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 carried
out under contract as part of the New and
Renewable Energy Programme, managed by the
Energy Technology Support Unit (ETSU) on
behalf of the Department of Trade and Industry.
The views and judgements expressed in this
report are those of the contractor and do not
necessarily reflect those of ETSU or the
Department of Trade and Industry.
EXECUTIVE SUMMARY
The Pelamis Wave energy Converter (WEC) is an innovative new concept for extracting
energy from ocean waves and converting it into a useful product such as electricity, direct
hydraulic pressure or potable water. The system is a semi-submerged, articulated
structure composed of cylindrical sections linked by hinged joints. The wave-induced
motion of these joints is resisted by hydraulic rams that pump high-pressure oil through
hydraulic motors via smoothing accumulators. The hydraulic motors drive electrical
generators to produce electricity. The complete machine is flexibly moored so as to swing
head-on to the incoming waves and derives its 'reference' from spanning successive wave
crests.
This 8-month project part funded by the UK DTI has concluded and fully documented all
aspects of the three-year research & development programme to rigorously examine the
viability of the Pelamis WEC. The results of this programme have been fully assessed by
WS Atkins Consultants Ltd to provide an independent opinion on the conclusions.
The main conclusion is that the Pelamis WEC is technically sound and, once the
development and demonstration phases are complete, should become economically
attractive.
DEVELOPMENT ISSUES SUCCESSFULLY ADDRESSED TO DATE
The following key technical issues have been successfully addressed by the research
programme to date:
1. Validation of the core survivability characteristics and mechanisms using a range of
model tests. These tests have shown that the system will be able to withstand storm
seas.
2. Validation of the power-capture potential of the concept using both numerical and
experimental techniques. This has shown that the system is effective at absorbing
power from the required range of small seas.
3. Analysis & design of an effective, high-efficiency power-capture and conversion
system. The power take-off system will allow high mechanical-electrical conversion
efficiencies of in excess of 80% to be achieved using proven, off-the-shelf
components.
4. Development and provisional testing of preliminary control systems & algorithms to
allow 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 of
representative 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 carried
out with non-specialist equipment using standard offshore practice.
I
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 to
date.
1. Full costing of the new prototype design showing that the first 500kW prototype will
have a cost of ~£1M (~£2,000/kW).
2. Development of reasoned cost estimates for a provisional 650kW series-production
design showing that in the medium-term costs will fall to approximately £500k
(~£750/kW).
3. Preliminary assessment of the likely installation, operation, maintenance and retrieval
costs for the prototype and production systems, including allowance for permitting,
site leases and insurance. This shows that annual costs for an early 25MW
installation will be of the order of 6% of capital cost per annum.
4. Assessment of the economics of early and longer-term Pelamis WEC installations
including a detailed sensitivity analysis covering the main parameters. This shows
that early demonstration schemes will generate electricity for approximately 67p/kWh. Longer-term estimates fall between 1.5-3.0p/kWh showing that the system
has the potential to compete directly with conventional and other renewable
generation technologies.
KEY REMAINING RISKS & DEVELOPMENT MILESTONES
The following remaining uncertainties must be tackled before the technology is ready to
be fully demonstrated through the construction and testing of the first full-scale Pelamis
WEC prototype:
1. Proof of the full control & data-acquisition system for the first full-scale prototype
2. Proof of the full-scale joint hydraulic & electrical systems
These two tasks are the immediate goals of the onward programme. The first full-system
is to be demonstrated using a 7th scale prototype to develop and prove all aspects of the
device technology apart from the full-scale engineering. This strategy minimises the
significant technical and financial risk posed by this key development hurdle.
Once the 7th scale system has been demonstrated work will focus on proving the
functionality 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 be
ready for the first full-scale prototype test.
II
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
3.2
3.3
3.4
3.5
3.6
Survivability…………………………………………………
Power Capture……………………………………………….
Power Take-off System Design……………………………...
Structural Design…………………………………………….
Mooring System……………………………………………..
Installation, Operation, Maintenance & Retrieval…………...
4
7
12
17
20
23
4. MACHINE COSTS……………………………………………………..
26
4.1
4.2
4.3
Prototype……………………………………………………..
Provisional Production Design………………………………
Installation, Operation, Maintenance & Retrieval…………...
26
28
29
5. SYSTEM ECONOMICS……………………………………………….
32
6. THE ONWARD PROGRAMME……………………………………...
38
7th Scale Prototype…………………………………………...
Full-scale Joint System Test…………………………………
Full-scale Prototype Machine………………………………..
38
39
40
7. OVERALL PROJECT CONCLUSIONS……………………………..
41
6.1
6.2
6.3
III
1. INTRODUCTION & AIMS
The aim of this project was to advance the Pelamis WEC concept and design to the stage
that 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 at
the beginning of 1998. Within this the specific project aims were as follows:
1. Extend the Pelamis WEC numerical simulation to increase realism and include nonlinear 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 the
numerical simulations and to further characterise the inherent power-limiting features
of the concept
3. Derive an updated set of key load cases for structural design
4. Carry out a detailed analysis on various structural configurations to allow drafting of
new designs for costing purposes
5. Advance our understanding of mooring loads and dynamics
6. Carry out an initial study on the likely installation, operation, maintenance and
retrieval requirements, and summarise and cost provisional procedures
7. Specify, produce, and fully cost new prototype and provisional production designs
using the results of the individual work topics
8. Carry out a preliminary assessment of the expected cost-of-energy of the prototype
and provisional production systems
9. Identify key areas for further study
10. 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:
Section 3:
Section 4:
Section 5:
Section 6:
Brief description of the Pelamis WEC concept and the working principle.
A summary of performance and technology issues and current status
including details of the methodology used to date, the key results and our
confidence in them, plus brief indications of further work required and
other future issues.
A summary of the provisional predictions of the cost of the system
including capital and operational costs, presented in a similar format to the
previous section.
The results of a preliminary study of the economics of Pelamis WEC
installations.
A summary of the onward programme to successful demonstration of the
Pelamis WEC.
1
2. THE PELAMIS WEC CONCEPT
The Pelamis WEC is a semi-submerged, articulated structure composed of cylindrical
sections linked by hinged joints (Figure 2.1). The wave-induced motion of these joints is
resisted by hydraulic rams which pump high pressure oil through hydraulic motors via
smoothing accumulators. The hydraulic motors drive electrical generators to produce
electricity. The complete device is flexibly moored so as to swing head-on to the
incoming waves and derives its 'reference' from spanning successive wave crests.
A novel joint configuration is used to induce a tunable cross-coupled resonant response
that greatly increases power capture in small seas. Control of the restraint applied to the
joints allows the resonant response to be 'turned-up' in small seas where capture
efficiency must be maximised or 'turned-down' to limit loads and motions in survival
conditions. Electrical power from all the joints is fed down a single umbilical cable to a
junction on the seabed. Several devices can be connected together and linked to shore
through 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 survivability
mechanisms are the use of length as the source of reaction (to allow the system to dereference in long storm waves) in conjunction with a finite diameter to induce full
submergence 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. The
moorings have a motion envelope large enough to accommodate extreme wave motions
in addition to the low frequency wave-group induced response.
2
3. PERFORMANCE & TECHNOLOGY
The following Sections summarise the work carried out on (and the current status of) the
following key development issues:
1.
2.
3.
4.
5.
6.
Survivability
Fundamental power capture
The power take-off & conversion system
Structural design
Mooring system analysis & design
Installation, operation, maintenance & retrieval
The following is presented for each topic:
1.
2.
3.
4.
5.
A summary of current status
A description of the methodology used
A summary of the key results
A brief appraisal of OPD’s confidence in these results
A summary of onward development tasks and other future issues
3
3.1 Survivability
Summary
The core survivability characteristics have been further examined using tank test models
in scale waves up to 28m in height. These tests have demonstrated that loads and
motions in extreme seas are comparable with those required for the device to reach rated
power. OPD are confident that the survivability characteristics of the Pelamis WEC
concept have been validated.
Methodology
The only reliable method to determine the survivability characteristics is to use model
tests 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 wave
heights 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 pass
at this is currently being implemented in the Pelamis WEC simulation as explained in the
following Section.
Other important loads have been estimated analytically or numerically, including peak
mooring, slamming, inertial and torsional loads.
Key Results
Tests at 80th scale in waves up to 28m at full-scale were carried out in the Edinburgh
Wide Tank. The 80th scale model had no joint restraint and the primary aim of the tests
was to determine the required clearance angle between adjacent segments to prevent
damage in the event of a complete hydraulic failure. Joint angles were tracked using
video stills – peak joint angles were seen to be manageable when compared with the joint
angles experienced in normal operation.
4
The 20th scale model was tested in full-scale wave heights of up to 17m in the City
University 55m flume. Extreme waves were generated using a ‘frequency-focussing’
technique, this results in steep, limiting height breaking waves. The maximum wave
height was limited to 17m because of the scale water depth of only 25m. The 20th scale
model has representative joint restraint and full joint moment and angle instrumentation.
These tests showed that joint moments and angles are similar to those experienced with
the model running at full power.
Joint excursion
A sweep of wave amplitudes was carried out with a wave period of 10s and wave
amplitudes of up to 12m. All joint angles and moments reached an asymptote at wave
height = 6 - 7m, or twice the section diameter (Figure 3.3).
H1
H2
H3
H4
0
2
4
6
8
Wave height (m)
10
12
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 absence
of 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 main
bending 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 means
that the contribution of drag terms to the overall loads is small at all scales. A high
degree of confidence can therefore be placed in these results as the main loads are
governed by Froude scaling (small models are valid). Loads due to viscous-drag do not
scale well but are generally small compared to the major loads and will generally be
higher at model scale compared to full scale.
All tests to date have been with joints restrained by passive dampers. There are potential
issues regarding the response of the joint control system in extreme cases but these are to
be investigated using the 7th scale demonstrator in the next phase of the development
programme.
5
Future Issues
Further tests at 33rd and 20th scales are planned by OPD and the current collaborative
EPSRC project with Strathclyde and Southampton Universities. These tests will include
further survivability cases and are to include slamming pressure tests.
The tests at 7th scale in the next phase of the programme will give further information
regarding survivability of the complete system including the behaviour of the joint
control systems.
6
3.2 Power Capture
Summary
The fundamentals of the power-capture characteristics of the Pelamis WEC have been
extensively studied across a wide range of wave conditions using various tank test
models and numerical techniques. Numerical results have been found to agree closely
with experimental results for wave amplitudes where optimisation of power capture are
important, agreement was generally better than 10%. In addition, the inherent
hydrodynamic power limiting features of the Pelamis WEC have been demonstrated
using tank test models. A far-field radiation pattern numerical model of the system has
shown that the ultimate power capture potential of the Pelamis is a factor of three higher
than for a pure heaving buoy of the same volume.
OPD is confident that the power absorption capability of the Pelamis WEC has been
confirmed.
Methodology
Power-capture has been determined using model tests at 35th, 33rd and 20th scales (Figures
3.4 & 3.5) in a wide range of regular and irregular waves. Power capture for the case of
linear (small) waves has been extensively studied using a purpose written numerical
model. The numerical model is a full 6 degree-of-freedom, multi-body, dynamic and
hydrodynamic simulation of the whole Pelamis WEC system. At present, linear bodydynamics and hydrodynamics are assumed. The former is a valid approximation even for
large waves, the latter is currently being addressed in the EPSRC funded hydrodynamics
programme. Numerical and experimental results have been correlated for a wide range of
wave 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 for
real sites using representative wave data. A 3-year data set from the UK Met Office
calibrated wind-wave 'hind-cast' model comprising some 2200, 12-hourly average wavespectra for a representative site were used for annual power capture prediction.
7
The numerical model is unable to predict the inherent power limiting due to hydrostaticclipping so the results are artificially limited at the nominal rated power of the system. A
representative minimum cut-in power has also been included in the annual power
prediction model to avoid distortion of the results due to contributions from very small
waves when the device would be idle. Overall power conversion efficiency as a function
of mean power is applied to all numerical predictions. Conversion efficiency across the
required range of power levels has been estimated using a detailed analysis of the
complete power take off system (see Section 3.3).
An independent numerical model developed by WS Atkins Consultants Ltd has shown
that the ultimate power capture potential of the Pelamis WEC is approximately three
times better than for a pure heaving machine of the same volume. This is due to
asymmetry of the wave radiation pattern produced by the machine. For more details
please 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 which
show that the Pelamis system can absorb significant amounts of power from the required
range of sea states. These tests have been used to validate the PEL suite of proprietary
simulation software. The PEL suite is a sophisticated numerical simulation of the
complete Pelamis WEC system including full six-degree of freedom dynamics, linear and
non-linear hydrodynamics and a model of the power take-off system.
Figure 3.6 – Experimental & numerical power capture for a range of sea states showing
good agreement for small wave amplitudes and long wavelengths and the desired,
progressive departure in agreement for larger, shorter waves.
8
Agreement between numerically and experimentally derived machine power capture is
generally better than 10% for small waves (wave height smaller than the section
diameter) where power capture efficiency is of prime importance. Agreement for longer
and medium wavelengths is generally better (within 5%) than for very short waves where
the segment length is significant compared to the wavelength. This is to be expected, the
effect does not become significant until wavelengths are shorter than 60m, where there is
little energy in real seas. The desired progressive departure in agreement between
numerical and experimental results is noted at larger wave-amplitudes. This is the result
of the inherent load and motion limiting characteristics of the Pelamis WEC due to local
full submergence and emergence of the segments in wave crests and troughs.
Experimental power-asymptotes in large waves were generally higher than the desired
rated-power of the full-scale machine. This is not a serious concern and will be trimmed
by choice of the static-ballasting level of the machine and through more appropriate
choice of applied joint damping levels. These effects are shown clearly in Figures 3.6 for
test 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 varying
the restraint applied to the joints. In addition, the 'Q' of the response can be
independently varied to maximise or minimise the size of the response (and therefore
power capture) to suit the prevailing sea state. These effects are shown in Figure 3.7 for
a range of 20th scale test results.
Figure 3.7 – Experimental capture widths for various control settings showing how the
system can be tuned to particular target frequencies and how the 'Q' of the response can
be varied to suit the prevailing wave height.
9
A key result is that experiments show that the maximum moments and angles required to
reach rated power are of the same order as those encountered in the survivability tests
described in the previous section. This is of vital importance so that the power take-off
system can be sized for their primary role rather than being significantly over-rated to
cope with seldom required survival loads and motions.
The numerical code outputs power for individual joints and the complete machine and
presents it in the form of a 'Power Matrix' as shown below in Figure 3.8. The table given
assumes 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 conversion
efficiency and the cut-in and rated power of the system. The power matrix format has
been chosen by OPD as the most effective way of summarising machine performance as
it allows easy computation of annual power capture for a given site using a table of
occurrence of wave conditions presented in the same format.
Hsig (metres)
Tpow (seconds)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
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
idle
idle
32
57
89
129
-
idle
22
50
88
138
198
270
-
idle
29
65
115
180
260
354
462
544
-
idle
34
76
136
212
305
415
502
635
739
750
-
idle
37
83
148
231
332
438
540
642
726
750
750
750
-
idle
38
86
153
238
340
440
546
648
731
750
750
750
750
750
-
idle
38
86
152
238
332
424
530
628
707
750
750
750
750
750
-
idle
37
83
147
230
315
404
499
590
687
750
750
750
750
750
750
idle
35
78
138
216
292
377
475
562
670
737
750
750
750
750
750
idle
32
72
127
199
266
362
429
528
607
667
750
750
750
750
750
idle
29
65
116
181
240
326
384
473
557
658
711
750
750
750
750
idle
26
59
104
163
219
292
366
432
521
586
633
743
750
750
750
idle
23
53
93
146
210
260
339
382
472
530
619
658
750
750
750
idle
21
47
83
130
188
230
301
356
417
496
558
621
676
750
750
idle
idle
42
74
116
167
215
267
338
369
446
512
579
613
686
750
idle
idle
37
66
103
149
202
237
300
348
395
470
512
584
622
690
idle
idle
33
59
92
132
180
213
266
328
355
415
481
525
593
625
Figure 3.8 – 'Power Matrix' for a 750kW machine
If wave data is available as actual measured conditions in a time-series format the PEL
numerical code can be used to directly calculate the annual average power output of the
system and outputs data in various formats. Probability of exceedence and seasonalvariability for the 500kW prototype machine for a site off the West Coast of Scotland are
shown in Figure 3.9. These show that on the site selected a 500kW machine would
produce 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
10
Confidence
A high degree of confidence can be placed in the experimental and numerical results. All
transducers were accurately calibrated before and after the test programmes. The
agreement between numerical and experimental results was generally excellent for the
range of wave amplitudes and wavelengths where linear wave theory is valid showing
that the numerical model is sound.
The good agreement also indicates that the numerical model may be used to predict
annual power capture with a good degree of confidence if sensible power clipping is
applied as described above. It should be noted that the numerical clip applied is lower
than 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 be
higher than predicted.
OPD is satisfied that it has characterised and validated the fundamental power capture
and survivability features of the Pelamis WEC. However, OPD is committed to a
vigorous ongoing numerical and tank test programme to extend the validity of numerical
predictions and gain further insight into the fundamental dynamics and hydrodynamics of
the concept.
Future Issues
Further extensive tank tests are planned within the EPSRC and OPD programmes to
extend the envelope of understanding of the power capture and survivability of the
Pelamis WEC concept. Experiments with the new 33rd scale model are to include
examination of the effect of limiting the peak applied joint moments on joint angles to
simulate the load limits of the hydraulic power take-off system.
The capability of the numerical model is currently being extended to include the nonlinear hydrodynamic effects that give rise to the inherent motion, load and power limiting
characteristics of the Pelamis. These new numerical models will be validated against
existing and new experimental data to complete the full device simulation capability.
11
3.3 Power Take-off System Design
Summary
The use of hydraulics for wave-energy conversion systems is attractive due to the high
forces and low velocities associated with wave-action. Also, hydraulic accumulators are
by far the most cost effective and efficient, short-term energy storage medium currently
available. However, the irregular, spectral nature of ocean waves presents a significant
problem for conventional hydraulic systems. Wave energy is characterised by high
instantaneous, wave-by-wave power peaks about a lower mean level. The phenomenon
of wave grouping introduces a further order-of-magnitude variation in short-term power
absorption – typical wave groups have periods of 50-100 seconds.
The large variation of instantaneous input power means that conventional hydraulic
transmission systems would have full-cycle conversion efficiencies less that 50% and
involve a number of high cost components. Both of these characteristics are clearly
undesirable and OPD have therefore focussed much effort on developing alternative
strategies that offer significantly higher conversion efficiency while using proven, lowcost components.
The result is a system with a flat conversion efficiency in excess of 80% across the
required range of power levels that minimises the number and type of complex
components. The system has been extensively modelled numerically and provisionally
tested using a single axis rig. The system has been fully specified and costed, including
all hydraulic, electrical, instrumentation and control elements, and an allowance for
assembly and testing. Estimates of component and system reliability have been made
based on manufacturers' data.
Methodology
Various candidate systems have been considered for the Pelamis WEC power take-off
and conversion system. The full-cycle conversion efficiency of a conventional
hydrostatic transmission system using off-the-shelf components was modelled using a
time-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 be
tested in detail before installation in the first full-scale prototype machine. These are the
key objectives of the immediate onward programme.
The functionality and conversion efficiency of the chosen system was extensively
modelled using a time-domain simulation of a single axis unit. The system was then
incorporated into the full time-domain Pelamis WEC simulation code to further
characterise system performance. The results were used to highlight areas where
efficiency may be increased and the design was modified accordingly.
12
The final configuration was modelled to determine losses at each stage, these include:
-
Flow losses along the inlet hydraulic pipe-work, manifold flow-path and
valves (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 a
complete system schematic which was used to draw up a full schedule of costs using
quotes from appropriate suppliers and sub-contractors.
A preliminary analysis of reliability and likely maintenance requirements was carried out
using manufacturers data.
Fundamental functionality of the primary power take-off system was proven using a
single 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 Figure
3.10 below. Each joint axis has an independent power take-off and conversion system
housed in a sealed compartment at the end of each unit. The individual units are
connected together by a common electrical bus running the length of the machine. The
voltage is stepped up for transmission to shore using a single transformer sited near the
cable exit.
Electrical Flexible link
11 kV Transformer
Hydraulic cylinder
Variable displacement hydraulic motor
Induction generator
Hydraulic accumulator
Figure 3.10 – Schematic of power take-off system
13
The rams resisting motion of the joints are configured to operate as large low-speed
pumps. These are ported to high-pressure accumulators that provide short-term storage
to smooth out wave-by-wave and wave-group associated fluctuations of input power, an
example of which is shown in Figure 3.11 (NB – It should be noted that this is a general
characteristic of wave energy, not only the Pelamis WEC system). A quasi-steady flow
of high-pressure oil is drawn by a variable-displacement hydraulic motor running at
constant speed. The motor controller is configured to deliver a constant torque (and
therefore a constant power) to a conventional induction generator. Plots of accumulator
pressure for an input of the form shown in Figure 3.11 and mean levels of 25, 50 and
75kW 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-off
system is summarised in Figure 3.13. Over 80% of the primary power absorbed (i.e. joint
moment times angular velocity) is converted into electrical power. The results also
indicate that the major area for future improvement of the system are the reduction of
losses in the hydraulic motor at low power levels and reduction of flow losses at high
powers through careful optimisation of the flow passages and valves.
14
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 individual
components should be high, and that maintenance requirements would be low. However,
these conclusions are based on limited information and thorough testing of a full-scale
power take-off unit is required to confirm reliability of the complete system – this is one
of the key objectives of the onward programme.
It should be noted that the system has very favourable characteristics compared to many
hydraulic systems in-service:
-
-
The system is housed within a sealed compartment limiting contact with
corrosive or abrasive substances
The system is completely sealed and is pressurised to eliminate ingress of
contaminants should they be encountered
Peak ram seal velocities are typically less than 0.1m/s which is low compared
with 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 degrees
centigrade over the year)
The system will not be stationary or stagnant for long periods of time (a
traditional source of unreliability)
A summary of the costing results is given in Section 4.
15
Confidence
Confidence in the functionality, overall efficiency and cost of the system are high.
Confidence in the predictions of system reliability and the likely maintenance
requirements are low at present.
However, there are many highly reliable, low-maintenance hydraulic systems of similar
or greater complexity in service in the industrial, marine, offshore and aerospace sectors.
OPD is therefore confident that a reliable system can be implemented through careful
design and appropriate testing.
Future issues
The functionality of the complete power take-off and control system is to be proven on
the forthcoming 7th scale demonstrator.
The functionality, conversion efficiency, reliability and maintenance requirements of the
full-scale system are to be thoroughly studied using a full-scale joint test rig in the
laboratory before the system is cleared for service in the first full-scale prototype
machine.
In the future, effort will focus on improving individual component efficiency, reducing
system part-count and cost, and reducing maintenance requirements to an absolute
minimum.
16
3.4 Structural Design
Summary
The design of the main structural elements of the Pelamis WEC is central to the
survivability and economics of the system. It is seen as essential that the prototype
structure is designed and built using established offshore practice and technology where
ever possible. However, it is recognised that in the longer-term cost-effective structures
will have to be fully optimised and will require a significant degree of subsequent
innovation and optimisation.
Considerable effort has been focussed on structural analysis and design to ensure that the
provisional structural designs produced have the required generous factors of safety and
are representative for costing purposes. A full set of operational and various failed
condition load cases have been defined using a mixture of model tests, numerical
predictions and analytical results.
Representative all-steel structures have been analysed and designed using the derived
load-cases. OPD are confident that the structures designed have generous factors of
safety 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 attachment
points into the main tubular structure
Efficient transmission of the main bending and shear loads along the length of
the machine
While structural efficiency is important, the ultimate aim is the most reliable and costeffective structure. The cost of the main structural elements is a major proportion of the
total machine cost. It is therefore important that representative structures are designed
from an early stage to ensure adequate confidence in the costing data. All efforts to date
have concentrated on all-steel structures. Steel has been chosen as the preferred material
for early devices to simplify structural analysis, design and instrumentation. It is likely
that significant cost savings will be possible in the future if other materials such as
concrete are used as the main structural material. Advice concerning appropriate
structural techniques has been sought from independent experts including WS Atkins
Consultants Ltd.
All key load cases have been carefully considered to provide reliable input data to the
structural design process. The primary operational loads including the overall bending
moments and shear forces have been derived from the experimental and numerical model
results. Additional secondary load cases such as mooring, slamming, inertial and
torsional loads have been derived from various independent numerical models. These
17
loads have been used to specify the power take-off system sizes and ratings. These
specifications have then been used to derive the actual peak operational loads to be
experienced by the main structural elements.
An attractive feature of using hydraulic power take-off systems is that applied loads are
limited by total piston area and maximum system pressure – this is advantageous from a
structural perspective. The effect of load limiting has been examined using the computer
simulation program. Even though the hydrodynamic loads are not currently limited
within the numerical model it has been confirmed that the effect of clipping the available
moment restraint does not have a significant effect on the resulting joint angles. This is
important from a survivability perspective. The various loads have been combined into a
standardised set of load-cases that were used for all structural analysis work.
Various structural configurations have been analysed using a range of techniques
including analytical and finite element methods. The models used for finite element
work included all the internal structure. All design factors of safety were related to the
fatigue limit of common offshore steels, typically ~150MPa.
As steel structures were assumed throughout, due regard was paid to appropriate
corrosion protection and shell thickness reduction.
Key Results
OPD has followed WS Atkins Consultants Ltd’s recommendation that a heavy, 25mm
wall tubular structure with minimal internal frames be adopted for the prototype. This
will give conservative factors of safety to reduce the chances of failure due to detail
design issues. Although a large weight of steel will be used for this configuration it is
anticipated that the overall 'one-off' fabrication costs will be significantly reduced.
An appropriate prototype structure has been designed and analysed. Structural analysis
of the chosen prototype structure has shown that peak stress levels in the main tube are
10-20 MPa for normal operating load cases giving design factors of safety (FoS) in
excess of 7 on the steel endurance strength (factor of >12 on yield strength). These
figures have been verified using simple analytical relationships. Stresses rise to 50MPa
at the bearing and ram attachment points giving a FoS of ~3 on the steel endurance
strength. However, it is anticipated that local stresses will be reduced by careful attention
to the detailed design of these areas. Worst case failed-mode stresses are approximately
double the normal operating limits.
A more optimal 12mm wall structure with additional internal stiffening was designed and
analysed for the provisional production design. Finite element analysis has shown that
general stress levels in the main tube are 20-40MPa for normal operating load cases
giving a FoS of ~4 on endurance strength. Local stresses rise to ~70MPa giving a fatigue
FoS >2. Again it is anticipated that local stresses will be reduced through careful detail
design. Again worst case failed mode stresses are approximately double the normal
operating limits. It should be noted that local buckling will become important for shell
18
thicknesses of 12mm and less, this was not analysed in detail but was addressed using
various empirical design rules. A full elastic buckling analysis would be required to
confirm that the internal structure assumed would be sufficient resist buckling failure in
highly loaded regions.
Confidence
OPD is confident that the load cases used are representative of the likely service
conditions. Confidence is also high that the designs produced are detailed enough for
accurate costing purposes.
However, the designs produced must still be viewed as representative only. The final
structure must be thoroughly analysed and designed by an expert structural engineer to
ensure that the target factors of safety are met and that concerns over local buckling are
addressed.
Future Issues
OPD will continue to expand and refine the load case list with new test and modelling
data. However, it is anticipated that all future structural analysis and design will be
carried out by specialist structural consultants.
19
3.5 Mooring System
Summary
Moorings are a key aspect of survivability of all WEC concepts and the Pelamis WEC is
no 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 practice
and technology where ever possible. However, significant differences between
conventional offshore mooring analyses and requirements have been identified which
necessitate detailed study of the main loading and dynamic characteristics of the Pelamis
WEC mooring system.
The loads and dynamic response of the Pelamis WEC mooring has been studied in detail
using a range of techniques. These have shown that the steady drift force experienced by
the system is mainly due to the momentum transfer associated with absorbing power
from the wave, there is little contribution from wave reflection and diffraction (unlike
other WEC configurations). The result of this is that predicted and measured operating
and extreme mooring loads are low. However, the unfavourable result of the form of the
Pelamis is that in-line damping of mooring motions is extremely low. Dynamic
simulations show that low frequency, wave group excited resonant response of the
mooring is the main design driver. However, preliminary model tests at 20th scales have
shown that designs produced and costed to date are conservative.
OPD has set out a sensible onward model test investigation of the key parameters
governing mooring loads and response.
Methodology
A large range of mooring configurations have been considered. A generic Pelamis WEC
mooring dynamic analysis simulation has been developed and used to study mooring
dynamics for a large range of cases, parametric studies of mooring stiffness and damping
have been carried out. Experiments have been carried out at 20th scale in the Trondheim
Ocean Wave Basin in a wide range of seas. Experimental and numerical results have
been correlated and generally show that the numerical predictions are conservative. An
estimate of line fatigue duty has been made using representative wave data.
Several provisional designs have been produced for costing purposes using the results of
the numerical dynamic simulations.
20
Key Results
The numerical simulations show that the Pelamis WEC mooring system will be
susceptible to large, low frequency wave-group excited responses. These responses
dominate mooring loads and motions it is therefore highly desirable to reduce their
contributions. This is a problem common to mooring systems for other floating
structures such as FPSOs. The dynamic response is strongly influenced by subtle
changes in the assumed quadratic mooring damping terms. Model tests will tend to overestimate the likely damping levels.
The measured dynamic response from experimental tests at 20th scale in irregular waves
is much smaller than predicted by the simulation, even when estimates of the damping
contributions from mooring lines and instrumentation wires are included. This
discrepancy is too large for scaled viscous effects and has been attributed to a
phenomenon known as 'wave-drift' damping which arises primarily from the Doppler
shifting of encountered waves due to the mooring response. This effect has also been
found to dominate the response of FPSO mooring systems. However, the precise
mechanisms involved are not necessarily the same. Fortunately, wave-drift damping
rises in parallel with the excitation force thereby ensuring that its effects are significant
across the entire range of wave conditions.
The numerical simulation was modified to include best-fit damping terms for the wave
drift damping component. The damping coefficients were varied to arrive at an
empirically derived best-fit response as shown in Figure 3.14. The contribution from
wave-drift damping is seen to be at least an order of magnitude greater than the viscous
and form drag terms (this is consistent with experience with FPSO moorings).
6
5
surge (metres)
4
3
2
1
0
-1
-2
0
100
200
300
400
500
600
700
time (seconds)
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 likely
scaling laws.
21
These findings are viewed as preliminary. Clearly such a dominant effect demands
further experimental work to improve understanding of the main loading and damping
mechanisms. The mooring designs produced for costing purposes assume load and
motion limits from the numerical motion predictions and are therefore likely to be very
conservative.
Confidence
OPD's confidence in experimentally and numerically derived mooring responses is
currently not high. Further work is clearly required to fully describe the key excitation
and damping terms. This is currently being addressed by further planned model tests at
33rd and 20th scale. It is particularly important to carry out similar tests at different scales
to determine the likely scaling factors for the excitation and damping terms. OPD
currently believes that Froude scaling will apply to the dominant excitation and wavedrift damping terms (ie small models are valid for these terms). The secondary quadratic
damping terms due to viscous drag will not scale well and care must be exercised as
model tests will generally over-predict these contributions.
However, it has been shown that the current numerical models used to specify the
moorings used for costing purposes significantly over predict loads and motions. OPD is
therefore confident that the moorings designed and costed are at least representative and
almost certainly conservative.
Future Issues
Further tests at 33rd and 20th scale will be carried out to improve understanding of the
main excitation and damping mechanisms. Further numerical models will be developed
and correlated with the experimental results until a satisfactory level of agreement is
reached. Experience with the 7th scale mooring loads and responses will be invaluable
for determining the appropriate scaling factors for full-scale mooring design.
22
3.6 Installation, Operation, Maintenance & Retrieval
Summary
Installation of the Pelamis WEC using standard offshore techniques poses few technical
or financial risks. In-service maintenance activities pose more of a problem. It is vital
that operation and maintenance costs are reduced to a minimum as they have a significant
effect on the delivered cost of energy. The machine has been designed from the outset to
require a minimum of routine maintenance.
OPD has determined provisional estimates of maintenance requirements and developed
preliminary techniques for carrying them out. It has been concluded that all maintenance
activities should be undertaken off-site at a suitable facility.
Rapid
attachment/detachment mooring and electrical connection systems to make this possible
have been examined.
Methodology
The Pelamis WEC was specifically conceived to minimise on-site work. The only major
task on-site is installation of the mooring and electrical interconnection. Most of the
equipment associated with this has a relatively low capital value. Mooring and
infrastructure installation activities can be started or stopped at short notice and much of
the work will not be strongly weather dependent. The high capital value Pelamis WEC
machine is only brought to site once the weather dependent activities have been
completed. It is anticipated that the time between arrival on site and hook up will be less
than one hour.
In-service maintenance activities pose more of a problem. OPD has secured the
involvement of an expert from the offshore sector with appropriate experience of
installing, operating, maintaining and retrieving offshore installation such as FPSOs. A
study of the provisional designs was made to determine the likely operation and
maintenance requirements. Until finalisation of the complete system this analysis can
only be provisional. Costs for appropriate support vessels were obtained from various
sources.
Initial work focussed on carrying out all routine maintenance on site with the machine on
its mooring. As a result of this work, and discussions with various key players in the
offshore sector, OPD concluded that maintenance on-site was unrealistic and have now
specified that all maintenance activities should be carried out off-site in a suitable
sheltered location. A mooring and electrical connection system with a simple attachment
and detachment mechanism with a broad weather window is required to make this
possible.
A provisional design for such a system has been developed. A first-pass summary has
been made of the required annual maintenance requirements for the prototype and initial
production systems.
23
Key Results
The initial installation of the system should pose few problems as it will use standard
offshore techniques and practices.
Of more concern are maintenance requirements and strategies. As mentioned above, all
efforts are now focussed on a system where all maintenance is carried out off-site. The
relative ease of removal of individual machines for maintenance and repair is seen as a
compelling advantage of floating, slack moored WEC systems such as the Pelamis, over
fixed or bottom standing WEC concepts and offshore wind-turbines. However, off-site
maintenance is only practical if the mooring and electrical system is designed so that can
be attached and detached across a broad range of sea states. It is seen as particularly
important that an individual machine can be safely removed in large seas in the event of a
component or system failure.
A suitable mooring and electrical connection system has been provisionally designed. It
is 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 that
reattachment will require lower sea states of ~1m significant wave-height.
A provisional study of the likely removal and reattachment 'windows' has been made
using wave data for a representative site. These are summarised in the table below:
Period
October - March
April - September
June - July
Whole Year
Removal (% of period)
60%
92%
98%
76%
Reattachment (% of period)
5%
31%
43%
18%
As can be seen it is likely that detachment can be readily achieved year-round, this is
very important for the removal of damaged or failed units. Reattachment prospects are
generally good during the summer but there may be lengthy delays reinstalling a machine
in winter. It should be noted that many of the more likely system failures such as failure
of the electrical switch-gear and bearing failures in the generator etc result in only a small
loss of energy from the complete device, therefore rapid removal and repair is not
important. More serious failures like hydraulic seal failure are less likely but must be
dealt 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 offshore
techniques and practices. Confidence in the estimated maintenance requirements of early
systems is low. True maintenance requirements will only be determined by installing and
operating a significant number of machines. However, OPD feel confident that the
maintenance requirements of the system will ultimately be low. It is anticipated that
system reliability and availability will be improved to the point where a single annual
24
maintenance programme in summer can be carried out readily and cheaply with the
minimum of weather related delays.
Future Issues
Clearly this topic requires further study. Laboratory testing of the complete power takeoff system will give early indications of the robustness and reliability of the system. In
addition, the first full-scale prototype will allow a comprehensive study of the
maintainability of the complete machine to be undertaken. However, it is likely that the
true maintenance requirements of the system will only be determined and minimised
from experience in the field with a number of machines.
Continued effort will also focus on broadening the weather window for removal and
reattachment, with particular emphasis placed on improving the prospects for reinstalling
units during the winter months.
25
4. MACHINE COSTS
4.1 Prototype
Summary
A new full-prototype design has been prepared incorporating all the modifications and
advances made since the Scottish Renewables Obligation tender application in August
1998. The new prototype design differs significantly from the SRO machine and as such
a full re-costing of the system was required to give accurate data for the onward R,D&D
programme. The new design includes a fully revised structure, power take-off and
conversion, mooring, and control and data-acquisition systems. The structure chosen was
a heavily over-engineered design as described in Section 3.4. The design life of the
system is 15 years.
The new ‘reference’ design has been fully costed using a similar methodology to the SRO
machine as described below. It is estimated that the full cost of the prototype system will
be ~£1M for a 500kW machine (~£2,000/kW).
Estimates were also made for installation and commissioning costs. A three-monthly
intervention cycle for inspection and/or maintenance and repair was assumed to allow
operation and maintenance costs to be defined. Further work will be undertaken in this
area before budgets are finalised for the onward programme.
Methodology
The structure was costed using updated quotes for the main tube elements from suitable
suppliers and detailed estimates of the costs of supply, forming, erection and welding of
all internal structural elements. Finishing costs to various offshore specifications were
obtained from various sources. As a cross-check, a budget estimate was obtained from an
offshore fabricator, this confirmed that the structural costing was representative.
All power take-off and conversion system components were costed using quotes from
appropriate suppliers and allowances for assembly and testing included. Similarly,
quotes were obtained for control and data-acquisition and mooring systems, and the
electrical connection components. Finally, estimates were made for installation of all
systems and components and final assembly of the complete machine.
26
Key Results
The main cost elements of the prototype machine can be summarised as follows:
-
Main structure (finished and ballasted)
Power take-off and conversion system
Instrumentation & control
Mooring system
TOTAL:
£550k
£320k
£30k
£95k
£995k
55%
32%
3%
10%
Confidence
Due to the reliance on quotes from suppliers and fabricators OPD are confident that the
costs derived are reliable.
Future Issues
Further work is required to accurately cost the likely Operation & Maintenance costs. As
the prototype will be part of the R,D&D programme and will be subject to a more
rigorous inspection programme, these costs are likely to be high when compared with a
commercial machine.
27
4.2 Provisional production design
Summary
Detailed cost estimates of a provisional production design are required to allow
assessment of the likely medium and long-term economics of the system. A full
provisional production machine was therefore specified, designed and costed using a
similar methodology to that described in the previous section. The design life for the
system 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 takeoff system chosen was similar to that of the prototype machine.
The costing exercise concluded that, without major technical advances, a 650kW
machine could be built in modest quantities for ~£500k (~£750/kW).
Methodology
The production design was costed using a similar methodology to that described in the
previous section, with reasoned allowances made for economies of scale. The design and
performance assessment assumed no major advances in the conceptual design or
operation of the system.
Key Results
The main cost elements of the system can be summarised as follows:
-
Main structure (finished and ballasted)
Power take-off and conversion system
Instrumentation & control
Mooring system (assuming shared components)
TOTAL:
£245k
£170k
£15k
£60k
£490k
50%
35%
3%
12%
Confidence
Confidence in the results of the costing process is moderate. Various reasoned
assumptions were made but overall OPD concludes that the resulting costs are
representative.
Future Issues
Refinement of the medium to long-term costs of the system will continue throughout the
onward R,D&D programme.
28
4.3 Installation, Operation, Maintenance & Retrieval
Summary
Provisional estimation of the likely operational costs of the system is vital to assess the
overall economics of the system. However, the true costs will only be determined by
further testing of the full-scale systems and through long-term tests of a complete fullscale machine. Operational costing for the prototype will be dominated by intervention
for inspection at regular intervals. Mobilising support vessels will therefore dominate the
in-service costs, an estimation was made to provide budgetary information for the onward
programme.
The main objective of studying of the likely operational costs was to provide reasoned
data for assessment of the onward economics of the system. Work therefore focussed on
the operational costs for the provisional production design deployed in a 25MW array
comprising 39 off 650kW machines sharing moorings and electrical interconnection and
tie-back.
All onboard systems have been specified and developed in discussion with potential
suppliers with an emphasis placed on design for maximum reliability and minimum
maintenance. For the purposes of costing it was assumed that the system would need one
annual intervention for scheduled maintenance with an occasional intervention for
unscheduled repair and a full mid-life service to replace all bearings, hoses and seals.
Preliminary schedules were drawn up for installation, maintenance, repair and
decommissioning of the complete 25MW array using standard offshore working
practices. 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 the
complete 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 for
appropriate support vessels, equipment and consumables.
The installation process is to be performed with a minimum of heavy equipment and
without divers. The latter is of particular importance as deployment in 50+ metres would
require 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 hour
working with an adequate allowance made for waiting on weather.
An annual maintenance schedule was drawn up for an individual machine. As explained
in Section 3.6, it was assumed that all maintenance activities would be carried out off-site
29
in sheltered water to minimise exposure to weather related delays. An estimate of the
likely failure rate of system components was made using manufacturers data with
consideration made for the service conditions allowing a probability-of-replacement to be
estimated for each main system component. This, in conjunction with an estimate of the
man-hours required to effect the replacement and the cost of the component, allowed an
average time and cost for maintenance of an individual machine to be derived. These
results 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 will
necessitate a full mid-life refit of all the main seals, hoses and bearings. An estimate of
the time and cost of this programme was made using a similar methodology to that
described above.
Finally, decommissioning and removal costs were estimated using similar techniques to
those used for deployment.
Key Results
Estimated installation costs for a 25MW array are summarised below (NB costs are for
installation of the system only and do not include the capital cost of the machines or
electrical 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 costs
include removal of the individual machines from the site, maintenance activities
(including manpower, components and consumables) and returning the machine to site):
- Annual scheduled maintenance programme
- Mid-life refit programme
- Unscheduled maintenance (spread over life of scheme)
- Mid-life electrical inter-connection maintenance
£M
0.4
2.5
1.2
0.5
Other miscellaneous operational costs are summarised below:
- Site-lease
- Reactive power charges
- Scheme insurance
~2% of scheme revenue per annum
~0.43p/kVARh
~2% of capital cost per annum
Finally, the decommissioning and removal costs for the complete scheme were estimated
to be £0.6M at the end of the project.
30
For a 15-year project, the installation, operational and removal costs represent an average
of ~£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 a
function of the Pelamis WEC system. The allowance made for insurance is seen to be
high, even for early schemes. It is anticipated that insurance costs will fall dramatically
as confidence in the system increases. By way of comparison typical insurance costs for
ships (which are much higher risk due to human factors and their mobility) are around
1% of capital cost per annum.
Confidence in the results of the Pelamis specific costing exercise is moderate. A
thorough attempt has been made to consider all the installation, operation and removal
costs of a representative scheme. However, more accurate estimation of the true costs of
installing and operating such an installation will only be possible after the actual costs of
the full-scale prototype programme are determined.
Future Issues
The onward programme will focus on determining the onward reliability of the system
and the likely long-term maintenance requirements. The single most important issue is
reliability to ensure that only one scheduled intervention per year is required.
31
5. SYSTEM ECONOMICS
Summary
Initial and onward energy-prices for the Pelamis system are predicted to be as follows:
SYSTEM
1. Scottish Renewables Order system
(2 x 400kW)
2. First ~25MW wave-farm installation
(39 x 650kW)
Anticipated
Date
Energy sale
price (p/kWh)
Scheme life
(years)
Discount rate
assumed (%)
2003
6.9
15
-
4.7
5.9
2.4 - 3.4
3.0 - 4.3
1.5 - 2.5
2.0 - 3.5
15
15
20
20
20
20
8%
15%
8%
15%
8%
15%
2004 - 2005
3. Costs for 25MW installation by 2010
2010
4. Long-term cost of 25MW installation
2015 - 2020
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 at
least 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 the
capital and operational cost data described in the preceding sections. A scheme size of
25MW was chosen to be representative of typical wind farms. However, it is likely that
larger installations would be considered in practice to maximise the economies of scale
through 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 North
Atlantic site with an annual average of 54kW/m. This give an annual energy
yield of ~90GWh.
- Availability of an individual machine of ~95%
- 15 year project length (in line with NFFO & SRO contracts)
32
- Discount rates of 8% and 15% (the Discount rate represents the overall project
rate-of-return)
Scheme capital costs assumed were as follows:
- Planning & approvals
- 39 x Pelamis capital cost
- Electrical connection cabling & equipment
TOTAL:
£M
0.5
19.1
2.9
22.5
(82%)
£M
0.7
1.6
2.7
5.0
(18%)
Installation costs were assumed to be as follows:
- Installation of the 25MW array
- Installation of electrical interconnections & tie-back
- Grid connection onshore
TOTAL:
SCHEME TOTAL: 27.5
(~£1.1M/MW)
Operational costs were assumed to be as follows:
-
£M
0.4
2.5
0.5
1.2
0.65
Annual maintenance programme
Mid-life refit programme
Mid-life electrical interconnection maintenance
Unscheduled maintenance/repair
Insurance
(per annum)
(year 7/8)
(year 7/8)
(over life)
(per annum)
In addition, generation specific costs were assumed as follows:
- Site-lease (Crown Estates Commission)
- Reactive power charges
2% of revenue per annum
0.43p/kVARh
These figures were used in a discounted cash-flow analysis to determine the selling price
of 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 expected
cost reductions as installed capacity rises according to the following expression:
Cm = C1 x m(lnTf/ln2)
Where:
Cm = Cost of mth unit
C1 = Cost of first unit
Tf = Technology Factor
33
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 the
wind energy industry has achieved a Tf of ~0.9. The wind energy industry is predicted to
maintain 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 25MW
installation as described in the previous Section. It was assumed a total world-wide
capacity of 2.5GW (~15% of current installed wind capacity) would be installed by 2010
to the profile shown in Figure 5.1 with major installation commencing from 2007-2008 as
the technology gains credibility. Upper and lower values for Tf of 0.90 and 0.95 and
discount rates of 8% and 15% were assumed to give reasonable upper and lower bounds.
Total w or ldw ide ins talle d capacity v's Tim e
2.5
Installed Capacity (GW)
2.0
1.5
1.0
0.5
0.0
2004
2005
2006
2007
2008
Tim e (ye ar )
2009
2010
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 installed
wind 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 and
performance 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
34
- 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 in
the Summary above):
SYSTEM
1. Scottish Renewables Order system
(2 x 400kW)
2. First ~25MW wave-farm installation
(39 x 650kW)
Anticipated
Date
Energy sale
price (p/kWh)
Scheme life
(years)
Discount rate
assumed (%)
2003
6.9
15
-
4.7
5.9
2.4 - 3.4
3.0 - 4.3
1.5 - 2.5
2.0 - 3.5
15
15
20
20
20
20
8%
15%
8%
15%
8%
15%
2004 - 2005
3. Costs for 25MW installation by 2010
2010
4. Long-term cost of 25MW installation
2015 - 2020
The reduction in the selling price of electricity generated by the Pelamis WEC system to
2010 (2.5GW) is shown in Figures 5.2 once again for a Tf of 0.90 and 0.95 and discount
rates of 8% and 15%. A Tf of 0.90 to 0.95 is seen as conservative but realistic. Wave
energy has the potential for larger gains in power capture efficiency solely through
improvements in control strategies than all other energy technologies. The theoretical
'capture-width' (power absorbed / incident power per metre of wave front) of the Pelamis
is approximately 60 metres in a 9second period wave (half a wave-length). Advanced
control 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 above
are based on very basic control strategies.
Extending the 'learning-by-doing' analysis out to 2020 with 40GW of world-wide
installed capacity gives sustained cost reductions as shown in Figure 5.3.
35
Total w orldw ide installed capacity v's Tim e
Cost of energy v'sTim e
2.5
6.0
5.0
Cost of energy (p/kWh)
Installed Capacity (GW)
2.0
1.5
1.0
4.0
3.0
2.0
Fasttech dev,8% D R
0.5
Slow tech dev,8% D R
1.0
Fasttech dev,15% D R
Slow tech dev,15% D R
0.0
2004
2005
2006
2007
2008
Time (year)
2009
0.0
2004
2010
2005
2006
2007
Time (year)
2008
2009
2010
Figure 5.2 – Results from the learning-by-doing analysis to 2010
Cost of energy v's Installed capacity (log capacity scale)
6.0
Cost of energy (p/kWh)
5.0
4.0
3.0
2.0
Fasttech dev,8% D R
Slow tech dev,8% D R
Fasttech dev,15% D R
1.0
0.0
0.01
Slow tech dev,15% D R
0.10
1.00
10.00
100.00
Installed capacity (GW)
Figure 5.3 – Results from the learning-by-doing analysis to 40GW installed
The results of the 'cross-check' analysis assuming expected cost-reductions and
improvements in performance (as described in the preceding section), are shown in the
Table below. Figures are given both with and without the 50% increase in annual energy
due to improved control-algorithms, as this is the most contentious assumption. They
agree well with the results of the 'learning-by-doing' analysis assuming 40GW installed
capacity.
CASE
Including
improved control
1.5 p/kWh
8% discount rate
2.0 p/kWh
15% discount rate
36
Excluding
improved control
2.2 p/kWh
3.0 p/kWh
Confidence
Confidence in these results is moderate. A thorough attempt has been made to estimate
the economics of an early 25MW scheme, this is seen as a good estimate of the opening
costs of the system if all key development hurdles are successfully passed. The longerterm predictions are less certain and involve significant assumptions about the technology
and market. However, the opening cost of energy of 6-7p/kWh is approximately half the
opening cost of wind energy and as such the longer term economics are seen as realistic
and realisable given sufficient initial market stimulus.
Future Issues
Near-term and longer-term energy prices will become more accurately defined as the
Pelamis WEC moves through the RD&D phase and initial commercial systems are
installed.
37
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 systems
model. This will allow the fully functional hydraulic, control and data-acquisition
systems for the first full-scale prototype to be developed and rigorously tested on a cheap
rugged 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 remaining
small enough to avoid the need for specialist handling equipment. The model will be a
cheap, rugged test platform with which to develop and prove all aspects of the full-scale
control and data acquisition systems. In addition, the model will allow various partial and
full systems failures to be simulated, tests which one would not dare conduct on a fullscale prototype. Damage or loss of the 7th scale model would not be disastrous, similar
mishaps at full-scale would be very serious and costly setbacks.
The 7th scale model will continue the OPD ethos of systematically tackling each aspect of
technical risk before committing to the next development stage. It is absolutely critical
that 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 Pelamis
WEC
2. Develop and demonstrate a robust preliminary Supervisory Control And Data
Acquisition (SCADA) system for the future full-scale technology demonstrator
3. Validate numerical simulations of the complete system
4. 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 scenarios
5. To thereby address these remaining key areas of technical uncertainty via a cheap,
robust but realistic full-system model
6. Allow OPD engineers to work closely with the project partners for the full-scale
programme
7. Give the OPD team valuable experience working with complex systems in the field
8. Provide additional data to characterise scale dependent effects such as drag loading
and mooring response
9. Provide additional data to allow further evaluation of the technical and economic
performance of the system
38
Once the 7th scale model is successfully demonstrated OPD will address the remaining
key aspect of technical risk for the first full-scale prototype - functionality, operability,
reliability and maintainability of the full joint hydraulic and electrical system. All other
elements of the full-scale prototype fall within the capabilities of the expertise of the
offshore sector. Responsibility for these aspects will be the preserve of appropriate
offshore consultants and contractors leaving OPD to concentrate on the machine's
systems.
6.2 Full-scale joint system test rig
This development task will focus on confirming the functional operation and reliability of
a full joint system for the Pelamis WEC prototype. A complex electro-hydraulic system
such as the Pelamis power take-off system would not be cleared for service in the
offshore or aerospace industries before it had completed a rigorous test programme in the
laboratory – 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 the
offshore demonstration phase. Various simplified test rig configurations were considered
but it was concluded that representative tests would only be possible using a rig with the
same 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 rams
mounted outside the power take-off cylinders. This will give them the necessary
mechanical advantage to overcome system friction and flow losses. A servo position
control drive-system will allow accurate simulation of in-service conditions. The drive
system has been specified to deliver the full angle, velocity, moment and continuous
power ratings of the full-scale joint.
The programme will have the following main objectives:
1.
2.
3.
4.
Build and test a full-scale Pelamis joint system
Confirm functionality of joint control modes
Confirm functionality of power conversion and electrical systems
Determine pressure drops through all hydraulic flow paths to ensure inlet cavitation
and local overpressure are avoided
5. Confirm the thermal stability of the system for a range of normal and failed operating
states
6. Confirm suitability of the chosen hydraulic fluid
7. Preliminary assessment of static and dynamic seal performance and likely service life
8. Determine full-cycle conversion efficiency of the complete system at a range of mean
power levels
9. Conduct a three-month cycle test to increase confidence in reliability before the first
offshore test
10. 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 joint
system
39
6.3 Full-scale prototype machine
Once all key elements of the system have been developed and tested the final phase of
the RD&D programme will the building, installation and testing of the first full-scale
prototype machine.
The programme will have the following objectives:
1. Build, install & test a full-scale, grid-connected Pelamis WEC prototype
2. 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
40
7. OVERALL PROJECT CONCLUSIONS
The main conclusions from the Pelamis WEC research and development programme to
date are as follows:
1. Core survivability characteristics and mechanisms have been confirmed using a range
of model tests. These tests have shown that the system will be able to withstand
storm seas.
2. The power-capture potential of the concept has been demonstrated using both
numerical and experimental techniques. This has shown that the system is effective
at 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 conversion
efficiencies of in excess of 80% to be achieved using proven, off-the-shelf
components.
4. Preliminary control systems & algorithms have been developed to allow the system to
optimise power capture across the required range of sea-states.
5. All key structural and hydrodynamic loads have been characterised to allow the
design of representative structures. Provisional cost-effective structures with
appropriate factors-of-safety have been designed and analysed.
6. Provisional mooring systems have been specified and designed including techniques
for rapid attachment and removal.
7. A preliminary examination of the anticipated installation, operation, maintenance and
retrieval requirements and procedures has been carried out, confirming that all
operations can be carried out with non-specialist equipment using standard offshore
practice.
8. A fully revised prototype design incorporating all of the issues indicated above has
been produced and costed. The 500kW prototype machine will have a cost of
approximately £1M (~£2,000/kW).
9. A provisional series-production design has been produced and costed to allow
estimation of the likely onward economics. It is estimated that in the medium term a
650kW series-production system will cost approximately £500k (~£750/kW).
10. A preliminary assessment of the likely installation, operation, maintenance and
retrieval costs for the prototype and production systems has been carried out,
including allowance for permitting, site leases and insurance. This shows that annual
costs 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 installations
including a detailed sensitivity analysis covering the main parameters. This shows
that 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 the
potential to compete directly with conventional and other renewable generation
technologies.
12. The key remaining technical risks have been identified to allow a responsible onward
programme to be formulated.
41