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Wednesday, 12 March 2014
16:30 - 18:00 Floating wind turbines
Science & Research  


Room: Llevant
Session description

The session covers design problems related to floating wind turbines and how current research is overcoming these hurdles through innovative platform concepts, experimental methods and mooring system analysis. In particular, technical and economic studies and analyses for three different novel concepts will be presented, including one vertical axis concept, a concrete platform design and a combined wind & wave energy device. In addition, a new methodology for experimental model testing with a focus on aerodynamics and control will be presented, as well as a detailed assessment on long-term mooring system loads.

Learning objectives

  • Learn about a novel experimental methodology for floating wind turbine aerodynamic and controller testing
  • Identify challenges and benefits of an innovative vertical axis concept
  • Understand the design and potential benefits and challenges of a concrete platform
  • Assess structural fatigue damage of a multi-modal wind/wave energy device
  • Learn about a new methodology capable of reproducing life cycle mooring loads
Lead Session Chair:
Denis Matha, University of Stuttgart, Germany

Co-chair(s):
Antoine Peiffer, Marine Innovation and Technology, United States
Climent Molins Universitat Politècnica de Catalunya, Spain
Co-authors:
Climent Molins (1) F P Alexis Campos (1) Frank Sandner (2) Denis Matha (2)
(1) Universitat Politècnica de Catalunya, Barcelona, Spain (2) University of Stuttgart, Stuttgart, Germany

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Dr. Molins is Associate Professor of Structural Engineering at the Universitat Politècnica de Catalunya (UPC). He has participated and led several research projects dealing with his main research areas: structural static and dynamic analysis of masonry constructions, structural analysis of precast segmental tunnel linings and steel fiber reinforced concrete members. He has published more than thirty papers in scientific journals and presented more than one hundred papers in conferences. As a practitioner, he was responsible more than one hundred structural projects from 1996 to 2004. Since 2011 he has been involved in the design of concrete floating platforms for WT.

Abstract

Monolithic concrete off-shore floating structure for wind turbines

Introduction

Floating platforms prototype concepts revealed in the ongoing projects, when compared to conventional fixed bottom offshore platforms, are much more expensive, with platform cost and mooring systems being the greatest cost drivers. Crucial to the future economic success and thus to the ability to exploit the great resources along coastlines with deep water, floating platforms need to become more cost effective.

Approach

A new concept of a SPAR floating platform is being developed in the KIC-Innoenergy project AFOSP (Alternative Floating Platform Designs for Offshore Wind Towers using Low Cost Materials) , being members of the consortium Gas Natural Fenosa, Universität Stuttgart and Universitat Politècnica de Catalunya. The main differentiating aspects with respect to other SPAR prototype are the monolithic nature of the whole structure, including both the platform and the tower, the use of posttensioned concrete as main material and the towing and erection process (UPC patent applications, 2011 and 2012)

The use of concrete has several advantages in comparison with steel solutions, being the strongest advantages the material and the maintenance costs and the service lifetime of the structure. The Oil & Gas industry experience has demonstrated the durability and the almost free maintenance of the concrete platforms, both for floating and for gravity base platforms.

A comparison between steel and concrete similar designs demonstrates that the material cost for the concrete prototype is one third of the steel one. Considering that the concrete O&G platforms are virtually free of maintenance, the real cost is less than 1/3 of the steel one, while the offshore tasks and the moorings systems have similar costs for both solutions. In addition, it has to be taken into account that the design service life can easily reach 50 years instead of 20, which is commonly used in steel off-shore platforms.

In this paper, results of the studies performed on such platform supporting the NREL 5MW wind turbine (Jonkman et al, 2009) are presented. Main sizes and thickness of the members, including material amounts and the estimation of costs will be presented, jointly with some details of the analyses.


Main body of abstract

As stated previously, a comparison between a concrete and a steel SPAR structures have been performed. In order to be comparable, both designs provide a similar restoring stiffness, which allows a tilt of 5º under the thrust force of 1.700kN applied at the rotor axis (+90m). The main dimensions are larger for the concrete one. The steel mass is 3,240,000 kg whilst the concrete mass is 12,190,000 kg. In order to maintain a similar restoring moment, the metacentric heights have to compensate that difference, being at the order of 17m for the concrete prototype and 25m for the steel one. The amount of ballast is larger for the steel one 9,700,000 kg than for the concrete one 8,960,000 kg. Assuming common materials costs for steel and concrete, the concrete prototype is about 1/3 of the cost of the steel one.

The floater is 120m long with external diameter of 13m while the tower is formed by a conical transition piece with diameter between 13m and 10m with a total length of 10m and a conical upper part with length of 87.6m with diameter comprised between 10m at the base and 4m at the top. The thickness of the sections are 50cm for the floater and transition pieces and 40cm for the tower. The draft of the structure at equilibrium position is 130m.

The most relevant structural parts have been checked, including nonlinear dynamic analysis for some critical load cases. Critical sections have been designed and the reinforcement (EC2, 2005) the passive and active reinforcement has been done, including a fatigue limit state checking according to DNV-OS-J101 (2013), obtaining a lifetime over 50 years.

From the worst load case computed, the most unfavourable section has been detected, being the bottom of the tower the section with the higher stresses along the structure. For that case, the bending moment acting on the section is around 910.000kN•m, resulting in a stress range of 20MPa. The most loaded section in the floater part presents bending moments around 1.060.000kN•m, resulting in a stress range close to 15MPa.

To ensure a long lifetime, the whole structure is forced to be compressed under this conditions, applying a total force for prestresed tendons of 395.000kN in the tower base and 360.000kN for the floater section. Since fatigue affects both compression and tension in concrete, a maximum compression should be limited. A compression level less than 25% of the fck is applied at any section, being possible to achieve fatigue lifetime over 50 years.

The amount of concrete for the designed prototype is around 3490m3 with a total amount of steel around 340,000 kg for passive reinforcement and 370,000kg for prestressing steel.

A modal analysis of the whole structure assuming hydrostatic and moorings stiffnesses as boundary conditions was performed. The first 3 modes correspond to the surge, roll/sway and heave directions, being 252s, 46s and 30s respectively. The period of the fourth mode, corresponding to the first structural, was 1.4s (0.7Hz), being over the 3p wind turbine eigenfrequency in the Campbell’s diagram. The second structural eigeperiod is 0.6s (1.7Hz).

Regarding the construction process, The SPAR design is probably the easiest concept to build in concrete, because its geometry is very simple and allows the use of simple formworks. Due to its simplicity, reinforcements can be added in an efficient way. The construction may be done horizontally in a dry dock. Once build, it can be transported by towing also in horizontal position with a required draft less than a half of its diameter, using a common tugboat.

When the structure is on the final position it should be partially flooded to put it in vertical position. In this case, one of the best options to reduce the maximum bending moment on the tower while erecting the structure is to partially sink the whole structure in such manner that when it becomes vertical, only a portion of the tower protrudes from MSL.

The installation of the wind turbine should be done before ballasting the structure with aggregates, maintaining the water ballast and, after the installation of the wind turbine, emerging the structure by pumping out water. The operation may be done without using large floating cranes.


Conclusion

The Spar type offshore floating structure including both the platform and the tower for supporting a 5 MW WT is much cheaper in posttensioned concrete than in steel. In spite that the concrete one is much heavier in terms of total mass, the amount of ballast is almost the same, since both require equivalent hydrostatic stiffness. According to the results herein presented the material costs of the concrete one is about one third of the steel one.

An addition advantage is that concrete allows the construction of a monolythic member from the bottom of the platform to the top of the tower, avoiding any connetion inbetween.

One of the critical aspects of designing offshore structures is the construction process. In this paper, the proposed process includes the construction of the whole structure in horizontal position inside a dry dock. After flooding the dry dock, the structure can be towed in almost horizontal position to the final place and, then, it has to be erected in a controlled manner by pumping in and out water ballasting and then substituting it by dense aggregates. For installing the WT, the ability of this structure to partially emerge and submerge hugely facilitates that operation.

In view of these promising results, the prototype is considered to be a possible real solution to achieve a cost-effective platform for deep water locations. The next step to provide a proof of concept is to test a scale model of such monolithic concrete floating wind platform at the Universitat Politècnica de Catalunya wave flume, at 1:100 scale, it is expected to provide some preliminary experimental results in the conference.



Learning objectives
Concrete spar type structures are a cost-effective durable solution for floating off-shore platforms for WT. The concept presented is monolythic from the bottom of the platform to the top of the tower, avoiding any connetion inbetween. Suitable construction process that makes possible this solution will be presented.


References
[1] EUROCODE 2: Design of concrete structures Part 1-1, 2005.

[2] D. N. Veritas, DNV-OS-J101: Design of offshore wind turbines, 2013.

[3] J. Jonkman, S. Butterfield, W. Musial y G. Scott, «Definition of a 5-MW Reference Wind Turbine for Offshore System Development,» National Renewable Energy Laboratory, Golden, US-CO, 2009.

[4] C. Molins, J. Rebollo and A. Campos, “Estructura flotante de hormigón prefabricado para soporte de aerogenerador”. Patent Application P201132097, 2011.

[5] C. Molins, J. Rebollo and A. Campos, “Proceso de instalación de estructura flotante tipo SPAR para soporte de aerogenerador formada por una única pieza única que incluya torre de soporte y elemento flotador”. Patent Application P201230199:, 2012.

[6] C. Molins, J. Rebollo and A. Campos, “Proceso de sustitución o remoción de aerogenerador en estructuras flotantes monolíticas tipo SPAR”. Patent Application P201230390:, 2012.