HomeMy WebLinkAboutAVEC Construction Operating Experience Toksook Bay 01-2008AVEC Construction and Operating
Experience
By Brent Petrie; Eric Marchegiani, P.E.; Chet Frost, P.E.; -AVEC
Benjamin Momblow, P.E.; –Coffman Engineers
David Myers –S.T.G., Inc.
Toksook Bay, Alaska
Jan2008
This project received financial support from the Alaska Village Electric Cooperative,
Denali Commission, Coastal Villages Region Fund, U.S. Department of Energy, and
the Alaska Energy Authority.
Background Information
•27 of AVEC’s 53 villages are in wind regimes of
class 4 or better.
•Given the characteristics of an NW/100, this
means that one machine should be able to produce
about 220,000 kWh per year.
•Given a diesel efficiency of 14 kWh/gallon
generated by our new diesel sets, this means that
one 100-kW wind turbine might displace about
15,746 gallons per year of diesel fuel use for
power generation. A mini-wind farm of three units
would displace about 47,238 gallons per year.
AVEC Systemwide Average Fuel Prices 1973 - 2006
$0.354$0.515$0.578$0.649$0.718$0.779$0.968$1.330$1.601$1.610$1.565$1.296$1.203$0.842$0.980$0.964$1.033$1.097$1.190$1.224$1.259$1.208$1.215$1.281$1.263$1.088$1.216$1.367$1.389$1.294$1.486$1.936$2.068$2.273$-
$0.250
$0.500
$0.750
$1.000
$1.250
$1.500
$1.750
$2.000
$2.250
$2.500
$2.750
1973197419751976197719781979198019811982198319841985198619871988198919901991199219931994199519961997199819992000200120022003200420052006Year
Cost
Many of AVEC’s
villages are in
Western Alaska
have Class 4 or
better wind
regimes.
Consider that in 2006 AVEC:
•Purchased 5.2 million gallons of diesel fuel
•Actively used nearly 550 fuel tanks
for storage
•Took on fuel in 170 separate deliveries
(including 44 by air)
•Has only one village –Minto –that
can be supplied by a fuel truck
•Continued to experience electric load
growth driven by new water and
sewer systems, airports, schools
and housing in the villages
This load growth increases fuel use
and fuel storage needs
Therefore, successful integration
of wind generation could mean
the following to AVEC:
•A hedge against increasing fuel costs
•A hedge against the increasing costs
of marine deliveries
•Extension of on-hand fuel supplies
which may translate to favorable
delivery scheduling by marine
transporters
•A reduction of the need to build
expensive, additional storage on
hard-to-acquire or difficult-to-
construct sites.
To do such efforts cost
effectively, we need to do
good planning and
coordinate efforts with
other construction projects
underway in the village.
•The recent bulk fuel tank farm
and power plant priorities of the
Denali Commission provide some
opportunity to coordinate
logistics and use specialty
equipment such as pile drivers
or cranes that may be on-site.
Access for
specialty
equipment
required to place
foundations and
erect turbines is a
challenge.
Poor roads, water and sewer lines,
boardwalks and existing overhead
power and phone lines present
obstacles and challenges.
Above ground water
and sewer lines are
often crossed with
timber bridges that
will only support an
ATV or snowmachine.
Boardwalks can be easily
damaged by heavy
equipment or melting
permafrost.
•They must not
settle, tilt or be
uplifted
•Pile foundations
(six to eight piles)
may extend 1/3 to
2/3 the height of
the tower into the
ground
Foundations in
permafrost are
a challenge
•Due to the significant capital expense of installing deep
foundations in permafrost conditions, the turbine owner wants
to make sure that the investment will consistently produce
power over the –year life of the equipment.
•Warming trends are affecting the expanse and depth of
permafrost.
•For example, the next slide shows that the mean annual
temperature in Bethel has increased from less than 28°F to
over 31°F in the 55 years from 1950 to 2005
University of Alaska Geophysical Institute Climate Trends
•Increasing mean temperatures may contribute to
diminished snow cover, increased surface water, and other
conditions that may lead to thawing in the underlying
permafrost.
•One manifestation of warming permafrost is the
disappearance of lakes, which is thought to be caused
when the permafrost under the lake thaws through its
entire thickness and allows the lake to drain into the
material below. (Science Magazine; June 03, 2005)
The following slide illustrates the process.
Such processes can
be noted today when
one flies over the
tundra and lake
expanses of Western
Alaska.
Diminished lake sizes on
Alaska's Yukon Flats are
evident between 1954 (top:
aerial photograph) and the
2000 (bottom: Landsat
ETM+ image). Image credit:
Institute of Arctic Biology,
University of Alaska
Fairbanks.
•In 2004, after two years of planning, AVEC and its construction manager, STG Inc., began mobilization of materials and equipment to the Toksook Bay wind project construction site.
•The site was underlain by frozen soils extending 5 to 15 feet deep over a tilted bedrock base.
•During the course of shipping the towers and piling, the turbine vendor alerted participants to a concern about the deep point of fixity and possible adverse harmonic conditions that might result.
Wind towers on land in
most of the world are
built with a ‘point of
fixity’ at the base of the
tower where it typically
rests on a massive
concrete foundation.
Point of
Fixity
Reinforced
Concrete Pad
100
ft
30-60 ft
In order to be
properly secured in
permafrost, wind
turbines may require
pilings in the ground
which are 1/3 to 2/3 of
the height of the
tower.
The tower foundation is
elevated to allow cold
air to pass over the
ground to keep it frozen
and to avoid heaving of
the tower base.
2 to 14 ft
Frozen ground at
surface in March
Frost line in
September/October
after seasonal thaw
One problem with Alaska
permafrost conditions is
that the point of fixity may
be below the ground
surface and may vary
throughout the year as the
frost line of the active
layer migrates.
2 -14 ft
No lateral support
when thawed
New ‘point of fixity’
When the active layer is
thawed, there is minimal to
no lateral support to the
piling near the base of the
tower.
Frozen/Solid Ground
In such conditions, the piles
act as an extension of the
tower.
The rotating turbine, and
strong wind forces can
create destructive
frequencies in the
‘extended’ tower.
This could be addressed by slowing down the
turbine and losing valuable energy, especially in
the late part of the year when the ground is
thawed to its deepest, and when wind speeds are
excellent for power production.
This approach loses energy and requires complex
monitoring of the system operation.
Alternative Solution
An alternative solution is to stiffen the foundation and damper
the structure by adding mass.
In the case of two projects involving six towers in permafrost
environments at Toksook Bay and Kasigluk, Alaska, pile
foundations were modified by adding a 130,000 pound,
concrete and steel mass between the tower base, and the piles.
In Kasigluk, foundation piles were used with 24 inch helices to
resist uplift and with thermal siphons to keep the ground
frozen. The objective was to reduce the thickness of the active
frost layer, and bring the point of fixity closer to the tower base.
Temperature Acquisition Cables (TAC) have been installed at
each tower in Kasigluk to monitor changes in ground
temperature and to detect thawing below each tower.
Wind site
Overview –Toksook Bay
5-15 feet of frozen silts lie
over tilted bedrock at the site.
•Holes pre-drilled
•Piles driven to refusal
•Piles later cut
Rock bolts would be placed
into the rock and tensioned
to the pile cap.
Additional Mass was added
by placing a rebar cage and
concrete in the pile.
Drilling out center of
piles to 20’ below end
of pile
Six piles for a single tower foundation
Piles with Met tower
The steel foundation
cap contains I-Beams
to connect the piles
and a ring to make the
tower base.
Steel Foundation Star
(Typical of 3)
Concrete and rebar was
incorporated into the tower
base and piles to add 130,000
pounds of dampening mass.
Rebar Cage to go
into a pile.
•18’ driven pile
•Drilled out 20’ below
pile end
•Installed rebar cage and
poured concrete (3,000
PSI)
•Design load 63 KIPS
•Tested up to 2 times
load 127 KIPS (0.019 in
movement)
•Tested up to 210 KIPS –
less than ¼” movement
Verification of pile testing
Drain
Conduit
Bolts
Rebar placement
Meter base and riser to
connect to overhead
distribution system
Forms were placed
underneath the
foundation star to
hold the concrete in
place until it cured.
Foundation Design Criteria
-Design Wind Speed = 130 mph (50 year)
-Overturning moment = 1,830,000 ft lb
-Total tower/turbine weight = 42,000 lb
Frequency Analysis
-Tower only natural frequency (supported on infinitely rigid base) = 1.15 Hz
-Minimum natural frequency for tower and foundation = 1.07 Hz
Based upon 5% over maximum rotor frequency plus 5% factor of safety
-Operating frequency of rotor –0.97 Hz (58 rpm)
The Wind Turbine Controller is
placed before the tower is set.
Danwin Tower Midway Platform
Danwin Tower Inside Flange/Bolt Holes
Tower/Turbine Specifications
-108 feet from ground level to center
of rotor
-Rotor diameter (3 blades) = 61 feet
View from the top!
13,950 lb nacelle
being prepared for
its lift to the top of
a 32 meter tower.
Cleaning the yaw gear teeth prior
to setting it on top of the tower.
A Nacelle and tower are placed
on a cured foundation.
A foundation awaiting
concrete is in the
foreground.
Second tower section
being put in place
Note wiring along
ladder and mid
point landing.
A rope light is used to
illuminate the inside of
the white tower.
Met tower
for power
confirmation.
Mast and
meter
base.
Meter BaseConcrete &
Steel Mass
Three wind turbines completed at Toksook Bay.
Blade pitching post.
Each tower has a
blade pitching
post to allow the
rotor to be
dismounted and
the blade pitch
adjusted.
Blade Extenders
Blade extenders
add 1 meter of
diameter (20
meters total) to
the swept are to
increase energy
capture.
Tightening hub to turbine
Blades with
blade
extenders
At Akula Heights (also called Kasigluk) three NW
100 Turbines were installed east of the village on
frozen sand and silt. No bedrock was encountered.
•Mass was added by filling the steel foundation star with
concrete, as in Toksook Bay.
•Because there was no bedrock, piles with 24 inch helices were
screwed into pilot holes predrilled into the ground.
•Thermal siphons were added between, and outward of the piles
to extract heat from the soil.
Special Equipment
was required to
twist the helical
piles into the ground
Insulation was
added below
the tower base
to resist
thawing.
Installing thermopiles during the Spring at Kasigluk
Installed thermopiles at Kasigluk
Temperature sensor strings, similar to the following,
have been installed at Kasigluk to monitor seasonal
and long term changes in permafrost temperatures.
TOP LEFT: Temperature
Acquisition Cable (TAC)
and Reader
BOTTOM LEFT: All TACs
are permanently labeled
with their serial number
which ties in with owner
information, last
calibration date, number
and spacing of sensors, &
sensor identifications.
RIGHT: An artists image
of what an installed TAC
looks like.TAC Images courtesy of
Beaded Stream
Lessons Learned
•Geotechnical information is critical
In areas of complex geology or a highly varied
active layer, it is useful to have information from
the actual turbine site in order to recognize local
variances.
When acquiring geotechnical information, also
acquire the permafrost temperature and, if
possible, install a temperature acquisition cable to
monitor temperatures up to the time of
construction.
Continue thermal monitoring of the turbine site
after construction.
•Materials
The lack of appropriate local aggregate for
concrete can increase costs. Appropriate
aggregate may have to be imported in addition to
importing cement and mixing equipment.
Waiting for the concrete to cure and then testing
it can consume valuable time and may require a
demobilization and remobilization of the crew.
An all steel foundation would require larger
components to provide desired stiffness, but may
reduce construction complexity by eliminating
tasks involving concrete. Portions of such a
foundation could be prefabricated and shipped to
the site.
•Frequency Modeling should include the pile
foundation.
In a permafrost environment the point of fixity
will rise and fall along the pile during the seasonal
change.
Under such conditions the piles may behave as an
extension of the tower and the turbine, tower and
foundation systems may interact to develop a
frequency that could be damaging.
•Mitigation
Adverse harmonics could be mitigated through:
•Adjusting turbine rpm, but slowing the turbine
would reduce valuable energy output.
•Addition of mass to dampen system response
at known frequencies.
•Stiffening the piles, pile caps, or tower.
2006. Institute of Arctic Biology,
University of Alaska Fairbanks.
National Aeronautics And Space
Administration. NASA. 13 July
2007
<http://landsat.gsfc.nasa.gov/g
raphics/news/sci0007lg.jpg>.
Fuller, Nicole R. Disappearing
Lakes. Live Science. 13 July
2007
<http://www.livescience.com/e
nvironment/050603_lakes_gone
.html>.
"Mean Annual Temperatures:
Bethel." Chart. 13 July 2007
<http://climate.gi.alaska.edu/Cli
mTrends/Change/betT.jpg>.
Smith, L C., Y Sheng, G M.
Macdonald, and L D. Hinzman.
"Disappearing Arctic Lakes."
Science June 2005. 13 July
2007
<http://www.sciencemag.org>.
"Temperature Change in Alaska
1971 -2000." Map. 13 July
2007
<http://climate.gi.alaska.edu/Cli
mTrends/Change/TempChange.
html>.
"Alaska Mainland Regions; 50
Meter Wind Power." Chart. US
Dept. of Energy, National
Renewable Energy Laboratory.
US Dept. of Energy, NREL,
2006.
Works Cited