Maurizio Collu

Conceptual design of a floating support structure for an offshore vertical axis wind turbine: the lessons learnt

The design of floating support structures for wind turbines located offshore is a relatively new field. In contrast, the offshore oil and gas industry has been developing its technologies since the mid 1950s. However, the significantly and subtly different requirements of the offshore wind industry call for new methodologies. An Energy Technologies Institute (ETI) funded project called NOVA (for Novel Vertical Axis wind turbine) examined the feasibility of a large offshore vertical axis wind turbine in the 10–20 MW power range. The development of a case study for the NOVA project required a methodology to be developed to select the best configuration, based on the system dynamics. The design space has been investigated, ranking the possible options using a multi-criteria decision making (MCDM) method called TOPSIS. The best ‘class’ or design solution (based on water plane area stability) has been selected for a more detailed analysis. Two configurations are considered: a barge and a semi-submersible. The iterations to optimise and compare these two options are presented here, taking their dynamics and costs into account. The barge concept evolved to the ‘triple doughnut-Miyagawa’ concept, consisting of an annular cylindrical shape with an inner (to control the damping) and outer (to control added mass) bottom flat plates. The semi-submersible was optimised to obtain the best trade-off between dynamic behaviour and amount of material needed. The main conclusion is that the driving requirement is an acceptable response to wave action, not the ability to float or the ability to counteract the wind turbine overturning moment. A simple cost comparison is presented.

Application and extension of the TOPSIS method for the assessment of floating offshore wind turbine support structures

The offshore wind industry is shifting its focus towards deeper water sites, more suited to floating rather than bottom fixed support structures. Floating support structures currently used for oil and gas platforms provide a starting point for the floating wind industry; however, the selection of an optimum structure is not trivial with several unique factors that contribute to its complexity. In this work programme, a methodology to rapidly assess several concepts for wind turbine floating support structures is proposed. Using the multi-criteria decision-making method, TOPSIS, configurations are rated for a range of attributes. In addition, two complementary methodologies have been developed that extend the TOPSIS capability by including an estimated impact of the uncertainties associated with each of the selected criteria, on the final choice of an optimum support structure. This methodology represents a robust yet flexible design tool to complement the early conceptual design process that is able to rapidly investigate a broad design space and narrow down the number of potential options suitable for floating wind turbines.

A comparison between the dynamics of horizontal and vertical axis offshore floating wind turbines

The need to further exploit offshore wind resources in deeper waters has led to a re-emerging interest in vertical axis wind turbines (VAWTs) for floating foundation applications. However, there has been little effort to systematically compare VAWTs to the more conventional horizontal axis wind turbine (HAWT). This article initiates this comparison based on prime principles, focusing on the turbine aerodynamic forces and their impact on the floating wind turbine static and dynamic responses. VAWTs generate substantially different aerodynamic forces on the support structure, in particular, a potentially lower inclining moment and a substantially higher torque than HAWTs. Considering the static stability requirements, the advantages of a lower inclining moment, a lower wind turbine mass and a lower centre of gravity are illustrated, all of which are exploitable to have a less costly support structure. Floating VAWTs experience increased motion in the frequency range surrounding the turbine [number of blades]×[rotational speed] frequency. For very large VAWTs with slower rotational speeds, this frequency range may significantly overlap with the range of wave excitation forces. Quantitative considerations are undertaken comparing the reference NREL 5 MW HAWT with the NOVA 5 MW VAWT.

FloVAWT: Further Progresses on the Development of a Coupled Model of Dynamics for Floating Offshore VAWTS

Interest in potential wind farm sites in deeper waters and further offshore has substantially increased recently, and in parallel an increased interest towards floating, rather than bottom-fixed, offshore wind turbines: the Energy Technologies Institute (UK) recently announced a plan to invest £25m in offshore floating wind turbine projects. Furthermore, a recent document by the UK LCICG (Low Carbon Innovation Coordination Group), demonstrated that the “Development and demonstration of new concepts such as floating foundations for water depths >60m”, has a value in meeting emissions targets at low cost of up to £13bn. The present article is a follow on with the previous article presented at OMAE 2013 [1], in which the progresses on the development of an aero-hydro-servo-elastic coupled model of dynamics for VAWT are illustrated, called FloVAWT. The further progresses presented consist in: a) the model, in particular the hydrodynamic module, has been now validated against experimental data provided by the DeepCwind project (see OC4) for the semi-submersible support structure configuration, b) the additional velocity component due to the 6 degree-of-freedom motion of the supporting floating structure are now taken into account within the aerodynamic module, while previously only the displacement imposed by the support structure was considered, c) a new module dedicated to the mooring system has been developed and validated, capable of modelling catenary mooring systems with a quasi-static, energy-based approach. Some of the new capabilities of the program are illustrated through a case study of a Darrieus-type VAWT rotor coupled with the OC4 semi-submersible support structure. Comparisons with the previous version of the program are presented, giving an insight on the relative importance of the additional aspects taken into account.Copyright

The longitudinal static stability of an aerodynamically alleviated marine vehicle, a mathematical model

An assessment of the relative speeds and payload capacities of airborne and waterborne vehicles highlights a gap that can be usefully filled by a new vehicle concept, utilizing both hydrodynamic and aerodynamic forces. A high-speed marine vehicle equipped with aerodynamic surfaces is one such concept. In 1904, Bryan & Williams (Bryan & Williams 1904 Proc. R. Soc. Lond. 73, 100–116 (doi:10.1098/rspl.1904.0017)) published an article on the longitudinal dynamics of aerial gliders, and this approach remains the foundation of all the mathematical models studying the dynamics of airborne vehicles. In 1932, Perring & Glauert (Perring & Glauert 1932 Reports and Memoranda no. 1493) presented a mathematical approach to study the dynamics of seaplanes experiencing the planing effect. From this work, planing theory has developed. The authors propose a unified mathematical model to study the longitudinal stability of a high-speed planing marine vehicle with aerodynamic surfaces. A kinematics framework is developed. Then, taking into account the aerodynamic, hydrostatic and hydrodynamic forces, the full equations of motion, using a small perturbation assumption, are derived and solved specifically for this concept. This technique reveals a new static stability criterion that can be used to characterize the longitudinal stability of high-speed planing vehicles with aerodynamic surfaces.

Progress on the experimental set-up for the testing of a floating offshore wind turbine scaled model in a field site:

This document describes design and realization of a small-scale field experiment on a 1:30 model of spar floating support structure for offshore wind turbines. The aim of the experiment is to investigate the dynamic behaviour of the floating wind turbine under extreme wave and parked rotor conditions. The experiment has been going on in the Natural Ocean Engineering Laboratory of Reggio Calabria (Italy). In this article, all the stages of the experimental activity are presented, and some results are shown in terms of motions and response amplitude operators. Finally, a comparison with corresponding results obtained using ANSYS AQWA software package is shown, and conclusions are drawn. The presented experimental set-up seems promising to test offshore floating structures for marine renewable energy at a relatively large scale in the Natural Ocean Engineering Laboratory field site.

Longitudinal static stability requirements for wing in ground effect vehicle

ABSTRACT The issue of the longitudinal stability of a WIG vehicle has been a very critical design factor since the first experimental WIG vehicle has been built. A series of studies had been performed and focused on the longitudinal stability analysis. However, most studies focused on the longitudinal stability of WIG vehicle in cruise phase, and less is available on the longitudinal static stability requirement of WIG vehicle when hydrodynamics are considered: WIG vehicle usually take off from water. The present work focuses on stability requirement for longitudinal motion from taking off to landing. The model of dynamics for a WIG vehicle was developed taking into account the aerodynamic, hydrostatic and hydrodynamic forces, and then was analyzed. Following with the longitudinal static stability analysis, effect of hydrofoil was discussed. Locations of CG, aerodynamic center in pitch, aerodynamic center in height and hydrodynamic center in heave were illustrated for a stabilized WIG vehicle. The present work will further improve the longitudinal static stability theory for WIG vehicle.

Design of floating offshore wind turbines

Abstract The floating wind industry is still in its infancy, with just a few scaled-down prototypes deployed around the world, but at the same time there is a lot of interest in this new field, since it is deemed to play a major role in the future of offshore wind. In the present chapter a classification methodology for floating offshore wind turbine (FOWT) systems is presented, followed by some considerations about their conceptual/preliminary design. In doing so, some of the basic concepts and principles are illustrated, starting from the theories developed for oil and gas offshore floating support structures, but adapting/modifying them specifically for FOWT. Then, the key issues in the design of FOWT are presented, such as the lack of design integration, the need for new and specific guidelines and standards, the limits of the numerical models available and the impact of the floating platform on turbine loading. A case study is then presented, and a look into the future trends that have emerged in the latest years is given.

Long-term global performance analysis of a vertical-axis wind turbine supported on a semi-submersible floating platform

This study examines the long-term reliability analysis of a floating vertical axis wind turbine (VAWT) situated off the Portuguese coast in the Atlantic Ocean. The VAWT, which consists of a 5-MW 3-bladed H-type rotor developed as part of the EU-FP7 H2OCEAN project, is assumed to be mounted on the OC4 semi-submersible floating platform. Given metocean conditions characterizing the selected turbine site, a number of sea states are identified for which coupled dynamics simulations are carried out using the FloVAWT design tool. Short-term turbine load and platform motion statistics are established for individual sea states that are analysed. The long-term reliability yields estimates of 50-year loads and platform motions that takes into consideration response statistics from the simulations as well as the metocean (wind-wave) data and distributions. Results can be used to guide future floating VAWT designs.Copyright

Techno-economic modelling analysis of microalgae cultivation for biofuels and co-products

The goal of microalgae for biofuels is not only to replace fossil fuel in quantity but also for the cost to be at parity with the existing fuel stock. Adopting this evolving technology would not only require a combination of technical and economic assessment, but also the confidence of key stakeholders. The main challenge is that there are multiple pathways and logistics related to the entire algal biofuel production chain, and each stage is subject to technical, economic, environmental and policy issues, making it difficult to determine the optimum option. Therefore there is a need for a holistic decision-making tool, which provides a clear choice of direction to all stakeholders. The study initially adopted a multi criteria decision-making approach called TOPSIS, using a case study of five alternatives of algal processes to produce either oil for transport or biogas for electricity generation via a wastewater treatment method. The TOPSIS technique is used to identify the most acceptable alternative that has the maximum distance from the negative ideal and the minimum distance from the positive ideal solution. The result shows the alternative 1 and 2 as the preferred ideal solution among the others. Because the method ranks alternatives according to attributes, it limits the technique to a more defined process and therefore it cannot provide an understanding of feedback effects such as policy, resource availability in a region, and possible scale-up issues affecting the process. These limitations make the TOPSIS a less powerful technique for assessing the overall microalgae supply chain. However it is feasible to adopt the TOPSIS approach and integrate it into a flexible Techno-Economic (TE) model to make a decision over defined processes.