Size matters

Thursday, 26 March, 2015 - 16:30
The quality of a monopile is largely dependent on the welding of the ­individual segments. (Photo: Vattenfall)
The quality of a monopile is largely dependent on the welding of the ­individual segments. (Photo: Vattenfall)

It’s “only” a steel pipe stuck in the seabed. But it is not quite that simple. OWI attempts to unravel the secrets of the monopile. Shaping and welding are the essential steps.

Hollywood in Rostock: in an ­episode of the German detective series “Polizeiruf 110” the TV cops investigate the murder of the boss of an offshore wind energy company. Pipe segments for offshore foundations at the premises of EEW Special Pipe Constructions were part of the set for the filming of the story. For technically interested viewers they were the real stars of the episode. A lot of know‑how goes into the pipe segments – both in the shaping technology and in the welding procedures.

Standard products still work

The pipes used have a diameter of up to ten metres and a wall ­thickness of up to 120 mm. Apart from the logistical problems caused by these large dimensions, the shaping technology required is anything other than ­trivial. There are not many firms that can do this – either the know-how or the equipment for such a power act is not available. The dimensions given above are, however, still the exception. The specifications for wind farms currently being built are around one third smaller than this.

At the end of May the energy companies Vattenfall and ­Stadtwerke München signed a delivery contract with EEW for 72 monopiles and transition pieces for the German offshore wind farm Sandbank. The depth of the water at the site is up to 34 m – the generally held ­opinion is that this is right on the limit for the use of a monopile. With an ­average ­diameter of the steel pipe of 6.5 m the total weight of the ordered monopiles and transition pieces comes to 78,000 t. For EEW the contract has come at the right time, securing the capacity utilisation of the ­production plant until mid 2015.

From a technical point of view this order is the current standard. The constructors generally use a harder steel such as S355 and produce the pipe sections with the same wall thickness throughout. In future, however, they will not get far with this standard, say the ­scientists from the Fraunhofer Institute for ­Manufacturing Engineering and Automation (IPA). The transition to two-figure megawatt capacities has already been initiated by the ­prototype of an 8 MW turbine; a 10 MW machine is on the drawing board.

Wind turbines with a higher ­performance are heavier and sweep a larger area with their rotors. In other words: the loads on the foundations are increasing – and exponentially rather than linearly. This leads the IPA scientists to the conclusion that the quasi-standard currently used by the industry will no longer suffice: foundations for 10 MW turbines are not economically realisable with the current constructions and ­materials.

New monopiles for the industry

The Fraunhofer IPA is therefore working together with the industry partner EEW and the TBI ­Technologie-Beratungs-Institut (Technology Consultancy ­Institute) on the development of high-strength foundation structures for the offshore industry (“HoGfos”). The project comes to a close at the end of 2014. There are already some suggestions for solutions:

• Development of a structurally ­optimised foundation design adapted to local load conditions (“tailored tubes”). An old but useful trick: the manufacturers of high-quality ­racing bicycles with steel frames used tubes that were conical on the inside. The sections of the tubes carrying a high load had walls that were several tenths of a millimetre thicker.

• Use of high-strength fine-grain steel (S690) and new UP-welding technology. “Statistically seen, S690 has a higher elastic limit and tensile strength”, says Robert Hein, Project Engineer at the Fraunhofer IPA. “The notch-rupture strength is unfortunately independent of the steel type, so there is no advantage gained from high-strength steel in this respect.”

• Development of a manufacturing and process technology optimised construction system, a task which has been requested by the in­dustry but so far neglected and, as a side-effect, has a high potential for ­reducing costs.

The quality of a monopile ­essentially depends on the welding of the individual segments. A single faulty welded seam can create enormous problems – not only for the welder but also for the component. Water collecting in defective seams is a common starting point for corrosion. In the longer term this kind of mistake can even cause the structural stability of an offshore wind turbine to become questionable.

Welding, which many ­people only understand as a simple ­“sizzling” procedure, actually requires extremely careful preparation and skilled execution. The components to be welded must be constructed to be suitable for this process and must be mechanically prepared. After setting up the welding equipment, the welders pre-heat the components. Especially with heavy components for the offshore wind energy industry, pre-heating is essential. Without additional heat, the components would not reach the correct welding temperature, which would have a negative effect on the quality of the seams.

Even after welding, a ­temperature of around 100 °C must be maintained for two to three hours. This prevents hydrogen embrittlement, which occurs when hydrogen collects in the matrix of the metal. This leads to hydrogen-induced corrosion with the formation of cracks. The warming of the component also reduces the possibility of cold cracks, which could develop in the component ­after welding.

Heating before and afterwards with a new burner

Linde AG has developed a process under the name Lindoflamm that can be used to heat the components before and after welding. Its central feature is a compressed air burner that is fuelled by acetylene. According to Linde, the special feature is the high performance of the primary flame, which, they say, guarantees preheating in exactly the right spot. “In comparison, for example, to propane, acetylene burns with a very high speed, which leads to a fast and concentrated transfer of heat into the metal”, explains Ronald ­Steusloff, Head of Thermal Applications at Linde, regarding the improvement over the burners previously used. “The flame temperature that can be achieved with an ace­tylene compressed air burner – around 2,400 °C – is significantly higher than the temperatures reached ­using  other gases and compressed air. With the Lindoflamm­ burner the steel can be heated up twice as quickly as with conventional burners­.”

The interesting ­application for the offshore wind energy industry is the use of the burner in the manufacture of monopiles. The welding of the segments takes place automatically. The burner, which Linde developed for EEW in Rostock, has a large number of burner jets located on a 5 m long tube. The distance between the jets is only a few centimetres. One of the problems in designing the burner was to get the same amount of gas to come out of each of the jets. This was necessary to ensure an even distribution of heat at the welding seam. In the meantime, Linde has developed a Lindoflamm burner for EEW for the longitudinal seams and another for circumferential welding. The maximum diameter is 10 m. So Rostock remains a spectacular address for the filming of the next TV series.

Jörn Iken

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