Technical and economical aspects of a floating offshore windfarm

Tecnomare UK

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TECHNICAL AND ECONOMICAL ASPECTS OF A FLOATING

OFFSHORE WINDFARM

K. Tong and C. Cannel!

Tecnomarc (UK) Ltd. Ekley House.

24-30 Great Titchfield Street

London \V I P 7AD

Paper for BWEA/DTI/Wind Engineering Seminar on "Prospects for Offshore Wind Energy'

ABSTRACT

The essential characteristics of a floating wind turbine system are discussed. An outline

of the design of the mooring system and power transmission system is also presented for a

notional 12 MW windfarm. A capital and operating cost model has been deveioped for

the windfarm which demonstrates that the concept could enable deeper water wind energy

resources to be exploited at costs consistent with those for current shallow water

developments.

1.

INTRODUCTION

All existing 'offshore' wind turbines, and most proposed 'offshore' windfarms are premised

on sites in relatively shallow water nearshore supported by bottom standing structures. However shallow water sites are not only limited in their availability, but also in their

potential to exploit fully the benefits of being 'offshore'.

Offshore windfarm development using floating support structures, being relatively

insensitive to water depth and seabed conditions, enables installation in plentiful numbers without the planning and space constraints found onshore. They can be further coupled to offshore sites with higher wind energy potential and turbine design with higher tip speed

The FLOAT project was undertaken by Tecnomare (UK) Ltd.. Garrad Hassan & Partners

Ltd.. and BMT Offshore Ltd. to assess the feasibility of using floating platforms for wind

turbines. The project was funded by DTI under the Wealth from the Oceans Programme.

The project aimed to provide technical definition of a specifically designed offshore windfarm system together with associated operational and economic consideration. Tecnomare (UK) Ltd. is the offshore engineering and design company responsible for design of the floater, mooring system, fabrication and installation considerations. IMR. and overall project management. Garrad Hassan is

the turbine design consultant

responsible for the turbine specifically for the floating windfarm. and electrical aspects.

BMT is the offshore consultant responsible for the environment data analysis and model

testing.

DESIGN SCENARIO

Two design scenarios were specified ut the beginning of the project to focus the research

while allowing reasonable assessment

of the sensitivity of the system

parameters. The environmental conditions study was based on wind resources, water

depth. distance to land. perceived energy market, and relatively benign environment. Two

locations around UK and European waters were nominated as a result : Northern Irish Sea

and Central Aegean Sea around theGreek Islands.

As a result of further in-depth environmental study, the design conditions were specified

as:

FLOAT WINDFARM SYSTEM DESIGN

The design was carried out in two phases. with a third phase mainly for design validation

by model testing. Phase I was a preliminary design study using simple approximation

methods for design, dynamics, materials, and cost assessment aiming to

select a

configuration for more detailed design and analysis in Phase 2. Detaileddynamic analysis

of the floater motions and the turbine system under the combined wind and wave action

were carried Out in Phase 2. Because of the novelty of the FLOAT system. heavily

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Tecnomare UK

influenced by both wind and wave actions, a comprehensive model test programme was conducted in Phase 3 to validate the design and identify any unexpected behaviour.

FLOATER SYSTEM DESIGN

A variety of possible floater configurations were investigated in the phase

including simple barge form to complicated four column semi-submersible hull form. Twin turbine configurations were also considered. A simple SPAR buoy concept of

concrete construction was selected tor Phase 2 studies.

The final selected design is shown in Fig. 1. The system consisted of a three bladed turbine rated at 1.4MW supported h' a steel tripod spaceframe tower at 45m above SWE.

The tower was bolted onto the deck of a concrete cylindrical buoy hull with a wider

bottom disk to improve the dynamic behaviour. The buoy was moored onto the seabed by

8 lines.

The critical characteristics of the tower were to avoid resonance at the nP. multiples of the

turbine rotation frequency, and the weight. which translates into cost.

compelling needs to reduce top weight. i.e. weight of the tower and turbine, to maximise overall floater stability.

The buoy was a simple tubular concrete shell construction selected to be easy and cheap to

fabricate.

The final design was a compromise between conflicting requirements

minimising size (cost), maximising stability, and minimising dynamic motion response.

After investigations on various materials of construction. concrete was considered most cost effective given the size of the buoy, material unit cost, fabrication cost and the

requirement for counter ballast for stability.

The design was adaptable to water depth between 75m and 500m. The mooring lines were catenary chain or taut wire synthetic fibre rope depending on the water depth. The primary function of the mooring system was to keep the buoy staying in position under the drifting action of wind and wave. lt does not affect the wave induced oscillatory motion of the buoy at all. Good offshore engineering practice of keeping the buoy in place when one line is broken resulted in 8 mooring lines used per buoy. The mooring

lines were connected to piled anchors at the seabed thus allowing multiple lines sharing a

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single anchor to reduce the system cost.

The turbine generated power at 3.3 kV which was carried by a dynamic cable throughthe

bottom of the buoy to a submerged buoy connector near the seabed. Power was gathered

to a submerged transformer by in farm static cables connected to each connector buoy.

Transmission to shore is via 33kV static cable after stepping up by the underwater

trans former.

TURBINE DESIGN

The turbine was specifically designed to exploit the benefits of offshore locations including higher wind speeds. no noise constraints - to aim at a machine with higher

overall efficiency. The possibility of complicated interaction with the floating support

structure was considered carefully. One of the consequences being the adoption of a free

yawing nacelle with downwind rotor.

The result was a lightweight rotor of carbon ribre construction with a combined nacelle weight of below 50 tonne. The machine was rated at 1.4MW at 12 ni/s hub wind speed.

cutting in at 6 mIs and cutting out at 26 m/s. The rotor was three bladed of 60m diameter operating downwind with 100 cone angle. A very high tip speed of 120m/s was achieved

with low solidity. The power train consisted of epicvclic gearbox driving an induction generator.

WINDFARM DESIGN

A typical windfarm consisting of 3 arrays

of 3 machines, as depicted in Fig. 2. was

Most anchors were shared by 2 or 4

mooring lines so reducing the cost of the piles and installation.

4.

COST ANALYSIS

Cost analyses were carried out to assess the economy of the FLOAT system. The analysis followed the fundamental principles of land based windfarm development cost analysis, and referenced to the recent RES and WEG studies, albeit for nearshore windfarms.

Tecnomare UK

The cost breakdown for a 9 buoy windfarm in Northern Irish Sea is shown in Fig. 3. The site was assumed 10 km from shore at lOOm water depth with a nominal farm output of

12.6 MW. The overall capital cost for the farm was approximately £30 million. Mooring

system was found to be the bigest cost element (28%) followed by buoy fabrication

19%) and turbines (18%). Power cables cost was also siznificant (15%).

For the alternative Aegean Sea site. assuming to be 5 km from shore at 300m water depth.

the overall development cost was found to he almost the same. Slight variations of

proportions were found.

Fig. 4 shows the cost of power in pence per kWh for the Northern Irish Sea site. The

fundamental assumptions were 20 'ear field life. 8% discount rate, income inflation of

5%, operating cost inflation ot I %. overall capacity factor of 41 .1%. plus I 0% profit. The

selling price was found to be around 6 p/kWh for 5% rate of return. rising to around 11 .5

p/kWh for 15% rate of return. However, the component of cost attributed to operating

cost remained virtually constant at around 2 p/kWh while the capital cost component

accounted for the selling price increase with rate of return.

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