It is generally assumed that fatigue strength decreases with an increasing wall thickness. To estimate the influence of wall thickness more exactly, however, there are different approaches depending on the national standards and the type of construction, showing a wide difference in the reduction factor.
When using high strength steel, smaller wall thicknesses usually suffice. Due to the fact that weld volume and weight can be reduced, especially structures with many welds become more economical.
In the context of the exploitation of alternative energies, many offshore wind parks are planned in the North Sea, Baltic Sea and Irish sea. Aggravated conditions such as large water depths, increasing heights of towers and hubs, as well as other environmental conditions require new solutions concerning the stability and the durability of offshore wind energy converters.
According to the current standards, the choice of the steel grade does not have an influence on the fatigue resistance. Now, the wall thickness can be reduced by the use of high strength steel. This has to be taken into account also with regard to different methods of post weld treatment which can extend the remaining fatigue life of a construction considerably.
A new research project has been brought into life, to address specific problems on stability and durability of such structures and develop new solutions. This includes the investigation of the influence of steel grade and wall thickness, the assessment of the remaining life and the problems involved with cylindrical and conical connections. Another important aspect is the modification of the existing fatigue design concept to the special requirements of offshore wind energy converters.
The capacity of the new generation of offshore wind energy converters goes up to 5 MW and several hundred of these converters are planned per park.
