Coupled Dynamic Analysis for Multi-Unit Floating Offshore Wind Turbine in Maximum Operational and Survival Conditions

Numerical tools for a single floating offshore wind turbine (FOWT) have been developed by a number of researchers, while the investigation of multi-unit floating offshore wind turbines (MUFOWT) has rarely been performed. Recently, a numerical simulator was developed by TAMU to analyze the coupled dynamics of MUFOWT including multi-rotor-floater-mooring coupled effects. In the present study, the behavior of MUFOWT in time domain is described through the comparison of two load cases in maximum operational and survival conditions. A semi-submersible floater with four 2MW wind turbines, moored by eight mooring lines is selected as an example. The combination of irregular random waves, steady currents and dynamic turbulent winds are applied as environmental loads. As a result, the global motion and kinetic responses of the system are assessed in time domain. Kane’s dynamic theory is employed to formulate the global coupled dynamic equation of the whole system. The coupling terms are carefully considered to address the interactions among multiple turbines. This newly developed tool will be helpful in the future to evaluate the performance of MUFOWT under diverse environmental scenarios. In the present study, the aerodynamic interactions among multiple turbines including wake/array effect are not considered due to the complexity and uncertainty.

Evaluation of the Dynamic-Response-Based Intact Stability Criterion for Floating Wind Turbines

As an alternative to the conventional intact stability criterion for floating offshore structures, known as the area-ratio-based criterion, the dynamic-response-based intact stability criteria was initially developed in the 1980s for column-stabilized drilling units and later extended to the design of floating production installations (FPIs). Both the area-ratio-based and dynamic-response-based intact stability criteria have recently been adopted for floating offshore wind turbines (FOWTs). In the traditional area-ratio-based criterion, the stability calculation is quasi-static in nature, with the contribution from external forces other than steady wind loads and FOWT dynamic responses captured through a safety factor. Furthermore, the peak wind overturning moment of FOWTs may not coincide with the extreme storm wind speed normally prescribed in the area-ratio-based criterion, but rather at the much smaller rated wind speed in the power production mode. With these two factors considered, the dynamic-response-based intact stability criterion is desirable for FOWTs to account for their unique dynamic responses and the impact of various operating conditions. This paper demonstrates the implementation of a FOWT intact stability assessment using the dynamic-response-based criterion. Performance-based criteria require observed behavior or quantifiable metrics as input for the method to be applied. This is demonstrated by defining the governing load cases for two conceptual FOWT semisubmersible designs at two sites. This work introduces benchmarks comparing the area-ratio-based and dynamic-response-based criteria, gaps with current methodologies, and frontier areas related to the wind overturning moment definition.

Sensitivity of a Wave Energy Converter Dynamics Model to Nonlinear Hydrostatic Models

A three dimensional time-domain model, based on Cummins equation, has been developed for an axisymmetric point absorbing wave energy converter (WEC) with an irregular cross section. This model incorporates a number of nonlinearities to accurately account for the dynamics of the device: hydrostatic restoring, motion constraints, saturation of the power-take-off force, and kinematic nonlinearities. Here, an interpolation model of the hydrostatic restoring reaction is developed and compared with a surface integral based method. The effects of these nonlinear hydrostatic models on device dynamics are explored by comparing predictions against those of a linear model. For the studied WEC, the interpolation model offers a large improvement over a linear model and is roughly two orders-of-magnitude less computationally expensive than the surface integral based method.

Control of an Open-Loop Hydraulic Offshore Wind Turbine Using a Variable-Area Orifice

The viability of offshore wind turbines is presently affected by a number of technical issues pertaining to the gearbox and power electronic components. Current work is considering the possibility of replacing the generator, gearbox and electrical transmission with a hydraulic system. Efficiency of the hydraulic transmission is around 90% for the selected geometries, which is comparable to the 94% expected for conventional wind turbines. A rotor-driven pump pressurises seawater that is transmitted across a large pipeline to a centralised generator platform. Hydroelectric energy conversion takes place in Pelton turbine. However, unlike conventional hydro-energy plants, the head available at the nozzle entry is highly unsteady. Adequate active control at the nozzle is therefore crucial in maintaining a fixed line pressure and an optimum Pelton turbine operation at synchronous speed. This paper presents a novel control scheme that is based on the combination of proportional feedback control and feed forward compensation on a variable area nozzle. Transient domain simulation results are presented for a Pelton wheel supplied by sea water from an offshore wind turbine-driven pump across a 10 km pipeline.

Model Tests of a Spar-Type Floating Wind Turbine Under Wind/Wave Loads

Wind power has great potential because of its clean and renewable production compared to the traditional power. Most of the present researches for floating wind turbine rely on the hydro-aero-elastic-servo simulation codes and have not been exhaustively validated yet. Thus, model tests are needed and make sense for its high credibility to master the kinetic characters of floating offshore structures. The characters of kinetic responses of the spar-type wind turbine are investigated through model test research technique. This paper describes the methodology for wind/wave model test that carried out at Deepwater Offshore Basin in Shanghai Jiao Tong University at a scale of 1:50. A Spar-type floater was selected to support the wind turbine in this test and the model blade was geometrically scaled down from the original NREL 5 MW reference wind turbine blade. The detail of the scaled model of wind turbine and the floating supporter, the test set-up configuration, the mooring system, the high-quality wind generator that can create required homogeneous and low turbulence wind, and the instrumentations to capture loads, accelerations and 6 DOF motions are described in detail, respectively. The isolated wind/wave effects and the integrated wind-wave effects on the floating wind turbine are analyzed, according to the test results.

Computation of Nonlinear Hydrodynamic Loads on Floating Wind Turbines Using Fluid-Impulse Theory

A hydrodynamics computer module was developed to evaluate the linear and nonlinear loads on floating wind turbines using a new fluid-impulse formulation for coupling with the FAST program. The new formulation allows linear and nonlinear loads on floating bodies to be computed in the time domain. It also avoids the computationally intensive evaluation of temporal and spatial gradients of the velocity potential in the Bernoulli equation and the discretization of the nonlinear free surface. The new hydrodynamics module computes linear and nonlinear loads — including hydrostatic, Froude-Krylov, radiation and diffraction, as well as nonlinear effects known to cause ringing, springing, and slow-drift loads — directly in the time domain. The time-domain Green function is used to solve the linear and nonlinear free-surface problems and efficient methods are derived for its computation. The body instantaneous wetted surface is approximated by a panel mesh and the discretization of the free surface is circumvented by using the Green function. The evaluation of the nonlinear loads is based on explicit expressions derived by the fluid-impulse theory, which can be computed efficiently. Computations are presented of the linear and nonlinear loads on the MIT/NREL tension-leg platform. Comparisons were carried out with frequency-domain linear and second-order methods. Emphasis was placed on modeling accuracy of the magnitude of nonlinear low- and high-frequency wave loads in a sea state. Although fluid-impulse theory is applied to floating wind turbines in this paper, the theory is applicable to other offshore platforms as well.

Comparison of Numerical Simulations With Experimental Measurements for the Response of a Modified Submerged Horizontal Cylinder Moored in Waves

Ocean industries such as oil and gas, defence, and marine renewables, face the challenge of costly and risky deployments and operations due to their complex and capital intensive nature. Numerical simulation tools are valuable assets that can be used to anticipate motions and stresses and therefore inform structural and operational design before deployment. Simulation tools that can capture all pertinent hydrodynamic phenomena increase their value by reducing design time, uncertainty, risk and capital associated with a deployment. Validation of numerical tools is critical to ensure accuracy and reliability. The following paper reviews a framework for simulation of moored, multi-body, floating systems, including the component models employed, the results of a model verification study, and the challenges encountered in the project. Tank test data of a moored horizontal cylinder was provided for the purposes of numerical tool validation.

Validation of the Index to Determine Design Parameters of a WEC for the Cost Optimization of a Wave Farm

An index to estimate the cost of electricity (COE) generated by a wave farm from the design parameters of a wave energy converter (WEC), such as the body size and the generator capacity, was examined to show the validity of index value in this study. The validation tests are performed for three different wave farm settings at three different locations. The result displays the potential of index to capture the trend of COE value especially when the wave farm size is small. The calculation result of COE reveals that the parameter combination to give better profitability is determined by the balance between WEC construction fee and installation fee. So, it would be different from the optimum size to have the best energy conversion efficiency. It also explains the shift of parameter combination to give the better profitability when the size of wave farm is changed. However, the index contains certain level of error because of the lack of this feature. Therefore, the error becomes larger when the size of wave farm becomes larger. As a result, it was found that the modification of the index is needed to improve the accuracy by including the cost related to the number of buoys in the wave farm.

New Extreme Model Applied to Mooring System Design Load Case Assessment

Offshore wind energy turbines are being deployed massively in the North Sea. Most of the latest developments are monopile based due to the local bathymetry. However, future offshore wind farms will be located at larger water depths. Mainly because the nearest sites to the shoreline will be already occupied, future wind farms will be in 60 m water depth at least. This is, approximately, the limit for fixed support structures, such as monopiles, tripods and jackets. Some developers have already identified this need and some prototypes are under testing, such as WindFloat and Hywind. Floating wind technology will face some challenges. One of the most important ones is how to moderate the cost of the platform and the mooring system. Consequently, it is necessary to reduce the uncertainty during design steps. In this paper, new extreme mixed model will be applied to mooring system design. This extreme model combines instrumental and reanalysis data in order to obtain more accurate design parameters, reducing the uncertainty and improving the input that is required for the structural design of these concepts.

A Vertical Axis Wave Turbine With Cup Blades

A new wave energy converter (WEC) design and some test results are discussed in this work. Among a variety of WEC technologies being explored to date, a huge majority employs wave-driven reciprocating motion (e.g., heave, pitch, sway, reciprocating bending or curving, etc.) to harness energy. It is well known that reciprocating WECs only work well at or near a predefined wave frequency, in a preferred alignment angle with the wave direction (except for the heave type), and in organized waves. But real ocean waves are chaotic and have daily changing frequencies and propagation directions. To circumvent those issues of the reciprocating WECs, a new unidirectional WEC concept — a vertical axis wave turbine — is explored in this research. The key component of the wave turbine is a rotor, which has a number of uniquely arranged hemispherical shells as blades. When the rotor is exposed in waves with its shaft vertically oriented, local water motion in any spatial directions (due to waves) can always drive the rotor for unidirectional rotation regardless of the wave type and propagation direction. In other words, the rotor can rely on omnidirectional water motion to realize its unidirectional rotation. A model wave turbine employing this rotor design has been tested in a wave flume. Upon a successful demonstration in simulated irregular waves, the rotor’s unidirectional performance was systematically characterized under various experimental conditions in simple waves.