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14 • DEEP FOUNDATIONS • NOV/DEC 2017 DEEP FOUNDATIONS • NOV/DEC 2017 • 15
For each stage of
the installation, a given
suction pressure is
applied to the bucket,
and the penetration of
the bucket continues
until the point when
the pushing force-soil
resistance equilibrium
is reached. The suction
pressure is then in-
creased (i.e., pushing force is greater than the soil resistance), and
the bucket will continue to penetrate into the soil until the next
stage of equilibrium is reached. This procedure is repeated until the
desired depth of penetration is achieved.
As discussed by Bang and Cho (2000), geotechnical engineers
must carefully control the suction pressure so that the complete
installation of a suction bucket is possible. The designer must
determine the correct bucket length-to-diameter ratio to ensure the
bucket is installed to its intended penetration depth. The soil
resistance corresponding to the bucket penetration dictates the
lower limit of the necessary suction pressure. That is, if the applied
suction pressure is less than the estimated value of soil resistance,
the vertical pushing force will be less than the soil resistance, which
will impede or prevent the penetration of the bucket. Conversely,
instability of the soil inside of the bucket dictates the upper limit of
the suction pressure. That is, if the suction pressure is too great, the
soil inside the bucket becomes unstable; consequently, this
instability will allow the soil to fill the inside of the bucket, which
will prevent the bucket from reaching the desired penetration
depth and the bucket installation will be incomplete.
Loading CapacityThe loading capacity of a suction bucket must consider three
separate loading conditions (i.e., vertical, horizontal and inclined
loading) and combinations thereof. The designer can estimate the
vertical compressional loading capacity based on conventional
theories for large diameter open-ended piles. The vertical tensile
loading capacity, however, requires consideration of three different
failure mechanisms:
1. Bucket slip, which occurs when the bucket itself slips out of the soil
2. Bucket pull-out, which occurs when the bucket and the soil
inside are pulled out simultaneously, as a unit
3. Reversed bucket bearing capacity, which occurs when the soil
outside the bucket experiences failure similar to shallow
foundations but in a reverse fashion
Bucket slip is a dominant failure mechanism for large-diameter
suction buckets and/or for Low-strength soils, whereas the bucket
pull-out mechanism is the dominant failure mechanism for small-
diameter suction buckets and/or high-strength soils. The least
resistance of the three mechanisms is selected as the final vertical
tensile loading capacity.
Geotechnical engineers have been installing innovative, permanent
underwater foundation systems utilizing suction buckets for the
offshore industry since the early 1980s (Senpere and Auvergne,
1982). This foundation system has been successfully used on
numerous and various types of offshore structures in a wide range
of environments. Suction buckets have numerous advantages
compared to conventional underwater foundation systems. The
more notable advantages include easy installation, large loading
capacity, low noise and retrievability. The application of reduced
water pressure inside the bucket facilitates the installation of the
suction buckets, where a suction pump attached at the top of the
bucket can accomplish the entire driving operation. Because of this
efficient operation, very large suction buckets can be driven into the
seafloor, which eliminates the use of a large number of small piles.
In Korea, one of the most economical and reliable renewable
energy sources is wind power; however, onshore wind power
generation has encountered many problems, such as the lack of
favorable sites and public concerns about noise and environmental
damage. Consequently, offshore wind power generation has
attracted increasing attention due to its abundant potential, high
efficiency of grid connection, and availability of easy expansion.
In November 2011, the government of Korea announced a
national offshore wind power development roadmap, which
targeted its offshore wind power generation with the goal of
Tri-Pod Suction Buckets for Offshore Wind Turbine Foundationbecoming one of the top three nations in the world for offshore
wind power generation. Korea Electric Power Corporation
Research Institute (KEPRI) is the main research arm of the Korea
Electric Power Corporation (KEPCO) and conducts research and
development on electricity, power generation, renewable energy
and other energy-related matters. KEPRI ultimately provides
forefront knowledge and leadership to KEPCO with respect to
energy and related areas for its near- and long-term needs. Financial
support for the overall project, of which the suction bucket
foundation system installed for the pilot phase wind turbine is
described below, was provided from the New and Renewable
Energy Program of the Korea Institute of Energy Technology
Evaluation and Planning funded by the Korean Government
Ministry of Trade, Industry and Energy.
This project is expected to include three stages: a pilot phase, a
demonstration phase and a dissemination phase. Currently, the
pilot phase is underway. As part of the pilot phase, KEPRI and
Advanced Construction Technology, Inc. (ADVACT) initiated the
installation of an offshore wind turbine in December 2014 and was
completed in October 2016. This 3 MW-capacity wind turbine,
with a hub height of about 80 m (262 ft) above the mean sea level,
was installed about 200 m (656 ft) offshore in a location where the
water was approximately 10 m (33 ft) deep. The length of each
blade on the wind turbine is about 48 m (157 ft).
COVER STORY Tri-Pod Suction BucketsSuction buckets were selected as the foundation structure mainly
because of their advantages compared to conventional underwater
foundation systems: simplicity, efficiency and environmental
friendliness. More technically, the tri-pod suction buckets were used
as the foundation for the wind turbine to convert the external
overturning moments into mostly axial tension or compression loads
applied at the top of suction buckets. To resolve all of the applied
loading (i.e., vertical and horizontal loads, and the overturning and
twisting moments) into equivalent vertical and horizontal loads
applied to each suction bucket, all practical load
combinations and directions of load application were
considered in the analyses from which the worst
loading condition was used in the final design of
the suction buckets. Per the final design, each
of the tri-pod suction buckets comprising the
foundation system is about 6 m (20 ft) in
diameter and 12 m (39 ft) in length.
Mechanism and InstallationA suction pump attached at the top of the bucket provides the
necessary reduction in water pressure inside the pile (i.e., outside
ambient water pressure minus water pressure inside the bucket) to
facilitate the entire installation operation. During installation, pump-
ing water out reduces the water pressure inside the bucket, which
creates a driving force that pushes the bucket down into the seafloor.
Therefore, the capacity of the suction pump must be greater than the
amount of seepage flow (i.e., the flow of water from outside of the
bucket to inside). If the pushing force is large enough to overcome
the soil resistance, the bucket will penetrate into the seafloor, and the
penetration of the bucket will cease at the point of equilibrium when
the pushing force is equal to the soil resistance.
2017 OPA WINNER
AUTHORSMoo Sung Ryu, KEPRI, Daejin Kwag, ADVACT , Dr. Jun-Shin Lee, KEPRI, Tae Hwan Lee, ADVACT, Dr. Sangchul Bang, P.E., South Dakota School of Mines and Technology, and Gerard T. Houlahan, P.E., Moffatt & Nichol
Fabrication of lower wind turbine system and suction buckets
Suction bucket mechanism
Positioning and installation of the suction bucket foundation
Positioning at installation site
14 • DEEP FOUNDATIONS • NOV/DEC 2017 DEEP FOUNDATIONS • NOV/DEC 2017 • 15
For each stage of
the installation, a given
suction pressure is
applied to the bucket,
and the penetration of
the bucket continues
until the point when
the pushing force-soil
resistance equilibrium
is reached. The suction
pressure is then in-
creased (i.e., pushing force is greater than the soil resistance), and
the bucket will continue to penetrate into the soil until the next
stage of equilibrium is reached. This procedure is repeated until the
desired depth of penetration is achieved.
As discussed by Bang and Cho (2000), geotechnical engineers
must carefully control the suction pressure so that the complete
installation of a suction bucket is possible. The designer must
determine the correct bucket length-to-diameter ratio to ensure the
bucket is installed to its intended penetration depth. The soil
resistance corresponding to the bucket penetration dictates the
lower limit of the necessary suction pressure. That is, if the applied
suction pressure is less than the estimated value of soil resistance,
the vertical pushing force will be less than the soil resistance, which
will impede or prevent the penetration of the bucket. Conversely,
instability of the soil inside of the bucket dictates the upper limit of
the suction pressure. That is, if the suction pressure is too great, the
soil inside the bucket becomes unstable; consequently, this
instability will allow the soil to fill the inside of the bucket, which
will prevent the bucket from reaching the desired penetration
depth and the bucket installation will be incomplete.
Loading CapacityThe loading capacity of a suction bucket must consider three
separate loading conditions (i.e., vertical, horizontal and inclined
loading) and combinations thereof. The designer can estimate the
vertical compressional loading capacity based on conventional
theories for large diameter open-ended piles. The vertical tensile
loading capacity, however, requires consideration of three different
failure mechanisms:
1. Bucket slip, which occurs when the bucket itself slips out of the soil
2. Bucket pull-out, which occurs when the bucket and the soil
inside are pulled out simultaneously, as a unit
3. Reversed bucket bearing capacity, which occurs when the soil
outside the bucket experiences failure similar to shallow
foundations but in a reverse fashion
Bucket slip is a dominant failure mechanism for large-diameter
suction buckets and/or for low-strength soils, whereas the bucket
pull-out mechanism is the dominant failure mechanism for small-
diameter suction buckets and/or high-strength soils. The least
resistance of the three mechanisms is selected as the final vertical
tensile loading capacity.
Geotechnical engineers have been installing innovative, permanent
underwater foundation systems utilizing suction buckets for the
offshore industry since the early 1980s (Senpere and Auvergne,
1982). This foundation system has been successfully used on
numerous and various types of offshore structures in a wide range
of environments. Suction buckets have numerous advantages
compared to conventional underwater foundation systems. The
more notable advantages include easy installation, large loading
capacity, low noise and retrievability. The application of reduced
water pressure inside the bucket facilitates the installation of the
suction buckets, where a suction pump attached at the top of the
bucket can accomplish the entire driving operation. Because of this
efficient operation, very large suction buckets can be driven into the
seafloor, which eliminates the use of a large number of small piles.
In Korea, one of the most economical and reliable renewable
energy sources is wind power; however, onshore wind power
generation has encountered many problems, such as the lack of
favorable sites and public concerns about noise and environmental
damage. Consequently, offshore wind power generation has
attracted increasing attention due to its abundant potential, high
efficiency of grid connection, and availability of easy expansion.
In November 2011, the government of Korea announced a
national offshore wind power development roadmap, which
targeted its offshore wind power generation with the goal of
Tri-Pod Suction Buckets for Offshore Wind Turbine Foundationbecoming one of the top three nations in the world for offshore
wind power generation. Korea Electric Power Corporation
Research Institute (KEPRI) is the main research arm of the Korea
Electric Power Corporation (KEPCO) and conducts research and
development on electricity, power generation, renewable energy
and other energy-related matters. KEPRI ultimately provides
forefront knowledge and leadership to KEPCO with respect to
energy and related areas for its near- and long-term needs. Financial
support for the overall project, of which the suction bucket
foundation system installed for the pilot phase wind turbine is
described below, was provided from the New and Renewable
Energy Program of the Korea Institute of Energy Technology
Evaluation and Planning funded by the Korean Government
Ministry of Trade, Industry and Energy.
This project is expected to include three stages: a pilot phase, a
demonstration phase and a dissemination phase. Currently, the
pilot phase is underway. As part of the pilot phase, KEPRI and
Advanced Construction Technology, Inc. (ADVACT) initiated the
installation of an offshore wind turbine in December 2014 and was
completed in October 2016. This 3 MW-capacity wind turbine,
with a hub height of about 80 m (262 ft) above the mean sea level,
was installed about 200 m (656 ft) offshore in a location where the
water was approximately 10 m (33 ft) deep. The length of each
blade on the wind turbine is about 48 m (157 ft).
COVER STORY Tri-Pod Suction BucketsSuction buckets were selected as the foundation structure mainly
because of their advantages compared to conventional underwater
foundation systems: simplicity, efficiency and environmental
friendliness. More technically, the tri-pod suction buckets were used
as the foundation for the wind turbine to convert the external
overturning moments into mostly axial tension or compression loads
applied at the top of suction buckets. To resolve all of the applied
loading (i.e., vertical and horizontal loads, and the overturning and
twisting moments) into equivalent vertical and horizontal loads
applied to each suction bucket, all practical load
combinations and directions of load application were
considered in the analyses from which the worst
loading condition was used in the final design of
the suction buckets. Per the final design, each
of the tri-pod suction buckets comprising the
foundation system is about 6 m (20 ft) in
diameter and 12 m (39 ft) in length.
Mechanism and InstallationA suction pump attached at the top of the bucket provides the
necessary reduction in water pressure inside the pile (i.e., outside
ambient water pressure minus water pressure inside the bucket) to
facilitate the entire installation operation. During installation, pump-
ing water out reduces the water pressure inside the bucket, which
creates a driving force that pushes the bucket down into the seafloor.
Therefore, the capacity of the suction pump must be greater than the
amount of seepage flow (i.e., the flow of water from outside of the
bucket to inside). If the pushing force is large enough to overcome
the soil resistance, the bucket will penetrate into the seafloor, and the
penetration of the bucket will cease at the point of equilibrium when
the pushing force is equal to the soil resistance.
2017 OPA WINNER
AUTHORSMoo Sung Ryu, KEPRI, Daejin Kwag, ADVACT , Dr. Jun-Shin Lee, KEPRI, Tae Hwan Lee, ADVACT, Dr. Sangchul Bang, P.E., South Dakota School of Mines and Technology, and Gerard T. Houlahan, P.E., Moffatt & Nichol
Fabrication of lower wind turbine system and suction buckets
Suction bucket mechanism
Positioning and installation of the suction bucket foundation
Positioning at installation site
16 • DEEP FOUNDATIONS • NOV/DEC 2017 DEEP FOUNDATIONS • NOV/DEC 2017 • 17
Assembling the wind tower structure
Assembly of nacelle Lifting and positioning of a blade
Installation of a blade
were installed near the top of the suction bucket on the inside and
outside and at about the same elevation. The bucket penetration
depth was measured primarily using an echo sounder, and was
supplemented by measurements on the outside surfaces of the
suction bucket. The inclination of the suction buckets was
measured during installation using two-way tilt meters that were
attached at the middle of the substructure. Thus, the tri-pod
structure’s tilt along any direction could be estimated, and the
anticipated maximum tilt could be predicted. Upon completion of
Initially, the suction buckets penetrated into the seafloor to a
depth of about 3 m (10 ft) under self-weight. At this location, the
generalized subsurface soil profile consists of interbedded sand and
clay layers. Thereafter, sequentially increased suction pressures were
applied to the inside of the suction buckets until the desired
penetration was achieved. The magnitudes of the lower and upper
limits of the suction pressure were estimated based on the analytical
solution developed by Bang and Cho (2000), and the limit suction
pressures were continuously updated as the buckets penetrated
deeper into the seafloor. With careful control of the suction pressure,
the tri-pod suction buckets were successfully installed to their design
depths using the suction pressures defined by those limits, which
confirmed that any soil heave inside the buckets was prevented.
Using only suction pumps attached to the top of each suction
bucket, the lower part of the wind turbine system was successfully
installed in approximately 10 hours. By using suction buckets for the
The estimation of the horizontal loading capacity of suction
buckets follows the procedures specifically developed for very large
diameter piles, which considers (Bang and Cho, 2000):
• A three dimensional failure wedge
• Development of vertical and circumferential soil shear stresses
on the surface of the suction bucket
• Variation of the normal stress of the soil around the
circumference of the bucket
• Progressive transition of the normal stress of the soil from the at-
rest state to the full passive state
The magnitude of the
normal stress of the soil
during the transition
from at-rest to passive
depends on the loca-
tion of the rotation of
the suction bucket and
the depth at which the
stresses are calculated.
This method provides
significant improve-
ment to the design of suction buckets as compared to the
conventional two-dimensional approaches, and can more accurately
estimate the loading capacity of very large diameter piles.
The determination of the inclined loading capacity typically
utilizes a failure envelope defined by combined external loading
conditions that cause the failure of the soil surrounding the suction
bucket. The external loading conditions are due to either vertical-
and-horizontal (V-H) loading or vertical-horizontal-moment
(V-H-M) loading, depending on the nature of the superstructure.
Typically, either an experimentally or analytically established
failure envelope defines the upper limit of the suction bucket
loading capacity under a given combined loading condition.
For this offshore wind turbine, the design of the foundation
system, including the dynamic analysis, was completed using a
comprehensive and integrated load analyses. In addition, the
natural frequency of the entire wind turbine after it was fully
assembled was measured. The design range of the natural
frequency of this wind turbine system is 0.285 Hz to 0.332 Hz. The
estimated and measured natural frequencies fall within these limits;
therefore, this wind turbine will be able to avoid any potential
resonance induced by external loads.
Offshore Installation and InstrumentationAll components of the lower part of the wind turbine system,
including the tri-pod suction buckets, were fabricated onshore,
and the assembly was then transported to the nearest wharf. A
tower crane barge lifted the assembly, which weighed
approximately 500 tons (454 tonne), and slowly moved it to the
designated site. After final positioning, the installation process of
the lower part of the wind turbine system was started.
foundation system, the installation time was significantly reduced,
especially when comparing this system to conventional offshore
wind turbine foundations (typical installation time of about 30 days).
The resulting cost savings amounted to about $1.5 million (U.S.).
For the installation of the suction buckets, the monitoring
program included measurement of the water pressures inside and
outside of each pile, the pile penetration depth into the seafloor, and
the pile inclination. A date logger was used to display and record
these measurements. Electric resistance type water-level meters
Suction buckets and lower structure after installation
Soil failure mode under horizontal load
16 • DEEP FOUNDATIONS • NOV/DEC 2017 DEEP FOUNDATIONS • NOV/DEC 2017 • 17
Assembling the wind tower structure
Assembly of nacelle Lifting and positioning of a blade
Installation of a blade
were installed near the top of the suction bucket on the inside and
outside and at about the same elevation. The bucket penetration
depth was measured primarily using an echo sounder, and was
supplemented by measurements on the outside surfaces of the
suction bucket. The inclination of the suction buckets was
measured during installation using two-way tilt meters that were
attached at the middle of the substructure. Thus, the tri-pod
structure’s tilt along any direction could be estimated, and the
anticipated maximum tilt could be predicted. Upon completion of
Initially, the suction buckets penetrated into the seafloor to a
depth of about 3 m (10 ft) under self-weight. At this location, the
generalized subsurface soil profile consists of interbedded sand and
clay layers. Thereafter, sequentially increased suction pressures were
applied to the inside of the suction buckets until the desired
penetration was achieved. The magnitudes of the lower and upper
limits of the suction pressure were estimated based on the analytical
solution developed by Bang and Cho (2000), and the limit suction
pressures were continuously updated as the buckets penetrated
deeper into the seafloor. With careful control of the suction pressure,
the tri-pod suction buckets were successfully installed to their design
depths using the suction pressures defined by those limits, which
confirmed that any soil heave inside the buckets was prevented.
Using only suction pumps attached to the top of each suction
bucket, the lower part of the wind turbine system was successfully
installed in approximately 10 hours. By using suction buckets for the
The estimation of the horizontal loading capacity of suction
buckets follows the procedures specifically developed for very large
diameter piles, which considers (Bang and Cho, 2000):
• A three dimensional failure wedge
• Development of vertical and circumferential soil shear stresses
on the surface of the suction bucket
• Variation of the normal stress of the soil around the
circumference of the bucket
• Progressive transition of the normal stress of the soil from the at-
rest state to the full passive state
The magnitude of the
normal stress of the soil
during the transition
from at-rest to passive
depends on the loca-
tion of the rotation of
the suction bucket and
the depth at which the
stresses are calculated.
This method provides
significant improve-
ment to the design of suction buckets as compared to the
conventional two-dimensional approaches, and can more accurately
estimate the loading capacity of very large diameter piles.
The determination of the inclined loading capacity typically
utilizes a failure envelope defined by combined external loading
conditions that cause the failure of the soil surrounding the suction
bucket. The external loading conditions are due to either vertical-
and-horizontal (V-H) loading or vertical-horizontal-moment
(V-H-M) loading, depending on the nature of the superstructure.
Typically, either an experimentally or analytically established
failure envelope defines the upper limit of the suction bucket
loading capacity under a given combined loading condition.
For this offshore wind turbine, the design of the foundation
system, including the dynamic analysis, was completed using a
comprehensive and integrated load analyses. In addition, the
natural frequency of the entire wind turbine after it was fully
assembled was measured. The design range of the natural
frequency of this wind turbine system is 0.285 Hz to 0.332 Hz. The
estimated and measured natural frequencies fall within these limits;
therefore, this wind turbine will be able to avoid any potential
resonance induced by external loads.
Offshore Installation and InstrumentationAll components of the lower part of the wind turbine system,
including the tri-pod suction buckets, were fabricated onshore,
and the assembly was then transported to the nearest wharf. A
tower crane barge lifted the assembly, which weighed
approximately 500 tons (454 tonne), and slowly moved it to the
designated site. After final positioning, the installation process of
the lower part of the wind turbine system was started.
foundation system, the installation time was significantly reduced,
especially when comparing this system to conventional offshore
wind turbine foundations (typical installation time of about 30 days).
The resulting cost savings amounted to about $1.5 million (U.S.).
For the installation of the suction buckets, the monitoring
program included measurement of the water pressures inside and
outside of each pile, the pile penetration depth into the seafloor, and
the pile inclination. A date logger was used to display and record
these measurements. Electric resistance type water-level meters
Suction buckets and lower structure after installation
Soil failure mode under horizontal load
installation, the records indicated that the final tilt of the tri-pod
suction buckets was no greater than 0.1 degrees from vertical. The
assembly of the superstructure components followed the
installation of the lower part, and was executed in three stages: the
tower, the nacelle and then the blades.
ConclusionsOne of the first offshore wind turbines utilizing tri-pod suction
buckets as its sub-surface foundation has been designed and
successfully installed in southwestern Korea. The design of suction
buckets followed the most up-to-date procedures that considered
three-dimensional effects of the bucket geometry and the soil
stresses acting on the bucket. In addition, progressive soil stress
transition from the at-rest state to the full passive state was
explicitly considered. It has been found that the suction bucket
foundation system is technically effective and can significantly
reduce both the construction time and the costs. Unless the
geological conditions prohibit the use of this foundation system, it
is expected that majority of the future wind turbine construction in
Korea and around the world will adopt this innovative offshore
foundation system.
Moo Sung Ryu is a senior engineer at KEPRI and the leader of the
SuCCESS project. Ryu’s main research area is the optimization and
improvement of foundations for offshore wind turbines.
Daejin Kwag is president of ADVACT, and is an expert in suction buckets
and suction anchors. Since 2004, he has installed more than 100 such
foundation structures for gravity-type breakwaters, temporary mooring
of immersed tunnel sections, and foundations of offshore meteorological
towers and wind turbines.
Dr. Jun-Shin Lee is director general and leader of the KEPRI’s Renewable
Energy Group, and is the current president of the Korea Wind Energy
Association, providing consultation and leadership for the renewable
energy technology and associated industry development in Korea.
Dr. Tae Hwan Lee is chairman of ADVACT, and has participated in a
variety of coastal and offshore projects for the past three decades, and has
served in engineering and design review committees for Korean
government ministries and public authorities.
Dr. Sangchul Bang, P.E., is professor emeritus of civil and environmental
engineering at South Dakota School of Mines and Technology, and has
more than 40 years of experience in teaching and research in various
areas of geotechnical engineering, including design and analysis of
suction buckets, suction anchors and mooring lines for various offshore
structures.
Gerald Houlahan, P.E., is vice president of Moffatt & Nichol, and has
more than 40 years of experience in design and construction engineering
of offshore platforms, towers and bridges, and offshore wind turbine
generators. He is a past chair of the DFI Marine Foundations Technical
Committee. Completed wind turbine
18 • DEEP FOUNDATIONS • NOV/DEC 2017