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http://www.pbworld.com/news_events/publications/network/ PB Network #65 92 Design and Construction Considerations for Offshore Wind Turbine Foundations By Sanjeev Malhotra, New York, New York; 1-212-465-5231; [email protected] The growing energy needs of the world and the sustainable nature of wind energy makes this sector a highly promising growth industry. The next generation of wind turbines that are on the drawing boards are gigantic in size, making them more cost-efficient, but also putting large demands on their support structures and foundations. As an increasing number of wind farms are being planned offshore in water depths of over 40 m (130 feet), the combination of water depth and increased windmill tower heights, turbine weights, and rotor blade diameters create loads that make foundation design very complex. This article summarizes various relevant foundation and geotechnical issues for offshore wind turbine tower foundations. Offshore foundations are exposed to additional loads that do not affect land-based towers. These include ocean currents, storm waves, ice and potential ship impacts. Currently, a lack of precedence in the U.S. leaves no established technical guidelines for the selection, design and construction of such structures. In addition, the established European practices may not be applicable to the environmental conditions in the U.S., which include deeper waters and greater wind, wave, and ice loading. Wind Turbine Tower System Configuration The components of a wind turbine system (Figure 1) include the: Foundation system, which is comprised of support structures that connect the foundation to the transition piece, and the foundation itself Transition piece, which connects the foundation system to the tower Tower, which is made of steel plate rolled into conical subsections that are cut and rolled into the right shape, and then welded together. Nacelle, which contains the key electro-mechanical components of the wind turbine, including the gearbox and generator Rotor blades, which are made using a matrix of fiberglass mats impregnated with polyester or epoxy Typical Support Structures Support structures for offshore wind towers can be categorized by their configuration and method of installation as described below. These foundations and associated water depths are shown in Figure 2. The typical sizes for offshore foundations and their construction sequence are presented in Table 1. Gravity Structures. These foundations resist the overturning loads solely by means of their own gravity. They are typically used at sites where installation of piles in the underlying seabed is difficult, such as on a hard rock ledge or on competent soil sites in relatively shallow waters. Gravity caissons are typically concrete shell structures. These structures are competitive when Research & Innovation Explore the Possibilities... Larger and larger wind turbines are being developed for off-shore wind farms. The loads from these huge turbines combined with the loads associated with off-shore facilities are creating new design and construction challenges related to the foundation systems. The author reports on his research of European foundation systems and tells how these can be adapted for off-shore conditions in the U.S.A. Figure 1: Components of a wind turbine system. Figure 2: Foundation types and typical water depths.

Design and Construction of Offshore Wind Turbine Foundations

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PB Network #65 92

Design and Construction Considerations for Offshore WindTurbine FoundationsBy Sanjeev Malhotra, New York, New York; 1-212-465-5231; [email protected]

The growing energy needs of the world and the sustainable nature of wind energy makes thissector a highly promising growth industry. The next generation of wind turbines that are onthe drawing boards are gigantic in size, making them more cost-efficient, but also putting largedemands on their support structures and foundations. As an increasing number of wind farmsare being planned offshore in water depths of over 40 m (130 feet), the combination of waterdepth and increased windmill tower heights, turbine weights, and rotor blade diameters createloads that make foundation design very complex.

This ar ticle summarizes various relevant foundation and geotechnical issues for offshore windturbine tower foundations. Offshore foundations are exposed to additional loads that do notaffect land-based towers. These include ocean currents, storm waves, ice and potential ship impacts.

Currently, a lack of precedence in the U.S. leaves no established technical guidelines for theselection, design and construction of such structures. In addition, the established Europeanpractices may not be applicable to the environmental conditions in the U.S., which includedeeper waters and greater wind, wave, and ice loading.

Wind Turbine Tower System Configuration

The components of a wind turbine system (Figure 1) include the:• Foundation system, which is comprised of support structures that connect the foundation

to the transition piece, and the foundation itself• Transition piece, which connects the foundation system to

the tower• Tower, which is made of steel plate rolled into conical

subsections that are cut and rolled into the right shape, and then welded together.

• Nacelle, which contains the key electro-mechanical componentsof the wind turbine, including the gearbox and generator

• Rotor blades, which are made using a matrix of fiberglass mats impregnated with polyester or epoxy

Typical Support Structures

Support structures for offshore wind towers can be categorizedby their configuration and method of installation as describedbelow. These foundations and associated water depths are shownin Figure 2. The typical sizes for offshore foundations and theirconstruction sequence are presented in Table 1.

Gravity Structures. These foundations resist the over turningloads solely by means of their own gravity. They are typicallyused at sites where installation of piles in the underlying seabedis difficult, such as on a hard rock ledge or on competent soilsites in relatively shallow waters. Gravity caissons are typicallyconcrete shell structures. These structures are competitive when

Research & InnovationExplore the Possibilities...

Larger and larger windturbines are beingdeveloped for off-shorewind farms. The loadsfrom these huge turbinescombined with theloads associated withoff-shore facilities arecreating new design andconstruction challengesrelated to the foundationsystems. The authorreports on his researchof European foundationsystems and tells howthese can be adaptedfor off-shore conditionsin the U.S.A.

Figure 1: Components ofa wind turbine system.

Figure 2: Foundation types and typical water depths.

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93 PB Network #65

the environmental loads are relatively low and the dead load is significant, or when additionalballast can be provided at a reasonable cost.

Monopile. This is a simple design in which the wind tower, made of steel pipe, is supportedby the monopile either directly or through a transition piece. The monopile consists of a steelpipe pile up to 6 m (20 feet) in diameter with wall thicknesses as much as 150 mm (6 inches).Depending on the subsurface conditions, the pile is typically driven into the seabed by eitherlarge impact or vibratory hammers, or the piles are grouted into sockets drilled into rock.

Compared to the gravity base foundation, the monopile has minimal and localized environmentalimpact. By far, the monopile is the most commonly used foundation for offshore wind turbines.

Guyed Monopile Towers. The limitation of excessive deflection of a monopile in deeperwaters is overcome by tying the monopile with tensioned guy wires.

Tripods. Where guyed towers are not feasible, tripods can be used to limit the deflectionsof the wind towers. The pre-fabricated frame is triangular in plan view and consists of steelpipe members connecting each corner. A jacket leg installed at each corner is diagonallyand horizontally braced to a transition piece in the center.

The tripod braced frame and the piles are constructed onshore and transported by bargeto the site. These foundations do not require any seabed preparation.

Braced Lattice Frame. A modification of the tripod frame, the lattice frame has morestructural members. The jacket consists of a 3-leg or 4-leg structure made of steel pipethat is interconnected with bracing to provide the required stiffness.

Suction Buckets. This design consists of a center column connected to a steel bucket throughflange-reinforced shear panels that distribute the loads from the center of the column tothe edge of the bucket. The steel bucket consists of a steel skir t extending down from ahorizontal base resting on the soil surface. The bucket is installed by means of suction andbehaves as a gravity foundation, relying on the weight of the soil encased by the steel bucket.

The stability of the system is ensured because there is notenough time for the bucket to be pulled out of the soil during awave passage. As the bucket is pulled up, a cavity is formedbetween the soil surface and the bottom of the bucket which cre-ates a suction pressure that resists the uplift loads.

Floating Tension Leg Platforms. These structures are floatedto the site and submerged by means of tensioned vertical anchorlegs. The base structure helps dampen the motion of the system.Installation is simple because the structure can be floated to the site and connected to anchor piles. The structure can besubsequently lowered by use of ballast tanks and/or tension systems. The entire structure can be disconnected from theanchor piles and floated back to shore for major maintenance or repair of the wind turbine.

Typical Foundations

Foundations anchor the support structures to the seabed, andtypically fall into the six types described below.

Gravity Caissons. This type of foundation has been used forseveral offshore wind farms in Europe. For economical fabricationof gravity caissons one requires a shipyard or a drydock near the

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Typical Water Type of Size Weight Depths ConstructionFoundation (m) (ton) (m) SequenceGravity Base 12 - 15 500 – 1000 0 – 15 (a) Prepare seabed (b) Placement (c) Infill ballast

Monopile 3 - 6 175 – 350 0 – 30 (a) Place pile (b) Drive pile

Monopile with 3 - 6 175 – 350 20 – 40 (a) Place pile Guy Wires (b) Drive pile

Tripod 15 – 20 125 – 150 20 – 40 (a) Place frame (b) Insert pile (c) Drive pile

Braced-Frame 10 – 15 200 – 400 20 – 50 (a) Place frame with Multiple (b) Insert pile Piles (c) Drive pile

Suction 10 – 20 150 – 400 0 – 30 (a) Place base Bucket (b) Suction installation

Tension Leg 10 – 20 100 – 400 >50 (a) Drive anchor Platform pile or suction bucket (b) Float tension leg platform (c) Install anchor cables

Table 1: Basic Sizing andConstruction Sequencingfor Offshore Wind TurbineFoundations.

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site (Figure 3) so the massive foundation structures can be floated out to the site and sunk.

Site preparation and placement required for gravity caissons typically involves dredgingseveral meters of generally loose, soft seabed sediment and replacing it with compacted,crushed stone in a level bed. Special screeds and accurate surveying is required for this task.

Driven Pipe Pile. The driven steel pipe pile option is an efficient foundation solution indeep waters. The typical method of offshore and near-shore installation of piled structuresis to float the structure (monopile, tripod or braced frame) into position and then to drivethe piles into the seabed using hydraulic hammers. The handling of the piles requires theuse of a crane of sufficient capacity, preferably a floating crane vessel (Figure 4).

Use of open-ended driven pipe piles allows the sea bottom sediment to be encased insidethe pipe, thus minimizing disturbance. The noise generated during pile driving in themarine environment might cause a short-term adverse impact to aquatic life, however, butbecause the number of piles is typically small, these adverse impacts are only short-termand relatively minor.

Recent innovations in the pile driving industry, such as the bubble cur tain, offer a wayto mitigate noise impacts. A bubble cur tain involves pumping air into a network of perforated pipes surrounding the pile. As the air escapes, it forms an almost continuouscur tain of bubbles around the pile, preventing the sound waves from being transmittedinto the surroundings.

Post-Grouted Closed-end Pile in Predrilled Hole. A closed-ended steel pipe pile isplaced into a predrilled hole and then grouted in place. This option is used often for off-shore pile foundations less than 5 m in diameter and offers significant advantages overthe cast-in-place drilled shaft option, including advance fabrication of the pile, better qualitycontrol, and much shorter construction time on the water. This option requires a speciallyfabricated large diameter reverse circulation drill (Figure 5). It also requires handling andplacement of a long, large-diameter pile, of considerable weight. Closed-end piles can befloated to the site and lowered into the drill hole by slowly filling them with water.

Drilled Shafts or Bored, Cast-in-Place Concrete Pile. The installation of bored, cast-in-placeconcrete pile (Figure 6) requires driving a relatively thin-walled (25 mm) casing throughthe soft sediment to the underlying denser material (if necessary to establish a seal), thendrilling through and below the casing to the required base elevation. Bending resistanceis provided by a heavy reinforcing cage utilizing high strength, large diameter bars, withdouble ring, where necessary. The casing provides excavation support, guides the drillingtool, contains the fluid concrete, and serves as sacrificial corrosion protection. Thisapproach requires a large, specially fabricated reverse circulation drill.

Composite “Drive-Drill-Drive” Pile. This procedure requires an adaptation of existingdrilling and piling techniques and involves a combination of drive-drill-drive sequence toachieve the design depth.

Suction Caissons. Like piles, suction caissons (Figure 7) are cylindrical in shape buthave larger diameters (10 m to 20 m) and subsequently shallower penetration depths.These caissons are closed at the top. They are installed by sinking into the seabed andthen pumping the water out of the pile using a submersible pump (Figure 8). Pumpingthe water creates a pressure difference across the sealed top, resulting in a downward

Figure 3: Gravity base foundation being constructedfor Nysted Offshore Wind Farmat Rodsand, Denmark. (Courtesy of Bob Bittner, Ben C. Gerwick, Inc.)

Figure 4: Monopile 50 m (165feet) long with a 4 m (13-feet)diameter being installed at theNorth Hoyle Wind Farm, UK. (Courtesy of RWE npower)

Figure 5: Reverse circulationdrill

Figure 6: Construction of drilled shaft. From left to right, install casing and auger drill, placereinforcement cage, and pour concrete by tremie.

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hydrostatic force on the pile top. The hydrostatic pressure thus developedpushes the pile to the design depth. Once the design depth is achieved, thepumps are disconnected and retrieved.

Suction caissons are expected to be particularly suitable for foundations in thetype of soft cohesive sediments found around the U.S. coasts. These foundationscannot be used in rock, in gravel or in dense sand.

Suction caissons are less expensive to install because they do not require underwater pile drivers. At the end of a wind turbine’s life, a suction caisson can be removed completely from the seabed, unlike piled foundations. This provides room for recycling.

General construction characteristics of the various foundation types are presented in Table 2.

Conclusions

The increasing windmill tower and turbine sizes and installations in deeper watershave clearly demonstrated a need for more innovative and cost-effective foundations.There is room for improvement in all areas; in design, through the innovative useof composite materials, support structures and foundations; and in constructionprocesses, through improvements in drilling techniques, fabrication, and transportation.

The need for high-capacity foundations that can be installed indeep water with limited accessibility and with little disturbance tothe existing environment can also be fulfilled by new technologiesand process improvements. Environmental impact can be mitigatedby the use of geotextiles for scour protection, and the use of abubble cur tain for noise mitigation.

I have drawn upon the experience of European offshore wind farmdevelopments and the current practices of the U.S. offshore oilplatform industry to amalgamate a set of guidelines for selectionand design of support structures and foundations for offshore windturbine towers. Additional information on support structures andfoundations, as well as construction, maintenance, and site selectionconcerns, is available in the on-line version of this ar ticle.1 �

Figure 7: Suction caissons for an offshoreplatform being transported to site in theGulf of Mexico. (Courtesy of E. C. Clukey).

Caisson

MooringCable Differential

Pressure

CompletedInstallation

Self-WeightInstallation

Phase Suction Installation Phase

Construction Tripod/Braced Tension LogPhase Gravity Base Monopile Frame PlatformOnshore On land and No On land and No constraintFabrication close to site constraint close to site to to be economical be economical Transport Float to site or Float to site On barge Float to siteOffshore on barge or on barge or on bargePre-placement Seabed None None NoneActivities preparation requiredPlacement Lift or float over Lift and sink Lift and sink Lift and sinkFixing Tower to Bolt to Grout to Grout to tripod Tie to tensionSubstructure substructure piling central member cableInstallation of Requires No hindrance Requires No hindranceTower and specialized cranes to lifting specialized to lifting Turbine and large barges cranes

Figure 8: Installation of suction caisson.

Table 2: Construction Characteristics of Offshore WindTurbine Foundations

Because Kuwait, along with other Middle Eastern countries like Egypt, Jordan, and Syria, are of semi-conservative culture,many work fields are dominated by men. However, if men work side by side with women respectfully and women maintainself confidence to the maximum level, many problems can be avoided. Looking at the future, the Kuwaiti government hasrecently granted its local women the right to vote and run for office. In addition, there are now two female ministers—for the Ministry of Health and the Ministry of Education. You can say that women here are practicing their right to voteand are rising to the top in many positions. �

Being a Female Engineer in Kuwait (continued from page 85)

Acknowledgements: The author wishes to thank Dr. George Munfakh for constantly promoting innovation; Mr. Phil Rice, Mr. Frank Pepe, Jr., and Mr. VahanTanal for encouraging this work; and Mr. Raymond Castelli for his review and valuable suggestions. The author is grateful to Dr. E.C. Clukey, Mr. Bob Bittner,Elsam, and RWE npower for the various construction photographs, and to Mr. Pedro Silva for creating the illustrations of the various wind turbine foundations.

Sanjeev Malhotra, a supervising geotechnical/seismic engineer who has been involved with foundation design of a few offshore wind farms. A project manager andprofessional associate, he joined PB in 1999.

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