Umbilical Fatigue Analysis for a Wave Energy Converter

Unique umbilical designs for wave energy converters (WECs), including the ability to handle significantly larger motions and loads over long deployments, are often required when conventional marine umbilical designs for offshore oil and gas and offshore wind may not meet the design and cost needs of wave energy technologies. This study details a fatigue analysis of a dynamic power umbilical attached to a two-body floating point absorber WEC system, using the sea states provided for the PacWave testing facilities. The 6 degrees of freedom motion time history for the WEC was simulated, and the motions of the attachment point for the umbilical on the WEC and respective sea states were used to analyze the dynamic motions and fatigue of the connected power umbilical to predict the fatigue life. The results show that the fatigue damage observed is more significant in shallow water, and extensive fatigue damage may occur because of the larger curvature response of the umbilical. The umbilical configurations departing at 90 deg off incoming waves were found to have the highest fatigue life attributed to less extension or compression of the umbilical. However, additional bend stiffener/limiter features may need to be incorporated into the buoyancy section and touchdown regions to minimize curvature-induced fatigue.

Tuning of Nacelle Feedback Gains for Floating Wind Turbine Controllers Using a Two-DOF Model

Designing a collective blade pitch controller for floating offshore wind turbines (FOWTs) poses unique challenges due to the interaction of the controller with the dynamics of the platform. The controller must also handle the competing objectives of power production performance and fatigue load management. Existing solutions either detune the controller with the result of slowed response, make use of complicated tuning methods, or incorporate a nacelle velocity feedback gain. With the goal of developing a simple control tuning method for the general FOWT researcher that is easily extensible to a wide array of turbine and hull configurations, this last idea is built upon by proposing a simple tuning strategy for the feedback gain. This strategy uses a two degree-of-freedom (DoF) turbine model that considers tower-top fore-aft and rotor angular displacements. For evaluation, the nacelle velocity term is added to an existing gain scheduled proportional-integral controller as a proportional gain. The modified controller is then compared to baseline land-based and detuned controllers on an example system for several load cases. First-pass results are favorable, demonstrating how researchers can use the proposed tuning method to efficiently schedule gains for adequate controller performance as they investigate new FOWT configurations.

Loads on a Point-Absorber Wave Energy Converter in Regular and Focused Extreme Wave Events

Accurate modeling and prediction of extreme loads for survivability is of crucial importance if wave energy is to become commercially viable. The fundamental differences in scale and dynamics from traditional offshore structures, as well as the fact that wave energy has not converged around one or a few technologies, implies that it is still an open question how the extreme loads should be modeled. In recent years, several methods to model wave energy converters in extreme waves have been developed, but it is not yet clear how the different methods compare. The purpose of this work is the comparison of two widely used approaches when studying the response of a point-absorber wave energy converter in extreme waves, using the open-source CFD software OpenFOAM. The equivalent design-waves are generated both as equivalent regular waves and as focused waves defined using NewWave theory. Our results show that the different extreme wave modeling methods produce different dynamics and extreme forces acting on the system. It is concluded that for the investigation of point-absorber response in extreme wave conditions, the wave train dynamics and the motion history of the buoy are of high importance for the resulting buoy response and mooring forces.

Possibilities for a Novel Wave Power Generator Using Dielectric Elastomers

As industrialization, worldwide population growth, and improvements in the living standards in developing countries continue, demands for energy, food, and water, likewise surge. This in turn accelerates global warming, and its resultant extreme weather effects. Among the measures proposed to meet the growing energy demands, the use of renewable energy is gaining more and more attention. In particular, wave power generation is attracting a great deal of attention as an effective use of ocean energy. However, current wave generators are large and very expensive relative to their output. Furthermore, they cannot generate power efficiently with wave directivity, small amplitude waves and so on. For these reasons, widespread use is very limited. In order to solve these problems, this paper discusses the possibility of a recently developed wave power generator that uses a newly developed dielectric elastomer (DE) as a new way to harvest renewable energy. We also discuss the technical breakthrough of building a mega power generation system using DEs.

Motion Decay Simulations of a Moored Wave Energy Converter

Significant cost reductions are required for marine renewable energy to become competitive. Aside from the deployment of arrays, one key area that has been identified as having potential for cost reductions is the mooring system. A challenge, therefore, is to design mooring systems which can satisfy their primary role of station keeping while being affordable and durable. This paper presents the effects of three different mooring configurations on the motion behavior of a buoy type wave energy convertor, considering nonlinear mooring-induced fluid-structure interactions, such as the associated viscous effects. To simulate motion decay, an overset mesh method that coupled a dynamic mooring model with the Navier-Stokes equations flow solver OpenFOAM was adopted. The mooring configurations comprised an all catenary system, a catenary system with buoys, and a catenary system with buoys and clump weights. The favorable agreement between the simulations and experimental measurements validated the coupled numerical approach for simulating different mooring configurations. The mooring systems influenced not only restoring force characteristics, but also total damping of the system, which demonstrated the importance of considering mooring-induced damping when investigating moored offshore structures.

Lifting Technique of Composite Bucket Foundation for Offshore Wind Turbines

The onshore pre-fabrication technology for composite bucket foundations takes “prefabrication-assembly-lifting” as the core concept. The practice of pre-fabrication of upper and lower structures is prefabricated respectively. In the research of hoisting engineering technology, combined with the structural form and construction requirements of composite bucket foundation, the assembly scheme of the upper prestressed concrete transition section and the lower steel bucket and the hoisting scheme of integral foundation with compartments were designed. The finite element model in the lifting process of composite bucket foundation was established by the large-scale general finite element analysis software ABAQUS. For the optimization analysis of the lifting point arrangement during hoisting process, the number, position and arrangement form of lifting points are simulated and analyzed. The results show that the maximum value of the principal stress of the concrete transition section structure appears in the assembly stage with the lower steel bucket, and the structure checking calculation should be carried out as the most unfavorable lifting condition in construction; the peak point of structural stress is at the junction of girder and secondary beams and inner ring beams of concrete roof, which belongs to the weak position of force transmission. In construction, it should be paid attention to as the key part of monitoring to ensure composite bucket foundation is under reasonable stress and the stability in the lifting process. The research results can provide guidance and reference for the future batch production and standardization production construction for composite bucket foundations.

Nonlinear Time-Domain Simulation of the Heaving-Buoy Type Wave Energy Converter by Using Three-Dimensional Potential Numerical Wave Tank Under Irregular Wave Conditions

The aim of this paper is to evaluate the hydrodynamic performance of a heaving buoy type wave energy converter (WEC) and power take-off (PTO) system. To simulate the nonlinear behavior of the WEC with PTO system, a three-dimensional potential numerical wave tank (PNWT) was developed. The PNWT is a numerical analysis tool that can accurately reproduce experiments in physical wave tanks. The developed time-domain PNWT utilized the previously developed NWT technique and newly adopted the side wall damping area. The PNWT is based on boundary element method with constant panels. The mixed Eulerian-Lagrangian method (MEL) and acceleration potential approach were adopted to simulate the nonlinear behaviors of free-surface nodes associated with body motions. The PM spectrum as an irregular incident wave condition was applied to the input boundary. A floating or fixed type WEC structure was placed in the center of the computational domain. A hydraulic PTO system composed of a hydraulic cylinder, hydraulic motor and generator was modeled with approximate Coulomb damping force and applied to the WEC system. Using the integrated numerical model of the WEC with PTO system, nonlinear interaction of irregular waves, the WEC structure, and the PTO system were simulated in the time domain. The optimal hydraulic pressure of the PTO condition was predicted. The hydrodynamic performance of the WEC was evaluated by comparing the linear and nonlinear analytical results and highlighted the importance accounting for nonlinear free surfaces.

Impact of a Wind Turbine Blade Pitch Rate on a Floating Wind Turbine During an Emergency Shutdown Operation

During the shutdown of a wind turbine, the turbine blades rotate from their typical operating angle to their typical idling angle (approximately 90 degrees) at a specific speed, called the blade pitch rate. This operation leads to rapid loss of thrust force on the turbine resulting in a corresponding heel response of the floating structure. This rapid variation of loads at the turbine also leads to large nacelle accelerations which are transferred to the bottom of the tower and consequently to the floating structure, making the turbine shutdowns, and specifically emergency shutdowns, of significance in the design and certification of the turbine, tower and floating structure. In case of an emergency shutdown (for instance due to a grid loss), the blades typically pitch from 0 degree to 90 degrees in approximately 20–35 seconds, whereas this time period can be more than 100 seconds in the case of a normal shutdown [6]. For fixed-bottom wind turbines, increasing the blade pitch rate leads to an increase of instantaneous loads at the nacelle and tower, leading to the emergency shutdown pitch rate being usually chosen to be as low as possible. In the case of a floating wind turbine, however, water/platform interaction effects such as wave induced damping on the floating platform, challenge this approach. Indeed, increasing the blade pitch rate can increase the effect of wave-induced damping on the floater and therefore reduce the loads on the overall structure. On the other hand, reducing the blade pitch rate during an emergency shutdown can reduce this damping effect and increase those loads, meaning that an optimal blade pitch rate for a fixed bottom turbine is not necessarily optimal for a floating wind turbine. This paper will examine the behavior of a floating offshore semi-submersible platform, the WindFloat, during turbine shutdown operations, with an emphasis on the blade pitch rate during an emergency shutdown.

Parametric Study of Catenary Mooring System for a Semisubmersible Floating Wind Turbine in Intermediate Water Depth

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.

Dynamic Responses of a Spar Type Floating Offshore Wind Turbine With Failed Moorings

The complex dynamic characteristics of Floating Offshore Wind Turbines (FOWTs) have raised wider consideration, as they are likely to experience harsher environments and higher instabilities than the bottom fixed offshore wind turbines. Safer design of a mooring system is critical for floating offshore wind turbine structures for station keeping. Failure of mooring lines may lead to further destruction, such as significant changes to the platform’s location and possible collisions with a neighbouring platform and eventually complete loss of the turbine structure may occur. The present study focuses on the dynamic responses of the National Renewable Energy Laboratory (NREL)’s OC3-Hywind spar type floating platform with a NREL offshore 5-MW baseline wind turbine under failed mooring conditions using the fully coupled numerical simulation tool FAST. The platform motions in surge, heave and pitch under multiple scenarios are calculated in time-domain. The results describing the FOWT motions in the form of response amplitude operators (RAOs) and spectral densities are presented and discussed in detail. The results indicate that the loss of the mooring system firstly leads to longdistance drift and changes in platform motions. The natural frequencies and the energy contents of the platform motion, the RAOs of the floating structures are affected by the mooring failure to different degrees.