PELAMIS WEC - CONCLUSION OF PRIMARY R&D

FINAL REPORT

ETSU V/06/00181/REP

DTI Pub Urn No 02/1401

Contractor

Ocean Power Delivery Ltd

UNRESTRICTED

The work described in this report was carriedout under contract as part of the New andRenewable Energy Programme, managed by theEnergy Technology Support Unit (ETSU) onbehalf of the Department of Trade and Industry.The views and judgements expressed in thisreport are those of the contractor and do notnecessarily reflect those of ETSU or theDepartment of Trade and Industry.

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EXECUTIVE SUMMARY

The Pelamis Wave energy Converter (WEC) is an innovative new concept for extractingenergy from ocean waves and converting it into a useful product such as electricity, directhydraulic pressure or potable water. The system is a semi-submerged, articulatedstructure composed of cylindrical sections linked by hinged joints. The wave-inducedmotion of these joints is resisted by hydraulic rams that pump high-pressure oil throughhydraulic motors via smoothing accumulators. The hydraulic motors drive electricalgenerators to produce electricity. The complete machine is flexibly moored so as to swinghead-on to the incoming waves and derives its 'reference' from spanning successive wavecrests.

This 8-month project part funded by the UK DTI has concluded and fully documented allaspects of the three-year research & development programme to rigorously examine theviability of the Pelamis WEC. The results of this programme have been fully assessed byWS Atkins Consultants Ltd to provide an independent opinion on the conclusions.

The main conclusion is that the Pelamis WEC is technically sound and, once thedevelopment and demonstration phases are complete, should become economicallyattractive.

DEVELOPMENT ISSUES SUCCESSFULLY ADDRESSED TO DATE

The following key technical issues have been successfully addressed by the researchprogramme to date:

1. Validation of the core survivability characteristics and mechanisms using a range ofmodel tests. These tests have shown that the system will be able to withstand stormseas.

2. Validation of the power-capture potential of the concept using both numerical andexperimental techniques. This has shown that the system is effective at absorbingpower from the required range of small seas.

3. Analysis & design of an effective, high-efficiency power-capture and conversionsystem. The power take-off system will allow high mechanical-electrical conversionefficiencies of in excess of 80% to be achieved using proven, off-the-shelfcomponents.

4. Development and provisional testing of preliminary control systems & algorithms toallow the system to optimise power capture across the required range of sea-states.

5. Specification of all main structural and hydrodynamic loads to allow the design ofrepresentative structures.

6. Design of cost-effective structures with appropriate factors-of-safety.7. Analysis and provisional design of appropriate mooring systems including techniques

for rapid attachment and removal.8. A preliminary examination of the anticipated installation, operation, maintenance and

retrieval requirements and procedures, confirming that all operations can be carriedout with non-specialist equipment using standard offshore practice.

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9. Production of a full prototype design incorporating all of the issues indicated above.10. Production of a provisional series-production design for estimating onward

economics.11. Identification of the key remaining risks and the formulation of a responsible onward

development programme.

The following key economic issues have been addressed by the research programme todate.

1. Full costing of the new prototype design showing that the first 500kW prototype willhave a cost of ~£1M (~£2,000/kW).

2. Development of reasoned cost estimates for a provisional 650kW series-productiondesign showing that in the medium-term costs will fall to approximately £500k(~£750/kW).

3. Preliminary assessment of the likely installation, operation, maintenance and retrievalcosts for the prototype and production systems, including allowance for permitting,site leases and insurance. This shows that annual costs for an early 25MWinstallation will be of the order of 6% of capital cost per annum.

4. Assessment of the economics of early and longer-term Pelamis WEC installationsincluding a detailed sensitivity analysis covering the main parameters. This showsthat early demonstration schemes will generate electricity for approximately 6-7p/kWh. Longer-term estimates fall between 1.5-3.0p/kWh showing that the systemhas the potential to compete directly with conventional and other renewablegeneration technologies.

KEY REMAINING RISKS & DEVELOPMENT MILESTONES

The following remaining uncertainties must be tackled before the technology is ready tobe fully demonstrated through the construction and testing of the first full-scale PelamisWEC prototype:

1. Proof of the full control & data-acquisition system for the first full-scale prototype2. Proof of the full-scale joint hydraulic & electrical systems

These two tasks are the immediate goals of the onward programme. The first full-systemis to be demonstrated using a 7th scale prototype to develop and prove all aspects of thedevice technology apart from the full-scale engineering. This strategy minimises thesignificant technical and financial risk posed by this key development hurdle.

Once the 7th scale system has been demonstrated work will focus on proving thefunctionality and operability of a full-scale joint power take-off system in the laboratory.

Once these two key development milestones have been passed the Pelamis WEC will beready for the first full-scale prototype test.

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TABLE OF CONTENTS

EXECUTIVE SUMMARY……………………………………………….. I

TABLE OF CONTENTS…………………………………………………. III

1. INTRODUCTION & AIMS…………………………………………… 1

2. THE PELAMIS WEC CONCEPT……………………………………. 2

3. PERFORMANCE & TECHNOLOGY………………………………. 3

3.1 Survivability………………………………………………… 43.2 Power Capture………………………………………………. 73.3 Power Take-off System Design……………………………... 123.4 Structural Design……………………………………………. 173.5 Mooring System…………………………………………….. 203.6 Installation, Operation, Maintenance & Retrieval…………... 23

4. MACHINE COSTS…………………………………………………….. 26

4.1 Prototype…………………………………………………….. 264.2 Provisional Production Design……………………………… 284.3 Installation, Operation, Maintenance & Retrieval…………... 29

5. SYSTEM ECONOMICS………………………………………………. 32

6. THE ONWARD PROGRAMME……………………………………... 38

6.1 7th Scale Prototype…………………………………………... 386.2 Full-scale Joint System Test………………………………… 396.3 Full-scale Prototype Machine……………………………….. 40

7. OVERALL PROJECT CONCLUSIONS…………………………….. 41

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1. INTRODUCTION & AIMS

The aim of this project was to advance the Pelamis WEC concept and design to the stagethat the programme is ready to proceed to the 7th scale full-system demonstration phase.All work has built on preceding work carried out by OPD since the concept's inception atthe beginning of 1998. Within this the specific project aims were as follows:

1. Extend the Pelamis WEC numerical simulation to increase realism and include non-linear joint restraint to allow arbitrary power take-off systems to be modelled

2. Carry out further model tests to extend the range of results for validation of thenumerical simulations and to further characterise the inherent power-limiting featuresof the concept

3. Derive an updated set of key load cases for structural design4. Carry out a detailed analysis on various structural configurations to allow drafting of

new designs for costing purposes5. Advance our understanding of mooring loads and dynamics6. Carry out an initial study on the likely installation, operation, maintenance and

retrieval requirements, and summarise and cost provisional procedures7. Specify, produce, and fully cost new prototype and provisional production designs

using the results of the individual work topics8. Carry out a preliminary assessment of the expected cost-of-energy of the prototype

and provisional production systems9. Identify key areas for further study10. Present the case that the technology is ready to move on to the next development

stage

The main body of this report is set out as follows:

Section 2: Brief description of the Pelamis WEC concept and the working principle.Section 3: A summary of performance and technology issues and current status

including details of the methodology used to date, the key results and ourconfidence in them, plus brief indications of further work required andother future issues.

Section 4: A summary of the provisional predictions of the cost of the systemincluding capital and operational costs, presented in a similar format to theprevious section.

Section 5: The results of a preliminary study of the economics of Pelamis WECinstallations.

Section 6: A summary of the onward programme to successful demonstration of thePelamis WEC.

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2. THE PELAMIS WEC CONCEPT

The Pelamis WEC is a semi-submerged, articulated structure composed of cylindricalsections linked by hinged joints (Figure 2.1). The wave-induced motion of these joints isresisted by hydraulic rams which pump high pressure oil through hydraulic motors viasmoothing accumulators. The hydraulic motors drive electrical generators to produceelectricity. The complete device is flexibly moored so as to swing head-on to theincoming waves and derives its 'reference' from spanning successive wave crests.

A novel joint configuration is used to induce a tunable cross-coupled resonant responsethat greatly increases power capture in small seas. Control of the restraint applied to thejoints allows the resonant response to be 'turned-up' in small seas where captureefficiency must be maximised or 'turned-down' to limit loads and motions in survivalconditions. Electrical power from all the joints is fed down a single umbilical cable to ajunction on the seabed. Several devices can be connected together and linked to shorethrough a single seabed cable.

Figure 2.1 – Artists impression of a Pelamis WEC wave-farm

The core theme of the Pelamis concept is survivability. The fundamental survivabilitymechanisms are the use of length as the source of reaction (to allow the system to de-reference in long storm waves) in conjunction with a finite diameter to induce fullsubmergence and emergence in large, steep waves, thereby limiting loads and motions.The system is slack moored and does use mooring reaction in order to absorb power. Themoorings have a motion envelope large enough to accommodate extreme wave motionsin addition to the low frequency wave-group induced response.

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3. PERFORMANCE & TECHNOLOGY

The following Sections summarise the work carried out on (and the current status of) thefollowing key development issues:

1. Survivability2. Fundamental power capture3. The power take-off & conversion system4. Structural design5. Mooring system analysis & design6. Installation, operation, maintenance & retrieval

The following is presented for each topic:

1. A summary of current status2. A description of the methodology used3. A summary of the key results4. A brief appraisal of OPD’s confidence in these results5. A summary of onward development tasks and other future issues

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3.1 Survivability

Summary

The core survivability characteristics have been further examined using tank test modelsin scale waves up to 28m in height. These tests have demonstrated that loads andmotions in extreme seas are comparable with those required for the device to reach ratedpower. OPD are confident that the survivability characteristics of the Pelamis WECconcept have been validated.

Methodology

The only reliable method to determine the survivability characteristics is to use modeltests at various scales. Survivability issues have been assessed using model tests at 80th

in waves of up to 28m height at full-scale (Figure 3.1), and at 20th scale in scale waveheights of up to 17m (Figure 3.2).

Figure 3.1 Figure 3.2

The generalised numerical solution to extreme wave loads is very complex. A first passat this is currently being implemented in the Pelamis WEC simulation as explained in thefollowing Section.

Other important loads have been estimated analytically or numerically, including peakmooring, slamming, inertial and torsional loads.

Key Results

Tests at 80th scale in waves up to 28m at full-scale were carried out in the EdinburghWide Tank. The 80th scale model had no joint restraint and the primary aim of the testswas to determine the required clearance angle between adjacent segments to preventdamage in the event of a complete hydraulic failure. Joint angles were tracked usingvideo stills – peak joint angles were seen to be manageable when compared with the jointangles experienced in normal operation.

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The 20th scale model was tested in full-scale wave heights of up to 17m in the CityUniversity 55m flume. Extreme waves were generated using a ‘frequency-focussing’technique, this results in steep, limiting height breaking waves. The maximum waveheight was limited to 17m because of the scale water depth of only 25m. The 20th scalemodel has representative joint restraint and full joint moment and angle instrumentation.These tests showed that joint moments and angles are similar to those experienced withthe model running at full power.

A sweep of wave amplitudes was carried out with a wave period of 10s and waveamplitudes of up to 12m. All joint angles and moments reached an asymptote at waveheight = 6 - 7m, or twice the section diameter (Figure 3.3).

Figure 3.3

Although the model was not instrumented to include slamming pressure measurement,slamming pressures and loads will be small due to the small section diameter and absenceof flat panels. Future model tests are planned at 20th scale to examine slamming effects.Torsional and other extreme motion related loads are small compared to the mainbending moments and shear forces.

Confidence

The Pelamis WEC is almost entirely driven by hydrostatic forces (buoyancy and weight).The small cross sectional area and low-drag form presented to the incoming waves meansthat the contribution of drag terms to the overall loads is small at all scales. A highdegree of confidence can therefore be placed in these results as the main loads aregoverned by Froude scaling (small models are valid). Loads due to viscous-drag do notscale well but are generally small compared to the major loads and will generally behigher at model scale compared to full scale.

All tests to date have been with joints restrained by passive dampers. There are potentialissues regarding the response of the joint control system in extreme cases but these are tobe investigated using the 7th scale demonstrator in the next phase of the developmentprogramme.

0 2 4 6 8 10 12Wave height (m)

Join

t ex

curs

ion

H1

H2

H3

H4

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Future Issues

Further tests at 33rd and 20th scales are planned by OPD and the current collaborativeEPSRC project with Strathclyde and Southampton Universities. These tests will includefurther survivability cases and are to include slamming pressure tests.

The tests at 7th scale in the next phase of the programme will give further informationregarding survivability of the complete system including the behaviour of the jointcontrol systems.

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3.2 Power Capture

Summary

The fundamentals of the power-capture characteristics of the Pelamis WEC have beenextensively studied across a wide range of wave conditions using various tank testmodels and numerical techniques. Numerical results have been found to agree closelywith experimental results for wave amplitudes where optimisation of power capture areimportant, agreement was generally better than 10%. In addition, the inherenthydrodynamic power limiting features of the Pelamis WEC have been demonstratedusing tank test models. A far-field radiation pattern numerical model of the system hasshown that the ultimate power capture potential of the Pelamis is a factor of three higherthan for a pure heaving buoy of the same volume.

OPD is confident that the power absorption capability of the Pelamis WEC has beenconfirmed.

Methodology

Power-capture has been determined using model tests at 35th, 33rd and 20th scales (Figures3.4 & 3.5) in a wide range of regular and irregular waves. Power capture for the case oflinear (small) waves has been extensively studied using a purpose written numericalmodel. The numerical model is a full 6 degree-of-freedom, multi-body, dynamic andhydrodynamic simulation of the whole Pelamis WEC system. At present, linear body-dynamics and hydrodynamics are assumed. The former is a valid approximation even forlarge waves, the latter is currently being addressed in the EPSRC funded hydrodynamicsprogramme. Numerical and experimental results have been correlated for a wide range ofwave cases at the various model scales, agreement is generally better than 10%.

Figure 3.4 – 20th scale model Figure 3.5 – 33rd scale model

The numerical simulation was extended to allow prediction of annual power capture forreal sites using representative wave data. A 3-year data set from the UK Met Officecalibrated wind-wave 'hind-cast' model comprising some 2200, 12-hourly average wave-spectra for a representative site were used for annual power capture prediction.

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The numerical model is unable to predict the inherent power limiting due to hydrostatic-clipping so the results are artificially limited at the nominal rated power of the system. Arepresentative minimum cut-in power has also been included in the annual powerprediction model to avoid distortion of the results due to contributions from very smallwaves when the device would be idle. Overall power conversion efficiency as a functionof mean power is applied to all numerical predictions. Conversion efficiency across therequired range of power levels has been estimated using a detailed analysis of thecomplete power take off system (see Section 3.3).

An independent numerical model developed by WS Atkins Consultants Ltd has shownthat the ultimate power capture potential of the Pelamis WEC is approximately threetimes better than for a pure heaving machine of the same volume. This is due toasymmetry of the wave radiation pattern produced by the machine. For more detailsplease see "The Pelamis WEC – May be good in practice but will it work in theory?",RCT Rainey, Proc. 16th IWWWFB, Hiroshima, 2001.

Key Results

Data has been collected from various test programmes at 35th, 20th and 33rd scales whichshow that the Pelamis system can absorb significant amounts of power from the requiredrange of sea states. These tests have been used to validate the PEL suite of proprietarysimulation software. The PEL suite is a sophisticated numerical simulation of thecomplete Pelamis WEC system including full six-degree of freedom dynamics, linear andnon-linear hydrodynamics and a model of the power take-off system.

Figure 3.6 – Experimental & numerical power capture for a range of sea states showinggood agreement for small wave amplitudes and long wavelengths and the desired,progressive departure in agreement for larger, shorter waves.

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Agreement between numerically and experimentally derived machine power capture isgenerally better than 10% for small waves (wave height smaller than the sectiondiameter) where power capture efficiency is of prime importance. Agreement for longerand medium wavelengths is generally better (within 5%) than for very short waves wherethe segment length is significant compared to the wavelength. This is to be expected, theeffect does not become significant until wavelengths are shorter than 60m, where there islittle energy in real seas. The desired progressive departure in agreement betweennumerical and experimental results is noted at larger wave-amplitudes. This is the resultof the inherent load and motion limiting characteristics of the Pelamis WEC due to localfull submergence and emergence of the segments in wave crests and troughs.Experimental power-asymptotes in large waves were generally higher than the desiredrated-power of the full-scale machine. This is not a serious concern and will be trimmedby choice of the static-ballasting level of the machine and through more appropriatechoice of applied joint damping levels. These effects are shown clearly in Figures 3.6 fortest results from the 20th scale model in the Glasgow University 77m tank.

The system can be independently tuned for a range of target wave frequencies by varyingthe restraint applied to the joints. In addition, the 'Q' of the response can beindependently varied to maximise or minimise the size of the response (and thereforepower capture) to suit the prevailing sea state. These effects are shown in Figure 3.7 fora range of 20th scale test results.

Figure 3.7 – Experimental capture widths for various control settings showing how thesystem can be tuned to particular target frequencies and how the 'Q' of the response canbe varied to suit the prevailing wave height.

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A key result is that experiments show that the maximum moments and angles required toreach rated power are of the same order as those encountered in the survivability testsdescribed in the previous section. This is of vital importance so that the power take-offsystem can be sized for their primary role rather than being significantly over-rated tocope with seldom required survival loads and motions.

The numerical code outputs power for individual joints and the complete machine andpresents it in the form of a 'Power Matrix' as shown below in Figure 3.8. The table givenassumes irregular sea spectra of a standard two-parameter Pierson-Moskowitz form.Results can be produced for other spectral definitions or for regular waves as required.As stated earlier the PEL simulation code includes due allowance for power conversionefficiency and the cut-in and rated power of the system. The power matrix format hasbeen chosen by OPD as the most effective way of summarising machine performance asit allows easy computation of annual power capture for a given site using a table ofoccurrence of wave conditions presented in the same format.

Figure 3.8 – 'Power Matrix' for a 750kW machine

If wave data is available as actual measured conditions in a time-series format the PELnumerical code can be used to directly calculate the annual average power output of thesystem and outputs data in various formats. Probability of exceedence and seasonal-variability for the 500kW prototype machine for a site off the West Coast of Scotland areshown in Figure 3.9. These show that on the site selected a 500kW machine wouldproduce 205kW on average resulting in an average annual capacity factor of 41%.

Figure 3.9 – Performance of a 500kW machine for a site off the West Coast of Scotland

Tpow (seconds)5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0

0.5 idle idle idle idle idle idle idle idle idle idle idle idle idle idle idle idle idle

1.0 idle 22 29 34 37 38 38 37 35 32 29 26 23 21 idle idle idle

1.5 32 50 65 76 83 86 86 83 78 72 65 59 53 47 42 37 33

2.0 57 88 115 136 148 153 152 147 138 127 116 104 93 83 74 66 59

2.5 89 138 180 212 231 238 238 230 216 199 181 163 146 130 116 103 92

3.0 129 198 260 305 332 340 332 315 292 266 240 219 210 188 167 149 132

3.5 - 270 354 415 438 440 424 404 377 362 326 292 260 230 215 202 180

4.0 - - 462 502 540 546 530 499 475 429 384 366 339 301 267 237 213

4.5 - - 544 635 642 648 628 590 562 528 473 432 382 356 338 300 266

5.0 - - - 739 726 731 707 687 670 607 557 521 472 417 369 348 328

5.5 - - - 750 750 750 750 750 737 667 658 586 530 496 446 395 355

6.0 - - - - 750 750 750 750 750 750 711 633 619 558 512 470 415

6.5 - - - - 750 750 750 750 750 750 750 743 658 621 579 512 481

7.0 - - - - - 750 750 750 750 750 750 750 750 676 613 584 525

7.5 - - - - - 750 750 750 750 750 750 750 750 750 686 622 593

8.0 - - - - - - - 750 750 750 750 750 750 750 750 690 625Hsi

g (m

etr

es

)

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Confidence

A high degree of confidence can be placed in the experimental and numerical results. Alltransducers were accurately calibrated before and after the test programmes. Theagreement between numerical and experimental results was generally excellent for therange of wave amplitudes and wavelengths where linear wave theory is valid showingthat the numerical model is sound.

The good agreement also indicates that the numerical model may be used to predictannual power capture with a good degree of confidence if sensible power clipping isapplied as described above. It should be noted that the numerical clip applied is lowerthan experimental power asymptotes which implies that the clips used are conservative.This implies that the likely average annual power capture of the system is likely to behigher than predicted.

OPD is satisfied that it has characterised and validated the fundamental power captureand survivability features of the Pelamis WEC. However, OPD is committed to avigorous ongoing numerical and tank test programme to extend the validity of numericalpredictions and gain further insight into the fundamental dynamics and hydrodynamics ofthe concept.

Future Issues

Further extensive tank tests are planned within the EPSRC and OPD programmes toextend the envelope of understanding of the power capture and survivability of thePelamis WEC concept. Experiments with the new 33rd scale model are to includeexamination of the effect of limiting the peak applied joint moments on joint angles tosimulate the load limits of the hydraulic power take-off system.

The capability of the numerical model is currently being extended to include the non-linear hydrodynamic effects that give rise to the inherent motion, load and power limitingcharacteristics of the Pelamis. These new numerical models will be validated againstexisting and new experimental data to complete the full device simulation capability.

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3.3 Power Take-off System Design

Summary

The use of hydraulics for wave-energy conversion systems is attractive due to the highforces and low velocities associated with wave-action. Also, hydraulic accumulators areby far the most cost effective and efficient, short-term energy storage medium currentlyavailable. However, the irregular, spectral nature of ocean waves presents a significantproblem for conventional hydraulic systems. Wave energy is characterised by highinstantaneous, wave-by-wave power peaks about a lower mean level. The phenomenonof wave grouping introduces a further order-of-magnitude variation in short-term powerabsorption – typical wave groups have periods of 50-100 seconds.

The large variation of instantaneous input power means that conventional hydraulictransmission systems would have full-cycle conversion efficiencies less that 50% andinvolve a number of high cost components. Both of these characteristics are clearlyundesirable and OPD have therefore focussed much effort on developing alternativestrategies that offer significantly higher conversion efficiency while using proven, low-cost components.

The result is a system with a flat conversion efficiency in excess of 80% across therequired range of power levels that minimises the number and type of complexcomponents. The system has been extensively modelled numerically and provisionallytested using a single axis rig. The system has been fully specified and costed, includingall hydraulic, electrical, instrumentation and control elements, and an allowance forassembly and testing. Estimates of component and system reliability have been madebased on manufacturers' data.

Methodology

Various candidate systems have been considered for the Pelamis WEC power take-offand conversion system. The full-cycle conversion efficiency of a conventionalhydrostatic transmission system using off-the-shelf components was modelled using atime-stepping code to provide a reference against which other systems could be judged.

An essential theme of the Pelamis WEC is that it can be implemented using 100%available technology. All candidate systems studied use off-the-shelf components.However, the chosen system has a number of novel operating principles that must betested in detail before installation in the first full-scale prototype machine. These are thekey objectives of the immediate onward programme.

The functionality and conversion efficiency of the chosen system was extensivelymodelled using a time-domain simulation of a single axis unit. The system was thenincorporated into the full time-domain Pelamis WEC simulation code to furthercharacterise system performance. The results were used to highlight areas whereefficiency may be increased and the design was modified accordingly.

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The final configuration was modelled to determine losses at each stage, these include:

- Flow losses along the inlet hydraulic pipe-work, manifold flow-path andvalves (also used to confirm that inlet cavitation would not be a problem)

- Friction losses in the seals in the hydraulic rams- Flow losses through the outlet manifold flow-path, valves and pipework (also

used to confirm that local over-pressure in the rams would not be a problem)- Losses in the variable displacement hydraulic motor used to drive the

generator- Electrical losses in the generator itself

Instrumentation and control requirements were then studied in detail to arrive at acomplete system schematic which was used to draw up a full schedule of costs usingquotes from appropriate suppliers and sub-contractors.

A preliminary analysis of reliability and likely maintenance requirements was carried outusing manufacturers data.

Fundamental functionality of the primary power take-off system was proven using asingle 7th scale hydraulic cylinder on a test rig in the laboratory.

Key Results

A generalised schematic of the complete power conversion system is shown in Figure3.10 below. Each joint axis has an independent power take-off and conversion systemhoused in a sealed compartment at the end of each unit. The individual units areconnected together by a common electrical bus running the length of the machine. Thevoltage is stepped up for transmission to shore using a single transformer sited near thecable exit.

Figure 3.10 – Schematic of power take-off system

Hydraulic cylinder

Induction generator

Variable displacement hydraulic motor

Hydraulic accumulator

11 kV Transformer

Electrical Flexible link

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The rams resisting motion of the joints are configured to operate as large low-speedpumps. These are ported to high-pressure accumulators that provide short-term storageto smooth out wave-by-wave and wave-group associated fluctuations of input power, anexample of which is shown in Figure 3.11 (NB – It should be noted that this is a generalcharacteristic of wave energy, not only the Pelamis WEC system). A quasi-steady flowof high-pressure oil is drawn by a variable-displacement hydraulic motor running atconstant speed. The motor controller is configured to deliver a constant torque (andtherefore a constant power) to a conventional induction generator. Plots of accumulatorpressure for an input of the form shown in Figure 3.11 and mean levels of 25, 50 and75kW are shown in Figure 3.12.

Figure 3.11 – Example of instantaneous power input to a single joint

Figure 3.12 – Accumulator pressure for an input of the form in Figure 3.9

The component and full-cycle conversion efficiency of a complete joint power take-offsystem is summarised in Figure 3.13. Over 80% of the primary power absorbed (i.e. jointmoment times angular velocity) is converted into electrical power. The results alsoindicate that the major area for future improvement of the system are the reduction oflosses in the hydraulic motor at low power levels and reduction of flow losses at highpowers through careful optimisation of the flow passages and valves.

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Figure 3.13 – Component & overall power take-off efficiency for a range of power levels

The preliminary study of system reliability concluded that reliability of the individualcomponents should be high, and that maintenance requirements would be low. However,these conclusions are based on limited information and thorough testing of a full-scalepower take-off unit is required to confirm reliability of the complete system – this is oneof the key objectives of the onward programme.

It should be noted that the system has very favourable characteristics compared to manyhydraulic systems in-service:

- The system is housed within a sealed compartment limiting contact withcorrosive or abrasive substances

- The system is completely sealed and is pressurised to eliminate ingress ofcontaminants should they be encountered

- Peak ram seal velocities are typically less than 0.1m/s which is low comparedwith typical applications

- The are no large positive or negative pressure spikes- The hydraulic motor and generator will not experience large torque or speed

transients- The system will run at a constant temperature due to the abundant source of

cooling water (seawater temperature typically only varies by a few degreescentigrade over the year)

- The system will not be stationary or stagnant for long periods of time (atraditional source of unreliability)

A summary of the costing results is given in Section 4.

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Confidence

Confidence in the functionality, overall efficiency and cost of the system are high.Confidence in the predictions of system reliability and the likely maintenancerequirements are low at present.

However, there are many highly reliable, low-maintenance hydraulic systems of similaror greater complexity in service in the industrial, marine, offshore and aerospace sectors.OPD is therefore confident that a reliable system can be implemented through carefuldesign and appropriate testing.

Future issues

The functionality of the complete power take-off and control system is to be proven onthe forthcoming 7th scale demonstrator.

The functionality, conversion efficiency, reliability and maintenance requirements of thefull-scale system are to be thoroughly studied using a full-scale joint test rig in thelaboratory before the system is cleared for service in the first full-scale prototypemachine.

In the future, effort will focus on improving individual component efficiency, reducingsystem part-count and cost, and reducing maintenance requirements to an absoluteminimum.

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3.4 Structural Design

Summary

The design of the main structural elements of the Pelamis WEC is central to thesurvivability and economics of the system. It is seen as essential that the prototypestructure is designed and built using established offshore practice and technology whereever possible. However, it is recognised that in the longer-term cost-effective structureswill have to be fully optimised and will require a significant degree of subsequentinnovation and optimisation.

Considerable effort has been focussed on structural analysis and design to ensure that theprovisional structural designs produced have the required generous factors of safety andare representative for costing purposes. A full set of operational and various failedcondition load cases have been defined using a mixture of model tests, numericalpredictions and analytical results.

Representative all-steel structures have been analysed and designed using the derivedload-cases. OPD are confident that the structures designed have generous factors ofsafety and are representative enough to allow meaningful costings to be developed.

Methodology

The main structure of the Pelamis WEC performs two key functions:

- Efficient distribution of the heavy point loads at ram and bearing attachmentpoints into the main tubular structure

- Efficient transmission of the main bending and shear loads along the length ofthe machine

While structural efficiency is important, the ultimate aim is the most reliable and cost-effective structure. The cost of the main structural elements is a major proportion of thetotal machine cost. It is therefore important that representative structures are designedfrom an early stage to ensure adequate confidence in the costing data. All efforts to datehave concentrated on all-steel structures. Steel has been chosen as the preferred materialfor early devices to simplify structural analysis, design and instrumentation. It is likelythat significant cost savings will be possible in the future if other materials such asconcrete are used as the main structural material. Advice concerning appropriatestructural techniques has been sought from independent experts including WS AtkinsConsultants Ltd.

All key load cases have been carefully considered to provide reliable input data to thestructural design process. The primary operational loads including the overall bendingmoments and shear forces have been derived from the experimental and numerical modelresults. Additional secondary load cases such as mooring, slamming, inertial andtorsional loads have been derived from various independent numerical models. These

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loads have been used to specify the power take-off system sizes and ratings. Thesespecifications have then been used to derive the actual peak operational loads to beexperienced by the main structural elements.

An attractive feature of using hydraulic power take-off systems is that applied loads arelimited by total piston area and maximum system pressure – this is advantageous from astructural perspective. The effect of load limiting has been examined using the computersimulation program. Even though the hydrodynamic loads are not currently limitedwithin the numerical model it has been confirmed that the effect of clipping the availablemoment restraint does not have a significant effect on the resulting joint angles. This isimportant from a survivability perspective. The various loads have been combined into astandardised set of load-cases that were used for all structural analysis work.

Various structural configurations have been analysed using a range of techniquesincluding analytical and finite element methods. The models used for finite elementwork included all the internal structure. All design factors of safety were related to thefatigue limit of common offshore steels, typically ~150MPa.

As steel structures were assumed throughout, due regard was paid to appropriatecorrosion protection and shell thickness reduction.

Key Results

OPD has followed WS Atkins Consultants Ltd’s recommendation that a heavy, 25mmwall tubular structure with minimal internal frames be adopted for the prototype. Thiswill give conservative factors of safety to reduce the chances of failure due to detaildesign issues. Although a large weight of steel will be used for this configuration it isanticipated that the overall 'one-off' fabrication costs will be significantly reduced.

An appropriate prototype structure has been designed and analysed. Structural analysisof the chosen prototype structure has shown that peak stress levels in the main tube are10-20 MPa for normal operating load cases giving design factors of safety (FoS) inexcess of 7 on the steel endurance strength (factor of >12 on yield strength). Thesefigures have been verified using simple analytical relationships. Stresses rise to 50MPaat the bearing and ram attachment points giving a FoS of ~3 on the steel endurancestrength. However, it is anticipated that local stresses will be reduced by careful attentionto the detailed design of these areas. Worst case failed-mode stresses are approximatelydouble the normal operating limits.

A more optimal 12mm wall structure with additional internal stiffening was designed andanalysed for the provisional production design. Finite element analysis has shown thatgeneral stress levels in the main tube are 20-40MPa for normal operating load casesgiving a FoS of ~4 on endurance strength. Local stresses rise to ~70MPa giving a fatigueFoS >2. Again it is anticipated that local stresses will be reduced through careful detaildesign. Again worst case failed mode stresses are approximately double the normaloperating limits. It should be noted that local buckling will become important for shell

19

thicknesses of 12mm and less, this was not analysed in detail but was addressed usingvarious empirical design rules. A full elastic buckling analysis would be required toconfirm that the internal structure assumed would be sufficient resist buckling failure inhighly loaded regions.

Confidence

OPD is confident that the load cases used are representative of the likely serviceconditions. Confidence is also high that the designs produced are detailed enough foraccurate costing purposes.

However, the designs produced must still be viewed as representative only. The finalstructure must be thoroughly analysed and designed by an expert structural engineer toensure that the target factors of safety are met and that concerns over local buckling areaddressed.

Future Issues

OPD will continue to expand and refine the load case list with new test and modellingdata. However, it is anticipated that all future structural analysis and design will becarried out by specialist structural consultants.

20

3.5 Mooring System

Summary

Moorings are a key aspect of survivability of all WEC concepts and the Pelamis WEC isno exception. The Pelamis mooring system must perform a number of key functions.These include:

- Maintain the device in station- Provide reaction against steady & unsteady loads- Orient the device in an appropriate way to the incoming waves- Allow a means for transfer of electrical power to the seabed- Allow for easy attachment/detachment in a range of wave conditions

It is essential that the mooring design is developed using established offshore practiceand technology where ever possible. However, significant differences betweenconventional offshore mooring analyses and requirements have been identified whichnecessitate detailed study of the main loading and dynamic characteristics of the PelamisWEC mooring system.

The loads and dynamic response of the Pelamis WEC mooring has been studied in detailusing a range of techniques. These have shown that the steady drift force experienced bythe system is mainly due to the momentum transfer associated with absorbing powerfrom the wave, there is little contribution from wave reflection and diffraction (unlikeother WEC configurations). The result of this is that predicted and measured operatingand extreme mooring loads are low. However, the unfavourable result of the form of thePelamis is that in-line damping of mooring motions is extremely low. Dynamicsimulations show that low frequency, wave group excited resonant response of themooring is the main design driver. However, preliminary model tests at 20th scales haveshown that designs produced and costed to date are conservative.

OPD has set out a sensible onward model test investigation of the key parametersgoverning mooring loads and response.

Methodology

A large range of mooring configurations have been considered. A generic Pelamis WECmooring dynamic analysis simulation has been developed and used to study mooringdynamics for a large range of cases, parametric studies of mooring stiffness and dampinghave been carried out. Experiments have been carried out at 20th scale in the TrondheimOcean Wave Basin in a wide range of seas. Experimental and numerical results havebeen correlated and generally show that the numerical predictions are conservative. Anestimate of line fatigue duty has been made using representative wave data.

Several provisional designs have been produced for costing purposes using the results ofthe numerical dynamic simulations.

21

Key Results

The numerical simulations show that the Pelamis WEC mooring system will besusceptible to large, low frequency wave-group excited responses. These responsesdominate mooring loads and motions it is therefore highly desirable to reduce theircontributions. This is a problem common to mooring systems for other floatingstructures such as FPSOs. The dynamic response is strongly influenced by subtlechanges in the assumed quadratic mooring damping terms. Model tests will tend to over-estimate the likely damping levels.

The measured dynamic response from experimental tests at 20th scale in irregular wavesis much smaller than predicted by the simulation, even when estimates of the dampingcontributions from mooring lines and instrumentation wires are included. Thisdiscrepancy is too large for scaled viscous effects and has been attributed to aphenomenon known as 'wave-drift' damping which arises primarily from the Dopplershifting of encountered waves due to the mooring response. This effect has also beenfound to dominate the response of FPSO mooring systems. However, the precisemechanisms involved are not necessarily the same. Fortunately, wave-drift dampingrises in parallel with the excitation force thereby ensuring that its effects are significantacross the entire range of wave conditions.

The numerical simulation was modified to include best-fit damping terms for the wavedrift damping component. The damping coefficients were varied to arrive at anempirically derived best-fit response as shown in Figure 3.14. The contribution fromwave-drift damping is seen to be at least an order of magnitude greater than the viscousand form drag terms (this is consistent with experience with FPSO moorings).

Figure 3.14 – Experimental & numerical (smooth line) mooring response

Wave drift damping is generally assumed to be well behaved with scale. However,further tests at 33rd, 20th and ultimately 7th scale are required to determine the likelyscaling laws.

0 100 200 300 400 500 600 700-2

-1

0

1

2

3

4

5

6

time (seconds)

surg

e (m

etre

s)

22

These findings are viewed as preliminary. Clearly such a dominant effect demandsfurther experimental work to improve understanding of the main loading and dampingmechanisms. The mooring designs produced for costing purposes assume load andmotion limits from the numerical motion predictions and are therefore likely to be veryconservative.

Confidence

OPD's confidence in experimentally and numerically derived mooring responses iscurrently not high. Further work is clearly required to fully describe the key excitationand damping terms. This is currently being addressed by further planned model tests at33rd and 20th scale. It is particularly important to carry out similar tests at different scalesto determine the likely scaling factors for the excitation and damping terms. OPDcurrently believes that Froude scaling will apply to the dominant excitation and wave-drift damping terms (ie small models are valid for these terms). The secondary quadraticdamping terms due to viscous drag will not scale well and care must be exercised asmodel tests will generally over-predict these contributions.

However, it has been shown that the current numerical models used to specify themoorings used for costing purposes significantly over predict loads and motions. OPD istherefore confident that the moorings designed and costed are at least representative andalmost certainly conservative.

Future Issues

Further tests at 33rd and 20th scale will be carried out to improve understanding of themain excitation and damping mechanisms. Further numerical models will be developedand correlated with the experimental results until a satisfactory level of agreement isreached. Experience with the 7th scale mooring loads and responses will be invaluablefor determining the appropriate scaling factors for full-scale mooring design.

23

3.6 Installation, Operation, Maintenance & Retrieval

Summary

Installation of the Pelamis WEC using standard offshore techniques poses few technicalor financial risks. In-service maintenance activities pose more of a problem. It is vitalthat operation and maintenance costs are reduced to a minimum as they have a significanteffect on the delivered cost of energy. The machine has been designed from the outset torequire a minimum of routine maintenance.

OPD has determined provisional estimates of maintenance requirements and developedpreliminary techniques for carrying them out. It has been concluded that all maintenanceactivities should be undertaken off-site at a suitable facility. Rapidattachment/detachment mooring and electrical connection systems to make this possiblehave been examined.

Methodology

The Pelamis WEC was specifically conceived to minimise on-site work. The only majortask on-site is installation of the mooring and electrical interconnection. Most of theequipment associated with this has a relatively low capital value. Mooring andinfrastructure installation activities can be started or stopped at short notice and much ofthe work will not be strongly weather dependent. The high capital value Pelamis WECmachine is only brought to site once the weather dependent activities have beencompleted. It is anticipated that the time between arrival on site and hook up will be lessthan one hour.

In-service maintenance activities pose more of a problem. OPD has secured theinvolvement of an expert from the offshore sector with appropriate experience ofinstalling, operating, maintaining and retrieving offshore installation such as FPSOs. Astudy of the provisional designs was made to determine the likely operation andmaintenance requirements. Until finalisation of the complete system this analysis canonly be provisional. Costs for appropriate support vessels were obtained from varioussources.

Initial work focussed on carrying out all routine maintenance on site with the machine onits mooring. As a result of this work, and discussions with various key players in theoffshore sector, OPD concluded that maintenance on-site was unrealistic and have nowspecified that all maintenance activities should be carried out off-site in a suitablesheltered location. A mooring and electrical connection system with a simple attachmentand detachment mechanism with a broad weather window is required to make thispossible.

A provisional design for such a system has been developed. A first-pass summary hasbeen made of the required annual maintenance requirements for the prototype and initialproduction systems.

24

Key Results

The initial installation of the system should pose few problems as it will use standardoffshore techniques and practices.

Of more concern are maintenance requirements and strategies. As mentioned above, allefforts are now focussed on a system where all maintenance is carried out off-site. Therelative ease of removal of individual machines for maintenance and repair is seen as acompelling advantage of floating, slack moored WEC systems such as the Pelamis, overfixed or bottom standing WEC concepts and offshore wind-turbines. However, off-sitemaintenance is only practical if the mooring and electrical system is designed so that canbe attached and detached across a broad range of sea states. It is seen as particularlyimportant that an individual machine can be safely removed in large seas in the event of acomponent or system failure.

A suitable mooring and electrical connection system has been provisionally designed. Itis estimated that a machine may be removed in significant wave-heights of up to 2-3m,common for Single Point Mooring Buoy pick-up and release. However, it is likely thatreattachment will require lower sea states of ~1m significant wave-height.

A provisional study of the likely removal and reattachment 'windows' has been madeusing wave data for a representative site. These are summarised in the table below:

Period Removal (% of period) Reattachment (% of period)

October - March 60% 5%April - September 92% 31%June - July 98% 43%Whole Year 76% 18%

As can be seen it is likely that detachment can be readily achieved year-round, this isvery important for the removal of damaged or failed units. Reattachment prospects aregenerally good during the summer but there may be lengthy delays reinstalling a machinein winter. It should be noted that many of the more likely system failures such as failureof the electrical switch-gear and bearing failures in the generator etc result in only a smallloss of energy from the complete device, therefore rapid removal and repair is notimportant. More serious failures like hydraulic seal failure are less likely but must bedealt with swiftly if further damage to the system is to be avoided.

Confidence

Confidence in the installability of the system is high since this will use standard offshoretechniques and practices. Confidence in the estimated maintenance requirements of earlysystems is low. True maintenance requirements will only be determined by installing andoperating a significant number of machines. However, OPD feel confident that themaintenance requirements of the system will ultimately be low. It is anticipated thatsystem reliability and availability will be improved to the point where a single annual

25

maintenance programme in summer can be carried out readily and cheaply with theminimum of weather related delays.

Future Issues

Clearly this topic requires further study. Laboratory testing of the complete power take-off system will give early indications of the robustness and reliability of the system. Inaddition, the first full-scale prototype will allow a comprehensive study of themaintainability of the complete machine to be undertaken. However, it is likely that thetrue maintenance requirements of the system will only be determined and minimisedfrom experience in the field with a number of machines.

Continued effort will also focus on broadening the weather window for removal andreattachment, with particular emphasis placed on improving the prospects for reinstallingunits during the winter months.

26

4. MACHINE COSTS

4.1 Prototype

Summary

A new full-prototype design has been prepared incorporating all the modifications andadvances made since the Scottish Renewables Obligation tender application in August1998. The new prototype design differs significantly from the SRO machine and as sucha full re-costing of the system was required to give accurate data for the onward R,D&Dprogramme. The new design includes a fully revised structure, power take-off andconversion, mooring, and control and data-acquisition systems. The structure chosen wasa heavily over-engineered design as described in Section 3.4. The design life of thesystem is 15 years.

The new ‘reference’ design has been fully costed using a similar methodology to the SROmachine as described below. It is estimated that the full cost of the prototype system willbe ~£1M for a 500kW machine (~£2,000/kW).

Estimates were also made for installation and commissioning costs. A three-monthlyintervention cycle for inspection and/or maintenance and repair was assumed to allowoperation and maintenance costs to be defined. Further work will be undertaken in thisarea before budgets are finalised for the onward programme.

Methodology

The structure was costed using updated quotes for the main tube elements from suitablesuppliers and detailed estimates of the costs of supply, forming, erection and welding ofall internal structural elements. Finishing costs to various offshore specifications wereobtained from various sources. As a cross-check, a budget estimate was obtained from anoffshore fabricator, this confirmed that the structural costing was representative.

All power take-off and conversion system components were costed using quotes fromappropriate suppliers and allowances for assembly and testing included. Similarly,quotes were obtained for control and data-acquisition and mooring systems, and theelectrical connection components. Finally, estimates were made for installation of allsystems and components and final assembly of the complete machine.

27

Key Results

The main cost elements of the prototype machine can be summarised as follows:

- Main structure (finished and ballasted) £550k 55%- Power take-off and conversion system £320k 32%- Instrumentation & control £30k 3%- Mooring system £95k 10%

TOTAL: £995k

Confidence

Due to the reliance on quotes from suppliers and fabricators OPD are confident that thecosts derived are reliable.

Future Issues

Further work is required to accurately cost the likely Operation & Maintenance costs. Asthe prototype will be part of the R,D&D programme and will be subject to a morerigorous inspection programme, these costs are likely to be high when compared with acommercial machine.

28

4.2 Provisional production design

Summary

Detailed cost estimates of a provisional production design are required to allowassessment of the likely medium and long-term economics of the system. A fullprovisional production machine was therefore specified, designed and costed using asimilar methodology to that described in the previous section. The design life for thesystem was 15-20 years in line with modern wind energy plant.

The design uses a more optimised structure as described in Section 3.4. The power take-off system chosen was similar to that of the prototype machine.

The costing exercise concluded that, without major technical advances, a 650kWmachine could be built in modest quantities for ~£500k (~£750/kW).

Methodology

The production design was costed using a similar methodology to that described in theprevious section, with reasoned allowances made for economies of scale. The design andperformance assessment assumed no major advances in the conceptual design oroperation of the system.

Key Results

The main cost elements of the system can be summarised as follows:

- Main structure (finished and ballasted) £245k 50%- Power take-off and conversion system £170k 35%- Instrumentation & control £15k 3%- Mooring system (assuming shared components) £60k 12%

TOTAL: £490k

Confidence

Confidence in the results of the costing process is moderate. Various reasonedassumptions were made but overall OPD concludes that the resulting costs arerepresentative.

Future Issues

Refinement of the medium to long-term costs of the system will continue throughout theonward R,D&D programme.

29

4.3 Installation, Operation, Maintenance & Retrieval

Summary

Provisional estimation of the likely operational costs of the system is vital to assess theoverall economics of the system. However, the true costs will only be determined byfurther testing of the full-scale systems and through long-term tests of a complete full-scale machine. Operational costing for the prototype will be dominated by interventionfor inspection at regular intervals. Mobilising support vessels will therefore dominate thein-service costs, an estimation was made to provide budgetary information for the onwardprogramme.

The main objective of studying of the likely operational costs was to provide reasoneddata for assessment of the onward economics of the system. Work therefore focussed onthe operational costs for the provisional production design deployed in a 25MW arraycomprising 39 off 650kW machines sharing moorings and electrical interconnection andtie-back.

All onboard systems have been specified and developed in discussion with potentialsuppliers with an emphasis placed on design for maximum reliability and minimummaintenance. For the purposes of costing it was assumed that the system would need oneannual intervention for scheduled maintenance with an occasional intervention forunscheduled repair and a full mid-life service to replace all bearings, hoses and seals.Preliminary schedules were drawn up for installation, maintenance, repair anddecommissioning of the complete 25MW array using standard offshore workingpractices. Costs were estimated on the basis of these assumptions.

Finally, allowance was made for planning and permitting, site leases from, for example,the Crown Estates Commission and a reasonable estimate of insurance costs for thecomplete scheme including electrical tie back.

Annual maintenance costs for the complete array were estimated to be 6% of capital cost.

Methodology

All installation, operational and removal costs were derived from industry quotes forappropriate support vessels, equipment and consumables.

The installation process is to be performed with a minimum of heavy equipment andwithout divers. The latter is of particular importance as deployment in 50+ metres wouldrequire a full saturation-diving spread and support vessel, the cost of which is prohibitive.An installation schedule was drawn up assuming a summer deployment and 24 hourworking with an adequate allowance made for waiting on weather.

An annual maintenance schedule was drawn up for an individual machine. As explainedin Section 3.6, it was assumed that all maintenance activities would be carried out off-site

30

in sheltered water to minimise exposure to weather related delays. An estimate of thelikely failure rate of system components was made using manufacturers data withconsideration made for the service conditions allowing a probability-of-replacement to beestimated for each main system component. This, in conjunction with an estimate of theman-hours required to effect the replacement and the cost of the component, allowed anaverage time and cost for maintenance of an individual machine to be derived. Theseresults were then used to estimate an annual maintenance cost for the entire scheme.

It is anticipated that the service conditions and high duty-cycle of the system willnecessitate a full mid-life refit of all the main seals, hoses and bearings. An estimate ofthe time and cost of this programme was made using a similar methodology to thatdescribed above.

Finally, decommissioning and removal costs were estimated using similar techniques tothose used for deployment.

Key Results

Estimated installation costs for a 25MW array are summarised below (NB costs are forinstallation of the system only and do not include the capital cost of the machines orelectrical components etc):

£M- Securing site-lease, EIA, permits etc 0.5- Installation of 25MW array 0.7- Installation of electrical interconnections & tieback 1.6

The expected maintenance costs for the 25MW array are summarised below (NB costsinclude removal of the individual machines from the site, maintenance activities(including manpower, components and consumables) and returning the machine to site):

£M- Annual scheduled maintenance programme 0.4- Mid-life refit programme 2.5- Unscheduled maintenance (spread over life of scheme) 1.2- Mid-life electrical inter-connection maintenance 0.5

Other miscellaneous operational costs are summarised below:

- Site-lease ~2% of scheme revenue per annum- Reactive power charges ~0.43p/kVARh- Scheme insurance ~2% of capital cost per annum

Finally, the decommissioning and removal costs for the complete scheme were estimatedto be £0.6M at the end of the project.

31

For a 15-year project, the installation, operational and removal costs represent an averageof ~£1.6M per annum or ~6% of the scheme capital cost.

Confidence

Confidence in the planning, site-lease and reactive power charges is high as they are not afunction of the Pelamis WEC system. The allowance made for insurance is seen to behigh, even for early schemes. It is anticipated that insurance costs will fall dramaticallyas confidence in the system increases. By way of comparison typical insurance costs forships (which are much higher risk due to human factors and their mobility) are around1% of capital cost per annum.

Confidence in the results of the Pelamis specific costing exercise is moderate. Athorough attempt has been made to consider all the installation, operation and removalcosts of a representative scheme. However, more accurate estimation of the true costs ofinstalling and operating such an installation will only be possible after the actual costs ofthe full-scale prototype programme are determined.

Future Issues

The onward programme will focus on determining the onward reliability of the systemand the likely long-term maintenance requirements. The single most important issue isreliability to ensure that only one scheduled intervention per year is required.

32

5. SYSTEM ECONOMICS

Summary

Initial and onward energy-prices for the Pelamis system are predicted to be as follows:

SYSTEM AnticipatedDate

Energy saleprice (p/kWh)

Scheme life(years)

Discount rateassumed (%)

1. Scottish Renewables Order system (2 x 400kW)

2003 6.9 15 -

2. First ~25MW wave-farm installation (39 x 650kW)

2004 - 20054.75.9

1515

8%15%

3. Costs for 25MW installation by 2010 20102.4 - 3.43.0 - 4.3

2020

8%15%

4. Long-term cost of 25MW installation 2015 - 20201.5 - 2.52.0 - 3.5

2020

8%15%

Confidence in these predictions reduces as the time-frame considered increases.However, OPD is confident that the prediction of the opening costs of the system are atleast representative if all development milestones are achieved.

Methodology

The assumptions used for the four cases given above are summarised below:

1. SRO3 system

- Contracted price- Economics & technology passed tender scrutiny

2. First 25MW wave farm

The economics of early multi-MW 'wave-farm' installations were analysed using thecapital and operational cost data described in the preceding sections. A scheme size of25MW was chosen to be representative of typical wind farms. However, it is likely thatlarger installations would be considered in practice to maximise the economies of scalethrough shared moorings, installation, operation, maintenance and grid connection.

The following parameters were assumed for the case study:

- 39 x 650kW Pelamis WEC machines (as described in Section 4.2)- Machine capacity factor of 41% derived using the frequency domain numerical

model as described in Section 3.2 and three years wave-data for a NorthAtlantic site with an annual average of 54kW/m. This give an annual energyyield of ~90GWh.

- Availability of an individual machine of ~95%- 15 year project length (in line with NFFO & SRO contracts)

33

- Discount rates of 8% and 15% (the Discount rate represents the overall project rate-of-return)

Scheme capital costs assumed were as follows: £M

- Planning & approvals 0.5- 39 x Pelamis capital cost 19.1- Electrical connection cabling & equipment 2.9

TOTAL: 22.5 (82%)

Installation costs were assumed to be as follows: £M

- Installation of the 25MW array 0.7- Installation of electrical interconnections & tie-back 1.6- Grid connection onshore 2.7

TOTAL: 5.0 (18%)

SCHEME TOTAL: 27.5 (~£1.1M/MW)

Operational costs were assumed to be as follows: £M

- Annual maintenance programme 0.4 (per annum)- Mid-life refit programme 2.5 (year 7/8)- Mid-life electrical interconnection maintenance 0.5 (year 7/8)- Unscheduled maintenance/repair 1.2 (over life)- Insurance 0.65 (per annum)

In addition, generation specific costs were assumed as follows:

- Site-lease (Crown Estates Commission) 2% of revenue per annum- Reactive power charges 0.43p/kVARh

These figures were used in a discounted cash-flow analysis to determine the selling priceof electricity for the two test discount rates of 8% and 15%.

3. Costs for 25MW installation by 2010

A commonly used 'learning-by-doing' economic analysis was used to predict expectedcost reductions as installed capacity rises according to the following expression:

Cm = C1 x m(lnTf/ln2)

Where: Cm = Cost of mth unitC1 = Cost of first unitTf = Technology Factor

34

Typically, Tf for industrially processed systems is 0.85-0.95. A low technology factor(~0.85) represents fast learning with a resulting rapid fall in costs, a high Tf (~0.95)represents a slower rate of cost reduction. Experience has shown that over 20 years thewind energy industry has achieved a Tf of ~0.9. The wind energy industry is predicted tomaintain a Tf of < 0.95 with 20GW of installed capacity to date.

For this analysis it was assumed that the cost of the first unit (C1) is the early 25MWinstallation as described in the previous Section. It was assumed a total world-widecapacity of 2.5GW (~15% of current installed wind capacity) would be installed by 2010to the profile shown in Figure 5.1 with major installation commencing from 2007-2008 asthe technology gains credibility. Upper and lower values for Tf of 0.90 and 0.95 anddiscount rates of 8% and 15% were assumed to give reasonable upper and lower bounds.

Figure 5.1 – Assumed installation profile

4. Long-term cost of a 25MW installation

The analysis presented above was extended to a world-wide installed capacity of 20GW(ie similar to current installed wind capacity) by 2015 and 40GW (predicted installedwind by 2010) by 2020, assuming a similar range of Tf and discount rates.

As a cross check, a study was carried out to identify likely long-term cost reductions andperformance improvements on the first 25MW wave-farm installation.

Cost reductions assumed were as follows:

- Structure: 33% reduction mainly due to anticipated move to alternative materials- Power systems: 25% reduction due to design optimisation & economies of scale- Finishing/corrosion protection: 50% reduction due to move to alternative materials for structure- Cabling costs: 33% reduction due to design optimisation, use of DC systems & specialist installation equipment- Grid connection costs: No reduction assumed as early systems will occupy best sites

Total w orldw ide ins talle d capacity v's Tim e

0.0

0.5

1.0

1.5

2.0

2.5

2004 2005 2006 2007 2008 2009 2010

Tim e (year)

Inst

alle

d C

apac

ity

(GW

)

35

- Installation costs: 33% reduction due to specialist equipment & techniques- Insurance: significant reductions (50-75%) as confidence in the technology rises- O&M: 50% reduction due to improving reliability, design optimisation & specialist equipment- Reactive power: Effectively eliminate due to probable use of DC transmission systems- Crown Estates charges: Likely to be reduced if large capacity installed, will be negligible if more than 12miles offshore

Improvements in performance:

- Improved power system efficiency: from ~80% to ~90% through design optimisation- Improved control algorithms: ~50% increase in annual energy capture

Key Results

The key results of the economic analysis are summarised in the Table below (as given inthe Summary above):

SYSTEM AnticipatedDate

Energy saleprice (p/kWh)

Scheme life(years)

Discount rateassumed (%)

1. Scottish Renewables Order system (2 x 400kW)

2003 6.9 15 -

2. First ~25MW wave-farm installation (39 x 650kW)

2004 - 20054.75.9

1515

8%15%

3. Costs for 25MW installation by 2010 20102.4 - 3.43.0 - 4.3

2020

8%15%

4. Long-term cost of 25MW installation 2015 - 20201.5 - 2.52.0 - 3.5

2020

8%15%

The reduction in the selling price of electricity generated by the Pelamis WEC system to2010 (2.5GW) is shown in Figures 5.2 once again for a Tf of 0.90 and 0.95 and discountrates of 8% and 15%. A Tf of 0.90 to 0.95 is seen as conservative but realistic. Waveenergy has the potential for larger gains in power capture efficiency solely throughimprovements in control strategies than all other energy technologies. The theoretical'capture-width' (power absorbed / incident power per metre of wave front) of the Pelamisis approximately 60 metres in a 9second period wave (half a wave-length). Advancedcontrol strategies will be required to approach this in irregular seas. The capture-widths(typically 5-15m, dependent on sea state) used to derive the economics presented aboveare based on very basic control strategies.

Extending the 'learning-by-doing' analysis out to 2020 with 40GW of world-wideinstalled capacity gives sustained cost reductions as shown in Figure 5.3.

36

Figure 5.2 – Results from the learning-by-doing analysis to 2010

Figure 5.3 – Results from the learning-by-doing analysis to 40GW installed

The results of the 'cross-check' analysis assuming expected cost-reductions andimprovements in performance (as described in the preceding section), are shown in theTable below. Figures are given both with and without the 50% increase in annual energydue to improved control-algorithms, as this is the most contentious assumption. Theyagree well with the results of the 'learning-by-doing' analysis assuming 40GW installedcapacity.

CASE Includingimproved control

Excludingimproved control

8% discount rate 1.5 p/kWh 2.2 p/kWh15% discount rate 2.0 p/kWh 3.0 p/kWh

Total worldwide installed capacity v's Time

0.0

0.5

1.0

1.5

2.0

2.5

2004 2005 2006 2007 2008 2009 2010

Time (year)

Inst

alle

d C

apac

ity

(GW

)

Cost of energy v'sTime

0.0

1.0

2.0

3.0

4.0

5.0

6.0

2004 2005 2006 2007 2008 2009 2010

Time (year)

Co

st o

f en

erg

y (p

/kW

h)

Fast tech dev, 8% D R

Slow tech dev, 8% D R

Fast tech dev, 15% DR

Slow tech dev, 15% DR

Cost of energy v's Installed capacity (log capacity scale)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0.01 0.10 1.00 10.00 100.00

Installed capacity (GW)

Co

st o

f en

erg

y (p

/kW

h)

Fast tech dev, 8% D R

Slow tech dev, 8% D R

Fast tech dev, 15% D R

Slow tech dev, 15% D R

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Confidence

Confidence in these results is moderate. A thorough attempt has been made to estimatethe economics of an early 25MW scheme, this is seen as a good estimate of the openingcosts of the system if all key development hurdles are successfully passed. The longer-term predictions are less certain and involve significant assumptions about the technologyand market. However, the opening cost of energy of 6-7p/kWh is approximately half theopening cost of wind energy and as such the longer term economics are seen as realisticand realisable given sufficient initial market stimulus.

Future Issues

Near-term and longer-term energy prices will become more accurately defined as thePelamis WEC moves through the RD&D phase and initial commercial systems areinstalled.

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6. THE ONWARD PROGRAMME

The onward programme is summarised below:

6.1 7th scale prototype

The next key task is to prove of the full Pelamis WEC concept using an 7th scale systemsmodel. This will allow the fully functional hydraulic, control and data-acquisitionsystems for the first full-scale prototype to be developed and rigorously tested on a cheaprugged platform.

The model scale of 1:7 has been chosen to match the wave climate in the Firth of Forth.This scale is large enough for functionally realistic systems to be tested while remainingsmall enough to avoid the need for specialist handling equipment. The model will be acheap, rugged test platform with which to develop and prove all aspects of the full-scalecontrol and data acquisition systems. In addition, the model will allow various partial andfull systems failures to be simulated, tests which one would not dare conduct on a full-scale prototype. Damage or loss of the 7th scale model would not be disastrous, similarmishaps at full-scale would be very serious and costly setbacks.

The 7th scale model will continue the OPD ethos of systematically tackling each aspect oftechnical risk before committing to the next development stage. It is absolutely criticalthat as little immature technology as possible is incorporated into first full-scale device.The step to a full-scale demonstrator must be as pure an engineering exercise as possible,rather than an uncertain part of the research & development process.

This programme will have the following key objectives:

1. Build, commission and demonstrate a 7th scale full-system model of the PelamisWEC

2. Develop and demonstrate a robust preliminary Supervisory Control And DataAcquisition (SCADA) system for the future full-scale technology demonstrator

3. Validate numerical simulations of the complete system4. Test the complete SCADA system in a broad range of conditions in active control

mode, passive 'fail-safe' control mode and for a range of partial failure scenarios5. To thereby address these remaining key areas of technical uncertainty via a cheap,

robust but realistic full-system model6. Allow OPD engineers to work closely with the project partners for the full-scale

programme7. Give the OPD team valuable experience working with complex systems in the field8. Provide additional data to characterise scale dependent effects such as drag loading

and mooring response9. Provide additional data to allow further evaluation of the technical and economic

performance of the system

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Once the 7th scale model is successfully demonstrated OPD will address the remainingkey aspect of technical risk for the first full-scale prototype - functionality, operability,reliability and maintainability of the full joint hydraulic and electrical system. All otherelements of the full-scale prototype fall within the capabilities of the expertise of theoffshore sector. Responsibility for these aspects will be the preserve of appropriateoffshore consultants and contractors leaving OPD to concentrate on the machine'ssystems.

6.2 Full-scale joint system test rig

This development task will focus on confirming the functional operation and reliability ofa full joint system for the Pelamis WEC prototype. A complex electro-hydraulic systemsuch as the Pelamis power take-off system would not be cleared for service in theoffshore or aerospace industries before it had completed a rigorous test programme in thelaboratory – wave energy systems should not be treated any differently.

A full joint system test is the only effective way to minimise the overall risk of theoffshore demonstration phase. Various simplified test rig configurations were consideredbut it was concluded that representative tests would only be possible using a rig with thesame configuration, geometry and load/motion limits as the full-scale joint.

The rig will be actuated by an independent hydraulic servo system using a pair of ramsmounted outside the power take-off cylinders. This will give them the necessarymechanical advantage to overcome system friction and flow losses. A servo positioncontrol drive-system will allow accurate simulation of in-service conditions. The drivesystem has been specified to deliver the full angle, velocity, moment and continuouspower ratings of the full-scale joint.

The programme will have the following main objectives:

1. Build and test a full-scale Pelamis joint system2. Confirm functionality of joint control modes3. Confirm functionality of power conversion and electrical systems4. Determine pressure drops through all hydraulic flow paths to ensure inlet cavitation

and local overpressure are avoided5. Confirm the thermal stability of the system for a range of normal and failed operating

states6. Confirm suitability of the chosen hydraulic fluid7. Preliminary assessment of static and dynamic seal performance and likely service life8. Determine full-cycle conversion efficiency of the complete system at a range of mean

power levels9. Conduct a three-month cycle test to increase confidence in reliability before the first

offshore test10. Allow the OPD team to work closely with the full-scale hydraulics contractor and to

gain experience of assembling, testing, operating and maintaining the full jointsystem

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6.3 Full-scale prototype machine

Once all key elements of the system have been developed and tested the final phase ofthe RD&D programme will the building, installation and testing of the first full-scaleprototype machine.

The programme will have the following objectives:

1. Build, install & test a full-scale, grid-connected Pelamis WEC prototype2. Confirm all key performance parameters including:

- survivability- power capture- power conversion efficiency- power quality- power delivery to shore- system reliability, availability, operability and maintainability

3. Provide key design drivers for optimising the system for production units

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7. OVERALL PROJECT CONCLUSIONS

The main conclusions from the Pelamis WEC research and development programme todate are as follows:

1. Core survivability characteristics and mechanisms have been confirmed using a rangeof model tests. These tests have shown that the system will be able to withstandstorm seas.

2. The power-capture potential of the concept has been demonstrated using bothnumerical and experimental techniques. This has shown that the system is effectiveat absorbing power from the required range of small seas.

3. A high-efficiency power-capture and conversion system has been analysed in detail.The power take-off system will allow high mechanical-electrical conversionefficiencies of in excess of 80% to be achieved using proven, off-the-shelfcomponents.

4. Preliminary control systems & algorithms have been developed to allow the system tooptimise power capture across the required range of sea-states.

5. All key structural and hydrodynamic loads have been characterised to allow thedesign of representative structures. Provisional cost-effective structures withappropriate factors-of-safety have been designed and analysed.

6. Provisional mooring systems have been specified and designed including techniquesfor rapid attachment and removal.

7. A preliminary examination of the anticipated installation, operation, maintenance andretrieval requirements and procedures has been carried out, confirming that alloperations can be carried out with non-specialist equipment using standard offshorepractice.

8. A fully revised prototype design incorporating all of the issues indicated above hasbeen produced and costed. The 500kW prototype machine will have a cost ofapproximately £1M (~£2,000/kW).

9. A provisional series-production design has been produced and costed to allowestimation of the likely onward economics. It is estimated that in the medium term a650kW series-production system will cost approximately £500k (~£750/kW).

10. A preliminary assessment of the likely installation, operation, maintenance andretrieval costs for the prototype and production systems has been carried out,including allowance for permitting, site leases and insurance. This shows that annualcosts for a 25MW installation will be of the order of 6% of capital cost per annum.

11. Assessment of the economics of early and longer-term Pelamis WEC installationsincluding a detailed sensitivity analysis covering the main parameters. This showsthat early demonstration schemes will generate electricity for approximately 6p/kWh.Longer-term estimates fall between 1.5-3.0p/kWh showing that the system has thepotential to compete directly with conventional and other renewable generationtechnologies.

12. The key remaining technical risks have been identified to allow a responsible onwardprogramme to be formulated.