tri-pod suction buckets for offshore wind turbine foundation · suction buckets, where a suction...

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


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


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

tri-pod suction buckets for offshore wind turbine foundation · suction buckets, where a suction...

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