The required performance standards of wind turbine components destined for the offshore market present manufacturers with tough challenges. One company, Linde Gases, explains how its latest gas technology is helping wind turbine manufacturers successfully attain the welding integrity needed to protect towers from cracks and saltwater corrosion.

Lee Hermitage, Linde Gases, Germany

Offshore wind farms such as Lillgrund in Sweden are posing fresh challenges in the field of industrial welding Source: Siemens

Man has harnessed the power of the wind for thousands of years to advance industrial and social development, from the windmills used in the seventh century for irrigation pumping and milling grain, to the early wind turbines developed in the UK and the USA in the 1800s to generate electricity for farms and homesteads. Now in the 21st century, against a backdrop of dwindling supplies of fossil fuels, global focus on wind energy has reached an unprecedented intensity.

The Belgium-based Global Wind Energy Council (GWEC), a wind industry trade association providing a representative forum for the global wind energy sector, says wind energy is the only power generation technology that can deliver the necessary cuts in carbon dioxide (CO2) in the critical period up to 2020, when greenhouse gas emissions must peak and begin to decline to avoid harmful climate change. GWEC believes the present 158.5 GW of installed global wind capacity will save more than 190 million tonnes of CO2 every year.

Stanford University report

In 2005, researchers at Stanford University’s Global Climate and Energy Project conducted an evaluation of the global potential of wind power, using five years of data from the US National Climatic Data Centre and the Forecasts Systems Laboratory. After collecting measurements from 7500 surface and 500 balloon-launch monitoring stations to determine global wind speeds at 80 metres above ground level, they found that nearly 13 per cent had an average wind speed above 6.9 m/s, sufficient for economical wind power generation. The report concluded that enouch wind power exists to supply all the world’s energy needs, although many practical barriers would have to be overcome to realize this potential.

GWEC says the growth of the market for wind energy is today being driven by a number of factors, including the wider context of energy supply and demand, the rising profile of environmental issues, especially climate change, and the impressive improvements in the technology itself. These factors have combined in many regions of the world to encourage political support for the industry’s development.

Most industries, including renewable energy, have been badly affected by the recent economic crisis. Investment in renewable energy has fallen by around 50 per cent, according to GWEC. The wind industry, however, proved largely resistant to the crisis, with global installed capacity growing by 31.7 per cent in 2009. In the UK alone, there are now 263 wind farms in operation delivering 4.5 GW of electricity. This represents enough electricity to power more than 2.5 million homes.

The wind power capacity installed in the European Union (EU) by the end of 2009 produces 74.7 GW of electricity, equal to about 4.8 per cent of the EU’s electricity demand. By comparison, in 2000, less than 0.9 per cent of the EU electricity demand was met by wind power.

Europe, North America and Asia are the top three regions driving global wind development. According to GWEC, over the past ten years, global wind power capacity has continued to grow at an average cumulative rate of more than 30 per cent. Another record year was recorded in 2009, with over 38 GW of new installations bringing the world total to more than 158 GW.

In 2009, the USA maintained its position as the number one market in wind power with 35 064 MW installed capacity. The total new capacity installed in 2009 was 10 GW. In Canada, 950 MW of new installed capacity were installed bringing the total to 3319 MW. This is sufficient energy to power more than one million Canadian homes – or about 1.1 per cent of the country’s total electricity production.

The growth in Asian markets has been described as breathtaking, with more than a third of the 38 GW of new wind power capacity added globally in 2008 being installed in this region. China’s total capacity has doubled for the fifth year in a row to reach 25 805 MW, making this country the second largest wind market in the world.

Other Asian countries with new capacity additions in 2009 include India (1271 MW, taking the total to 10.9 GW) Japan (178 MW, for a total of 2.1 GW), South Korea (112 MW, taking the total to 348 MW) and Taiwan (78 MW, for a total of 436 MW).

Offshore Offshore Wind Technology

A wind farm comprises a group of turbines in the same location to produce electric power. Turbines are interconnected with a medium-voltage power collection system (usually 34.5 kV) and communications network. At a substation, this medium-voltage electrical current is increased in voltage via a transformer for connection to the high-voltage transmission system.

Turbines used in wind farms for the commercial production of electric power are usually three-bladed and are pointed into the wind by computer-controlled motors. They have high tip speeds of over 320 km/hour, high efficiency and low torque ripple, which contribute to good reliability.

The blades are usually light grey to blend in with the clouds and range in length from 20 to 40 metres or more. The tubular steel towers range in height from 60 to 90 metres. And all turbines are equipped with shutdown features to avoid damage when exposed to very high wind speeds.

Wind power has grown into an important player in the world’s energy markets, with the 2009 market for turbine installations worth around €45 billion ($58 billion). In China, for example, by the end of 2009 there were almost 80 wind turbine manufacturers. Despite the global financial crisis, the 17 per cent per annum growth trend for wind turbine manufacturers until 2012 is still on track.

Performance standards

Offshore wind energy turbines are often situated in some of the roughest and most inhospitable oceans, and have to be able to withstand enormous loads from huge waves and mighty swells. High quality steel and the most up-to-date production methods are necessary for wind farms operating in such harsh conditions.

In this context, welding becomes particularly important as the huge steel towers and support stilts that the turbines are composed of are manufactured from several individual steel segments. A faulty weld seam on a single component can have catastrophic consequences. Cracks or dangerous saltwater corrosion could lead to a rupture in the steel components.

Performance standards for wind turbine components, particularly those operating offshore, therefore confront manufacturers with tough challenges. The Linde Group describes this as a new horizon.

In the case of standard steel constructions, fully automatic systems for the heat treatment of weld seams have been in place for some time. But for segments of wind turbine towers – steel plates which can be up to 4 metres wide and 12 cm thick – such systems are as yet non-existent.

Welding in this application is a complicated affair. To begin with, the thick metal pieces need to be pre-heated. If this is not done, the large, cold steel plates will lose heat too quickly and the metal will not be completely melted in the welding zone, making a secure connection impossible. Pre-heating will also prevent the build-up of cold cracks that can occur because of hydrogen exposure or internal stress inthe component.

This is particularly important when treating high-strength steels. After the weld, these materials must be post-heated for around two to three hours to diffuse rogue hydrogen atoms in the weld seam. For manufacturers who have to maintain a fast production speed, it is vital to quickly reach a pre-heated temperature of greater than 100 °C.

Providing a Welding Solution

The Lindoflamm concept, which is based upon Linde’s pioneering acetylene-based burner, has proved to be an ideal technology for this application, delivering pre-heating and post-heating treatment solutions for heavy steel manufacturers and fabricators.

An important characteristic of acetylene is the high heat intensity of the primary flame. This results in a focused flame, pre-heating only in the weld area and increasing the speed at which the weld area is heated by as much as two thirds compared with other fuel gases, and also significantly saving on total process cost.

As opposed to a propane gas flame, for instance, acetylene gas burns with a very precise, pointed, so-called ‘primary flame cone’, which drives the heat directly into the metal. In addition, the flame temperatures that can be reached with the associated acetylene-compressed air torch – approximately 2400 °C – are significantly higher than those achievable using other oxy-fuel gases in combination with air. The new burner is therefore capable of heating up the steel twice as fast as conventional methods.

Lindoflamm has made Linde a leader in pre-heating and post-heating welding applications for offshore wind turbine towers and jackets, based on its collaborative work with steel experts EEW Special Pipe Constructions GmbH of Germany to develop welding technology for wind turbines.

EEW has a long history of manufacturing components for offshore construction in the gas and oil industries, but wind farm components for a 30-year lifespan proved a new challenge. Similar to a domestic stove-top gas burner, the burner Linde has developed for EEW’s Rostock facility has several nozzles. But instead of forming a ring, these nozzles are on a 5-metre pipe, out of which a flame shoots at intervals of a few centimetres.

A graph providing a comparison of fuel gas flame temperatures Source: Linde Gases

The challenge was to design this long burner in such a way that the same amount of gas comes out of each nozzle, allowing for a uniform temperature throughout. It was also important to keep the number of gas feed pipes to a minimum in order to reduce the complexity in construction.

In collaboration with gas flow experts from the Dresden University of Technology in Germany, researchers at Linde have designed a unit assembly system featuring segments for almost any burner length. This special design facilitates the steady flow of gas.

For the Rostock plant, the Linde team has designed a burner suited to so-called ‘longitudinal welds’, as well as a curved segment burner for circular steel segments (circumferential welds) with a diameter of up to 7 metres. Although most companies work on pipes with a maximum diameter of 1.2 metres, at the Rostock facility EEW starts at 1.6 metres.

Both machines are fully automatic, which is primarily necessary for the post-weld heating, which should begin immediately after the welding. The new acetylene burners are fitted with several temperature sensors, which regulate the heat precisely. In addition, the burners automatically turn on or off, which keeps the temperature within the desired range during the post-heating process. The benefits of the Lindoflamm approach versus other heating methods are outlined in Table 1.

 

EEW has become Linde’s reference customer for this application. Germany leads the world in technology research for wind farm component manufacture and Linde established an applications centre in Germany back in 2000 to ensure it remains on the leading edge of global research and development. The technology it has developed and which is now being used to manufacture wind turbine tower and jackets at the Rostock facility reflects several years of teamwork.

Offshore wind is a new industry and everyone is seeking new standards in construction. Linde is delighted to have long-term experience as a platform to deliver world-class standards in the future.

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