[Title: ]{.underline}**[NSF/Tokyo Report: Report On Ocean Wave Energy
Conversion Projects]{.underline}**

## [Date: 4/23/98]{.underline}

Replaces: None

[The National Science Foundation\'s offices in Tokyo and in Paris
periodically report on developments abroad that are related to the
Foundation\'s mission. These documents present facts for the use of NSF
program managers and policy makers; they are not statements of NSF
policy.]{.underline}

[Chiba-san: Please send this report to the ssr alias, ,
dheinric@nsf.gov, aoyama-san at STA]{.underline}

[\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--]{.underline}

[Special Scientific Report #98-08 (April 09, 1998)]{.underline}

[Dr. ]{.underline}***[Umesh Korde]{.underline}***[ prepared the
following report. In May of 1997, Dr.
]{.underline}***[Korde]{.underline}***[ began a 24-month Science and
Technology Agency (STA) fellowship at the Japan Marine Science and
Technology Center (JAMSTEC). His host is Dr. Yukihisa Washio of the
Coastal Research Department at JAMSTEC. Dr.
]{.underline}***[Korde]{.underline}***[ can be reached via email at
uak@jamstec.go.jp.]{.underline}

**[\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*]{.underline}**

[Report On Ocean Wave Energy Conversion Projects]{.underline}

[Summary]{.underline}

[This report presents a brief overview of the recent wave-energy
developments in Japan, and summarizes recent work on the \"Mighty
Whale\" floating prototype at the Japan Marine Science and Technology
Center (JAMSTEC). It also outlines a part of the author\'s project on
active control methods to optimize energy conversion by floating devices
in irregular waves. ]{.underline}

[Introduction]{.underline}

[Serious research on wave-energy extraction methods began in several
countries during the 1970\'s. Wave-energy projects in Japan began
drawing attention in the late \'70\'s, after JAMSTEC tested the world\'s
first large-scale offshore floating prototype \"Kaimei\" in the Sea of
Japan. The \"vessel\" powered 9 generators on board. These were mounted
above lengthwise chambers open to the sea at the bottom. Wave action
caused the internal water levels to rise and fall, forcing an
alternating airflow that was used to drive air turbines. The main idea
was first used by Masuda (e.g. Masuda, 1985) in the 1960\'s, to power
light buoys serving as navigational aids to shipping. ]{.underline}

[The \"oscillating water column\" (OWC) method has become very popular
since the Kaimei tests, and forms the core of a number of prototypes of
current interest in Japan and elsewhere. These include the \"Mighty
Whale\" offshore floating prototype, which has been under development at
JAMSTEC since 1987. Sea-trials on this device are scheduled to begin in
July 1998. ]{.underline}

[Kaimei and Mighty Whale differ from other large-scale prototypes in
Japan and elsewhere in being floating, as opposed to shore/seafloor
supported, devices. Other recent large-scale developments in Japan
include the Caisson-type Oscillating Water Column prototype, the
Pendulor prototypes, the Constant-Pressure Manifold device, and the
Water-Valve Rectifier device. ]{.underline}

[Recent results of ongoing projects worldwide can be found in EU (1996).
]{.underline}

[Developments in Japan]{.underline}

[1. Caisson-type Oscillating Water Column]{.underline}

[This was developed by the Port and Harbor Research Institute of the
Ministry of Transport. The principal dimensions of each caisson were
20.9 m X 24.3 m X 27.0 m. The operating water depth was approximately 18
m. Each caisson had 4 openings on its front surface facing the
predominant wave direction. Corresponding with the frontal openings,
partition walls divided the internal space into 4 chambers. Incident
waves caused the water level inside each to rise and fall. The piston
action of the oscillating water column (OWC) forced reciprocating
airflow over power absorbing air turbines. Self-rectifying (i.e.
spinning in the same direction in a reciprocating airflow) \"Wells\"
turbines (1.34 m diameter, 16 blades) in tandem configuration were used.
The generators were each rated at 60 kW. The prototype tests were
conducted in the Sea of Japan, near Sakata Port in Yamagata Prefecture.
]{.underline}

[2. \"Pendulor\" Device]{.underline}

[This device is being developed by Muroran Institute of Technology and
Cold-Region Port and Harbor Research Center. A hinged plate with frontal
dimensions 2.0 m X 6.5 m is mounted inside a caisson (with the hinge at
the top). Wave action causes oscillation of the plate (\"pendulor\"),
and the pendulor compresses fluid in a hydraulic power take off. The
second-generation prototype uses active control for efficient energy
conversion. Tests are underway off Muroran Port in Hokkaido.
]{.underline}

[3. Constant-Pressure Manifold Device]{.underline}

[This device was operational from 1988 to 1997 near Kujukuri beach,
Chiba Prefecture. It was developed and operated by Takenaka Komuten Co.
The device had 10 cylindrical OWC chambers all connected to a
constant-pressure air manifold through one-way valves. Additional
one-way valves on the OWC chambers allowed only upward water column
motion to generate sufficient airflow. The constant-pressure manifold
enabled use of a constant-RPM radial air turbine. This was coupled to a
30 kW generator to produce a smooth AC output at 200 Volts, 50
Hz.]{.underline}

[4. Water-Valve Rectifier Device]{.underline}

[The device has been developed by Tohoku Power Co., and Mitsui
Engineering and Shipbuilding Co. This is another OWC-based device
producing smooth, constant voltage AC output. The device spans 20 m, and
is mounted in a breakwater serving the Haramachi coal-fired power
station in Fukushima Prefecture. Rectification and smoothing in this
case are both provided by a water-valve placed above the OWC chamber.
The water-valve directs airflow to an impulse turbine with tandem
rotors. The rated generator output is 130 kW. The device has been in
operation since 1996. Current experiments are expected to conclude in
spring 1998. ]{.underline}

[5. Offshore Floating Device \"Mighty Whale\"]{.underline}

[This device has been developed by JAMSTEC, under funding from the
Science and Technology Agency (STA). Envisioned applications include
fish farming and coastal-water aeration. Work on this device has been in
progress since 1987, and the final prototype design is based on results
from a number of laboratory tests at progressively increasing scales.
The prototype tends to resemble a whale in appearance, and is 50 m X 30
m X 12 m in overall dimensions. It is designed to float at a draft of 8
m at even keel, and is to be moored in a water depth of 40 m. The device
has three oscillating water column chambers distributed breadth-wise.
]{.underline}

[Prototype construction is in steel and structural design is in
accordance with NK (Japan Classification Society) Part P regulations for
special-purpose floating platforms. The device has been in construction
at the Ishikawajima Harima Heavy Industries (IHI) Aioi Shipyard in Hyogo
Prefecture. A 1.7 m diameter, Aluminum-alloy, biplane (i.e. with tandem
rotors), self-reciprocating Wells turbine is provided for each chamber.
Each rotor has 8 camberless blades with the NACA 0021 profile. Four
induction generators are used on board. Two of the four generators are
rated at 30 kW, one at 50 kW, and one at 10 kW. A 7.5 kW air compressor
is also provided. The total rated capacity is 110 kW, and automatically
controlled switching circuitry selects the appropriate combination of
generators at any given time. ]{.underline}

[Mooring is by means of 6 lines (each with intermediate weights)
terminating in box-type anchors. The test site is just outside the mouth
of Gokasho Bay off Mie Prefecture. Experiments are scheduled to begin in
July 1998 and continue through July 2000. More than 48 parameters are to
be monitored and analyzed on board. Many of the results are expected to
be transmitted to shore. In addition to generating technologically
important data, the device is also expected to serve as an offshore
platform for measurements of interest to Oceanographic and Environmental
sciences. ]{.underline}

[NSF/STA Project: Optimal control of floating wave energy
devices]{.underline}

[Offshore floating devices such as the Mighty Whale have at least three
advantages over shore/seafloor fixed devices: First, significant economy
results from the fact that floating hulls experience considerably lower
impact loads in extreme wave conditions. Second, available wave energy
generally increases with increasing water depth (exceptions are
shallow-water regions where refractions due to favorable bottom
topography can lead to localized focusing of energy). Third, rigid-body
motion of the hull can increase energy absorption in certain wave
conditions, due to increased relative motion between OWCs and hull.
]{.underline}

[Part of this project concerns the third effect. For axisymmetric,
primarily heaving buoys supporting OWCs \[e.g. McCormick (1976), Masuda
(1985)\], the buoy heave motion is generally found to increase energy
absorption. However, theory and laboratory experiments on a Kaimei-type
floating OWC device (Maeda, et al. 1985) have shown the rigid-body
motion of the hull to reduce energy absorption at most wave frequencies
of interest. Similar observations have also been made with Mighty Whale
laboratory models.]{.underline}

[For this reason, a system is being developed as a part of this work,
whereby the rigid-body motion of the Mighty Whale hull can be utilized
to supplement the energy already being generated by the OWCs. The design
constraints are: to achieve this without the use of actuated moorings or
tethers, to retain the original advantages of a floating hull, and to
minimize any energy required to operate such a system.]{.underline}

[The system exploits favorable interaction of two or more coupled
oscillators (spring-mass systems) whereby one of the masses is locked
into zero displacement at a certain frequency. This phenomenon is often
used in \"dynamic vibration absorbers\", and can be utilized in heave
compensation systems for drill ships. Real-time control of at least one
of the oscillators is necessary in the case of a floating wave-energy
device. This is because ocean waves are irregular (when not very large,
an irregular wave can be considered to be a superposition of a large
number of sinusoidal waves of different frequencies), and the phenomenon
above occurs at a single frequency. Control using correctly applied
reactive forces allows the phenomenon to be extended to a range of
frequencies (Korde, 1998). Thus, when waves and hull motions are not
very large, such control produces good hull-motion compensation in
irregular waves. ]{.underline}

[Passive versions of the system were tested on a 1/62.5 scale model of
the Mighty Whale device; as well as on a \"point-absorber\" type
axisymmetric buoy consisting of an OWC in a long tube inside a flotation
collar. In both cases the system was arranged to provide compensation
against hull-heave for a platform placed near the roof of the model,
across the vertical airflow passage. With the hull heaving and the
platform stationary the piston action of the platform was expected to
increase the magnitudes of overall airflow and gage pressure in the
chamber. Improvement in energy absorption was noticed for both devices
near the frequencies at which effective motion compensation was
achieved. The tests also highlighted some essential design aspects to be
considered prior to building an active system. ]{.underline}

[Experiments on the active system are expected to begin shortly once the
control hardware is fully fabricated and tested. The active tests are to
be carried out with the compensator arranged in the surge mode, because
this is the dominant mode for the Mighty Whale at low frequencies, and
because considerably more energy is carried by incident waves at low
frequencies. For precise quantification of the additional
energy-absorption enabled by the system, the model is to be constrained
(from above) to pure surge motion. A Scott-Russell mechanism to achieve
this is currently being tested. Calculations based on numerically
computed hydrodynamic parameters are being carried out to
validate/interpret the experimental results.]{.underline}

[Acknowledgements]{.underline}

[I thank STA and NSF for supporting my stay in Japan. Interaction with
JAMSTEC researchers has been most valuable.]{.underline}

[References]{.underline}

[EU (1996) Proc. Second European Wave Power Conf., Lisbon,
Portugal]{.underline}

[Korde, U.A.(1998), Active heave compensation on drill ships in
irregular waves, Ocean Engineering, in press]{.underline}

[Maeda, H., Kinoshita, T., Kudo, K. and Masuda, K. (1985), Optimization
of wave energy absorber of an attenuator OWC device with an air turbine,
Proc. IUTAM Symp. On Hydrodynamics of Wave Energy Devices, Springer
Verlag, Berlin]{.underline}

[Masuda, Y. (1985), An experience with wave power generation through
tests and improvement, Proc. IUTAM Symp. On Hydrodynamics of Wave Energy
Devices, Springer Verlag, Berlin]{.underline}

[McCormick, M.E.(1976), A modified linear analysis of a wave-energy
conversion buoy, Ocean Engineering, v. 3, no. 3, pp.
133-144]{.underline}

[Washio, Y., Osawa, H., Imai, M., Korde, U.A., Ito, H.(1998), The
offshore floating wave energy device \"Mighty Whale\", Proc. 1st Coastal
Environment Science and Technology Panel Meeting of the U.S.-Japan Conf.
On Utilization of Natural Resources, Hayama, Japan]{.underline}