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Alternative solutions for offshore foundations

EUROPE: Offshore wind turbines are largely fixed to the seabed with a permanent or semi-permanent support structure. Some models float with the aid of a floater body and mooring system --tension leg or otherwise -- but fixed foundations remain by far the most common for offshore turbines.

Transition pieces sit on top of monopiles at Belwind wind farm
Transition pieces sit on top of monopiles at Belwind wind farm

A wind-turbine’s support structure — basically everything below the nacelle — is usually divided into two or three parts. The foundation at the bottom transfers the turbine loads into the soil, then may come a so-called transition part to which a standard tubular steel tower is fitted on top, and the nacelle yaw bearing is attached.

Five basic fixed substructure/foundation topologies have been used so far: the most common monopiles; the jackets, as used by Repower; Bard Tripiles; tripods as used by Areva Wind; and gravity- based structures, as used by Siemens and Repower.

The first four of these require pile ramming during installation, which causes a loud sound that is initially dull but gradually changes into an intense sharp noise carried over a long distance. This loud ramming faces public resistance because of the potential harmful marine health effects on fish and sea mammals in close vicinity to the construction site. The industry is seeking design solutions that are focused on either noise dampening or eliminating the need for ramming.

Design developments and innovations have emerged especially from the first three types.
 
Market leader

Steel monopile-type foundations have dominated the market for years. These straightforward designs usually comprise two distinct components. A three- to six-metre diameter and 50- to 60-metre long thick-wall steel pipe is typically rammed 20 to 30 metres into the seabed, depending on water depth, soil conditions and turbine size. The second component is a transition piece (TP), a hollow structure that, in the most common layout, slides with a loose fit over the pile. The steel tower, often a standard product, is finally mounted to the horizontal flange on top of the TP.

A less common but functionally comparable design solution is a monopile with an internal TP, which — as a claimed advantage — minimises the surface exposed to sea current and wave loading. This latter solution has so far been applied only once, in the 108MW Dutch Egmond aan Zee wind farm.

The main function of a TP is to compensate for a potential pile inclination error that could occur during the ramming process. TPs also contain weld-on additions such as mountings for an access ladder, a J-tube for cable passage and, quite often, cathodic-corrosion protection packs to fend off rust. The gap between the pile and TP is filled with a fast-curing mixture of cement-like material and water, named grout, which produces a rigid single assembly. Unfortunately there have been serious design-related issues with a substantial number of failing grout connections in the past. This has caused the TPs to slide vertically, negatively affecting their structural integrity.

One extensively researched remedy, from the German Hanover Leibniz University, involves welding so-called shear keys to the opposing metal surfaces in order to enhance the grout connection’s shear-load-bearing capacity. This has been applied in some wind farms, including Arklow Bank off Ireland, completed in 2004, with no know problems.

An alternative substitutes a grouted connection by two cone-shaped end pieces. The first is a transition piece top section with a slight outer coning, the second a bottom pile section with external cone surface. The connection is achieved by dropping the upper part over the lower part from a slight distance. Former Dutch firm Windmaster already successfully applied this method in the 1990s with its 750kW onshore turbines.

Concrete innovation

Dutch civil engineering contractor Ballast Nedam has developed an innovative concrete drilled monopile, which eliminates pile ramming altogether. The piles comprise multiple prefabricated concrete cylindrical-shaped rings, which can be assembled at various lengths with the aid of pre-tensioning cables. The technology development formed part of a major research project and as an integrated system involves Ballast Nedam’s huge floating vessel HLV Svanen. At the heart of this installation method is a rotating drill head with adjustable diameter that is lowered into the pile. Each pile bottom is further fitted with a steel add-on and a steel nose cone slightly bigger in diameter, which tapers into a rather sharp edge towards the soil contact area.

During gravity-supported pile penetration a self-hardening drilling fluid is injected into the gap between the concrete pile and the bored hole made in the seabed. Both measures aim to ease pile seabed penetration by gradually sinking into the seabed.

If the drill head hits a difficult-to-penetrate soil layer, it is lowered slightly further into the pile past the steel nose cone.This external position enables the drill-head diameter to be enlarged so it matches the outer nose cone diameter, allowing the pile to easily pass this soil obstruction. Once the drilling process is completed with the monopile sticking out roughly 3.5m above sea level, the drill head is removed. Finally, a conical-shaped dual-function ‘anti-icing’ top and tower-mounting flange is placed on top. The concrete monopile technology has yet to be employed in a wind farm project.

Jacket foundations are three- or four-legged lattice-steel structures consisting of welded tubular members that extend from the seabed to above the water. Piles are driven through pile sleeves, connected to each jacket leg, into the seabed to fix the structure against the combined forces of waves and wind thrust on the rotor. An alternative is pre-piling where the piles are driven first and the jacket is inserted into the piles.

Jackets are robust, open and relatively lightweight structures, but traditional design and manufacturing involves multiple welded joints and typically several concentrated welds in confined areas. Places where multiple pipes and other structural elements join can experience higher stress levels, whereas heat generated during welding might cause additional local material stress and enhance component distortion risks.

German specialists Weserwind and Repower have developed an innovative solution called a Variobase jacket to resolve these issues. They found a four-legged foundation more cost-effectiveness with a better overall performance for deep-water offshore substructures in combination with Repower 5M or 6M turbines.
 
Welding advances

Weserwind developed cast steel nodes for different positions within the structure and of a corresponding shape. The cast components come in three versions — X-Node, Double-K-Node and Double-Y-Node — and enable the use of standard tubular steel interconnecting pipes and a robotic welding process offering advanced manufacturing and cost-saving advantages.

Furthermore, the fact that heat from the welding process is now redistributed over a much-wider area adds to overall product quality through much-reduced material distortion. However, a disadvantage is the relatively high cost of these cast steel nodes.

So far, one onshore Variobase prototype with a 5MW Repower turbine has been built in Bremerhaven, Germany. Conventional jackets have been applied for several other Repower projects, including Beatrice (water depth 45m), Alpha Ventus (30m), and Ormonde (17-30m).

As well as developing and improving existing foundation designs, new solutions are being explored. One of these is Bard’s Tripile, which consists of three monopiles, each with a standard 3.9-metre outer diameter and varying in length and pile wall-thickness depending on water depth and soil conditions. The TP accommodates the three downward facing legs, each one fitting into a monopile. The power electronics sit in the centre of the TP, and the tower slides over them. A grout connection joins the piles and TP.

A gravity-based substructure is a hollow concrete foundation that sits on the seabed filled with sand, gravel or rocks. The steel tubular tower is mounted on the flange of a transition piece on top.

Tripods are a three-legged structure, the legs pointing downwards into the water each one being fixed to a monopile. Again, a transition piece sits on top.

No unified vision

A lack of standardisation and a reluctance from wind-turbine suppliers to share strategic design-related information with foundation developers are hailed as the main contributing factors to the current high cost of foundations. So, custom-developed foundations continue to be developed individually for most wind-farm projects, even though the wind turbines above are already semi-standard products.

An integrated design approach for both turbine and foundation is said to offer significant potential for materials and other cost savings. Yet the unwillingness to share information continues as many believe that combining two individually optimised main components will prove inferior to an integrated overall design of the product.

Dutch firm 2-B Energy is, however, working on a 6MW two-bladed downwind offshore turbine that sits on a lattice-type truss tower. This is a strategy that combines the foundation and tower into an integrated design that extends from the seabed to the nacelle bottom — a so-called truss tower.

Others see bright future prospects in so-called floating-to-fixed solutions, where fully commissioned offshore wind turbines including support structure are towed to wind-farm construction sites and lowered onto the seabed. For major service or reconditioning actions, the installations can be brought back in floating mode and towed to a sheltered harbour and exchanged with a replacement unit.

Further into the future lies the prospect of fully floating installations. Only time will tell which solutions will turn into real winners.

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