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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Floating wind turbines' taking place on Wednesday, 12 March 2014 at 16:30 -18:00. The meet-the-authors will take place in the poster area.

Juan Amate Iberdrola Engineering & Construction, Spain
Co-authors:
Juan Amate (1) P Carlos Oriol (1) Marta Caicoya (1) Victor De Diego (1) Laura Garcia (1) Pablo Gomez (1) F Bernardino Couñago (1) - - (1)
(1) Iberdrola Engineering&Construction, Madrid, Spain

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Presenter's biography

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

Juan Amate López is Head of Offshore Technology at Iberdrola Engineering & Construction (IEC). He has been working in IEC since 2004 coordinating both Ocean Líder (the biggest R+D project on Ocean Renewable Energies, with a total budget of 30 M€ and a grant of 15 M€ from the Spanish government) and Flottek, two large R+D initiatives where IEC´s TLP (Tension-legged-Platform) concept has been developed, as well as an aero-hydrodynamic software tool, hybrid solutions of wind-wave-current floating generation devices and dynamic cables & connectors for these applications

Abstract

Iberdrola E&C tension leg platform: a smart way to drive costs down

Introduction

TLPs could become a promising concept solution for the offshore wind market due to its improved dynamic behaviour & optimized cost trends. Historically the major problem for TLPs has been its transport&installation process. IEC has developed a disruptive design taking cost as main design driver. Also Installation, operability&maintainability and H&S are taken into account since the early stages of the design and an ad-hoc solution for the transport&installation of the whole platform+turbine+wavedevice has been developed and extensively tested. This presentation will focus on presenting key learnings of the design&test process and a Technical&Economical analysis.

Approach

IEC´s TLP design is the result of two different R&D projects (Ocean Lider and Flottek) in which two platforms (2 - 5 MW) have been developed in order to deal with the extreme conditions of Galician and Aberdeen coasts. A specific coupled software tool (aerodynamical+structural+hydrodynamical) has been developed and validated via code to code and model comparison and used to optimize the overall design.
First step of the design process was identifying carefully the main requirements and design parameters intervening. The main objective was fixing the overall forms of the platform, taking into account manufacturing (steel wall thickness, central body diameters, drydock requirements, etc), transport&installation and operation&maintenance (TLP dynamic behavior maximize operational limits at any operation concerned vs overall costs) activities. The use of naval standardized manufacturing methods&procedures was considered critical towards reducing costs. Transport&Installation were designed to eliminate the necessity of using expensive HLVs. Offshore Wind and Oil&Gas standards were selected to be followed during the design process.

Once the conceptual design was completed, a numerical modelling of the selected TLP platform (including the Installation system) was conducted using the fully-coupled time-domain in-house software. Its primary purpose was to take into account simultaneously both aerodynamic and hydrodynamic effects on the structure in order to evaluate the seakeeping performance of the selected platform. Simulations for operational, survival and transportation conditions (towing) were performed. Also for the installation process, a specific stability analysis for different transitional draughts is conducted. Based on strictly rigorous and unbiased criteria, the best solution according to the defined design bases is finally selected.
An iterative design loop approach were applied, where in several steps the internal scantling, final dimensions and systems configuration of the platform were completely defined and minutely characterized. This iterative process is extremely necessary in order to get to the final solution. In each step the scantling was optimized and therefore inertias were recalculated and reintroduced into the seakeeping simulations. Several structural and seakeeping simulations were finally conducted under a wide range of metocean conditions (upto 31 meters waves) and situations to completely check the final TLP design.



Main body of abstract

The TLP design/concept consists of a central cylindrical column and four pontoons symmetrically distributed on its bottom. In the top of the central column, a conical frustum allows a smooth transition between the main cylinder diameter and the WTG tower diameter.
Each of the outer ends of the four pontoons incorporates two porches which allow the connection of the two tension lines per pontoon. These two lines per pontoon assure a complete redundancy of the mooring system and guarantee stability in operational and even survival conditions even in case of break of two lines in different pontoons. Mooring lines can be fabricated of steel or synthetic material depending on the specific conditions of the site and available supply chain.

The lower ends of the mooring lines will be preferably connected to “cluster suction/driven/drilled piles” allowing the installation of the two mooring lines rapidly and accurately. By separating the mooring line installation from the platform installation process, two different weather windows would be used and thus maximizing overall available installation days.
A new method for transporting and installing the wind turbine pre-assembled onshore had to be completely developed. In order to increase the stability during transportation and installation and to avoid the use of expensive HLVs, a system of re-usable floaters will be temporarily connected to the ends of the pontoons. These floaters incorporate a variable ballast system which allows actively adjusting the platform draught according to the different requirements of each stage (float off, transportation or installation) and conferring the additional stability required to perform these temporary operations. This procedure allows fully transportation and installation of the wind turbine with standard tug vessels and increased operational limits (upto 3-5m Hs), reducing significantly installation costs, specially when applying this technology in large offshore wind projects.
Available Drydocks dimensions would allow us on establishing some additional restrictions on the size of the platform in order to have the possibility to construct and transport efficiently.

A model testing campaign were conducted at two different Hydrodynamic Centers (the CEHIPAR and the CEHINAV Model Testing Facilities). Tests under operational and survival conditions were performed considering even wind&wave misalignment. Misalign was proved to be critical for extreme conditions, due to the reduced aerodynamical damping. Influence of tidal range (three different conditions were tested HAT, MSL and LAT) were also checked. Damaged conditions are also considered in order to verify that platform compartmentalization is sufficient (any single compartment at or below the waterline is considered to be flooded due, for instance, to a collision). Also accidental conditions where 1 or 2 mooring lines were broken are simulated. Platform was stable even in survival conditions with two mooring lines (in different pontoons) were broken.
Towing tests were also performed, including a study on different towing velocities (3-5 knots) and precise measurements of the bollard pull required to perform transport operations under different sea states (upto 5m Hs). Motions and acceleration of the system were also registered under regular and irregular wave seas.

The primary objectives of the model tests were to:
• Verify numerical hydrodynamic computations and platform motion response.
• Verify model predictions under extreme metocean conditions (upto 31m waves).
• Verify transportation&Installation.
• Measure mooring line tensions.
• Verify dynamic behaviour under accidental conditions (line break, flooded compartment).
Results from these tests are carefully analysed and the platform performance is finally checked.
Free decay and regular wave tests were used to calibrate the hydrodynamic model. The system’s viscous damping was clearly underestimated and a velocity-dependent expression was introduced in the model to explicitly simulate the damping. Tendon tensions were explicitly measured and checked, and proved to be extremely well predicted.

Costs & Risks were a main design driver during all the design process and all practical decisions were taken in order to decrease fabrication, installation and O&M costs & risks. Different estimations of the final cost of the solutions were done based on current shipyard and Oil&Gas cost-rates. Finally a detailed study was conducted to specifically compute the total LCOE for a wind farm of fifty units using IEC´s TLP as floating solution. LCOE conservative estimations placed IEC´s TLP solution as a very competitive solution, specially taking into account the TLP development learning curve.



Conclusion

The proposed method uses an ad-hoc group of reusable floaters as an economical and feasible solution to the main traditional problem associated to TLPs, its transportation and installation processes. In addition, the proposed design methodology cares, since early stages, about the whole life cycle of the technology. The design process focus on the logistic chain, the manufacturing and assembly process, the transport and installation operations and also O&M and H&S requirements with the aim to reduce costs and optimize holistically the concept.
Floaters allow transportation and installation to be performed using standard and inexpensive vessels under sea states up to 3-5m Hs. Mooring system installation is independent from TLP Installation leading to reduced weather windows, and therefore a significant increase of the number of available days to perform offshore operations.
The most extensive collection of basin tests ever done in the offshore wind industry proved that the technology dynamical behavior is extremely good under the most severe sea state conditions. These tests also give very important feedback to calibrate the computational models and also allow to re-optimize the design in next development stages. Motions of the platform on heave, pitch and roll are found insignificant and may allow the installation of current WTG, without the need for manufacturers to perform significant modifications on their original designs.
Due to this great dynamic, safety at sea during O&M operations may be increased and access operation facilitated. In addition to the limited motions, the simplicity of the structure design and the reduced number of additional complex systems (i.e. no need for active ballast during operation) may reduce significantly O&M costs and risks. Fatigue on dynamic cables has proved to be acceptable and no springing or ringing has been detected on mooring lines during the tests neither simulations. The redundant mooring system allows fully operation in case of one line out of service and contributes to guarantee low risk operation. Finally, it is worth highlighting its steel weight/cost optimization (5MW platform weights 1053tn) specially when using large WTGs.
Full-scale demonstration will be next step.



Learning objectives
• • An ad-hoc solution for installing and transporting TLPs has proved as an economical and feasible method and to reduce offshore wind costs

• An hybrid conceptual design with a wave energy device was developed

• A design methodology that takes into account since the early stages the logistic chain, manufacturing, transport & installation and O&M requirements is necessary to achieve a cost effective solution.

• Basin tests methodology developed to calibrate the computational models