Figure 9 Technician carrying out geotechnical survey in laboratory, courtesy of Igeotest.
Function
Geotechnical site investigations are conducted following the geophysical survey to use the information obtained to target soil and rock strata boundaries, engineering properties and specific sea floor features.
What it costs*
About £5 million for a 1 GW wind farm.
Who supplies them
Suppliers include Fugro, G-tec, Gardline, Geoquip Marine, Horizon. OWC, Rydar Geotechnical and Sulmara.
Key facts
Geotechnical studies are predominantly intrusive and include such methods as boreholes with soil/rock sampling, and cone penetration testing (CPT).
Geotechnical investigation is generally the most expensive part of wind farm survey work, making it a substantial at-risk investment for developers. Typically the geotechnical surveys are performed in phases to add value to the project risk mitigation process.
Geotechnical surveys require specialised equipment and skilled personnel. The scope of the investigation depends on the type of foundation being considered and the variability in the sea bed characteristics.
Boreholes and CPTs to depths in the order of 50-70 m are carried out to investigate the physical characteristics of the sea bed. Surface push CPTs are also used as a rapid method to gather sea bed soil stratigraphy. Cable routes are typically investigated using vibrocores and CPTs to a depth of 5 m.
Offshore laboratories are used to obtain basic soil parameters and the samples taken are then returned to an offshore laboratory for detailed testing. Often soil dynamics tests are performed to monitor the soil behaviour under the constant dynamic loading on the foundation by the wind, waves and current.
Resultant data from the geotechnical surveys are combined with results of the geophysical survey, to improve the geological model prior to the design and installation of foundations. Geotechnical data is also used at a later date in combination with heavy lift jack-up vessel information to determine the risks and feasibility of conducting heavy lift construction activities.
Fixed offshore wind farms require geotechnical data to depths of 50 to 70 m to inform the design of monopile or jacket foundations.