Exploring the trends in foundation design

Most offshore wind turbines use monopile foundations, but alternative foundation designs are gradually gaining ground. We evaluate the choices available to developers depending on their own preferences and perceptions, site conditions and turbine size

Bard’s high cost tripiles incorporate three monopiles
Bard’s high cost tripiles incorporate three monopiles

During the past decade the vast majority of offshore wind farms have been installed in modest water depths, relatively close to shore, and using 2-3.6MW class turbines. But things are changing.

The average installed capacity of turbines deployed on new wind farms in European waters grew by 14.2% to 4MW in the first half of this year compared with the same period in 2011. Siemens’ 3.6MW turbines enjoyed an 82% market share within this period. A continued shift towards larger turbines, and especially the 4–6MW+ class, is expected over the coming years. This, together with other factors, will have a significant impact on foundation choice.

Steel monopiles have enjoyed great popularity over the past decade and have managed to build a substantial track record. These relatively uncomplicated structures in a semi-standard design comprise a steel pile rammed into the seabed and an adjustable transition piece (TP) that loosely fits over the pile top section.

A pile typically has a three- to six-metre diameter and is 50 to 60 metres long. Wall thickness ranges between 40mm and 65mm depending on water depth, turbine size and seabed conditions. These piles are usually rammed 20 to 30 metres deep into the seabed.

The main function of a TP is to compensate for minor pile inclination faults during the ramming, as wind turbines must be vertical for correct functioning in wind. A firm connection is established by applying fast-curing concrete (grout) within the annular cylindrical gap, being the interface between TP and pile.

TPs typically incorporate add-ons like an access ladder, J-tubes for infield cable guidance, boat landing supports and cathode protection packs.

A recent innovation at Germany’s Riffgat project incorporates an E-module inside the TP rather than mounting it atop. This three-storey platform contains the transformer, switchgear and power electronic AC/DC inverter. In Siemens’ SWT-3.6-120 turbines the DC/AC rectifier is located inside the nacelle.

Monopiles are usually fitted with an external TP, but an internal TP was applied in 2006 on the Dutch OWEZ wind farm. The contractor claims reduced wave loading as a key benefit of this design solution.

Serious pile slippage issues were first reported in 2010, with failing grout connections identified as the root cause for both monopile variants.

The issue prompted a comprehensive industry search for remedial solutions. Germany’s Hannover Leibniz University has extensively pioneered the use of shear keys welded onto the opposing cylindrical surfaces. This measure aims to enhance the load-bearing capacity of grout connections. The method has become one of the accepted remedies now being applied in the industry.

Another novel alternative design solution is eliminating a grouted connection between pile and TP and substituting it by matching external and internal coning steel surfaces. A firm impact connection between the two components is achieved by dropping the upper TP with inner coning from a slight distance over the lower matching pile, shaped like a pencil point. For the Riffgat project, Belgian company G&G International implemented an intermediate design solution comprising a conically shaped pile and TP, both featuring shear keys and joined by a grout connection.  

Jackets

Jacket foundations mostly come as four-legged lattice-steel structures, with individual piles driven through corner pile sleeves providing anchorage to the seabed. An alternative is pre-piling, whereby the piles are driven in first followed by jacket placement.

Jackets are known as robust open structures with relatively favourable mass. Traditional designs, on the other hand, incorporate many concentrated welding joints and places where multiple pipes and other structural elements join. This can exacerbate material stress related to concentrated heat build-up, and result in component distortion. According to some specialists, changes in the properties of materials could also accelerate corrosion damage.

WeserWind and Repower, both of Germany, have developed an innovative Variobase jacket. WeserWind has developed cast steel nodes for the various connections, aiming to counteract material stress-related issues while enabling a much higher degree of production automation and robotised manufacture.

The cast components come in X-Node, Double-K-Node, and Double-Y-Node and allow the use of standard straight-cut tubular steel pipes. René Surma, WeserWind’s head of sales, explains that one four-legged Variobase prototype has been operating successfully for several years at an onshore location in Bremerhaven. Furthermore, a four-legged structure was found to be superior in terms of cost-effectiveness and overall performance for deep-water offshore projects using Repower 5M/6M turbines.

A jacket design variant called twisted jacket and developed by US-based Keystone Engineering was introduced in 2011. Described as an inward battered guided structure, it comprises a three-legged support angled around a central pile. A consortium called Smart Wind aims to incorporate twisted jackets into future UK Round 3 offshore wind projects.

Speaking of the future of Variobase and manufacturing advancement, Surma says: "We are fighting hard to secure new projects with such jackets and also, in general, future supply chains that are geared towards easier, faster and more cost-effective production methods." In its new Bremerhaven factory, WeserWind can manufacture different foundation types in series and in parallel lines. "At the moment, we are manufacturing 80 tripod foundations for two major German projects, both with 5MW Areva Multibrid turbines," he adds.

Tripods consist of a central steel shaft connected with three pairs of conical braces to three cylindrical steel tubes through which piles are driven into the seabed, ensuring a secure attachment to the seafloor. Similar to jackets, they have become a proven concept for projects characterised by water depths of 30 to 40 metres and with larger turbines of 5MW and upwards. The Alpha Ventus project in Germany is so far the only operating wind farm that has used tripod foundations.

Surma thinks that jackets and tripods do not differ much in terms of design complexity, and both require thorough computer-aided design and optimising processes. He sees a trend towards jacket foundations, both as three- and four-legged structures. The former requires bigger pipes, larger wall thicknesses and perhaps also higher-grade steel.

The company has further specialised in a jacket TP design whereby the flange provides the rather complex interface with a tubular steel tower.

Continuous progress is being made in various fields, from design to production technology and automation. "For the automated welding of tripods, and especially the complex eclipse shapes, we have developed a welding robot that has been tested and demonstrated at laboratory scale but is not yet employed for serial production," says Surma.

He says the company’s Best Fit project is at the same stage of development. Best Fit is a technology that determines the best tolerance match between two welded components. Surma says it can be applied in future to both jacket and tripod manufacture.

As one of Europe’s main offshore foundations suppliers, WeserWind has gained extensive experience building tripods, jackets and Bard’s own tripile. Surma estimates that, at 35-metre water depth and comparable turbine size, the mass of an optimised tripod would be about 150 tonnes higher compared with a four-legged jacket. He expects tripiles to suffer a 30% mass penalty compared with jackets for similar conditions.

Bard’s tripile consists of three relatively small vertical monopiles and an interlinking star-shaped TP. Each monopile features a standard 3.9-metre outer diameter and varying lengths and wall thicknesses depending on water depth and soil conditions.

For the 400MW Bard Offshore I North Sea project, currently under construction, the pile length is 90 metres. Piles are individually rammed into the seabed with the aid of a precision guiding frame, followed by fitting the transition piece with three downward-facing legs, each sliding into an individual pile. A grout connection completes the installation process.

A tripile innovation that can save both time and cost is an integrated winch system for pulling in the ingoing and outgoing electricity transmission cables, eliminating the need for divers.

A TP redesign project has resulted in mass reduction of about 10% from the original 495 tonnes. Bard had plans to apply tripile foundations in several North Sea projects, but the company’s future is now uncertain.

Despite Bard’s future-oriented offshore wind farm development concept, including the standardised pile feature, wind experts appear generally sceptical about the chances of future market success for the tripile design, due to cost and mass considerations.

Pile ramming

All the substructure types described above involve pile ramming during the installation process. However, this method faces increasing public resistance due to its potentially harmful effects on the fish and sea mammals in close proximity to the site. Industry, meanwhile, is looking for remedial solutions to dampen noise or eliminate the need for ramming altogether.

Dutch civil engineering contractor Ballast Nedam has developed an innovative concrete drilled monopile, which eliminates pile ramming. The piles comprise multiple prefab concrete cylindrical shape rings, which can be assembled to various lengths with the aid of pre-tensioning cables.

At the heart of the installation method is a rotating drill head with adjustable diameter, which is lowered inside the pile. During gravity-supported pile penetration, a self-hardening drilling fluid acting as grout is injected into the annular gap created by the concrete pile while sinking into the seabed. Once the drilling is completed, the drill head is removed.Finally, a conical-shaped dual-function anti-icing top and tower-mounting flange is put on top of the pile well above the sea surface. Ballast Nedam’s concrete monopile technology has not yet been employed in a wind farm project.

Gravity based

A minority of offshore wind farms in operation feature gravity-based concrete foundations. Gravity-based foundations typically comprise reinforced concrete filled with ballast to ensure they become heavy enough to withstand the overturning moment created by the turbine. Seabed preparation is an integral part of the total installation process.

Most projects with gravity-based foundations have been built in shallow waters with 2-2.3MW turbines. An exception is the first phase of Thornton Bank, which consists of six 5MW Repower turbines installed about 28 kilometres off the Belgian Zeebrugge coastline at a depth of 12–27.5 metres. The foundation base is 21 metres wide and the total mass is about 21,000 tonnes.

Experts continue to disagree on the main trends and ultimate solutions for the future of offshore wind turbine foundations. One expert, who prefers not to be named, points to the fact that only 18 months ago it seemed that foundations for turbines with capacities of 5MW and up would definitely be jackets. Two major jacket-manufacturing projects based on a conventional design have turned out 10-15% more expensive than anticipated, he said. Tripods have also proved heavier and more expensive to produce than anticipated.

Three French projects, all destined for water depths of around 30 metres, were initially planned with jacket foundations, the source claims. Two of the three projects awarded to EDF-Dong and using Alstom’s 6MW turbines might now be built with monopoles, and the third with a gravity-based solution. If this is the case, an XL monopile sufficiently stiff to accommodate 5-7MW class turbines at 30- to 35-metre water depth is needed.

Nonetheless, the search for alternatives will continue to drive industry players to develop novel jacket solutions. Their motivation rests on the need for standardisation and industrialisation, and a much higher level of automation, including the use of robots.  


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