This paper presents a parametric design study of the mooring system for a floating offshore wind turbine. We selected the OC4 DeepCwind semisubmersible floating wind turbine as the reference structure. The design water depth was 50 m, which was the transition area between the shallow and deep waters. For the floating wind turbine working in this water area, the restoring forces and moments provided by the mooring lines were significantly affected by the heave motion amplitude of the platform. Thus, the mooring design for the wind turbine in this working depth was different from the deep-water catenary mooring system. In this study, the chosen design parameters were declination angle, fairlead position, mooring line length, environmental load direction, and mooring line number. We conducted fully coupled aero-hydro dynamic simulations of the floating wind turbine system in the time domain to investigate the influences of different mooring configurations on the platform motion and the mooring tension. We evaluated both survival and accidental conditions to analyze the mooring safety under typhoon and mooring fail conditions. On the basis of the simulation results, this study made several design recommendations for the mooring configuration for floating wind turbines in intermediate water depth applied in China.

To predict the short-term motion responses of floating offshore wind turbine under extreme wind-wave excitation, a numerical model based on the two-phase flow finite volume method was developed. In this paper, uni-directional irregular waves composed of 100 cosine waves with equal frequency interval were simulated by the wave forcing technique, resulting in the measured spectrum in accordance with the target spectrum. Then, a 100-seconds wave segment containing the maximum wave height was selected for fully coupled dynamic analysis of the OC4-DeepCwind system in CFD, and the results were compared with those of FAST under the same wind and wave sea state. It was found that the motion responses of heave and pitch motion responses predicted by two methods agree well. The second-order slow drift force generated in CFD led to the difference in surge motion. The predicted sway, roll, and yaw motions by these two methods were also compared. In addition, significant differences between two methods’ predictions on mooring tension were found.

A novel design of offshore twin counter-rotating vertical axis wind turbines (VAWTs) with deflector is proposed in this paper. We investigate the performance of the twin VAWTs by the two-dimensional computational fluid dynamic method with the Spalart-Allmaras turbulence model. Then, the performances of twin VAWTs with three kinds of deflectors are compared. The results show that installing the front deflector leads to significant improved aerodynamic performance. To better understand the simulation results, we introduce a simple and effective method to obtain the blade’s angle of attack. The mechanism of enhanced performance by deflector is pointed out, based on the information of the blade’s local angle of attack and flow field. Finally, a guideline on the design of deflector for the twin vertical axis wind turbines is provided.

The prediction of roll motion of a ship section with bilge keels is particularly difficult because the flow separation and vortex shedding under the hull significantly affect the behavior of roll damping. To predict the roll damping and roll motion directly, the numerical models must simulate the fluid viscosity. Recently, Reynolds-averaged Navier–Stokes (RANS) method and Discrete Vortex Method (DVM) have been applied in this area and show promising results. In this paper, we will use both methods to simulate the free roll-decay motion of a ship section with bilge keels. The numerical predictions of the roll time histories will be compared with experimental measurements. Besides, the numerically-predicted vorticity distributions at different time instants near a bilge keel will be shown and compared. Moreover, the computation times for both numerical methods will also be reported. In this work, we will conduct the comparison for a number of cases that are with different bilge-keel heights and bilge-keel installation angles. Thus, the accuracies and the computational efficiencies will be evaluated comprehensively.

The process of ice-structure interaction is a complex problem which is influenced by the properties of both ice and the structure. In this paper, the material point method (MPM) is introduced to simulate the interaction between an ice sheet and a cylinder structure. MPM is efficient in solving history dependent and large deformation problems and has shown advantage in hyper-velocity impact and landslide issues, etc.. The constitutive relation of ice is based on elasto-viscous-plastic model with the Drucker-Pragers yield criterion. Ice follows the Maxwell elasto-viscous model before the yield criterion is reached and fails when the plastic strain surpasses the failure strain. Meanwhile, the constitutive model used in this work considers the effect of the Young’s modulus, Poisson’s ratio, density, temperature, cohesive force and internal friction angle of ice. A series of simulations are conducted and the results are in accord with existing theories. According to the comparison, the influences of ice temperature and penetration speed of the structure on the global ice load are testified. The numerical tests have proven the feasibility of MPM in simulating the interaction between an ice sheet and a cylinder structure. Future work in ice-structure interaction problems with MPM is also discussed.

The prediction of roll motion of a ship with bilge keels is particularly difficult because of the nonlinear characteristics of the viscous damping. Flow separation and vortex shedding caused by bilge keels significantly affect the roll damping and the magnitude of the roll response. To predict free response of roll, the Slender-Ship Free-Surface Random Vortex Method (SSFSRVM) developed in Seah & Yeung (2008) [1] was employed. It is a fast free-surface viscous-flow solver designed to run on a standard desktop computer. It features a quasi-three dimensional formulation that allows the decomposition of the three-dimensional hull problem into a series of two-dimensional computational planes, in which the two-dimensional free-surface Navier-Stokes solver FSRVM [2] can be applied. This SSFSRVM methodology has recently been further developed to model multi-degrees of freedom of free-body motion in the time domain. In this paper, we will first examine the effectiveness of SSFSRVM modeling by comparing the time histories of free roll-decay motion resulting from simulations and experimental measurements. Furthermore, the detailed vorticity distribution near a bilge keel obtained from the numerical model will also be compared with the experimental PIV images. Next, we will report, based on the time-domain simulation of the coupled hull and fluid motion, how the roll decay coefficients and the flow field are altered by the span of the bilge keels. Plots of vorticity contour and vorticity iso-surface along the three-dimensional hull will be presented to reveal the motion of fluid particles and vortex filaments near the keels. It is appropriate and an honor for me to present this roll-damping research in the Emeritus Professor J. R. Paulling Honoring Symposium. It was from “Randy” that I first acquired the concept of equivalent linear damping. Even more so, I am very grateful for his teaching, guidance and friendship of many years. — R. W. Yeung

The performance of an unsymmetrical rolling cam as an ocean-wave energy extractor was studied experimentally by Salter (1974) and then analyzed from the hydrodynamics standpoint by a number of workers in the 70’s (e.g. Evans, 1976). The analysis was carried out on the basis of inviscid-fluid theory and the energy-absorbing efficiency was found to approach 100%. This well-known result did not account for the presence of viscosity, which alters not only fluid damping but also, to some extent, the added-inertia characteristics. How fluid viscosity may alter these conclusions and reduce the energy-extraction effectiveness is examined in this paper, for two rolling-cam shapes: a smooth “Eyeball Cam” with a simple mathematical form and a “Keeled Cam” with a single sharp-edged bilge keel. The solution methodology involved the Free-Surface Random-Vortex Method (FSRVM), reviewed by Yeung (2002). Frequency-domain solutions in inviscid fluid were first sought for these two shapes as baseline performance metrics. As expected, without viscosity, both shapes perform exceedingly well in terms of extraction efficiency. The hydrodynamic properties of these two shapes were then examined in a real, viscous fluid, under a high Reynolds-number assumption. The added moment of inertia and damping are noted to be changed, especially for the Keeled Cam. With the power-take-off (PTO) damping chosen based on the viscous-fluid results, time-domain solutions are developed to understand the behavior of the rolling motion, the effects of PTO damping, and the effects of the cam shapes. These assessments can be effectively made with FSRVM as the computational engine, even at motion of fairly large amplitude, for which an actual system may need to be designed.

Fluid viscosity is known to influence hydrodynamic forces on a floating body in motion, particularly when the motion amplitude is large and the body is of a bluff shape. While these hydrodynamic force or force coefficients have been predicted traditionally by inviscid-fluid theory, much recent advances had taken place in the inclusion of viscous effects. Sophisticated RANS (Reynolds-Averaged Navier Stokes) software are increasingly popular. However, they are often too elaborate for a systematic study of various parameters, geometry or frequency, where many runs with extensive data grid generation are needed. The Free-Surface Random-Vortex Method (FSRVM), developed at UC Berkeley in the early 2000, offers a middle-ground alternative, by which the viscous-fluid motion can be modeled and yet allowing vorticity generation be either turned on or turned off. The heavily validated FSRVM methodology is applied in this paper to examine how the draft-to-beam ratio and the shaping details of two-dimensional cylinders can alter the added inertia and viscous damping properties. A collection of four shapes is studied, varying from rectangles with sharp bilge corners to a reversed-curvature wedge shape. For these shapes, basic hydro-dynamic properties are examined, with the effects of viscosity considered. With the use of these hydrodynamic coefficients, the motion response of the cylinders in waves are also investigated. The origin of viscous damping is clarified. It is a pleasure and honor for the authors to contribute to the Jo Pinkster Symposium, held in his honor in OMAE-2011.