Mean Wave Drift Forces on a Barge-Type Floating Wind Turbine Platform with Moonpools

Barge-type platforms with moonpools are a promising type of foundation for floating offshore wind turbines due to their good seakeeping performance. In this paper, the mean wave drift force on a barge-type vertical-axis floating wind turbine with multiple moonpools was investigated through physical model testing and numerical calculations using WAMIT. The focus was on the characteristics of mean drift load and its optimization potential. The present numerical results indicated that the application of moonpools was useful in reducing horizontal mean drift force at specific frequencies, and the reason was ascribed to the significant radiation effect of the resonant water oscillations in moonpools. The observed reduction effect on mean drift force was shown to be dependent on the viscous damping of moonpool resonance. The experimental results showed that the maximum response of the mean sway drift force was reduced by the gyroscopic effect of rotations of the vertical-axis wind turbine, and this reduction effect became stronger as the rotating speed of the wind turbine increased, but was weakened as wave amplitude increased. The comparisons between experimental data and potential flow predictions indicated that viscous effects should be taken into account to reasonably estimate the mean wave drift forces on barge-type floating wind turbines.

Mean Wave Drift Force on a Barge-Type Floating Wind Turbine With Moonpools

The barge-type foundation with moonpool(s) is a promising type of platform for floating offshore wind turbines, since the moonpool(s) could improve the hydrodynamic performance at particular frequencies and reduce the costs of construction. In this paper, the horizontal mean drift force and yaw drift moment of a barge-type platform with four moonpools are numerically and experimentally investigated. Physical model tests are carried out in a wave tank, where a 2MW vertical-axis wind turbine is modelled in the 1:100 scale. By varying the rotating speed of the turbine and the mass of the blades, the gyroscopic effects due to turbine rotations on the mean drift forces are experimentally examined. The wave diffraction and radiation code WAMIT is used to carry out numerical analysis of wave drift force and moment. The experimental results indicate that the influence of the rotations of a vertical-axis wind turbine on the sway drift force is generally not very significant. The predictions by WAMIT are in reasonable agreement with the measured data. Numerical results demonstrate that the horizontal mean drift force and yaw drift moment at certain frequencies could be reduced by moonpool(s).

Calibration of a Time-Domain Hydrodynamic Model for A 12 MW Semi-Submersible Floating Wind Turbine

Design optimization of mooring systems is an important step towards the reduction of costs for the floating wind turbine (FWT) industry. Accurate prediction of slowly-varying horizontal motions is needed, but there are still questions regarding the most adequate models for low-frequency wave excitation, and damping, for typical FWT concepts. To fill this gap, it is fundamental to compare existing load models against model tests results. This paper describes a calibration procedure for a three-columns semi-submersible FWT, based on adjustment of a time-domain numerical model to experimental results in decay tests, and tests in waves. First, the numerical model and underlying assumptions are introduced. The model is then validated against experimental data, such that the adequate load models are chosen and adjusted. In this step, Newman’s approximation is adopted for the second-order wave loads, using wave drift coefficients obtained from the experiments. Calm-water viscous damping is represented as a linear and quadratic model, and adjusted based on decay tests. Additional damping from waves is then adjusted for each sea state, consisting of a combination of a wave drift damping component, and one component with viscous nature. Finally, a parameterization procedure is proposed for generalizing the results to sea states not considered in the tests.

Sensitivity of a Floating Bridge Global Responses to Different Wave Drift Force Models

Floating bridges are a promising solution for replacing ferries in the crossing of Norwegian fjords. Their design involves the adoption of accurate, but at the same time efficient models for the loads the structure is subjected to. Wave drift forces at the bridge’s pontoon may excite the bridge’s lower horizontal modes, with consequences to the loads on the bridge and mooring lines. Newman’s approximation is normally adopted to calculate the wave drift forces in such applications. A common simplification is to assume that the pontoons are fixed in the calculation of wave drift coefficients, while it is known that wave frequency motions may significantly influence drift loads. This paper evaluates the consequences of this simplification, in comparison to coefficients obtained considering the pontoons’ motions. First, the effect of the bridge deflection, due to mean drift, on the pontoon’s motions, is evaluated. It is found that this effect is negligible. Then, the RAOs are used in the calculation of wave drift coefficients, showing very different results than those obtained with fixed pontoons. Time-domain simulations are then performed with wave drift coefficients calculated with both approaches, with focus on the bridge girder moments and mooring line tensions. It is shown that using wave drift coefficients obtained with fixed pontoon is a non-conservative simplification, depending on sea state and wave incidence direction.

Investigation of nonlinear static displacements during the ship’s motions in various confined waterways

В статье проводится исследование нелинейных статических перемещений, возникающих в случае качки судна на мелководье, качки судна параллельно вертикальной стенке и при совместной качке двух судов на мелководье. Определение статических перемещений осуществляется на основании определения соответствующих сил волнового дрейфа по методам, разработанным в предшествующих работах. Данные методы основаны на применении метода интегральных уравнений и зеркальных отображений для случая качки судна параллельно вертикальной стенки. Проведенное исследование в отечественной практике является новым. В статье приводятся результаты расчетов нелинейных статических перемещений, возникающих при вертикальной, бортовой и килевой качки различных типов судов. Проводится исследование влияния различных факторов на их величины, а именно: изменения относительной глубины фарватера, изменения расстояния между судном и вертикальной стенкой, изменения расстояния между судами, курсового угла. Показано увеличение значений нелинейных статических углов крена и дифферента, а также вертикальных перемещений при уменьшении относительной глубины, уменьшении расстояния между судами и расстояния между судном и стенкой. Приведено сравнение значений статических перемещений, возникающих в различных стесненных фарватерах при прочих равных условиях. The article investigates nonlinear static displacements arising in the case of a ship’s motions in shallow water, motions of a ship parallel to a vertical wall and during coupled motions of two ships in shallow water. The determination of static displacements is carried out on the basis of determining the corresponding forces of wave drift according to the methods developed in previous works. These methods are based on the application of the method of integral equations and mirror images for the case of the ship’s motions parallel to the vertical wall. The research carried out in domestic practice is new. The article presents the results of calculations of nonlinear static displacements occurring during heaving, rolling and pitching of various types of ships. A study of the influence of various factors on their values is being carried out, namely: changes in the relative depth of the waterway, changes in the distance between the ship and the vertical wall, changes in the distance between ships, heading angle. An increase in the values of nonlinear static angles of roll and trim, as well as vertical displacements with a decrease in the relative depth, a decrease in the distance between ships and the distance between the ship and the wall, is shown. A comparison of the values of static displacements arising in various confined waterways, all other things being equal, is given.

Experimental and Numerical Analysis of Wave Drift Force on KVLCC2 Moving in Oblique Waves

In this study, experimental and numerical methods were applied to estimate surge and sway wave drift forces and yaw drift moment acting on KVLCC2, advancing in oblique wave. An experiment was carried out in the ocean engineering basin of the Korea Research Institute of Ships and Ocean Engineering (KRISO). A series of regular wave tests under various heading conditions were conducted to investigate ship motion responses and wave drift forces. A Rankine panel method based on potential flow was adopted in the numerical analysis, and the direct pressure integration method that integrates second-order pressure on the hull surface was applied to compute wave drift force. Through this study, validation data of wave drift force acting on KVLCC2 was established, and the computation capability of the potential-based numerical method was systematically analyzed.