Design and Construction of Offshore Wind Turbine Foundations

http://www.pbworld.com/news_events/publications/network/

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.

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 promising growth industry. industry. The next generation of wind wind turbines that are on the drawing boards boards are gigantic in size, making them more cost-efficie cost-efficient, nt, but also putting putting large demands on their their support structures and foundations. foundations. As an increasing increasing number of wind wind farms are being planned offshore offshore in water depths of over 40 m (130 feet), the combination of water depth and increased windmill windmill tower tower heights, turbine weights, weights, and rotor blade diameters diameters create loads that make foundation design very complex. This article summarizes various relevant foundation and geotechnical issues for offshore wind turbine tower tower foundations. foundations. Offshore foundation foundationss are exposed to additional additional loads that do not affect aff ect land-based land-based towers. towers. Thes Thesee include ocean ocean current currents, s, storm waves, waves, ice and potentia potentiall ship impacts. impacts. Currently, a lack of precedence in the U.S. leave Currently, leavess no established technical technical guidelines for the selection,, design and construction selection construction of such such structures. structures. In addition, addition, the establish established ed European European practices may not be applicable applicable to the environme environmental ntal conditions conditions in the U.S., U.S., which include include deeper waters waters and greater greater wind, 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 suppor suppor t structures that connect the foundation foundation to the transition transition piece, and the foundation foundation itself • Transition piece, which connects connects the foundation foundation system to the tower • Tower, which is made of steel plate rolled rolled into conical subsections subsect ions 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, includin includingg the gearbox and generator • Rotor blades, which are made using using a matrix of fiberglass 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 below foundations and associated associated water depths are shown shown in Figure 2. The typical sizes sizes for offshore offshore foundations 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 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 competent soil sites in relative relatively ly shallow shallow waters. Gravi Gravity ty caissons caissons are typically typically concrete shell shell structures. These structures structures are competitive when Figure 2: Foundation types and typical water depths.

PB Network # 65 65

92

The world’s largest digital library

Try Scribd FREE for 30 days to access over 125 million titles without ads or interruptions! Start Free Trial Cancel Anytime.



http://www.pbworld.com/news_events/publications/network/ R&II R&

the environmental environmental loads are relatively low and the dead load is significant, significant, or when additional ballast can be provided at a reasonable cost. Monopile. This is a simple simple design in which the wind tower tower,, made of steel pipe, is supported by the monopile either directly or through a transition piece. The monopile consists of a steel pipe pile up to 6 m (20 feet) in diameter with wall thicknesses as much as 150 mm (6 inches). Depending on t he subsurface conditions, the pile is typically drive drivenn into into the seab seabed ed by eith either er large impa impact ct or or vibratory vibratory hammers, hammers, or the piles ar e grouted into sockets drilled into r ock.

Compared to the gravity Compared gravity base foundation foundation,, the monopile monopile has minimal minimal and localized environmental impact. By far, far, the monopile monopile is the most most commonly used foundation foundation for offshore wind wind turbines. Guyed Monopile Towers. Towers. The limitation of excessive deflection of a monopile in deeper waters is overcome by tying the monopile with tensioned guy wires.

deflections Tripods. Where guyed towers are not feasible, tripods can be used to limit the deflections of the wind towers. towers. The pre-fabricated frame is triangular in plan view and consists of steel pipe members connecting connecting each corner. corner. A jacket leg installed installed at each corner is diagonally and horizontally braced to a transition piece in the center. center. The tripod braced frame and the piles are constructed onshore and transported by barge to the the site. These foundations foundations do not require any seabed preparation. Braced Lattice Frame. A modification modification of the tripod frame, the lattice frame has more structural members. members. The jacket jacket consists consists of a 3-leg or 4-leg 4-leg structure made of steel steel pipe that is interconnected with bracing to provide the required stiffness.

Table 1: Basic Sizing and Construction Sequencing for Offshore Wind Turbine Foundations.

Type of Foundation Foundation

Size (m)

Weight (ton)

Suction Buckets. This design consists of a center column connected to a steel bucket through flange-reinforced shear panels that distribute the loads from the center of the column to the edge edge of the bucket. The steel bucket bucket consists consists of a steel skirt extending down from from a horizontal base resting on the soil surface. The bucket is installed by means of suction and behaves behav es as a gr avity foundation, foundation, relying on the weight of the soil encased by the steel bucket. Typical Water Depths (m)

Construction Sequence

Gravity Base 12 - 15 500 – 1000

0 – 15

(a) Pr Prepare epare seabed (b) Placement (c) Infill ballast

Monopile

3-6

175 – 350

0 – 30

(a) Place pile (b) Drive pile

Monopile with Guy Wires

3-6

175 – 350

20 – 40 (a) Place pile (b) Drive pile

15 – 20

125 – 150

20 – 40 (a) Place frame (b) Insert pile (c) Drive pile

Tripod

Braced-Frame 10 – 15 200 – 400 Braced-Frame with Multiple Piles

20 – 50 (a) Place frame (b) Insert pile (c) Drive pile

Suction

0 – 30

10 – 20 150 – 400

(a) Place base

The stability of the system is ensured because there is not enough time for the bucket to be pulled out of the soil during a wave passage. As the bucket is pulled pulled up, up, a cavity cavity is formed between the soil surface and the bottom of the bucket which creates a suction pressure that resists the uplift loads. Floating Tension Tension Leg L eg Platforms. Pla tforms. These structures are floated to the site and submerged by means of tensioned vertical anchor legs. The base structure structure helps dampen dampen the motion motion of the the system. Installation is simple because the structure can be floated to the site site and connected connected to anchor anchor piles. The structure can be subsequently lowered by use of ballast tanks and/or tension systems. The entire entire structure can be disconnect disconnected ed from the anchor piles and floated back to shore for major maintenance or repair of the wind turbine.




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 dredging several meters of generally loose, soft seabed sediment and replacing replacing it with compacted, crushed stone stone in a level bed. Speci Special al screeds and and accurate surveying surveying is required required for this task.

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

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

Use of open-ended driven pipe piles allows the sea bottom sediment to be encased inside the pipe, pipe, thus minimizing minimizing disturbance. The noise noise generated during pile driving in the marine environment might cause a short-term adverse impact to aquatic life, howeve howeverr, but because the number number of piles is typically small, small, these adverse impacts are only only short-term and relatively minor. Recent innovations innovations in the pile pile driving industry, such as the bubble bubble curtain, offer a way to mitigate mitigate noise impacts. A bubble bubble curtain involves pumping air into into a network network of perforated perforat ed pipes surroundin surroundingg the pile. As the air escapes, escapes, it forms an almost continu continuous ous curtain of bubbles around the pile, preventing the sound sound waves from being transmitted transmitted into the surroundings.

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

Figure 5: 5: Reverse circulation drill

Post-Grouted Closed-end Pile in Predrilled Hole. A closed-ended steel pipe pile is placed into a predrilled hole hole and then then grouted in in place. This option option is used often for offshore pile foundations less than 5 m in diameter and offers significant advantages over the cast-in-place cast-in-place drilled shaft option, option, including advance advance fabrication of the pile, better quality control, and much shorter shorter construction time on on the water water. This option option requires requires a specially specially fabricated large diameter reverse circulation drill (Figure 5). It also requires handling handling and placement place ment of a long, long, large-d large-diamet iameter er pile, of considerab considerable le weight. weight. Close Closed-end d-end piles piles can be floated to the site and lowered into the dr ill hole by slowly filling filling them with water. water. Drilled Shafts or Bored, Cast-in-Place Concrete Pile. The installation installation of bored, cast-incast-in-place place concrete pile (Figure 6) requires driving a relatively thin-walled (25 mm) casing through the soft sediment sediment to the underlying denser material (if necessary to establish a seal), then drilling through and below the casing to the required base elevation. Bending resistance resistance is provided by a heavy heavy reinforcing reinforcing cage utilizing utilizing high strength, large diameter bars, with double doub le ring, where necessa necessary. ry. The casing casing provides provides excavatio excavationn support, guide guidess the drilling drilling tool, to ol, conta contains ins the fluid fluid concrete, concrete, and serves as sacrificial sacrificial corrosio corrosionn protection. protection. This approach requires a large, specially fabricated reverse reverse circulation drill. Composite “Drive-Drill-Drive” Pile. This procedure requires an adaptation of existing drilling and piling techniques and involves involves a combination of drive-drill-dr drive-drill-drive ive sequence to achieve the design depth.

shape but Suction Caissons. Like piles, suction caissons (Figure 7) are cylindrical in shape




hydrostatic force on the pile top. The hydrostatic hydrostatic hydrostatic pressure pressure thus developed developed pushes the pile pile to the the design design depth. Once the the design depth isis achieved, achieved, the pumps are disconnected and retrieved. Suction caissons are expected to be particularly suitable for foundations in the type of soft soft cohesive cohesive sediments found found around the U.S. coasts. These foundations foundations cannot be used in in rock, in gravel or in in dense sand.

Figure 7: Suction caissons for an offshore offshore platform being transported to site in the Gulf of Mexico. (Courtesy of E. C. Clukey).

Mooring Cable

Suction caissons are less expensive to install because they do not require underwater under water pile pile drivers. At the end of a wind turbine’ turbine’ss life, a suction suction caisson caisson can be removed removed completely completely from the seabed, seabed, unli unlike ke piled foundati foundations. ons. This provides room for recycling. General construction characteristics of the various foundation types are presented in Table 2.

Differential Pressure

Conclusions

Caisson

Completed Installation

Self-Weight Installation Phase

The increasing windmill tower and turbine sizes and installations in deeper waters have clearly demonstrated a need for more innovative innovative and cost-effective cost-effective foundation foundations. s. There is room for improvement improvement in all areas; in design, through the innovativ innovativee use of composite materials, support str uctures and foundations; foundations; and in const constructi ruction on processes proce sses,, thro through ugh improveme improvements nts in drilling techniques techniques,, fabri fabricatio cation, n, and transpor tation.

Suction Installation Phase

Figure 8: Installation of suction caisson.

Construction Phase Gravity Base Onshore Fabrication Transport Offshore Pre-placement Activities

Monopile

Tripod/Braced Tension Log Frame Platform

On land and No On land and No constraint close to site constraint close to site to to be economical be economical Float to site or Fl oat to site On barge Float to site on barge or on barge or on barge Seabed preparation required

None

None

None

Placement Lift or float over Lift and sink Lift and sink Lift and sink Fixing Tower to Bolt to Grout to Grout to tripod Tie to tension Substructure substructure piling central member cable Installation of Requires No hindrance Tower and specialized cranes to lifting Turbine and large barges

Requires specialized cranes

No hindrance to lifting

Table 2: Construction Characteristics of Offshore Wind Turbine Foundations

The need for high-capacity foundations that can be installed in deep water with limited accessibility and with little disturbance to the existing environment can also be fulfilled by new technologies and process improv improvements. ements. Enviro Environmental nmental impact impact can be be mitigated mitigated by the use of geotextiles geotextiles for scour protection, protection, and the use of a bubble curtain for noise mitigation. I have drawn upon the experience of European offshore wind farm developments developme nts and the current practices of the U.S. offshor offshoree oil platform industry to amalgamate a set of guidelines for selection and design of support structures and foundations for offshore wind turbine towers. Additiona Additionall information information on support structures and foundation foun dations, s, as well as constructio construction, n, mainte maintenance, nance, and site selectio selectionn 1 concerns, is available available in the on-line version version of this article. 

Acknowledgements : The author wishes to thank Dr. George Munfakh for cons tantly promot ing innovation; Mr. Phil Rice, Mr. Frank Pepe, Jr., and Mr. Vahan Tanal for encouraging this work; and Mr. Raymond Castell i for his review and valuable suggestio ns. 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 and professional associate, he joined PB in 1999. 1

http://www.pbworld.com/news_events http://www.pbwor ld.com/news_events/publications/ne /publications/network/issue_65/65_index.as twork/issue_65/65_index.asp p

Being a Female Engineer in Kuwait

(continued from page 85)

Design and Construction of Offshore Wind Turbine Foundations

Recommend Documents