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.

Appraisal of the IEC Technical Specification for Assessment of Wave Energy Resources

This article describes and presents results from research focused on appraising the new technical specification (TS) for the assessment of wave energy resources developed by technical committee 114 of the International Electro-technical Commission (IEC-TC-114). The new IEC TS is appraised through an extensive pilot application to the waters off the west coast of Vancouver Island, British Columbia, Canada. A series of wave models are developed and used to simulate the wave conditions and estimate the wave energy resource over the study area. The accuracy of the various resource estimates derived from the model outputs is assessed through comparison with measurements from a directional wave buoy. Furthermore, sensitivity analyses are conducted to determine the main sources of error and uncertainty impacting the precision of resource assessments obtained following the IEC methodology. Preliminary results indicate that the IEC TS can be applied to the estimation of wave energy resources with a reasonable level of effort and accuracy.

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.

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.

Systematic Testing of the Fatigue Performance of Submerged Small-Scale Grouted Joints

With an increasing demand for renewable energy, offshore wind farms become more and more important. Within the next 15 years the German government intends to realize offshore wind farms with a capacity of 15 GW of electrical energy. This corresponds to approximately 3000 to 4000 new turbines. The grouted joint is a common structural detail for the connection between substructure and foundation piles in offshore wind turbine structures. For lattice substructures such as jackets, the connection is located just above the seabed and is permanently surrounded by water. Prior investigations by Schaumann et al. showed that the surrounding water may have an impact on the fatigue performance of grouted joint specimens. Thus far, very few results of submerged fatigue tests on grouted joint specimens are published and their statistical reliability is insecure. Within this paper, 24 individual test results are presented. Regarding test parameters, the focus is set on two different applied load levels, two different loading frequencies and two different grout materials. All parameters are varied in a factorial experiment and are statistically evaluated. The evaluation shows that load level and loading frequency have a significant effect on the fatigue performance of the connection. Moreover, both effects are significantly correlated. For the used grout materials no significant impact is visible, which can be explained by their similarity regarding mechanical properties and micro structure. Furthermore, the mean displacement and the stiffness degradation of the specimens during fatigue tests are discussed in detail in the paper. In conclusion, previously published results on the fatigue performance of submerged small scale grouted joint specimens can be confirmed. Load level as well as loading frequency can be stated as most relevant parameters for the fatigue performance.

Effect of Pile-Soil Interaction on Structural Dynamics of Large MW Scale Offshore Wind Turbines in Shallow-Water Western GOM

The effect of pile-soil interaction on structural dynamics is investigated for a large offshore wind turbine in the hurricane-prone Western Gulf of Mexico (GOM) shallow water. The offshore wind turbine has a rotor with three 100-meter blades and a mono-tower structure. Loads on the turbine rotor and the support structure subject to a 100-year return hurricane are determined. Several types of soil are considered and modeled with a distributed spring system. The results reveal that pile-soil interaction affects dynamics of the turbine support structure significantly, but not the wind rotor dynamics. Designed with proper pile lengths, natural frequencies of the turbine structure in different soils stay outside dominant frequencies of wave energy spectra in both normal operating and hurricane sea states, but stay between blade passing frequency intervals. Hence potential resonance of the turbine support structure is not of concern. A comprehensive Campbell diagram is constructed for safe operation of the offshore turbine in different soils.

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.