displacement and velocity of each element would be obtained to calculate the response amplitude and frequency for the lift coefficient and damping. Expect for CF VIV, the mean drag force is also considered, which would be magnified by CF VIV.The model test at Delta Flume of Delft Hydraulics is simulated using the proposed approach, and the CF VIV responses and the mean drag displacement are predicted. The results match well with the measured data.Copyright

Response of Beams Under the Impact of Freak Waves

Recently the research on freak waves has focused on the formation mechanism as well as the experimental and numerical simulation, however the study of freak waves’ action on marine structures which is often confined to numerical methods is still not much. As beams are often studied as the simplified model of plates for structural safety assessment, in this paper, the response of a beam which is hit by a 2-D freak wave is studied. The freak wave is generated in a numerical wave tank (NWT) which solves the 2-D incompressible Navier-Stokes equations. The freak wave is based on the data of real sea condition in the Sea of Japan. An efficient wave absorbing method which satisfies the mass conservation is applied in the numerical wave tank. The influence of the beam’s motion on the freak wave fluid field is also considered in this paper, as well as different boundary conditions of the beam. It is found that the natural frequency has a great impact on the response of the beam.© 2014 ASME

Characteristic Analysis of VIV-Induced Fatigue Damage of Top Tensioned Risers Based on Simplified Model

In this study, a simplified empirical model for fatigue analysis of top-tensioned risers due to vortex-induced vibration (VIV) in a nonuniform current is presented. Compared with previous studies on this subject, this model is able to take into account the main intrinsic nature of VIV for low mass ratio structures in lock-in regions, added mass, and nonlinear fluid damping. Based on the simplified model, a closed-form solution of fatigue damage is suggested for the case of a riser with uniform mass and cross-section oscillating in a uniform flow. Fatigue damage characteristics of the top-tensioned risers due to VIV are investigated to determine the effects of various ocean conditions and riser dimensions on fatigue damage.

Added mass variation effect on vortex-induced vibration for flexible risers based on force-decomposition model

ABSTRACT Vortex-induced vibration (VIV) due to periodic vortex shedding around the cylindrical structures is a typical and complicated fluid-structure interaction issue. In this paper, an improved time domain numerical simulation model, which can take the variation effect of added mass into account, is proposed to predict VIV for flexible risers. Originated from forced vibration experimental data, a time-varying function of both response frequency and amplitude is used to describe hydrodynamic forces, and added mass coefficient is considered variable with response frequency. Modal analysis serving VIV hydrodynamic forces calculation is implemented at the beginning of each time step, and dominant frequency of every element will be updated based on real-time structural natural frequency. By validating with model tests of two flexible risers, the proposed model is proved to show better performance for VIV prediction than traditional method (assuming Ca = 1). The added mass variation effect on VIV is discussed qualitatively and quantitatively.

Frequency Domain Approach for the Coupled Analysis of Floating Wind Turbine System

ABSTRACT This paper presents a simple frequency domain approach to predict the coupled response of floating wind turbine based on AQWA, and then investigates the effect of damping induced by wind, current and mooring system on the floating foundation response. This approach is validated against the experiment of DeepCwind floating offshore wind turbine in literatures. The results show reasonable agreement, and also indicate that the drift damping should be elaborated for the drift response. Based on this approach, the effect of wind and current on the floating wind turbine response is parametrically investigated. These aspects have rare effect on the wave frequency response, but affect the drift response due to the introduction of damping. The comparison between quasi-static and dynamic mooring line models reveals that the dynamics and added mass of mooring line play negligible roles, but the drag force significantly damps down the surge drift response.

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.

Numerical Modeling of Unbonded Flexible Risers Under Combined Symmetrical Loads

This paper presents a numerical method to predict the behavior of unbonded flexible risers under symmetrical loads, and takes an eight-layer unbonded flexible riser for the case study. In order to accurately simulate geometric properties of the riser and contact interaction between layers, carcass and zeta layers are modeled as the actual complicated cross-sections, and all layers are modeled by solid element, both contact and friction between layers are taken into consideration. ABAQUS/Explicit is adopted to avoid convergence problem caused by geometric and contact nonlinearities, mass scaling is adopted to avoid long computational time caused by the very detailed finite element model of carcass and zeta layers. Load cases considered in this paper are external pressure coupled with axial tension, axial compression under different lay angles of helical strips and boundary conditions. The results show that numerical results agree well with theoretical results, lay angles of tensile armor layers have a great impact on the axial displacement, but boundary conditions have little effect on the axial displacement.Copyright

VIV-induced fatigue damage study of helical wires in catenary unbonded flexible riser in time domain

ABSTRACT Helical wire is a key component of unbonded flexible riser, and is vulnerable to fatigue failure. The present study simplifies the flexible riser into a beam element with constant axial and bending stiffness, and then investigates the vortex-induced vibration (VIV)-induced fatigue damage of the helical wire in a catenary flexible riser using a time domain VIV approach. The simplification of the flexible riser is based on the equivalence of axial and bending stiffness. The former can be iteratively calculated based on the integrated axisymmetric formulation in which the interlayer contact and separation are taken into account, while the latter is simply taken as the combination of all layers’ bending stiffness and keeps constant since helical wire may not slide (i.e. full-sticking) under intact outer sheath due to high external pressure. Based on the simplification, this study compares the VIV characteristics of the flexible riser under full-sticking and full-sliding conditions of helical wires since the latter associated with conservative results is often applied, and then parametrically investigates the fatigue damage of the helical wire. The results indicate that the critical position is located near touchdown point, and seabed stiffness, helical wire lay angle and the top end position have a significant effect on the fatigue damage.

Hybrid RANS/LES simulation of sloshing flow in a rectangular tank with and without baffles

ABSTRACT Turbulence modelling is a key issue to be addressed in the numerical study of violent liquid sloshing. To improve the predictive accuracy of RANS and the computational efficiency of LES, a new hybrid RANS/LES approach is developed in this paper. This model is first validated against the available experimental data for liquid sloshing in a tank without baffle. The numerical results are compared to those obtained by experiment and other three turbulence models. The performances of present hybrid method are similar with that of the classical LES model, which can provide very accurate solutions. Then, the model is employed to study liquid sloshing in a tank with a vertical baffle and a horizontal baffle. The comparisons of numerical and experimental pressure histories further confirm the capability of the present method for sloshing simulation.

Fatigue Damage Study of Helical Wires in Catenary Unbonded Flexible Riser Near Touchdown Point

This study presents an analytical model of flexible riser and implements it into finiteelement software ABAQUS to investigate the fatigue damage of helical wires near touchdown point (TDP). In the analytical model, the interlayer contact pressure is simulated by setting up springs between adjacent interlayers. The spring stiffness is iteratively updated based on the interlayer penetration and separation conditions in the axisymmetric analysis. During the bending behavior, the axial stress of helical wire along the circumferential direction is traced to determine whether the axial force overcomes the interlayer friction force and thus lead to sliding. Based on the experimental data in the literature, the model is verified. The present study implements this model into ABAQUS to carry out the global analysis of the catenary flexible riser. In the global analysis, the riser–seabed interaction is simulated by using a hysteretic seabed model in the literature. The effect of the seabed stiffness and interlayer friction on the fatigue damage of helical wire near touchdown point is parametrically studied, and the results indicate that these two aspects significantly affect the helical wire fatigue damage, and the sliding of helical wires should be taken into account in the global analysis for accurate prediction of fatigue damage. Meanwhile, different from the steel catenary riser, high seabed stiffness may not correspond to high fatigue damage of helical wires. [DOI: 10.1115/1.4036675]