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NEAR SHORE FLOATING OSCILLATING WAVE COLUMN Prototype development Powered By Docstoc

Prototype development and

CONTRACT NUMBER: V/06/00201/00/00

URN NUMBER: 05/581
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                                      Page 2 of 22

  1. The Near Shore Floating OWC Buoy project has investigated three different
     types of OWC device – Sloped Buoy, Backward Bent Ducted Buoy and Spar
     Buoy and considered their power capture efficiency, cost to build and
     operating economics for bulk generation of electricity in an Atlantic
     environment. All three forms of buoy assumed the use of air turbine power
     take-off systems.
  2. The Spar Buoy was considered to be the lowest risk and most economic
     configuration of buoy for the proposed purpose and this design was
     developed further as detailed in Section 1. Other geometries of buoy, while
     giving greater power capture generally linked to better tuning to the
     predominant wave period, had poorer economics driven by higher cost of
     build, deployment or mooring.
  3. Investigation of the optimum size of the Spar buoy was carried out involving
     consideration of construction, deployment, mooring and grid connection.
  4. Construction of the buoy using both steel and concrete techniques was
     investigated. The conclusion was that the concrete design would give savings
     in the multiple buoy construction scenario but that there were more risks
     associated with this material.
  5. A risk assessment was carried out on the Spar buoy solution (Sections 2 and
     3). There are still unresolved technical issues with free floating OWC buoys
     such as the difficulty of access due to large amplitude motions, even in
     modest sea states.
  6. For economics of power generation to achieve 6p/kWh with a multiple Spar
     buoy installation (assuming 100 units) would require for example a 40%
     improvement in pneumatic power capture (from 32% to 45%), a 30%
     improvement in pneumatic to electricity power conversion (from 50% to 65%)
     and a 35% reduction in cost of build, deployment and grid connection.
  7. The project concluded that, while the OWC Spar buoy offered a technically
     feasible means of generating power in combination with an air turbine power
     take-off system, the economics are not currently competitive with other forms
     of bulk renewable power generation due to the high added cost of
     deployment, mooring and cabling to suitable deep water sites. The project
     recommended further analytical investigation of buoy geometries for improved
     tuning in longer wave lengths to achieve the targeted 45% wave to pneumatic
     power capture. The report also recommends that the key risk areas of
     mooring / riser integrity and buoy accessibility could be validated through
     small scale testing of a device in inshore waters before going to a full scale
     prototype deployed offshore.
  8. The project investigated alternative means of power generation. One system
     involved the use of a piston placed within the oscillating water column to drive
     a hydraulic ram through relative motion between piston and the buoy
     structure. The second method involved placing a piston in series between an
     anchor line connecting the buoy to the seabed. From the initial investigations
     carried out it was concluded that neither option offered more potential than
     the OWC/air turbine solution.

                                   Page 3 of 22

Section 1   History of Project Development

Section 2   Project Definition

Section 3   Power Generation Economics and Sensitivities

Section 4   Lessons Learned

                           Page 4 of 22
Section 1


Overview of Investigations

The feasibility project was initiated in early 2002 when it was proposed that a floating
oscillating water column (OWC) device could be installed to the west of Lewis and be
connected to the electricity grid via a sub-sea cable.

Three types of devices were investigated as floating offshore OWC devices:

   •   Sloped Buoy

   Figure 1.1 – Sloped Buoy

The sloped buoy is a floating buoy with three parallel, immersed tail tubes that float at
an angle of some 45 degrees to the vertical. The length of the tail tubes constrains
the buoy to move in the direction of the sloped tubes. The tail tubes, which are open
to the sea at the bottom end contain a mass of sea water against which a moving
buoy can react, utilising the energy in both surge and heave motions.

                                      Page 5 of 22
   •   Backward Bent Duct Buoy (BBDB)

Figure 1.2 – Backward Bent Duct Buoy

The BBDB has a horizontal tail tube that uses the surge and pitch motion of the buoy
to create relative movement between the device and its constrained water columns.

   •   Spar Buoy

Figure 1.3 – Spar Buoy

The Spar Buoy has a predominant heave motion and generates pneumatic power
through the relative motion between the water column in the vertical draught tube

                                    Page 6 of 22
that is open at its base to the sea and the buoy’s whole body motion, which is
designed to be out of phase with the water column motion.

Each type of buoy was tested in Wavegen’s wave tank and optimised in terms of
damping from the power take-off system to achieve best power capture. The Spar
Buoy has the advantage of being symmetrical and equally efficient at capturing
energy from all directions. Both the Sloped Buoy and BBDB are particularly efficient
in one direction but their efficiency falls off when waves approach them from an angle
to the normal operating direction. It was not considered cost effective to tether the
devices with the provision for them to weathervane, both because of the cost of the
weather-vaning arrangement and also because it was not certain that the device
would wish to align itself with the prevailing wave conditions when also presented
with current and wind, possibly originating from different directions. Therefore, for
power capture performance each device was moored by a four point spread mooring
system into the predominant wave direction and the fall off in performance was
assessed for wave directions other than the predominant direction. The analysis was
carried out using wave rose information for a possible west of Lewis deployment

The relative performance of the three devices in terms of pneumatic power generated
is shown in Figure 1.4.

Figure 1.5 presents the relative capital costs of building and deploying the three
types of device. The BBDB because of its considerable size and hence cost
becomes a relatively expensive device. The Spar Buoy, because of its relatively
simple construction becomes the most cost effective device of the three considered.

Different mooring systems were tested in the tank and also tried out on numerical
simulation package Orcaflex in order to specify a solution that would hold the buoys
in all weathers and also not interfere with their movement and hence power capture.
The preferred solution was a four point mooring configuration using a combination of
wire rope and nylon. The system was designed to allow the buoy to move freely in
order to capture energy efficiently but with a limitation of a 500 metre watch circle set
as a limiting zone that the buoy was allowed to move from its specified location.
Investigations were carried out into normal operation and a one line broken scenario
where minimum excursions help with respect to riser design and the proximity of
other designs or obstructions.

The optimum size of the device was investigated to determine the optimum power
capture given consideration for capital cost. Significant work was undertaken in our
wavetank to optimise the shape of the buoy to achieve maximum performance. In
addition cost studies were undertaken to optimise the size of the device and
maximise the potential of the available construction facilities. A major limiting factor in
sizing the device is the size and/or weight of the structure and the available facilities
for lifting and deploying a device into the water.

Materials for buoy construction were considered and given the size of the device the
logical alternatives are steel, concrete and possibly a steel-concrete hybrid solution.
Steel was considered as the most likely alternative for a prototype device. Steel has
the advantage of being an easy material to work with for design changes that might
wish to be made to the prototype. It is also relatively inexpensive for a one-off
structure. Its main disadvantage is corrosion. Concrete looks a more interesting
option for a multiple build production scenario. Savings in the order of 30% can be

                                       Page 7 of 22
made in concrete structures by utilising reusable shuttering. This material also has
the advantage of being less affected by corrosion from sea water.

A number of other alternatives were looked at briefly and they included sprayed
concrete and steel-concrete hybrids such as bi-steel. Bi-steel was considered for use
for water column structures as it provided a rigid column without the requirement for
external stiffening which would interfere with the water flow either inside or outside
the column. All of these alternatives showed some merit for particular applications or
for particular parts of a structure.

Comparison between Power Capture of Floating Wave Energy Devices

Models were based on full-scale devices, which were considered to be buildable,
targeting realistic body form, mass distribution, centre of gravity (COG) and mass
radius of gyration (ROG). Each model underwent a degree of performance

The purpose of the tests was to provide device performance for annual average
wave conditions which could then be compared against the cost of build as part of a
design evaluation matrix, with the ultimate aim of making an informed decision with
respect to device selection.

The results from the power capture tests are presented in Figure 1.4.

The models were tested in an annual average Lewis wave climate (Hs=3.25m) for
two peak wave periods and for long crested and short crested seas. The choice of
peak period depends on whether the average peak period (10.5s) or the most
common period (8.7s) associated with the average Hs is selected. The table also
makes reference to performance in uni-directional and omni-directional seas. Results
in omni-directional seas were achieved by taking the BBDB and Sloped Buoy and
measuring their captured power in the same Lewis sea state (with and without
spreading) for a range of vessel headings in the wave tank, namely 0, 30, 60 and 90
degrees. The measured power outputs were then factored by the percentage time
that waves impinged on the devices from these directions based on wave rose data
for that sea area. Because of its symmetry the Spar Buoy was only tested for one

Based on pure power capture performance the BBDB is superior to the Sloped buoy
and Spar buoy, the comparison being for devices with the same capture width. The
Spar Buoy performed very well in operating seas with an average pneumatic output
of 177kW for the shorter prescribed peak period, although this performance dropped
off for the longer peak period (10.5s). It is interesting to note that the performance of
the Spar Buoy actually increases when short-crested seas were applied, whereas the
other devices have been shown to be sensitive to short-crestedness.

                                      Page 8 of 22
Pnuematic Performance Comparison           SLOPED                         SPAR
for systems on spread mooring in            BUOY BBDB2                    BUOY
Lewis annual average conditions
    Spectral                Sea     Spread
     Model     Hs Tp Directionality Seas     (kW) (kW)                     (kW)
Bretschneider 3.25 8.7      UNI      Crest    175  183                     177
Bretschneider 3.25 10.5     UNI      Crest    144  166                     119
Bretschneider 3.25 10.5     UNI     Cos^10    138  142                     130
Bretschneider 3.25 10.5   OMNI       Crest    133  133                     119
Bretschneider 3.25 10.5   OMNI      Cos^10    126  122                     130

              Figure 1.4 Comparison of Pneumatic Power Capture

The fall off in power output with the BBDB from 166kW in uni-directional seas to
122kW in spread seas taking into account a range of predominant sea directions
clearly demonstrates the sensitivity of this device to real sea conditions.

The relative economic potential of the devices becomes clearer when consideration
is given to cost of build (Figure 1.5).

                                    Sloped Buoy        BBDB           Spar Buoy

kW (pneumatic)                          126             122               130
PTO Efficiency                          45%             45%               45%
kW (electrical)                          57              55                59

Rated Power (3 x annual average)         171            165               177
(a) Normalised Rated Power               0.97           0.93              1.0

(b) Relative CAPEX                       1.20           1.51              1.0

Ranking Value [(a)/(b)]                  0.81           0.62              1.0
                           Figure 1.5 Cost Comparison

                                    Page 9 of 22
Other Attributes for Consideration

Attributes other than economic were also considered for the three designs of buoy.
The list of assessment areas is given in Table 1.1 below.

              Description                                 Criteria
   1    Survivability                 Performance in survival seas.
                                      Snatch loads on moorings.
                                      Propensity to capsize in extreme seas.
   2    Susceptibility to wind,       Impact of wind and current on the performance
        current (in association       of the device.
        with selected mooring
   3    Access and                    Movement of the device for carrying out work
        maintainability               onboard.
                                      Ease of access for boarding and evacuation.
   4    Taking power away             Excursions of mooring/power umbilical system.
                                      Interference between the mooring system and
                                      the power umbilical
   5    Potential market              Any identified differences in market potential
                                      between the devices.
   7    Potential for increase in     Scope for increasing the power output or
        power/decrease in cost        reducing the cost of building and deploying the
   8    Normalised cost of power      The predicted cost of building the full scale
        generation                    device versus the predicted power generated,
                                      normalised against the value for the best
   9    Suitability for 3-D seas      Is the device uni-directional or will it operate in
                                      3-D seas?
   10   Accommodate different         Is the device restricted to a single type of power
        PTO options                   take off (eg Wells turbine) or can other methods
                                      be used (eg hydraulics).
   11   Ease of deployment            The ease and cost of getting the device from the
                                      construction site to its final installation location.
   12   Confidence in predicted       Confidence that calculations and wave tank
        performance                   testing results can be scaled up to full size.
   13   Low environmental             Consider visual impact, noise, potential for
        impact                        problems such as release of hydraulic oil,
                                      impact of mooring system on the seabed, etc.
   16   Ease of decommissioning       Issues with decommissioning the device
                                      generally related to the material of construction
                                      and ease of recovery and disposal.
                            Table 1.1: Assessment Criteria

The relative performance of the devices against the listed criteria is given in Table
1.2. The Spar Buoy was considered at least as good as the other two designs for
nearly all criteria and in particular for lack of susceptibility to wind and current,
suitability for 3-D seas and for the accommodation of different power take-off options.

Survivability was considered to be the most important attribute on the list and the
Spar Buoy, which exhibited the best motion characteristics in extreme waves, was
considered superior to the other two designs.

                                    Page 10 of 22
However, the Spar Buoy was seen to have poor performance in three specific areas;

   •    Access for maintenance is poor due to the large heave amplitude even in low
        sea states, a characteristic that contributes to its good power production.
   •    The inability of the device to be tuned to different sea conditions without
        changing its constructed size.
   •    The difficulty of deployment due to its large draught when in its vertical
        operating condition. Solutions for tow out and recovery in the horizontal mode
        need investigation.

                Description           Sloped Buoy       BBDB Buoy        Spar Buoy
   1     Survivability                 Acceptable         Poor             Good
   2     Susceptibility to wind,       Acceptable         Poor             Good
         current (in association
         with selected mooring
   3     Access and                        Poor         Acceptable          Poor
   4     Taking power away             Acceptable       Acceptable       Acceptable
   5     Potential market                Limited          Limited         Universal
                                       deployment       deployment       deployment
   7     Potential for tuning             Poor           Medium             Poor
         device to sea conditions
   8     Normalised cost of power          1.22             1.05             1.0
   9     Suitability for 3-D seas          Poor             Poor            Good
   10    Accommodate different             Poor             Poor            Good
         PTO options
   11    Ease of deployment               Poor          Acceptable          Poor
   12    Confidence in predicted       Acceptable       Acceptable       Acceptable
   13    Low environmental                Good              Good            Good
   16    Ease of                       Acceptable       Acceptable       Acceptable
                           Table 1.2: Analysis of Devices

Following a structured assessment against the above criteria the Spar Buoy was
chosen on both economic and practical considerations as the preferred design to
take forward.

A number of Spar Buoy models were then tested in the wave tank to optimise the
shape and size of a suitable prototype design.

                                    Page 11 of 22
Buoy Geometry

Having decided upon the Spar Buoy as the preferred design of buoy a considerable
amount of time was expended at the wave tank investigating the optimum shape and
size of buoy. In addition, a mathematical model of buoy performance was developed
and this was benchmarked against tank test results. The various models of Spar
Buoy were tested for power capture and for motion response. The design produced
is considered to have the optimum shape, size and centre of gravity.
Various models were also tested in the wave tank to provide details on mooring
loads, along with Orcaflex mathematical modelling, and structural loads in various
sea states.

Power Take Off

The analysis in this report has assumed a fixed pitch Wells turbine would be installed
as the power take-off system.

Two other types of power take-off systems were also investigated. The two systems

Piston reference system:     This involves an inertial reference piston positioned in
                             the water column which can be mechanically coupled to
                             the buoy through a hydraulic piston power take-off.

Ground reference system:     This system extracts power from the buoy’s motion
                             relative to a fixed reference the seabed rather than its
                             relative movement to some internal water column.

Work into the piston reference system concluded that this mechanism would not offer
improved power capture efficiency with the floating Spar Buoy due to the high losses
caused by flow between the piston and the tube at what were considered acceptable
engineering build tolerances.

Conceptual investigations into the ground reference system for power take off
involving numerical modelling showed some promising results but the performance
could not be replicated with a full 6 degrees of freedom model in the wave tank. The
advantage of a ground reference system over an oscillating water column is its ability
to capture power over a wider range of wave periods and the potential to incorporate
some form of ‘latching’ as the floating buoy can be held by the geo-fixed power take-
off system to be release when it can extract maximum power from the wave.
However, the complexity of such a system to cope with mooring loads and tidal range
should not be underestimated.

                                   Page 12 of 22
Section 2


This section describes the preferred Spar Buoy solution.

1 - Description:
OWC Power Buoy is designed for offshore power generation and is a free floating,
soft moored “point absorber” device that can generate power through a number of
different mechanisms.
The geometry consists of a tubular free flooded draught tube, the upper portion of
which is surrounded by a buoyancy can.

2 - Location:
Any offshore location where there is adequate depth of water (preferably greater than
2 x buoy draught to limit extreme event mooring loads). Were the prototype device to
be sited in about 50m water, the buoy draught would be limited to 25 metres.
The device has high survivability and can operate in extreme wave climates such as
those that exist west of Lewis.
The device is axisymetric and can operate in conditions where wind, waves and
current are not aligned.
The seabed has to accommodate the anchoring system. If adequate sediment exists
this can be drag-in or suction can anchor systems. If there is no sediment then
gravity anchors, or drilled and grouted piles or rock anchors can be used.

3 - Structure:
A number of options exist:
   1. Stiffened plate steel buoy and thick walled unstiffened steel plate draught
      tube with heavy steel section at base of ballast tube for stability. This is a
      relatively high cost solution and should only be considered for smaller size
      devices when alternative construction materials will not work.
   2. Concrete buoy connected to reinforced concrete draught tube. This is
      potentially the lowest cost solution provided initial set-up costs are covered by
      a sufficiently large production run. The requirement to paint the concrete
      structure to prevent water absorption and to prevent corrosion needs to be

                                   Page 13 of 22
4 – Survivability
Model tests and simulations have been carried out to demonstrate that the buoy does
not suffer extreme motions in survival conditions. The model tests look at waves only
while the Orcaflex simulation includes the influence of wind and current as well as
waves. Large seas break over the buoy and the extreme motions of the buoy can
cause it to plunge below the surface. However, the buoy always recovers and does
not trip over its mooring system.
Loss of the buoy could arise through flooding of the buoyant annulus, failure of the
mooring system or structural damage through extreme wave loads or grounding on
the seabed.
The buoy is designed such that it can survive the damage of any one watertight
segment where the damage extends vertically without limit.
Failure of the mooring system was addressed.
Damage of the main buoy structure due to extreme wave loads is addressed during
the design. Damage to the turbine machinery housing is also a possibility. However,
it is of smaller diameter than the buoy and will attract lower wave loads. It will be
designed such that failure of the housing will not lead to failure of the buoy’s intact
Damage though grounding on the seabed could arise if the buoy motions are greater
than expected.

5 - Power Capture Performance:
The basic mechanism for extracting power from waves involves the relative motions
of the heaving buoy and its entrained water column. Alternatively, the absolute
heaving motion of the buoy relative to a fixed reference point, the sea bed, may be
an alternative basic mechanism.
The device power capture performance is tuned such that one power capture peak
occurs at the buoy heave resonant period while a second generally lesser peak
occurs at the water column resonant period. Changes in buoy and draught tube
dimensions alter the wave periods at which these peaks occur and also the
magnitude of the peaks.
The power capture performance is generally unaffected by spread energy seas or by
bi-direction seas due to its axi-symmetric geometry.

                                   Page 14 of 22
6 – Mooring / Anchoring:
The concept involves installing multiple buoys in water depths of between 50m to
80m. A number of anchoring solutions exist and the project investigated drag-in and
pile anchor solutions. Drag-in anchors require sufficient depth of sediment at the site.
Pile anchors only become economically attractive with large wave farm
developments involving multiple installations.
Detailed modelling and tank testing has shown that a soft mooring is required both to
reduce mooring loads and also to allow the device to perform. The downside of soft
mooring is the long lines and hence large footprint required. The current solution
uses a normal catenary system. Alternatives such as buoys and sinkers have been
investigated to achieve a reduction in footprint area and cost.

7 – Power Generation

Air Turbine Solution: The relative motion of buoy and water column compresses air
which can, as with Wavegen’s shoreline OWC device LIMPET, be fed through a self
rectifying Wells turbine. This is mounted either vertically or horizontally above the
draught tube top plate and protected from the sea by a deckhouse structure. The
decision on mounting the turbine in either the horizontal or vertical position on the
buoy would be resolved during detailed design. Mounting the turbine vertically
provides a direct route for the airflow but the device is exposed to the larger waves
and is difficult to protect. Mounting horizontally provides a more compact design
solution but may give more losses.

8 – Power Conditioning
The generated power output from any of the above devices will be alternating current
of variable frequency and voltage. By passing the current through a double invertor
system (a.c. – d.c. – a.c.) a regular 50Hz a.c. supply can be generated. This can then
be transformed up to a higher voltage for transmission ashore and connection to the
An alternative option would be to invert the a.c. power to d.c. on the buoy and to
transmit the d.c. supply through an umbilical to a Power Control Buoy that would be
receiving d.c. supplies from a number of adjacent Power Buoys. The Power Control
Buoy would invert the collected d.c. inputs to a.c. 50Hz and transform up to a higher
voltage for transmission to shore. The invertor on the Power Buoy would be a
modular unit for maintenance by removal. It would require a cooling system. The d.c.
output could pass through a voltage regulator to charge onboard batteries for
supplying communication and control equipment and for driving ancillary equipment,
e.g. bilge and ballast pumps.

                                    Page 15 of 22
9 - Power Export:
This will require a highly dynamic riser cable connected to the main subsea cable by
a wet or dry mateable connector and also provided with a weak link in the event of
mooring failure.

10 – Installation:
The device could be deployed in the horizontal position either through blanking off
the draught tube to give additional buoyancy or by adding temporary flotation. Once
on site the buoy would be rotated into the vertical through ballasting/flooding of
buoyancy spaces and connected to its mooring system. The final operation would
involve connection of the power umbilical.

11 – Monitoring:
The list below details the requirements for testing of a prototype device offshore.
           a. Measure power output from given seas. Requires wave data,
              measurement of air pressures and flows on buoy and units of
              electricity generated.
           b. Measure ability to gain access to the device by determining limiting
              sea states and the amount of time they are achieved.
           c. Measure loads and accelerations on the device using strain gauges
              and accelerometers. Measure loads on the draught tube cover plate
              structure with the air vent open (operating state) and closed-off
              (survival state).
           d. Measure loads on the mooring system using strain gauges and
              excursions of buoy using GPS.
           e. Identify any issues with the performance of the umbilical and
              associated connectors in a dynamic environment.
           f.   Identify maintenance issues plus any other issues causing downtime.
           g. Confirmation of operating and safety procedures.
           h. Measure marine growth and corrosion.
           i.   Record influence of draught and c of g on motions and power capture

                                    Page 16 of 22
Section 3


This section defines the expected economics for deploying multiple devices in a UK
west coast sea area with an annual average wave climate of 45kW/m.

Energy Production

The pneumatic powers captured for each cell in the wave scatter diagram (defined by
a wave height/wave period set assuming a Bretschnieder spectral distribution) was
derived from pneumatic power capture factor curve for the Spar 6 MkXI model
analysed against the power distribution for each spectra.

Capture factor for each cell is simply the average pneumatic power divided by the
incident wave energy given by P = 0.4906Hs2Te kW/m.

The capture factor for each cell is tabulated in Table 4.1 below:
   Hs         1     2.5    3.5    4.5     5.5     6.5     7.5     8.5     9.5    10.5    11.5    12.5   13.5   14.5   16.5    19
  0.25                                                  35.0%
  0.75                           5.0%   21.2%   34.9%   38.9%   34.3%   27.3%   21.6%   16.9%
  1.25                           7.8%   23.3%   35.9%   37.3%   32.9%   27.0%   21.1%   16.2%
  1.75                                  23.8%   34.8%   36.5%   32.9%   26.9%   21.0%   16.0%   12.4%
  2.25                                  23.6%   34.9%   37.0%   33.0%   27.3%   21.3%   16.3%   12.1%   9.1%   6.7%
  2.75                                  23.7%   35.2%   37.0%   32.9%   27.1%   21.1%   16.1%   12.1%   9.1%   6.8%
  3.25                                  23.5%   35.3%   36.8%   32.9%   27.2%   21.2%   16.2%   12.1%   9.1%   6.8%
  3.75                                          35.1%   36.8%   32.9%   27.2%   21.2%   16.2%   12.1%   9.1%   6.8%
  4.25                                                  36.9%   33.0%   27.2%   21.3%   16.3%   12.2%   9.1%   6.8%
  4.75                                                  36.8%   32.9%   27.1%   21.3%   16.3%   12.2%   9.1%   6.8%   3.9%
  5.25                                                  36.9%   32.9%   27.2%   21.4%   16.3%   12.2%   9.1%   6.8%   3.9%
  5.75                                                          33.0%   27.2%   21.4%   16.3%   12.2%   9.1%   6.8%   3.9%
  6.25                                                                  27.2%   21.4%   16.3%   12.2%   9.1%   6.8%   3.9%
  6.75                                                                  27.2%   21.4%   16.3%   12.2%   9.1%   6.8%   3.9%
  7.25                                                                          21.4%   16.3%   12.2%   9.1%   6.8%
  7.75                                                                          21.4%   16.3%   12.2%   9.1%   6.8%
  8.25                                                                          21.3%   16.3%   12.2%   9.1%   6.8%
  8.75                                                                                  16.3%   12.2%   9.1%   6.8%
  9.25                                                                                          12.2%   9.1%   6.8%
  9.75                                                                                                  9.1%   6.8%
  10.5                                                                                                  8.7%
Average     0.0%   0.0%   0.0%   6.4%   23.2%   35.2%   36.9%   33.1%   27.2%   21.3%   16.3%   12.2%   9.1%   6.8%   3.9%   0.0%

                          Table 4.1: Capture Factor Distribution

It will be seen that the capture factors vary only slightly with wave height in each
period band. It is therefore suggested that it is reasonable to use an average value
for each period band for the purposes of calculating energy production using the
standard frequency domain model.

The variation of capture factor (spectral) with period is presented in Figure 4.1. This
also shows the regular wave capture factor. The reduced peak capture efficiency with
the spectral response is due to the fact that in an irregular sea made up of a
distribution of waves about the reference period there is less opportunity for the
device to build up resonant motion as is the case when tests are carried out in
regular waves of a set frequency. The degree to which the spectral or regular wave
response is the most appropriate depends upon the degree of energy spread within
typical sea conditions for the chosen sea area.

                                          Page 17 of 22
                                                    OWC Buoy Capture Factor
                                         (Lundy Sea per WERATLAS, Bretshneider Spectra)







                    0.0   2.0      4.0        6.0           8.0         10.0     12.0         14.0     16.0   18.0   20.0

                                                    Spectral response          Regular wave response

                                Figure 4.1: Variation of Capture Factor

Energy production was calculated using Wavegen’s standard frequency domain
analysis spreadsheet.

        Using a frequency domain turbine optimisation program developed at
Wavegen it was possible to determine the average performance for an optimised
fixed pitch and variable pitch machine for the year, ref. Tables 4.2 and 4.3. A variable
pitch turbine is about twice as efficient as a fixed pitch machine. Both analyses
assume an average pneumatic input of 160kW.
        The incident flow variation to the turbine is represented by a gaussian
distribution. The aerodynamic part of the turbine model is based on average values
and is constructed using 2D aerofoil lift and drag characteristics. Blade proximity
pressure field effects are accounted for by using cascade corrections pre and post
stall. Mach number compressibility has been corrected for.

The following aerodynamic loss models were included:
            (1) Rotor tip and hub clearance losses.
            (2) Hub windage.
            (3) Duct losses due to reduced pressure recovery from axial diffusion and swirl
                recovery, caused by flow separation and skin friction effects.
            (4) Pressure loss across valves.
The following mechanical loss models were included:
            (1) Frictional loss in bearings as they become loaded and unloaded.
            (2) Blade actuation.

                                              Page 18 of 22
The following electrical loss models were included:
   (1) Generator
   (2) Inverter
   (3) Transformer
   (4) Grid limitations

The rotor speed is fixed and operational limits can be set so startup, shutdown
(incident power levels must be sufficient to overcome the fixed losses of the system)
and power limiting (the system can only absorb power up to the limits which are
imposed by the power absorption equipment and the grid) can be accounted for. The
control algorithm defining when the blades should pitch is assumed ideal. Once the
effective angle of attack on the blades has reached a pitch threshold, then the blades
pitch instantaneously to keep the effective angle of attack on the blades constant
thus preventing stall.

                                  Power Breakdown for Full Scale Fixed Pitch Turbine

             Generator Capacity          250                       kW

             Turbine Geometry
                    Dia.                  2.6                      m
                     h                   0.62
                  Solidity               0.64
                  Omega                  1300                     RPM

             Power Component         Average (kW)          % of Pneumatic in           System Efficiency, %
               Pneumatic in              160
                   Blade                  75                      46.9                        46.9
                Tip/hub loss               4                      2.5                         44.4
                 Duct loss                 9                      5.6                         38.8
             Hub windage loss             7.2                     4.5                         34.3
                Bearing loss              2.5                     1.6                         32.7
               Actuation loss              0                      0.0                         32.7
               Electrical loss             8                      5.0                         27.7
             Time delay effects            5                      3.1                         24.6

              Blade Windage               20                      12.5

              Table 4.2: Power Breakdown for Fixed Pitch Turbine

                                          Page 19 of 22
                                       Power Breakdown for Full Scale Variable Pitch Turbine

                 Generator Capacity             250                      kW

                  Turbine Geometry
                         Dia.                   1.3                       m
                          h                     0.7
                       Solidity                 0.5
                       Omega                   2500                     RPM

                  Power Component          Average (kW)           % of Pneumatic in        System Efficiency, %
                    Pneumatic in               160
                        Blade                  140                      87.50                     87.50
                     Tip/hub loss                2                       1.25                     86.25
                      Duct loss                 22                      13.75                     72.50
                  Hub windage loss             3.05                      1.91                     70.59
                     Bearing loss                3                       1.88                     68.72
                    Actuation loss              0.6                      0.38                     68.34
                    Electrical loss            7.14                      4.46                     63.88
                  Time delay effects            22                      13.75                     50.13

                   Blade Windage               5.66                      4.13

                 Table 4.3: Power Breakdown for Variable Pitch Turbine

Capital and Operating Costs

Costs for construction and deployment for a field of 100 buoys are summarised in
Table 4.4 below. These costs are based on a fixed pitch Wells turbine power take-off

             Description                              Qty        Unit           Rate (£K)      Total (£K)
Development, legal & Finance                                1    Sum              1,000.0         1,000.0
Project Design & Management etc.                       100       Sum                   50.0       5,000.0
Buoy Construction                                      100       Sum                  526.5      52,650.0
Moorings and deployment                                100       Sum                  302.0      30,200.0
Infrastructure                                              1    Sum              2,500.0         2,500.0
Total Project Cost (£K)                                                                          91,350.0
                        Table 4.4 – Total Project Development Costs

Note that the above does not include a contingency as it is assumed this will be
created through efficient procurement.

A detailed evaluation of operating costs has not been carried out and so for the
purposes of this evaluation it is assumed that operating costs can be contained within
levels predicted for offshore wind turbines (US$ 0.01/kWh) (Ref. 5) but factored up to
account for the lower energy production per device.

A 3MW wind turbine will produce approximately 9986MWh per annum, indicating an
annual O & M cost of 0.01 x 9986 x 1000 = US$99,860 per device. This is equal to

                                               Page 20 of 22
Energy Production Costs

Energy production costs are assessed by taking the total annual project cost and
dividing this by the total project energy production. Total costs include both O & M
costs and the cost of capital calculated in order to provide a desired return on
investment over the project life.

For the purposes of this analysis, the required return on investment is assumed at
8% (real, pre-tax) and the project life is assumed as 25 years.

The energy production costs present both what we could expect to realistically
achieve today and the reduction in cost with series production. It is anticipated that
over time and through experience construction costs could be significantly lowered.
Volume production of the components could have a significant impact on this. It is
also expected that major improvements will be made in respect of capturing the
pneumatic energy available by design of the buoy and chamber and also
improvements in the efficiency of converting pneumatic energy into electricity by the
use of more efficient turbines such as the variable pitch turbine. The likely scenario is
that the unit cost of energy will decrease rapidly as more and more devices are
installed similar to the progress made by the wind industry.

                                                Spar Buoy (11.0m diameter)
                                                operated in a 45kW/m
       Spar Buoy (11.0m diameter)               environment.
       operated in a 45kW/m                     Assumes 100% improvement in
       environment.                             energy conversion (from 10%
       Assumes no improvement in                wave-to-wire to 20% wave-to-
       technology with numbers                  wire) through the adoption of
       produced                                 VPT technology

                  Installed Cost of                        Installed Cost of
       No. of     Capacity Energy               No. of     Capacity Energy
       Devices    (MW)        (p/kWh)           Devices    (MW)        (p/kWh)
                1        0.14     149.00                 1        0.28     76.00
                5        0.70      55.00                 5        1.40     28.00
               10        1.40      42.00                10        2.80     22.00
               50        7.00      32.00                50      14.00      16.40
              100      14.00       30.00               100      28.00      15.30
              500      70.00       27.00               500     140.00      13.90
                              Table 4.5 – Cost of Energy

                                     Page 21 of 22
Section 4


The project has investigated three different options of buoy geometry for a near-
shore floating OWC device and has concluded that, with our current understanding of
buoy performance and construction costs, further development work is necessary to
make the device competitive in today’s market for bulk electrical power generation.
There are also a number of other issues, for example access onto a dynamic buoy,
that currently pose a significant threat to the success of the device and that have not
been fully resolved.

The main findings of the project are:

   •   Each of the three devices has different performance characteristics.
   •   The sloped buoy and BBDB perform relatively well when seas normal to the
       device are considered, however, the Spar Buoy is the only device of the three
       that is omni-directional.
   •   The influence of wind and current forces superimposed upon the wave regime
       means that the buoys will not necessarily align with the wave direction in
       mixed conditions.
   •   The construction costs are very different for the three types of buoy geometry
       for devices that see the same width of wave front.
   •   Access onto these offshore floating buoys will be extremely limited so that a
       maintenance strategy has to be defined that requires little or no intervention.
   •   Concrete is seen as a potential material for reducing construction costs for a
       large number of devices. However, steel offers a number of advantages for a
       single prototype device.
   •   Sprayed concrete was investigated as a possible option for weight reduction
       but raised concerns over material strength properties.
   •   Both the Spar Buoy and the Sloped Buoy have the complication of having to
       be deployed from site in the horizontal position and upended to their
       operating position at the final location. This also has implications for removal
       from site for maintenance work.
   •   A number of mooring options were considered and in general mooring
       configurations that allow the buoy to move freely to avoid load and perform
       effectively are necessary but the downside is that this requires a considerable
       amount of sea space.
   •   It is suggested that the most energetic wave climates are not necessarily the
       most cost effective sites for wave energy devices by virtue of the fact that the
       larger extremes pose significant loading problems but the energy in these
       extremes cannot be economically captured.
   •   A highly dynamic riser cable is required to connect the buoy to the main sub-
       sea cable. This type of dynamic riser currently poses significant problems for
   •   Two other types of power take off system were investigated as possible
       alternatives to the OWC Wells turbine configuration. However, it was
       concluded that neither of these options could offer any improved power
       capture efficiency with a floating buoy.
   •   All three types of device would have low environmental impact and visual
       impact. Their main impact would be interference to shipping and fishing
       activities in the designated areas.

                                   Page 22 of 22

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