Study on the Dynamic Response for Floating Foundation of Offshore Wind Turbine

The wind resources for ocean power generation are mostly distributed in sea areas with the distance of 5–50km from coastline, whose water depth are generally over 20m. To improve ocean power output and economic benefit of offshore wind farm, it is necessary to choose floating foundation for offshore wind turbine. According to the basic data of a 600kW wind turbine with a horizontal shaft, the tower, semi-submersible foundation and mooring system are designed in the 60-meter-deep sea area. Precise finite element models of the floating wind turbine system are established, including mooring lines, floating foundation, tower and wind turbine. Dynamic responses for the floating foundation of offshore wind turbine are investigated under wave load in frequency domain.

Hydrodynamic Design and Analysis of Horizontal Axis Marine Current Turbines With Lifting Line and Panel Methods

This paper presents the computational models used by the authors at MARETEC/IST for hydrodynamic design and analysis of horizontal axis marine current turbines. The models combine a lifting line method for the optimization of the turbine blade geometry and an Integral Boundary Element Method (IBEM) for the hydrodynamic analysis. The classical lifting line optimization is used to determine the optimum blade circulation distribution for maximum power extraction. Blade geometry is determined with simplified cavitation requirements and limitations due to mechanical strength. The application of the design procedure is illustrated for a two-bladed 300 kW marine current turbine with a diameter of 11 meters. The effects of design tip-speed-ratio and the influence of blade section foils on power and cavitation inception are discussed. A more complete analysis may be carried out with an IBEM in steady and unsteady flow conditions. The IBEM has been extended to include wake alignment. The results are compared with experimental performance data available in the literature.

Optimization of Mooring Configuration Parameters of Floating Wave Energy Converters

Floating point absorbers devices are a large class of wave energy converters for deployment offshore, typically in water depths between 40 and 100m. As floating oil and gas platforms, the devices are subject to drift forces due to waves, currents and wind, and therefore have to be kept in place by a proper mooring system. Although similarities can be found between the energy converting systems and floating platforms, the mooring design requirements will have some important differences between them, one of them associated to the fact that, in the case of a wave energy converter, the mooring connections may significantly modify its energy absorption properties by interacting with its oscillations. It is therefore important to examine what might be the more suitable mooring design for wave energy devices, according to the converters specifications. When defining a mooring system for a device, several initial parameters have to be established, such as cable material and thickness, distance to the mooring point on the bottom, and which can influence the device performance in terms of motion, power output and survivability. Different parameters, for which acceptable intervals can be established, will represent different power absorptions, displacements from equilibrium position, load demands on the moorings and of course also different costs. The work presented here analyzes what might be, for wave energy converter floating point absorber, the optimal mooring configuration parameters, respecting certain pre-established acceptable intervals and using a time-domain model that takes into account the non-linearities introduced by the mooring system. Numerical results for the mooring forces demands and also motions and absorbed power, are presented for two different mooring configurations for a system consisting of a hemispherical buoy in regular waves and assuming a liner PTO.

Concept of Seafloor Mineral Processing for Development of Seafloor Massive Sulfides

Seafloor Massive Sulfides (SMS), which were formed by deposition of precipitates from hydrothermal fluids vented from seafloor, is one of unconventional mineral resources beneath deep seafloors in the world. The authors have proposed the concept of seafloor mineral processing for development of SMS, where useful minerals included in SMS ores are separated on seafloor to be lifted while the remaining gangue is disposed on seafloor in appropriate ways. To apply column flotation, one of conventional methods in mineral processing, to seafloor mineral processing, the authors carried out simulating experiments of column flotation on deep seafloor using ores including copper, iron, lead and zinc as metallic elements. Prior to the experiments at high pressures, preparatory experiments at the atmospheric pressure were carried out to find out the optimum condition of the properties of pulp, a mixture of feed ore, water and chemical reagents. In flotation experiments at high pressures, formation and overflow of froth layer by bubbling were observed at 1MPa in both of pulps with pure water and artificial seawater. The analytical data showed that the concentration of metallic elements such as copper and zinc in the concentrates recovered from the experiments was higher than that in the feed ores while the concentration of silicon and calcium, which are assigned to gangue, in the concentrates was lower than that in the feed ores. These results suggest that column flotation can be applied to operation on seafloor.

Expected Values of Wave Power Absorption Around the Japanese Islands Using OWC Types With Projecting Walls

This paper describes performance improvement of wave power absorption by using a new concept. Basic system proposed is an oscillating water column (OWC) type. An artificial harbor surrounded by projecting walls is installed. The type is called as PW-OWC in this paper. Standing waves occur in the artificial harbor, the absorbing device consequently has a resonance frequency differing from that of OWC. From the effect, the system is able to absorb wave power in very wide range of the wave frequency. From the experimental results, PW-OWC types are very good performance of wave power absorption comparing with conventional OWC types. In addition, the performance of the PW-OWC type is insensibility to the nozzle ratio of an orifice. The performance can be easily improved by installing the harbor part even if the performance of a base OWC device is not good. Finally, we investigate the expected values of acquirable wave power in not only a year but also every season. The expected values of PW-OWC types are greater than that of conventional OWC ones in seas around the Japanese islands.

Cancellation of Non-Harmonic Waves Using a Cycloidal Turbine

We computationally investigate the ability of a cycloidal turbine to cancel two-dimensional non-harmonic waves in deep water. A cycloidal turbine employs the same geometry as the well established Cycloidal or Voith-Schneider Propeller. It consists of a shaft and one or more hydrofoils that are attached eccentrically to the main shaft and can be independently adjusted in pitch angle as the cycloidal turbine rotates. We simulate the cycloidal turbine interaction with incoming waves by viewing the turbine as a wave generator superimposed with the incoming flow. The generated waves ideally are 180° out of phase and cancel the incoming wave downstream of the turbine. The upstream wave is very small as generation of single-sided waves is a characteristic of the cycloidal turbine as has been shown in prior work. The superposition of the incoming wave and generated wave is investigated in the far-field and we model the hydrofoil as a point vortex. This model has previously been used to successfully terminate regular deep water waves as well as intermediate depth water waves. We explore the ability of this model to cancel non-harmonic waves. Near complete cancellation is possible for a non-harmonic wave with components designed to match those generated by the cycloidal turbine for specified parameters. Cancellation of a specific wave component of a multi-component system is also shown. Also, step changes in the device operating parameters of circulation strength, rotation rate, and submergence depth are explored to give insight to the cycloidal turbine response characteristics and adaptability to changes in incoming waves. Based on these studies a linear, time-invarient (LTI) model is developed which captures the steady state wave frequency response. Such a model can be used for control development in future efforts to efficiently cancel more complex incoming waves.

Modelling Wave Energy Resources in the Irish West Coast

In order to assess the potential wave energy extraction, a study is made to validate a model that can be used to characterize Ireland’s wave climate in a more extensive study. The target area is the Irish West Coast, known for having the highest average wave power in Europe. The wave conditions in the coastal area were characterized by coupling the wave models SWAN and WAVEWATCH III. Validation tests are carried out with buoy data so that the model’s performance can be evaluated. The wave parameters considered for the comparisons in the time domain are significant wave height and mean period, and the spatial distribution of wave energy is examined in a case study. Theoretical values of wave power are obtained for sites close to the coast and in particular for the two tests sites of Galway and Belmullet.

Wind Turbine Installation Vessel of a New Generation

A new type of the wind turbine installation vessel is developed. The concept utilizes the Small Waterplane Area Twin Hull (SWATH) vessel. High transit speed, excellent seakeeping, and quick installation sequence allow placing a large number of wind turbines within short time minimizing the time of offshore construction works. The paper deals with the design development of this concept. Such major subjects as seakeeping, model testing in seakeeping tank, wind turbine landing sequence, and the workability are covered in the paper. Special attention will be given to the design of the active motion compensation system applied in the hoisting system. Dynamic behavior of this system is studied. The motions of the vessel are also compensated by dedicated active anti roll and anti-pitch systems. These systems were also tested in a seakeeping tank.

Introducing Wave Regeneration by Wind in a Mild-Slope Wave Propagation Model MILDwave to Investigate the Wake Effects in the Lee of a Farm of Wave Energy Converters

The increasing energy demand, the need to reduce greenhouse gas emissions and the shrinking reserves of fossil fuels have all enhanced the interest in sustainable and renewable energy sources, including wave energy. Many concepts for wave power conversion have been invented. In order to extract a considerable amount of wave power, single Wave Energy Converters (abbreviated as WECs) will have to be arranged in arrays or ‘farms’ using a particular geometrical layout, comprising large numbers of devices. As a result of the interaction between the WECs within a farm, the overall power absorption is affected. In general, the incident waves are partly reflected, transmitted and absorbed by a single WEC. Also, the wave height behind a large farm of WECs is reduced and this reduction may influence neighbouring farms, other users in the sea or even the coastline (wake effects of a WEC farm). The numerical wave propagation model MILDwave has been recently used to study wake effects and energy absorption of farms of WECs, though without taking into account wave regeneration by wind in the lee of the WEC-farm which can be significant in large distances downwave the WECs. In this paper, the implementation of wave growth due to wind in the hyperbolic mild-slope equations of the wave propagation model, MILDwave is described. Several formulations for the energy input from wind found in literature are considered and implemented. The performance of these formulations in MILDwave is investigated and validated. The modified model MILDwave is then applied for the investigation of the influence of the wind on the wakes in the lee of a farm of wave energy converters.

Progressive Drift of Moored Floating Wind Turbines in a Wind Farm: Improved Mooring Capacity Model

A wide range of platform concepts have been investigated for a floating wind turbine. So far analysis and design of motion characteristics of the platform is main research concern. One key research area less focused is floating platform related risk. If the wind energy would be one of the major sources of electric power supply, wind farms which are comprised of large number of floating wind turbines must be deployed in the ocean. Wind turbines are relatively closely arranged in a wind farm. In such an arrangement, a wind turbine accidentally started drifting will have some possibility to collide with floater and moorings of neighboring moored floating wind turbines, and might initiate another drift which might cause progressive drifting of wind turbines. In the previous report, a scenario of progressive drifting of wind turbines was investigated and associated risk was formulated. Quantitative risk of several arrangements of wind farm was estimated. Effects of arrangement of wind turbines in a wind farm and safety factor used in the design of moorings is discussed. Probability of initial drift was evaluated analyzing past records of accidents and design of mooring. In this research, strength of mooring system was modeled more precisely and probabilistic model was developed considering aged deterioration. Risk of progressive drifting was evaluated and safety factor required to realize a acceptable risk of a wind farm was discussed.