Feature

Avoiding stress fractures in towers through welding innovation


David Hopwood

Performance standards for wind turbine components, particularly those operating offshore, confront manufacturers with tough challenges. In this case study we look at how one organisation - the Linde Group - is offering help to wind turbine tower manufacturers.

Wind energy has grown into an important player in the world's energy markets, with the 2008 market for turbine installations worth about €36.5 billion. In China for example, more than 20 new turbine manufacturers entered the market in 2008, bringing the total number of manufacturers in China to 70. Despite the global financial crisis, the 17% per annum growth trend for wind turbine manufacturers until 2012 is still on track.

Turbines used in wind farms for commercial production of electric power are usually three-bladed and pointed into the wind by computer-controlled motors. They have high tip speeds of over 320 km/h (200 miles per hour), high efficiency, and low torque ripple, which contribute to good reliability.

The blades are usually coloured light grey to blend in with the clouds and range in length from 20 to 40 metres (65 to 130 feet) or more. The tubular steel towers range from 60 to 90m (200 to 300ft) tall. The blades rotate at 10-22 revolutions per minute. All turbines are equipped with shut-down features to avoid damage at high wind speeds.

Performance standards

Offshore wind energy turbines are often situated in some of the roughest and most inhospitable seas, 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 under 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 one or more the steel components.

The Linde Group's Ronald Steusloff describes the tough challenges faced by turbine manufacturers (especially those developing offshore units) as “a new horizon” in the industry's development:

“In the case of standard steel constructions, fully automatic systems for the heat treatment of weld seams have been in place for some time,” says Steusloff, “but for segments of wind turbine towers – steel plates which are up to 4m wide and 12cm 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, which can occur due to hydrogen exposure or internal stress in the 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 any rogue hydrogen atoms in the weld seam. For manufacturers who have to maintain a fast production speed, it is vital that they quickly reach a pre-heated temperature of greater than 100 degrees Celsius”, says Steusloff.

The Lindoflamm concept – which is based upon Linde's acetylene-based burner (Lindoflamm) is addressing 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 in the primary flame. This results in a focused flame, pre-heating only in the weld area, increasing the speed at which weld area is heated.

“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,” explains Steusloff. “In addition, the flame temperatures that can be reached with the associated acetylene-compressed air torch – approximately 2,400 degrees Celsius – 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.”

As a result of Lindoflamm, Linde is one of the pioneers in the field of pre- and post-heating welding applications for offshore wind turbine towers and jackets, based on its collaborative work with steel experts EEW Special Pipe Constructions to develop welding technology for the manufacture of wind farms.

EEW has a long history of manufacturing components for offshore construction in the gas and oil industries, but manufacturing wind farm components for a 30 year lifespan has proved to be an entirely new challenge.

Similar to a domestic stovetop gas burner, the burner that Linde has developed for EEW's Rostock plant has several burner nozzles. But instead of forming a ring, these nozzles are positioned on a 5m pipe, out of which a flame shoots at intervals of a few centimetres. 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 low, in order to reduce complexity in construction.

In collaboration with gas flow experts from the Technical University Dresden, Linde designed a unit assembly system comprising segments for almost any burner length. This design facilitates the steady flow of gas.

For the Rostock plant, the Linde team 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 7m.

“Most companies work on pipes with a maximum diameter of 1.2m. At our Rostock facility, we start at 1.6m”, comments Steusloff.

Both machines are operated fully automatically. This is necessary for the post-weld heating, which should begin immediately after the welding. The new Linde acetylene burners are fitted with several temperature sensors, which regulate the heat precisely. In addition, the burners automatically turn on or off, keeping the temperature within the desired range during the post-heating process.

The benefits of such precise process controls include not only the required quality assurance, but also a more economical use of acetylene as a fuel gas.

“EEW has become our reference customer for this application,” says Linde's Steusloff. “Germany leads the world in researching technology for wind farm component manufacture. Linde set up an applications centre in this country in 2000 to ensure that we remain on the leading edge of this global research and development. The technology we've come up with and which is now being used to manufacture wind turbine tower and jackets at Rostock, is the result of several years of teamwork.

“This is a very young industry and everyone is looking for new standards in offshore wind farm construction. We are thrilled to have acquired long term experience like this as a platform to deliver world class standards for the future.”

 

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