Reach for the skies!

The energy sector is the biggest global market for manufacturers of steel pipes, with the focus on two nonrenewable resources: oil and gas. However, tubular products are equally indispensable for renewable energy, especially wind power. While land-based towers are the most conspicuous, offshore towers use far more steel because they require substantial foundations hidden below the water’s surface.

Although 2014 was a record year in the German wind power sector, 2015 was yet another year with record figures. In 2014, 141 new turbines were built, and offshore output doubled compared with 2013. In 2015 the overall total increased even further; in that year, 546 offshore wind turbines were connected to the German grid, providing a total output of 2,282.4 megawatts (MW). According to Deutsche WindGuard in its “Offshore Wind Power Expansion Status in Germany,” 792 offshore wind turbines were contributing power to the German grid at the end of December 2015, for a total of 3,295 MW.

According to AG Energiebilanzen, offshore electric power turbines provided about 1.4 percent of Germany’s gross power supply in 2015, which met the electricity needs of more than 2 million households, Although offshore wind power has the smallest share in Germany’s power production (with the onshore volume being nearly 10 times as high), this sector is nevertheless an attractive one for the steel pipe industry.

Reeling in More Wind

The increasing attractiveness of offshore power is partly due to the lack of acceptance of onshore facilities, although this isn’t the only reason. The others relate to wind consistency.

First of all, the wind at sea is not just stronger but also more constant and regular than on land, which was confirmed in the “Wind Energy Report for Germany 2014,” a study published by the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES). Although both types of locations experience seasonal differences in the wind power supply (more in winter, less in summer), offshore wind is not as dependent on the time of the day as onshore because thermal convection has less of an impact on the open sea. In summary, offshore wind turbines deliver far more energy than corresponding onshore wind farms, and farshore facilities are more productive than those situated close to the coastline.

An efficiency principle applies to turbines in general, whether onshore or offshore: the bigger the better. The rotor diameter plays a major role for the output and yield of a wind turbine, as the surface of the rotor determines the share of captured wind flow and the share that can be converted to electric power by the turbine. Moreover, depending on the location, an increase in height also means a higher wind speed. Because the output generated by the wind is in proportion to the cube of the wind speed, the hub height has a major effect on a wind turbine’s yield.

This is accompanied by an increase in the nominal output of the wind turbines. According to IWES, the average nominal output of a newly installed offshore wind turbine has increased from 1.9 MW in 2000 to 3.6 MW in 2014. During the same period the average hub height rose from about 200 to 280 feet. Rotor blade lengths likewise have increased. In 2014 wind turbine rotors had an average diameter of 375 ft., compared with about 245 ft. in 2000. New models in the 6-MW class have diameters of nearly 500 ft., and some are even longer. In fact, the next generation of offshore wind turbines, which is currently under development, will have rotor diameters greater than 1,725 ft., with a nominal output of 6 to 7 MW.

The foundation of an offshore wind turbine is affected not only by a trend towards larger dimensions, but also by the water depth at the point of installation. Whereas the first wind farms were still built relatively close to the coast, where the water was quite shallow, today’s offshore facilities are further out in deeper water. According to IWES, German offshore turbines are situated, on average, 40 miles from the coast in water around 100 ft. deep.

The structure of an offshore wind turbine depends on its location. Gravity foundations, high-rise pile caps, and monopiles are used largely in coastal and shallow waters. Tripod and tripile foundations are particularly suited for locations far from the coast in in deeper waters.

Dealing With the Strains of Offshore Locations

Offshore wind turbines generally need to be more stable and robust than onshore. This is because they are exposed to more and bigger forces. As well as their own weights and high wind speeds, they need to cope with waves, currents (including high and low tides), and floating ice. Their foundation structures must be designed and dimensioned to withstand all the forces to which they are exposed over several decades.

The simplest way to anchor an offshore wind turbine in the seabed is to use monopiles—long pipes driven into the seabed by an installation vessel. Installation takes just a few hours, making them cost-effective. About half of the pile must be embedded for sufficient stability.

To bear the enormous weight of an entire structure, the pipes must have thick walls and large diameters (many feet). At a 5-MW facility, for instance, the gondola with its rotor and hub alone can weigh more than 400 tons. The turbine and the transition piece between the foundation and the turbine add several hundred tons.

To cope with such large volumes and to use monopiles at increasing depths, pipe manufacturers that serve this industry are continually entering new territory in terms of dimensions. At the beginning of the millennium, 16 ft. was about the largest possible diameter. By 2015 the standard piling diameter had grown to more than 19 ft., and the pile can be used at a depth of about 100 ft.

The Dutch Sif Group believes it should be possible to increase the length of a monopile to 400 ft., its diameter to 36 ft., and its weight to 2,000 tons. Anticipating such developments, the company has been expanding its production capacities so that it can eventually produce such pipes in series. In the near future, however, Sif believes that it is sufficient to make foundations with 30-ft. diameters, which are 325 ft. long and weigh 1,500 tons.

Other manufacturers likewise are producing extremely large monopiles. EEW Special Pipe Constructions GmbH, Rostock, Germany, produces large, longitudinally welded pipes up to 33 ft. in diameter, up to 400 ft. long, and weighing up to 1,500 tons. This currently is the most cost-effective foundation structure for offshore wind farms with turbines from 5 to 8 MW, at a depth of up to 130 ft. According to the manufacturer, this allows savings of up to 30 percent on foundations compared with a jacket design. Such savings are substantial; foundations account for 20 to 25 percent of the total cost in an offshore project.

Megamonopiles are also being studied by a workgroup at Dillinger Hüttenwerke, associated with the company’s subsidiary Steelwind Nordenham, to set up a special manufacturing facility for such foundations. The first monopile that was produced in Nordenham in September 2014 had a diameter of 25 ft. and weighed around 1,000 tons, the biggest of its kind. Yet it is possible to build monopiles up to 32 ft. in diameter, up to 400 ft. long, up to 6 in. in wall thickness, and up to 1,500 tons in weight. Such piles can be used in water up to 145 ft. deep.

High-strength Steel for High-stress Applications

Dillinger Hüttenwerke is also one of the main suppliers of heavy plates for the production of monopiles. To make these, the company has developed steels thicker than 4 in. with strength exceeding 72,500 pounds per square inch.

Weldability is critical because welding is one of the main processes for producing a monopile. First, the plate is formed into lengths of pipe segments, using three- or four-roller bending machines. Second, the inside and outside longitudinal seams of the individual lengths are welded on a welding line. Third, the finished individual lengths are taken to a production line, where they are welded together into a single, large segment. Alternatively, it is possible to mount smaller segments comprising two or three lengths and then combine them on the same large-segment production line.

Having reached this so-called growing line, the final component then is assembled. The individual lengths, or small segments, are aligned and the components are joined. The inner and outer circumferential seams then are welded together, using a column and boom and applying a submerged arc welding technique. In this way the components are gradually assembled to form the actual monopile, which then is taken to finishing and coating. Continuous, highly automated production results in a good level of capacity utilization and cost-effective manufacturing of offshore wind turbine foundations.

Innovative plants and machinery for the cost-effective manufacturing of monopiles and other foundation structures will be presented at the next staging of TUBE^®, the international tube and pipe expo held biennially in Düsseldorf, Germany. The next tradeshow runs April 16-20, 2018.

Messe Düsseldorf North America, 150 N. Michigan Ave., Suite 2920, Chicago, IL 60601, 312-781-5180, info@mdna.com, www.mdna.com