Response Of Floater Research Articles

A reduced order/simplified method for uncertainty quantification on the floating wind turbine design basis

The cost of floating wind turbines (FWTs) is currently higher than onshore and bottom fixed offshore wind turbines. An important step towards more cost-efficient FWT structures is understanding the uncertainties and how they affect the design performance. On the path towards developing a fully probabilistic design approach, an important initial step is defining the variables and uncertainties that significantly impact the final design performance. This study aims to conduct a design sensitivity analysis on a FWT system consisting of the VolturnUS Semisubmersible floater coupled with an IEA 15 MW turbine. First, a Python-based design evaluation framework is implemented that computes operational limit states as a function of design input parameters defining the floater design. The system responses and the loads are obtained using Horizontal Axis Wind turbine simulation Code 2nd generation (HAWC2). Time domain outputs of the design evaluation framework are compared to a frequency domain response using Quick Load Analysis of Floating Wind Turbines (QuLAF). Initial results provide that buoyancy column diameter and the floater radius have the highest effects on the general floater response.

Fully nonlinear wave interaction with 2D floating body including nonlinear sloshing tank

Two-dimensional fully-nonlinear numerical sloshing tank (FN-NST) simulation program is developed based on boundary element method (BEM), potential theory with artificial free-surface damping, and space-fixed (global) coordinate system. The free surface in sloshing tank is updated by the mixed Eulerian-Lagrangian (MEL) method with material node approach (Full Lagrangian approach). The developed potential-based nonlinear model was validated through comparisons with available experimental and CFD (computational fluid dynamics) results and they show good agreements. The nonlinear sloshing model is then further implemented in the fully nonlinear wave and floating-body simulation program to observe the effects of liquid sloshing motion. In the fully nonlinear program, the entire nonlinear fluid pressure is integrated over the instantaneous wetted body surface and position, which is subsequently compared with the corresponding linear theory and results. The second-order effects are also identified from the fully-nonlinear simulation. The floater Response Amplitude Operators (RAOs) for frozen- and liquid-cargo cases are also compared for various wave frequencies and amplitudes to better identify the role of liquid cargo in the stability of rotational motions. The effects of the vertical location of liquid tank and artificial free-surface damping are also investigated.

Dynamic response analysis of the TetraSpar floater in waves: Experiment and numerical reproduction

The initial proof of concept model scale test campaign for the TetraSpar floating wind turbine substructure is presented here along with a detailed response analysis and numerical reproduction. The tests were conducted at scale 1:60 in wind and waves with the pitch-regulated DTU 10 MW wind turbine. The floater was tested in two configurations: semi-submersible and spar. The experimental setup and program is described in detail followed by system identification for natural frequencies and damping. The responses of the floater in the two configurations to hydrodynamic loading are analysed and compared. The analysis includes irregular sea states and focused wave groups at both 0° and 30° heading. The hydrodynamic damping of the floaters was quantified in decay tests, showing a clear linear and second-order component. It was observed that the semi-submersible configuration had significantly larger motion response than the spar configuration in ultimate limit state wave conditions. Emphasis is placed on the mooring loads and the tensions in the support lines for the ballasted keel. The increased ballast of the spar keel led to larger loads in these support lines. Further, second- and higher-order wave forcing were observed in responses of both configurations. A numerical model based on first-order radiation-diffraction theory, second-order Newman loads and additional Morison viscous forcing is set up. The model damping is calibrated against the measurements at each sea state. It is demonstrated that after this calibration, the model is able to reproduce the floater response and tower top accelerations with good accuracy, both in the linear range and at the natural floater frequencies, with heave in the storm sea state as the exception. The dynamic tensions in the keel lines are found to depend strongly on the lines projection to the inline wave direction. Also this behaviour is reproduced accurately by the model, although with some under-prediction in one of the lines in the rated wind sea state, which is linked to differences in the experimental pre-tension for the six lines.

Modeling the coupled aero-hydro-servo-dynamic response of 15 MW floating wind turbines with wind tunnel hardware in the loop

Accurate modeling and design tools are required to lower the cost of floating wind and enable the rapid growth that is expected in the next years. This paper presents a hardware-in-the-loop (HIL) wind tunnel experiment, showing it is a valuable tool to investigate the coupled response of two 15 MW floating wind turbines. In the HIL experiment, the wind turbine is emulated with a physical scale model, that is a 1:100 version of the IEA 15 MW, with reference closed-loop control functionalities. Rotor aerodynamic loads are continuously measured and fed back to a numerical simulation of the floater response. The output of the simulation are platform motions that are recreated in the wind tunnel with a robot. The two floating wind turbines are tested with various wind, waves, and turbine control conditions. It is shown the turbine scaled model matches with good accuracy the response of the IEA 15 MW, thanks to the aerodynamic re-design of blades and the adopted closed-loop control strategy. The HIL control system reproduces the coupling between turbine and platform with very small errors. In operational conditions, aerodynamic damping of platform motion is sensitive to wind speed when the turbine is controlled in closed loop, and it is shown that this is due to the coupling between platform pitch and rotor. The amount of aerodynamic damping just below the rated wind speed is found to be more uncertain than in other wind conditions.

Numerical and experimental studies of heave damping and added mass of spar with heave plates using forced oscillation

Heave plates are widely used in floating offshore structures to reduce the heave motion by increasing heave damping and heave added mass, and these parameters are sensitive to the amplitude and frequency of heave motion. It is important to obtain the hydrodynamic parameters, namely, damping and added mass of the floater not only at its natural frequency, which can be obtained from free decay tests but also at other frequencies because the floater response is of interest over a wide range of amplitudes and frequencies. Forced oscillation tests in calm water can aid the investigation of these parameters at various motion frequencies and amplitudes. Heave damping and added mass of classic spar with heave plate are investigated in this study using experiments and numerical simulations of forced heave oscillation of a 1:100 scale model in calm water for various frequency-amplitude combinations. The least square method was used to determine the added mass and damping using three damping models, namely, linear, quadratic, and linear-plus-quadratic, and their applicability assessed. The effect of amplitude and frequency of oscillation on the parameters are discussed for various heave plate configurations with the aid of flow visualization from numerical simulations. Added mass effect is examined using flow visualization of the fluid acceleration field. The scale effect on the parameters is also addressed.

Influence of wakes and atmospheric stability on the floater responses of the Hywind Scotland wind turbines

AbstractAs the worlds first floating wind farm, one of the main technical challenges in the design phase of Hywind Scotland was the uncertainty related to wake interaction effects between the floating wind turbines. In this paper, we address this challenge by presenting an analysis of full‐scale measurements from the Hywind Scotland wind farm. Measurements of the floaters' roll, pitch and yaw motions are presented, both in the form of statistical data and response spectra. The floater responses of a turbine in free wind and a turbine in wake are compared, and the influence of different atmospheric stabilities on the floater motions is investigated. Despite the large distance between the turbines of 9 rotor diameters, wake effects in the measured floater motions are observed at the downstream turbine. Furthermore, both the wake responses and the free wind responses show a clear dependency on atmospheric stability. However, overall motions are small for all wind speeds, showing that the design performs satisfactorily in both free wind and wake conditions.

Detection of damaged mooring line based on deep neural networks

Since severe damage to the floating offshore structures due to the deterioration of their structural stability may lead to major disasters, it is necessary to detect mooring line damage at an early stage. However, most of the existing damage detection approaches of mooring line have difficulties to provide constant monitoring or to detect local damages to line. This study aims to develop a detection approach of a damaged mooring line in tension leg platform (TLP) based on deep neural networks (DNN). Simulation data with Charm3D was used for training and testing the DNN in the study because it is impractical to obtain actual data by intentionally damaging mooring lines that are in operation. The accuracies of the DNN model were significantly high (94.6%–99.3%) with noise level differing from 0% to 20%. The quite low false negative (FN) errors of 0.7%–5.4% for noise levels of 1–20% shows the potential of DNN-based structural health monitoring system to identify a damaged mooring line in TLP. The results of the study indicate that DNN-based damage detection approach with floater responses is applicable for even a local damage, and thus can prevent further damage or accident by early-stage detection.

Research on the dynamical responses of H-type floating VAWT considering the rigid-flexible coupling effect

Offshore floating wind turbines are classified into floating horizontal axis wind turbines (HAWTs) and floating vertical axis wind turbines (VAWTs) according to the orientation of rotating shaft. Compared with the traditional floating HAWTs, main advantages of floating VAWTs are the independence of wind direction and the lower position of power generating system, which reduce the installation and maintenance costs. Dynamical characteristics of floating VAWTs were widely studied in the preceding researches by tackling the wind turbine as a rigid body. However, the increasing size of wind turbine is beneficial to achieve higher cost performance. The elastic deformations of slender structures including blades and tower should be considered when the dynamical characteristics of the floating wind turbine are studied. In this paper, rigid-flexible coupling equations were established to investigate the dynamic behavior of H-type floating VAWT based on multi-body kinematics and dynamics. Natural motion performances of the wind turbine structures were presented and the phenomenon of “dynamic stiffening” was analyzed. Motions of the floater and vibrations of the tower and blades were computed under complex environmental loads. Time domain and frequency domain simulations were carried out to study the dynamic response of floater, tower and blades, respectively. Besides, the effect of aerodynamic loads and wave loads on the floating VAWT was researched. Load transfer performance through rigid and flexible structures was investigated.

Performance study of the QuLAF pre-design model for a 10 MW floating wind turbine

Abstract. This paper presents a comparison study of the simplified model QuLAF (Quick Load Analysis of Floating wind turbines) and a FAST model of the Technical University of Denmark (DTU) 10 MW reference wind turbine mounted on the LIFES50+ OO-Star Wind Floater Semi 10 MW floating substructure. The purpose is to investigate how accurate results can be obtained from this simplified model for different load cases. The two models are briefly presented and the limitations of QuLAF are discussed. These are (a) an under-prediction of the wave excitation loads for large sea states; (b) a simplified representation of the rotor-induced forcing and damping; (c) an over-predicted aerodynamic damping for the tower mode motion and (d) restriction to planar motion. All the limitations are linked to approximations applied for achieving the substantial model speedup relative to the state-of-the-art model. The comparative study is based on the planar version of design load cases (DLCs) 1.2, 1.3, 1.6, 2.1 and 6.1, and the overall analysis shows that the simplified model is generally good at estimating the bending moment at the tower base and the floater motions in heave and pitch. The largest tower-base bending moments are slightly over-predicted, but it is observed that while stronger wind leads to an over-prediction, stronger waves lead to an under-prediction. Thus, in DLC 1.6, where the largest load was obtained at 10.3 m s−1, a good match in tower-base bending moments between the two models is found. The nacelle acceleration, however, is generally under-predicted, which is linked to an over-prediction of the aerodynamic damping on the tower mode. Furthermore, the floater response in large sea states is influenced by the omission of viscous hydrodynamic drag forcing, which leads to an under-prediction of the wave excitation loads. A further investigation of the model limitations confirms these findings with respect to the tower mode damping and viscous drag loads, while the simplified approach to rotor-induced loads is found to provide remarkably accurate forcing results. Although a full design load basis evaluation with a state-of-the-art model must be carried out for the final design, the present results show the potential of applying simplified models in the preliminary design phase.

Sensitivity Analysis of Limited Actuation for Real-time Hybrid Model Testing of 5MW Bottom-fixed Offshore Wind Turbine

The present paper studies the effect of limited actuation for real-time hybrid model testing (ReaTHM® testing) of a bottom-fixed offshore wind turbine in operational, parked, and fault conditions. ReaTHM® testing is a new approach for conducting small-scale experimental campaigns and has recently applied to test a braceless semisubmersible floating wind turbine in MARINTEK ocean basin [1, 2, 3]. The aerodynamic loads on the wind turbine were applied based on simultaneous simulations coupled to the experiments while the wave loads and floater response were physically tested. The effects of actuation limitation on the ReaTHM® testing setup for the semisubmersible wind turbine were investigated previously [4] using numerical simulations, by not including some components of the aerodynamic loads or by inducing error (for example in the direction of the force actuation). In this paper, the same approach is used to investigate the sensitivity of a bottom-fixed 5MW offshore wind turbine to limited actuation. The consequences of limited actuation are also considered for fault conditions (grid loss, and blade seize with and without shutdown) due to the potential importance of fault events for the ultimate and accidental limit state analysis. For the operational turbine, most responses of interest were not strongly dependent on the studied limitations in actuation, but the aerodynamic pitch and yaw moments were important for fault cases.

Hydrodynamic Response of Floating Coupled Spar in Deep Sea

Various advanced floating structures were proposed and developed till date due to their cost effectiveness and productiveness in deep and ultra-deep water exploration. Spar has been seen as a latest and productive alternative amongst the all type of floating offshore platform. However, the precise behavior of fully coupled action of the platform in deeper water is still a great issue to be forecasted in actual sea environment. Dynamic analysis for fully coupled 3D model of floating spar platform structure can be a resourceful tool to predict the responses, where primary assemblage of Spar platform and mooring lines are considered in a joined compact coupled system. To define accurately the interaction between the Spar and mooring lines, coupled dynamic analyses are appropriate to get responses in deep sea. Considering the coupling effect of the platform and its mooring system, hydrodynamic analyses of a coupled Spar has been carried out in time domain. Also, Response Amplitude Operator (RAO) has been studied of every motion and mooring tension with Fast Fourier Transform method (FFT). The numerical simulation and motion investigations are completed with the commercial finite element (FE) package ABAQUS/AQUA. The purpose of this paper is to investigate the response of fully coupled Spar platform under regular ocean environment. Tension to the deformable mooring lines is equally distributed. Wave loading acts simultaneously on the rigid spar hull and deformable mooring line. Rigid body resonant motions influence the dynamic motion responses of Spar. Damping of the mooring line in coupled Spar-mooring system decreases the top tension of its mooring system after certain wave duration. The wave induced effects have been evaluated and discussed to investigate their interaction with the floating spar platform. RAO of the motion responses and mooring tension are consistence to predict motion of the Spar platform. It shows energy concentrate peaks of the floater's response with the frequency.

Performance validation and optimization of a dual coaxial-cylinder ocean-wave energy extractor

A point-absorber wave-energy extractor is developed, consisting of a dual coaxial-cylinder system, with the inner cylinder tension-tethered and an outer cylinder (floater) oscillating vertically. A permanent magnet linear generator (PMLG) is used as a power take-off (PTO) capturing wave energy from the relative motion of the two cylinders. The mathematical modeling of the system includes the coupling effects of the cylinder hydrodynamics and the PMLG behavior. It gives a rational and effective way of providing performance predictions and directions for optimization. The flat bottom shape of the floater is modified into a needle-like curved shape to minimize viscous losses, which leads to three-times increase in floater response, compared with the flat-bottom geometry and thus improved wave-energy capture. The behavior of the PTO in the presence of an appropriate supporting structure for the coaxial cylinders are investigated, and optimal operating conditions for energy extraction and mechanical to electrical conversion efficiency are determined. Experimental results of this coupled system in regular waves confirm the validity of the theoretical predictions and soundness of the engineering design. Optimizing the floater bottom shape and the operating conditions for energy extraction lead to a two-times increase in overall efficiency, even without any active control.

A Moored Arctic Floater in First-Year Sea Ice Ridges

First-year sea ice ridges are a major concern for structures operating in the Arctic offshore and will in many cases give the design mooring load. In this paper, the response of a moored conical floater, somewhat similar to the well-known Kulluk, is studied in first-year ridges. The study is based on model tests performed at Hamburg Ship Model Basin (HSVA) in several ridges with different properties. Mooring forces and floater response, resulting from interaction with different ridges, were compared with respect to ridge properties, ridge behavior, and simulated ice management. Clearance of accumulated rubble upstream the structure was the dominating physical process in the ridge–structure interaction. Accumulation of rubble caused large mooring forces. The amount of accumulated rubble depended on the ridge cross-sectional area, thus the mooring forces increased with ridge cross-sectional area. Large mooring forces were also experienced after the ridge was passed by the structure due to difficulties with clearing of accumulated rubble.

Real-time Hybrid Model Testing of Floating Wind Turbines: Sensitivity to Limited Actuation

Real-time hybrid model testing (ReaTHM) is a new approach for conducting small-scale experimental campaign [1–3]. In the case of a floating wind turbine in a wave basin, the aerodynamic loads on the wind turbine may be applied based on simultaneous simulations (coupled to the experiments), while the wave loads and floater response are physically tested. The objective of this paper is to demonstrate the effects of actuation limitation on the ReaTHM testing setup for a particular platform: numerical simulations are employed to examine the effects of not including some components of the aerodynamic loads (or of inducing error, for example in the direction of the force actuation).

Dynamic response analysis of floating offshore wind turbine with different types of heave plates and mooring systems by using a fully nonlinear model

A finite element model is developed for dynamic response prediction of floating offshore wind turbine systems considering coupling of wind turbine, floater and mooring system. The model employs Morison's equation with Srinivasan's model for hydrodynamic force and a non-hydrostatic model for restoring force. It is observed that for estimation of restoring force of a small floater, simple hydrostatic model underestimates the heave response after the resonance peak, while non-hydrostatic model shows good agreement with experiment. The developed model is used to discuss influence of heave plates and modeling of mooring system on floater response. Heave plates are found to influence heave response by shifting the resonance peak to longer period, while response after resonance is unaffected. The applicability of simplified linear modeling of mooring system is investigated using nonlinear model for Catenary and Tension Legged mooring. The linear model is found to provide good agreement with nonlinear model for Tension Leg mooring while it overestimates the surge response for Catenary mooring system. Floater response characteristics under different wave directions for the two types of mooring system are similar in all six modes but heave, pitch and roll amplitudes is negligible in tension leg due to high restraint. The reduced amplitude shall lead to reduction in wind turbine loads.

Floating Offshore Wind Turbines

Wind is a rapidly growing renewable energy source, increasing at an annual rate of 30%, with the vast majority of wind power generated from onshore wind farms. The growth of these facilities, however, is limited by the lack of inexpensive land near major population centers and the visual impact caused by large wind turbines.Wind energy generated from floating offshore wind farms is the next frontier. Vast sea areas with stronger and steadier winds are available for wind farm development and 5 MW wind turbine towers located 20 miles from the coastline are invisible. Current offshore wind turbines are supported by monopoles driven into the seafloor or other bottom mounted structures at coastal sites a few miles from shore and in water depths of 10-15 m. The primary impediment to their growth is their prohibitive cost as the water depth increases.This article discusses the technologies and the economics associated with the development of motion resistant floating offshore wind turbines drawing upon a seven-year research effort at MIT. Two families of floater concepts are discussed, inspired by developments in the oil and gas industry for the deep water exploration of hydrocarbon reservoirs. The interaction of the floater response dynamics in severe weather with that of the wind turbine system is addressed and the impact of this coupling on the design of the new generation of multi-megawatt wind turbines for offshore deployment is discussed. The primary economic drivers affecting the development of utility scale floating offshore wind farms are also addressed.