Pilot projects on serviceability limit considerations for offshore wind turbines - Geotechnical in-situ measurements and supplementary pilot projects on serviceability limit considerations for establishing offshore wind turbines
Technische Universität Braunschweig
Prof. Dr. Joachim Stahlmann
The interaction between the seabed and the dynamically stressed foundation element by wave and wind power are of fundamental importance for the dimensioning and stability of the wind turbine. Slight misalignments of the installation can lead to damaged bearings on the windmill.
At present there are only a few studies available on the establishment of offshore turbines a long distance from the coast in terms of their load-bearing behaviour and load impact.
In the case of FINO3 this interaction is being recorded metrologically for the first time on the whole 30-meter length over which it is embedded in the seabed. The aim of the measurements is to research the mechanical and dynamic phenomena in the seabed, in order to clarify the question as to what mechanisms must be assumed to be influential for investigating and proving stability.
134 geotechnical sensors were installed for this purpose on the monopile of the platform. The following parameters were recorded: total tensions and pore water pressures in the border area between subsoil and pillar, the changes in pillar inclination and accelerations and the sequence of forces in the pillar during driving and in the operating phase. The measurements started at the end of July 2008 and will be continued in the coming three years of the operating phase.
- acceleration and elongation/compression during the driving process
- changes in vertical bearing strength over depth
- progress of casing friction over the length of the bond
- peak pressure at the foot of the pillar
- size of foundation in a horizontal direction
- deformations in a horizontal direction
- crater depth
- temperature at every measuring point
Demands on metrology
- robustness of sensors in withstanding high pillar-driving energy
- careful welding work at the measuring stations and cable protection channels to ensure water-tightness and prevent destruction of cables and sensors
- very brief installation times for metrology as a whole → pre-assembly
- installation ashore → protection during transportation and installation
- high frequency measurement during driving (> 7 kHz, subsequently 1 Hz)
- minimal possible power consumption by all sensors and data loggers/PC
- low maintenance
Results are only available on the German website