Dynamic Analysis of a Floating Offshore Wind Turbine Under Extreme Environmental Conditions

This paper is concerned with the development of a Floating Offshore Wind Turbine (FOWT) utilizing spar-type floating foundation. In order to design such a structure, it is essential to evaluate the dynamic response under extreme environmental conditions. In this study, therefore, a dynamic analysis tool has been developed. The dynamic analysis tool consists of a multi-body dynamics solver (MSC.Adams), aerodynamic force evaluation library (NREL/AeroDyn), hydrodynamic force evaluation library (In-house program named SparDyn), and mooring force evaluation library (In-house program named Moorsys). In this paper, some details of the developed dynamic analysis tool are given. In order to validate the program, comparison with the experimental results, where the wind, current and wave are applied simultaneously, has been made. The comparison shows that satisfactory agreements between the simulation and the experimental results are obtained. However, when VIM (Vortex Induced Motion) occurs, the current loads and cross flow responses (sway and roll) are underestimated by the simulation since the simulation code does not account for the effect of VIM.

Fully Coupled Three-Dimensional Dynamic Response of a TLP Floating Wind Turbine in Waves and Wind

A dynamic model for a tension-leg platform (TLP) floating offshore wind turbine is proposed. The model includes three-dimensional wind and wave loads and the associated structural response. The total system is formulated using 17 degrees of freedom (DOF), 6 for the platform motions and 11 for the wind turbine. Three-dimensional hydrodynamic loads have been formulated using a frequency- and direction-dependent spectrum. While wave loads are computed from the wave kinematics using Morison’s equation, aerodynamic loads are modelled by means of unsteady Blade-Element-Momentum (BEM) theory, including Glauert correction for high values of axial induction factor, dynamic stall, dynamic wake and dynamic yaw. The aerodynamic model takes into account the wind shear and turbulence effects. For a representative geographic location, platform responses are obtained for a set of wind and wave climatic conditions. The platform responses show an influence from the aerodynamic loads, most clearly through a quasi-steady mean surge and pitch response associated with the mean wind. Further, the aerodynamic loads show an influence from the platform motion through more fluctuating rotor loads, which is a consequence of the wave-induced rotor dynamics. In the absence of a controller scheme for the wind turbine, the rotor torque fluctuates considerably, which induces a growing roll response especially when the wind turbine is operated nearly at the rated wind speed. This can be eliminated either by appropriately adjusting the controller so as to regulate the torque or by optimizing the floater or tendon dimensions, thereby limiting the roll motion. Loads and coupled responses are predicted for a set of load cases with different wave headings. Based on the results, critical load cases are identified and discussed. As a next step (which is not presented here), the dynamic model for the substructure is therefore being coupled to an advanced aero-elastic code Flex5, Øye (1996), which has a higher number of DOFs and a controller module.

Wave Energy Hyperbaric Converter: Small Scale Models, Prototype and Control Strategies

A device for electricity generation from ocean waves has been developed for installation on the Brazilian northeast coast. It is based on pumping modules and a two-stage accumulator linked to a hyperbaric chamber in order to provide a prescribed high pressure water flow, which operates a turbine coupled with an electrical generator. In the period between 2001 and 2007, wave tank testing of small-scale models have been carried out. A prototype with two pumping modules is presently installed in the Port of Pecém. This paper describes an overview of the device development since the first tests to the installation of the prototype. Wave climate analysis, design analysis and optimization studies have been developed and are briefly revised in this study. Furthermore, control strategies are proposed to optimize the electrical output considering the proper combination of both pressure and flow rate as a function of the predominant wave climate, when the applied pressure is constant and the generator is a synchronous machine.

Numerical Investigation of Flow Around a ROV With Crawler Based Driving System

The Remotely Operated Vehicle, so called “ROV” which has crawler based moving system is considered as one of the appropriate underwater vehicles for seafloor exploration or seabed resources development [1][2][3][4][5][6][7]. The advantages of crawler driven ROV are to be able to stay on a fixed sea bottom location and to be capable to do heavy works such as digging the seafloor. However, the ROV moving on the sea bottom with crawler based driving system easily turn over due to the buoyancy and hydrodynamic forces [8][9][10][11][12]. Therefore, it is important to know the moving capability of the ROV on the sea bottom for the design point of view. The authors have shown the condition for the normal running of the ROV which moves on horizontal and inclined flat sea bottom by means of a simple dynamic model [11]. Normal running means that the ROV runs without bow-up or stern-up situations and the crawlers touch the ground normally. The normal running condition of ROV indicates the constrained condition of the relation between gravity and buoyancy center locations for any given design parameters such as geometry, weight, displacement and running speed of the ROV. Though this method estimates the ROVs’ moving capability with acceptable accuracy, the hydrodynamic forces on the ROV and its application point are required for accurate estimation. In the previous research, those quantities are roughly estimated from the past experimental investigations. The present study investigated the flow around the crawler driven ROV which runs on seafloor with CFD (Computational Fluid Dynamics) analysis to evaluate the characteristics of hydrodynamic forces acting on the ROV. The open source CFD code, OpenFOAM [13] was applied for flow calculation and the results were validated with model experiments. By using the calculated hydrodynamic forces on ROV, the moving capability of ROV was evaluated with a method the authors had shown. The estimates of the running capability of the ROV by using the CFD calculations are quite different from past estimations in some running conditions.

Conceptual Design of TMR-Less Manganese Nodule Mining System and the Economy Examination

Manganese nodules on deep ocean floors have been interested in as future metal resources these forty years. The Total Materials Requirement (TMR) of the conventional proposed mining method, however, is very high because of the much lifted water with the nodules from the seafloor to the sea surface and the longer transportation from the mining site to the smelting plant. An innovative conceptual design of the TMR-less mining system is presented. The economy is examined and compared with the one of the conventional method.

Simulation of the Mooring Vessels in Tsunamis Using the MPS Method Considers Leading Wave and Backwash

The present paper describes the simulation of behavior of mooring vessels in tsunami using the 3-D MPS (Moving Particle Semi-implicit) Method for considering leading wave and backwash effect. The chain of a disaster is brought about by two kinds of tsunami. The chain of disaster means breaking the mooring tether, grounding on a wharf, drift to continental areas, the collision with building by leading wave and the outflow of the floating matter by backwash. In this research, the 3D-MPS method is applied, and the bore like wave is applied as an assumed tsunami. The expression of backwash is tried by water pillar collapse. The present calculation results are compared with the experimental results and the applicability of the MPS method is discussed. In addition, the vehicle is arranged on a wharf, and the chain of disaster is simulated.

Hydrodynamic Analysis of a Vertical Axisymmetric Oscillating Water Column Device Floating in Finite Depth Waters

A floating oscillating water column device (OWC) consists of a vertical cylinder, with a finite wall thickness, partly submerged as an open-bottom chamber in which air is trapped above the inner water free surface. The chamber is connected with the outer atmosphere by a duct housing an air turbine. Forced by incident waves from any direction, the water surface inside pushes the dry air above through a Wells turbine system to generate power. In the present contribution the volume flows, the wave forces, the added mass and damping coefficients and the mean second-order loads for various configurations of OWC devices are being presented. Finally, it is tested how differentiations in the device’s geometry (wall thickness, draught, shape of the chamber, turbine characterises) affect the inner pressure and as a result the absorbed power by the device.

An Optimum Design Concept for Offshore Cage Culture

There is an overwhelming support to move aquaculture cages into offshore waters. Some of the key drivers for moving into offshore waters are limitation of available space near the coast, conflicts within as well as other coastal users, prospect of limitless expansion in offshore sites, the potential for optimum growth conditions and the need to reduce the production cost by increasing the scale of operation. Therefore, by using a set of requirements derived earlier by the authors, the paper looks into the current offshore cage designing concepts in order to propose an optimum design concept for offshore aquaculture. With the help of an expert panel, representing various disciplines which are important for fish farm development, the assessment point towards a single point mooring cage concept as the best option for offshore aquaculture farming. This concept is demonstrated by the use of simple geometric relations and graphs, showing the relation between the total horizontal forces, FH, vertical force component, FV, as the cage submerge. While the contribution from the current, FHC, to the total horizontal force, FH, is kept constant, the contribution from the waves to the total horizontal force, FH, (as assumed to be 33%) is subjected to a reduction proportional to the factor e2kz, representing the reduction in water particle velocity as a function of depth. Further, in light of the requirements derived through the major stakeholders, this paper also propose an alternative classification of cages into two major categories, i.e. systems that are intended to resist and dissipate environmental forces and system that are designed to avoid environmental forces.

Actuator Saturation Control of Floating Wind Turbines

A state feedback aerodynamic controller is proposed for the stabilization and reduction of platform/tower pitch vibrations of a spar-type floating wind turbine, considering blade pitch saturation effects. The controller is synthesized from a linearized rigid body model developed for a NREL 5-MW offshore wind turbine operating at the above rated condition (region 3). Wind turbulence and wave induced loads are obtained from the blade element momentum (BEM) aerodynamic theory and Morison’s equation, respectively. The simulation results show that the proposed nonlinear control system yields significant vibration reduction in comparison to a proportional-integral controller.

The Effect of the Spectral Distribution of Wave Energy on the Performance of a Bottom Hinged Flap Type Wave Energy Converter

The power output from a wave energy converter is typically predicted using experimental and/or numerical modelling techniques. In order to yield meaningful results the relevant characteristics of the device, together with those of the wave climate must be modelled with sufficient accuracy. The wave climate is commonly described using a scatter table of sea states defined according to parameters related to wave height and period. These sea states are traditionally modelled with the spectral distribution of energy defined according to some empirical formulation. Since the response of most wave energy converters vary at different frequencies of excitation, their performance in a particular sea state may be expected to depend on the choice of spectral shape employed rather than simply the spectral parameters. Estimates of energy production may therefore be affected if the spectral distribution of wave energy at the deployment site is not well modelled. Furthermore, validation of the model may be affected by differences between the observed full scale spectral energy distribution and the spectrum used to model it. This paper investigates the sensitivity of the performance of a bottom hinged flap type wave energy converter to the spectral energy distribution of the incident waves. This is investigated experimentally using a 1:20 scale model of Aquamarine Power’s Oyster wave energy converter, a bottom hinged flap type device situated at the European Marine Energy Centre (EMEC) in approximately 13m water depth. The performance of the model is tested in sea states defined according to the same wave height and period parameters but adhering to different spectral energy distributions. The results of these tests show that power capture is reduced with increasing spectral bandwidth. This result is explored with consideration of the spectral response of the device in irregular wave conditions. The implications of this result are discussed in the context of validation of the model against particular prototype data sets and estimation of annual energy production.